Page 1 of 4 1234 LastLast
Results 1 to 35 of 116

Thread: My first build - AB763 Deluxe Reverb clone - opinions please

  1. #1
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2

    My first build - AB763 Deluxe Reverb clone - opinions please

    Hello guys, my name is Greg. I've recently built my very first amp, as the title says, and overall I'm very pleased with it and all of it's functions work as they should. It's a Weber 6A20 kit - built as a head unit. I'm using a ClassicTone 40-18087 OT. Otherwise a stock type build.

    My only issue is a bit of "brittle" break up right at the attack of a note, it sustains for a sec, then the brittleness goes away as the note sustains out.

    I've heard this sound on other AB763 type amps, but mine seems more brittle and a little more pronounced.

    It happens on either channel, normal or vibrato, reverb on or off, tremolo on or off, no matter the mic or speaker used. It's always there.

    Here is a soundclip running through the amp volume, guitar volume, reverb, and tremolo. You can clearly hear the brittle breakup, it almost sounds like a faint static on the attack of the notes. Sorry for the length and size of the clip, but I wanted to provide a high-quality example with lots of variation. Please download the wav file and listen and comment on what you think this may be.

    This sound clip was done using a Gibson SG with a P90 bridge pickup through a Marshall 4x12 cab with vintage Celestion G12-65s.
    https://jmp.sh/KXmIVIV

    Other than that, I'm very pleased with the amp and happy with the outcome for it being my first build. It sounds great overall, it's just got that hint of brittle breakup all the time.

    Any pics or specs you may need to help decipher this issue will be readily available upon request. I have tons of pics and pin voltages and all that stuff. I even have a scope at the ready - I'm not very good with it, but I'll give it a shot. Haha.

    Thanks for your time and I genuinely appreciate any guidance you can give me.

    0 Not allowed! Not allowed!

  2. #2
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    Can you post your power tube DC voltages at idle (no signal). Do you have a means of checking idle current of power tubes?

    1 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  3. #3
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by g1 View Post
    Can you post your power tube DC voltages at idle (no signal). Do you have a means of checking idle current of power tubes?
    Sure thing.....

    Power Tubes
    Tung Sol 6V6GT


    V7
    1: NA
    2: H
    3: 435
    4: 435
    5: -42.0
    6: 436
    7: H
    8: 19.5 mv - measured at 1 ohm resistor to ground

    V8
    1: NA
    2: H
    3: 435
    4: 436
    5: -42.2
    6: 436
    7: H
    8: 19.5 mv - measured at 1 ohm resistor to ground

    0 Not allowed! Not allowed!

  4. #4
    Senior Member
    Join Date
    Sep 2014
    Location
    se texas
    Posts
    172
    Thumbs Up/Down
    Received: 20/0
    Given: 7/0
    Rep Power
    6
    Hi, first I will say congrats that amp sounds pretty good to me, much of what you hear IMO is those speakers.
    try it with some real greenbacks you will be surprised.

    2 Not allowed! Not allowed!

  5. #5
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by dstrat View Post
    Hi, first I will say congrats that amp sounds pretty good to me, much of what you hear IMO is those speakers.
    try it with some real greenbacks you will be surprised.
    Thank you very much. I have some Greenbacks, it's pretty much the same thing no matter what speakers or mics I use.

    The sound doesn't really bother me too much because it doesn't show up in a mix or live, but it's eating at me because it just doesn't seem right. I've done everything in my power to do this build right. Everything else is great. Just that little brittleness has me scratching my head.

    0 Not allowed! Not allowed!

  6. #6
    Senior Member
    Join Date
    Sep 2014
    Location
    se texas
    Posts
    172
    Thumbs Up/Down
    Received: 20/0
    Given: 7/0
    Rep Power
    6
    um ya could be some slight oscillation which would come down to layout, grid wires (p2 - p7) on ax7 tubes can be fussy about how close they are to other higher level signal carrying wires. but I can only guess at this point. maybe a clear picture of the tube wiring etc may help.
    there are lots of knowledgeable folks here that will likely have better ideas then me. only spoke about the G12-65 speakers because they always seem brittle to my ears. crank it up and play it will smooth out over some time playing it also.

    yeah I do like the P90s!

    1 Not allowed! Not allowed!
    Last edited by dstrat; 06-30-2018 at 04:55 AM. Reason: comment

  7. #7
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by dstrat View Post
    um ya could be some slight oscillation which would come down to layout, grid wires (p2 - p7) on ax7 tubes can be fussy about how close they are to other higher level signal carrying wires. but I can only guess at this point. maybe a clear picture of the tube wiring etc may help.
    there are lots of knowledgeable folks here that will likely have better ideas then me. only spoke about the G12-65 speakers because they always seem brittle to my ears. crank it up and play it will smooth out over some time playing it also.

    yeah I do like the P90s!
    Thanks. My 65s are from 1980, so they should be broken in by now. Haha. The amp is brand spanking new though. I've been beating on it, but it still has that "new amp smell". I just finished it a few weeks ago.

    I suppose oscillation is a possibility. I've had it on my scope and the signal path looks normal to me from input all the way to speaker output, but I admit I'm not very well-versed with using oscilloscopes. I just got it pretty recently.

    As for the grid wires, I *think* they're pretty good. My heaters are done "Marshall style" with a bit of Valve Wizard thrown in, so there's no criss-crossing with signal and heater wiring. The layout of the board in relation to the tube sockets means I can't avoid a few wires crossing here and there. But over all the amp is very very stable and quiet. It's quieter than any of my Marshalls. And original Fenders looked like pure hell inside, so I feel okay about mine. Hahahaha.

    Anyway here are some pics.











    1 Not allowed! Not allowed!

  8. #8
    Senior Member
    Join Date
    Sep 2014
    Location
    se texas
    Posts
    172
    Thumbs Up/Down
    Received: 20/0
    Given: 7/0
    Rep Power
    6
    over all everything looks ok , I didn't look real close , this time of night my eyes are shot.
    give it some time it will mellow over time if played it regularly.

    are those tubes new ones or are like this real tung-sol? just curious
    Click image for larger version. 

Name:	tung-sol.png 
Views:	44 
Size:	895.9 KB 
ID:	49551.

    I have tried to cook this tube but it just keeps kicking.

    yeah you did a very nice job with that amp. I would be very happy!

    1 Not allowed! Not allowed!

  9. #9
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by dstrat View Post
    over all everything looks ok , I didn't look real close , this time of night my eyes are shot.
    give it some time it will mellow over time if played it regularly.

    are those tubes new ones or are like this real tung-sol? just curious
    Click image for larger version. 

Name:	tung-sol.png 
Views:	44 
Size:	895.9 KB 
ID:	49551.

    I have tried to cook this tube but it just keeps kicking.

    yeah you did a very nice job with that amp. I would be very happy!
    Thanks a lot!

    Mine are new production Tung Sols. Honestly, I'm not that into NOS vintage tubes. *Ducks for cover*

    I know they're nice and all, I have a few nice old 12AX7s, but my amps get ridden hard and for me new stuff seems to be fine....and cheaper. I blew up some TAD EL34s in a 50W Plexi cranked to the moon - like really blew them up. It was quite the light show. But overall I've had good luck with regular production tubes.

    0 Not allowed! Not allowed!

  10. #10
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,767
    Thumbs Up/Down
    Received: 2,607/4
    Given: 3,063/0
    Rep Power
    30
    Looks and sounds right to me If you wanted a darker sounding amp you should have built a different model. DR's aren't known to be "mellow"

    Here's a trick I use to kill the glassies on one model I build. Connect a 4.7pf/1000V ceramic cap from the plate to the grid on each input triode. Don't go larger or you will hear the affect on the overall EQ. Try it on one channel first (obviously) to see if it gives you what you want. It shaves off the shards without really killing the top end. Further, the effect is relative to gain. So the more you attack the more it works. Worth a try.

    It's worth noting that most really good players like that glassy top end and they alter their attack to suit what they're trying to do. Removing it for such players is limiting. YMMV. And FWIW I have the 4.7pf cap on my own amp (I'm not a really good player )

    2 Not allowed! Not allowed!
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  11. #11
    Senior Member
    Join Date
    Sep 2014
    Location
    se texas
    Posts
    172
    Thumbs Up/Down
    Received: 20/0
    Given: 7/0
    Rep Power
    6
    oh I just have a few old ones laying around I really like the sound of (compared to newer ones) but hey we have to use what we can get now days , I say don't worry be happy!

    3 Not allowed! Not allowed!

  12. #12
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by Chuck H View Post
    Looks and sounds right to me If you wanted a darker sounding amp you should have built a different model. DR's aren't known to be "mellow"

    Here's a trick I use to kill the glassies on one model I build. Connect a 4.7pf/1000V ceramic cap from the plate to the grid on each input triode. Don't go larger or you will hear the affect on the overall EQ. Try it on one channel first (obviously) to see if it gives you what you want. It shaves off the shards without really killing the top end. Further, the effect is relative to gain. So the more you attack the more it works. Worth a try.

    It's worth noting that most really good players like that glassy top end and they alter their attack to suit what they're trying to do. Removing it for such players is limiting. YMMV. And FWIW I have the 4.7pf cap on my own amp (I'm not a really good player )
    Hey thanks for the tip!

    I like bright. That's no problem. I do not want "dark". I just wasn't sure if that little bit of brittleness in my sound is supposed to be there or not. I'm a life-long Marshall guy and this is actually my first Fender type amp. I played a lot of Fenders in stores before I decided the Deluxe Reverb is what I wanted to build.

    This amp will primarily be used for semi-aggressive surf/rockabilly type tones. There will be lots of reverb and tremolo action happening with this thing!

    So for those caps, I will give it a try and see what happens. I've done something similar with high-gain mods on Marshalls to kill oscillation. Do they really need to be 1000v though? There's only ~200v on the plates. Just curious.

    1 Not allowed! Not allowed!

  13. #13
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,767
    Thumbs Up/Down
    Received: 2,607/4
    Given: 3,063/0
    Rep Power
    30
    500V would be fine. I suggest 1000V because you REALLY don't want that cap to ever fail short and apply HV to the input grid (which is connected to your guitar!!!).

    2 Not allowed! Not allowed!
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  14. #14
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by Chuck H View Post
    500V would be fine. I suggest 1000V because you REALLY don't want that cap to ever fail short and apply HV to the input grid (which is connected to your guitar!!!).
    Ha right. Okay thank you.

    I'll go pick up some caps tomorrow and see what happens.

    1 Not allowed! Not allowed!

  15. #15
    Member Emeritus Forever Steve A.'s Avatar
    Join Date
    May 2006
    Posts
    4,506
    Thumbs Up/Down
    Received: 129/0
    Given: 77/0
    Rep Power
    18
    Quote Originally Posted by Greg_L View Post
    Mine are new production Tung Sols. Honestly, I'm not that into NOS vintage tubes. *Ducks for cover*
    If you search around the forum here you will learn that no one still makes 6V6s anymore... the current supply are basically military tubes that have been tested and do work in Deluxe Reverbs, etc. You might see what is recommended here for DRs these days.

    While NOS tubes can be very pricey there are some sellers on eBay that sell used and tested old stock tubes, usually with a 30 day exchange period.

    Steve A.

    P.S. The head cabinet for your DR looks really cool. I wonder if it would fit my 2006 DRRI chassis...

    P.P.S. So how many hours have you run your amp? It seems to me that they mellow out a bit with time... perhaps the caps? Or maybe I just get tuned in and pick softer...

    You got this covered with your original from 1980 but Warehouse Guitar Speakers makes an ET65 Celestion clone that sells for $80. GC & MF sell the 8 & 16 ohm varieties with free shipping which is really cool since WGS's shipping charges have always been a deal killer for me.

    Not that I tend to go overboard (I do! I do!) but I've ordered 4 of them since discovering them last year — best.speaker.ever!

    3 Not allowed! Not allowed!
    Last edited by Steve A.; 06-30-2018 at 02:42 PM.

  16. #16
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by Steve A. View Post
    If you search around the forum here you will learn that no one still makes 6V6s anymore... the current supply are basically military tubes that have been tested and do work in Deluxe Reverbs, etc. You might see what is recommended here for DRs these days.

    While NOS tubes can be very pricey there are some sellers on eBay that sell used and tested old stock tubes, usually with a 30 day exchange period.

    Steve A.
    Interesting, thank you for chiming in. I'll look into that.

    P.S. The head cabinet for your DR looks really cool. I wonder if it would fit my 2006 DRRI chassis...
    Ha thanks. It's a Weber head cab. My wife chose the purple, I chose the Marshall grill cloth because ultimately I'm a Marshall kind of guy. I've since put a purple jewel over the pilot light and it looks pretty cool.

    P.P.S. So how many hours have you run your amp? It seems to me that they mellow out a bit with time... perhaps the caps? Or maybe I just get tuned in and pick softer...
    The amp is really really new. I've only put...I don't know...maybe few hours in total on it in little bursts. Did you hear my sound clip? If so, do you think that brittle sound is a normal it-just-needs-be-broken-in type of thing...or is it something I need to address? On the first start-up, I brought it up really slowly on a variac and light bulb limiter to "form the caps". Besides one of my own boneheaded mistakes right off the bat (I had the rectifier socket in backwards) the amp came alive and has worked flawlessly....except for this little brittle sound thing.

    You got this covered with your original from 1980 but Warehouse Guitar Speakers makes an ET65 Celestion clone that sells for $80. GC & MF sell the 8 & 16 ohm varieties with free shipping which is really cool since WGS's shipping charges have always been a deal killer for me.

    Not that I tend to go overboard (I do! I do!) but I've ordered 4 of them since discovering them last year — best.speaker.ever!
    I'm a huge fan of WGS speakers. Love them. And I love their business model. What's not to love about good quality and good prices? I only have two myself - Green Berets. They compare okay with real Greenbacks. If I'm really being picky, they're a little more mid-scooped and bright than real Greenbacks, but in a 4x12, in a room, turned up loud, they sound correct.

    I have lots of experience with other WGS speakers though through friends and bands that have come through my little studio. The Veteran 30s, Retro 30s, ET-65s,...all of them. Every WGS I've ever mic'd has been pretty nice.

    I've never tried any of their "American" speakers though. My plan all along was to build a Fender style head to be used with Marshall cabs. I've got an empty cab out in the garage. Maybe I need to make an "American" cab for this DR clone head.

    1 Not allowed! Not allowed!

  17. #17
    Member Emeritus Forever Steve A.'s Avatar
    Join Date
    May 2006
    Posts
    4,506
    Thumbs Up/Down
    Received: 129/0
    Given: 77/0
    Rep Power
    18
    Quote Originally Posted by dstrat View Post
    oh I just have a few old ones laying around I really like the sound of (compared to newer ones) but hey we have to use what we can get now days , I say don't worry be happy!
    Always a good idea to have some tubes around that we KNOW sound good...

    Steve A.

    3 Not allowed! Not allowed!

  18. #18
    Supporting Member tubeswell's Avatar
    Join Date
    Feb 2007
    Location
    Wellington NZ
    Posts
    4,202
    Blog Entries
    1
    Thumbs Up/Down
    Received: 133/0
    Given: 45/0
    Rep Power
    18
    I think I can hear what you’re describing, but only when I listen to your soundclip with headphones.

    A couple of possibilities spring to mind.
    1) The OT you’re using might be the cause. Either try another 25W 6k6 OT (or at least try a couple of feedback tricks to tame the ‘ice pick’ - like a 100pF speed-up cap in parallel with the 820R NFB resistor, or 220pF grid to cathode on each 6V6.)
    2) lead dress -try separating some of the pre amp socket leads a bit more - they look a bit bunched up with different sockets’ leads overlapping/right next to each other
    3) I take it you’re using a 12AT7 in the LTP?

    2 Not allowed! Not allowed!

  19. #19
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by tubeswell View Post
    I think I can hear what you’re describing, but only when I listen to your soundclip with headphones.

    A couple of possibilities spring to mind.
    Yes, the brittle sound is subtle, and it doesn't show up in a recorded mix or live in the room. I didn't even notice it until I solo'd some studio tracks and I was like wtf is that? The amp sounds great overall IMO, but that little noise caught my attention and now I'm obsessed with it.

    1) The OT you’re using might be the cause. Either try another 25W 6k6 OT (or at least try a couple of feedback tricks to tame the ‘ice pick’ - like a 100pF speed-up cap in parallel with the 820R NFB resistor, or 220pF grid to cathode on each 6V6.)
    I chose this Classic Tone 40-18087 OT for the multi-taps. I want to use this head with 16 ohm Marshall cabs. It's *supposed* to be the one for this sort of thing, but you might be right. Replacing the OT will be a last-resort kind of thing for me. I've seen some of the negative feedback tricks and I do have those on my radar if the simpler stuff doesn't work first.

    2) lead dress -try separating some of the pre amp socket leads a bit more - they look a bit bunched up with different sockets’ leads overlapping/right next to each other
    This is certainly doable. Poking around last night with a chopstick with the amp on a scope, I found that the bridge connecting the cathodes on my PI needs to be re-flowed. It's cutting out and crackly with the chopstick test. That's something that will be addressed today. I also found that my V4 tube was buggy. The heaters in V4 went out while gently poking around and never came back. V1, V2, and V3 went dark. I replaced V4 and things came back to life. The layout doesn't really lend itself to really clean lead dress on some tube sockets, but I will definitely try to improve on it. One positive is that the amp is very quiet at idle. So I at least don't have a bunch of heater hum coming through.

    3) I take it you’re using a 12AT7 in the LTP?
    Yes sir. JJ 12AT7.

    Thanks for your input!

    1 Not allowed! Not allowed!

  20. #20
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,454
    Thumbs Up/Down
    Received: 163/0
    Given: 2,485/0
    Rep Power
    4
    I did the same as you, first amp build AB763 Deluxe, started about a year ago, got sound out of it late last year. For various reasons (suggestions from friends/family), I have a Celestion Black Shadow C90 - 90 Watt - 12" speaker. When I first got it running, it was brutally 'ice pick' bright. I could not stand listening to it. I had some wiring/lead dress problems causing oscillations, cleaned that up (took two significant efforts), and it improved slightly. But a big change came from just playing the thing. It just mellowed out after some number of hours of use. The cap mfr said "it will take something like 100 hours to .." fully break in. Im no where near that now (maybe 50 or 60), but the amp is a lot more mellow. Maybe partly due to the beating I gave the output tubes when diagnosing the oscillation problems. Some suggested that the speaker loosening up helped as well. I wish I had good A/B recordings to show you the difference (all same tubes, same speaker, just playing and some wiring changes).

    2 Not allowed! Not allowed!

  21. #21
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by mikepukmel View Post
    I did the same as you, first amp build AB763 Deluxe, started about a year ago, got sound out of it late last year. For various reasons (suggestions from friends/family), I have a Celestion Black Shadow C90 - 90 Watt - 12" speaker. When I first got it running, it was brutally 'ice pick' bright. I could not stand listening to it. I had some wiring/lead dress problems causing oscillations, cleaned that up (took two significant efforts), and it improved slightly. But a big change came from just playing the thing. It just mellowed out after some number of hours of use. The cap mfr said "it will take something like 100 hours to .." fully break in. Im no where near that now (maybe 50 or 60), but the amp is a lot more mellow. Maybe partly due to the beating I gave the output tubes when diagnosing the oscillation problems. Some suggested that the speaker loosening up helped as well. I wish I had good A/B recordings to show you the difference (all same tubes, same speaker, just playing and some wiring changes).
    Thanks for the input. You may be right - I just haven't played it enough. I've given it some hard use, but it's been in short blasts. Basically as soon as I fired it up and verified that my voltages are in range and nothing blew up and all of the features worked, I put it to work in the studio. It definitely has not had anywhere near 100 hours of actual playing time on it.

    0 Not allowed! Not allowed!

  22. #22
    Member Emeritus Forever Steve A.'s Avatar
    Join Date
    May 2006
    Posts
    4,506
    Thumbs Up/Down
    Received: 129/0
    Given: 77/0
    Rep Power
    18

    Some mods for your amp...

    Quote Originally Posted by Greg_L View Post
    I chose this Classic Tone 40-18087 OT for the multi-taps. I want to use this head with 16 ohm Marshall cabs. It's *supposed* to be the one for this sort of thing, but you might be right. Replacing the OT will be a last-resort kind of thing for me. I've seen some of the negative feedback tricks and I do have those on my radar if the simpler stuff doesn't work first.
    I don't know if this is still applicable (or would apply to the regular OT from the Weber kit) but the unofficial rule of thumb with BF/SF amps was that you could go up or down by a factor of two when matching speakers to OT impedance. So with a nominal output transformer impedance of 8 ohms using a 4 or 16 ohm cab should be "good enough for rock'n'roll". In any case it never bothered me...

    I heard that there were a lot of Marshall OTs dying back in the 60s and 70s but that might have been caused by faulty impedance selector switches more than impedance mismatches.

    As for "negative feedback tricks" do you mean conjunctive filters or something else? Actually for an AB763-ish amp adding a 25k pot in series with the 820R negative feedback resistor is a really slick mod for opening up the sound a bit. Adding a 25k pot in series to ground with the 6K8 "midrange" resistor in the tone stacks is another cool mod. (You did try removing the 47pf bright cap on the volume pot, right?) For a nice midrange boost adding a 500pF to 1000pF cap across the 250pF treble cap is cool (my favorite is switching a 390pF cap across a 330pF treble cap, silver mica if you have 'em.)

    Adding a Vox AC-30 style Cut control after the 0.1uF PI coupling caps is another cool mod if you wire it up to a switched pot to remove it from the circuit completely when not needed. Or go hogwild with a PPI master volume control to match the output to the room...

    Steve A.

    1 Not allowed! Not allowed!
    Last edited by Steve A.; 07-02-2018 at 08:11 PM.

  23. #23
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by Steve A. View Post
    I don't know if this is still applicable (or would apply to the regular OT from the Weber kit) but the unofficial rule of thumb with BF/SF amps was that you could go up or down by a factor of two when matching speakers to OT impedance. So with a nominal output transformer impedance of 8 ohms using a 4 or 16 ohm cab should be "good enough for rock'n'roll". In any case it never bothered me...
    You're probably right but I didn't want to chance it.
    This is the OT I used:
    http://www.classictone.net/40-18087.html
    http://www.classictone.net/40-18087.pdf

    "Description: This is a 4/8/16 Ohm version of the classic! This is a nice, beefy, taller upgrade version of the original Deluxe / Deluxe Reverb output transformer but still has the same mounting centers for easy, drop in installation in these great classic amps. This is also a great general purpose 4/8/16 Ohm output transformer for many 20W or "so" type new OEM amp builds and D.I.Y. amp projects as well. Furthermore, it features an attractive black finish. Like the vintage era originals, it is paper layer wound for that ultimate tone!"

    I heard that there were a lot of Marshall OTs dying back in the 60s and 70s but that might have been caused by faulty impedance selector switches more than impedance mismatches.
    Marshalls had that little impedance selector plug that would fall out and then boom bye bye OT. I have a 73 Super Lead that still has the original impedance plug, and it scares the hell out of me sometimes, but I very rarely use anything other than 16 ohm cabs with that particular head, so it's set there and stays there. That head also does't get taken to gigs, so it's use is pretty much always at home in the studio and it never moves.

    As for "negative feedback tricks" do you mean conjunctive filters or something else? Actually for an AB763-ish amp adding a 25k pot in series with the 820R negative feedback resistor is a really slick mod for opening up the sound a bit. Adding a 25k pot in series to ground with the 6K8 "midrange" resistor in the tone stacks is another cool mod. (You did try removing the 47pf bright cap on the volume pot, right?) For a nice midrange boost adding a 500pF to 1000pF cap across the 250pF treble cap is cool (my favorite is switching a 390pF cap across a 330pF treble cap, silver mica if you have 'em.)

    Steve A.
    Those sound cool, thanks. More ideas. There are several little tweaks that I've thought about implementing, but I want to make sure the amp is 100% healthy and broken in first. The negative feedback mod I was referring to is simply strapping a 100pf cap across the 820R feedback resistor. It's supposed to allow super highs to bypass the resistor and get used in the negative feedback signal, thus cancelling themselves out. But first I'm going to try some different tubes and I've fixed my PI soldering. As it turns out, one leg of the little cathode jumper didn't get soldered with the cathode wire on pin 8. The wire itself got soldered, but the jumper didn't get melted in with it. Doh! Operator error. It was just laying in there making contact purely by chance. But that's fixed now and I'm going to give it a go and see what happens now.

    0 Not allowed! Not allowed!

  24. #24
    Member Emeritus Forever Steve A.'s Avatar
    Join Date
    May 2006
    Posts
    4,506
    Thumbs Up/Down
    Received: 129/0
    Given: 77/0
    Rep Power
    18
    Quote Originally Posted by mikepukmel View Post
    But a big change came from just playing the thing. It just mellowed out after some number of hours of use. The cap mfr said "it will take something like 100 hours to .." fully break in. Im no where near that now (maybe 50 or 60), but the amp is a lot more mellow. Maybe partly due to the beating I gave the output tubes when diagnosing the oscillation problems.

    Some suggested that the speaker loosening up helped as well. I wish I had good A/B recordings to show you the difference (all same tubes, same speaker, just playing and some wiring changes).
    I run practically all of my amp builds into the same old speakers and noticed that some of them really smooth out the more I play them. As for breaking in new speakers I read that some boutique builders wire them up to 24vac transformers for 12 to 36 hours... after using Ohm's Law to insert a series resistor make sure that they don't exceed rated wattage.

    Steve A.

    0 Not allowed! Not allowed!

  25. #25
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    Re: heater issue. Probably not related to the sound issue, but you mentioned V1 thru V4 heaters went out but not the others? If so I suspect a connection at V4 socket rather than the tube itself.
    A bad tube that was responsible for pulling down other pre tube heaters should have taken down the power tube heaters as well.

    1 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  26. #26
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by g1 View Post
    Re: heater issue. Probably not related to the sound issue, but you mentioned V1 thru V4 heaters went out but not the others? If so I suspect a connection at V4 socket rather than the tube itself.
    A bad tube that was responsible for pulling down other pre tube heaters should have taken down the power tube heaters as well.
    Okay good to know. Thanks. I'll revisit the heater wiring on V4.

    0 Not allowed! Not allowed!

  27. #27
    Supporting Member tubeswell's Avatar
    Join Date
    Feb 2007
    Location
    Wellington NZ
    Posts
    4,202
    Blog Entries
    1
    Thumbs Up/Down
    Received: 133/0
    Given: 45/0
    Rep Power
    18
    You say chopsticking some joints showed problems. Definitely give those bad solder joints a good reflow - they won’t be helping it sound good. Also, Keep the joints clean by making sure the iron tip is clean.

    1 Not allowed! Not allowed!

  28. #28
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by tubeswell View Post
    You say chopsticking some joints showed problems. Definitely give those bad solder joints a good reflow - they won’t be helping it sound good. Also, Keep the joints clean by making sure the iron tip is clean.
    Just did that. Reflowed a few pins, got everything nice and secure. Chopsticking shows no flutter or weirdness now. That much of it is good to go at this point.

    So here's what I've done:
    All new tubes - all of the chinese came-with-the-kit tubes are replaced.
    Reflowed a few bad solders - all that's good.
    Checked for oscillation to the best of my ability - no problems found there.
    Chopsticking the lead dress showed no change.
    Heated up the bias a little bit to 23ma.

    Tone test - still the same. Lol.

    The overall tone is indeed better. The tubes made a difference. But that brittleness is still there. Here's what I've found in addition to the previous stuff:

    The brittleness I'm hearing is still on each channel, with each input. Bright switches on or off. Nothing has changed there.
    With V3 reverb driver completely removed, the brittleness is still there. V3 has no effect in or out.
    The brittleness is the same with the treble rolled all the way off, or all the way up. The tone gets darker and brighter as expected with the treble control, but the level and amount of that brittle sound is unchanged lurking within the base tone.
    With a parametric EQ in the DAW I've kind of narrowed the noise to around the 3.2khz range. That's where the sound seems the most pronounced.

    So at this point, I'm going to try the cap across the negative feedback resistor and see what happens..

    0 Not allowed! Not allowed!

  29. #29
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    More update:

    The 100pf cap across the negative feedback resistor yielded no change.
    I also tried the "copper cap" which is a solid-state rectifier tube replacement. No change.

    0 Not allowed! Not allowed!

  30. #30
    Supporting Member tubeswell's Avatar
    Join Date
    Feb 2007
    Location
    Wellington NZ
    Posts
    4,202
    Blog Entries
    1
    Thumbs Up/Down
    Received: 133/0
    Given: 45/0
    Rep Power
    18
    You could try going to a 220pF cap.

    Other than that, my hunch is it’s likely to be OT or lead dress related.

    If it is HF oscillation caused by inter winding capacitance in that OT, you could try one more thing which is slugging the dominant FB loop pole with 220pF grid to cathode on each 6V6.

    0 Not allowed! Not allowed!

  31. #31
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by tubeswell View Post
    You could try going to a 220pF cap.

    Other than that, my hunch is it’s likely to be OT or lead dress related.

    If it is HF oscillation caused by inter winding capacitance in that OT, you could try one more thing which is slugging the dominant FB loop pole with 220pF grid to cathode on each 6V6.
    Well check this out and tell me what you think....


    That's V2b grid. Vibrato channel, vol knob around 5-6 is where this happens. I'm injecting a 1k sine about 300 mv pk to pk into input 1, and that's the weirdness I'm getting on the 2nd stage.

    Here's a little video sweeping the volume knob on V2.



    What do yall think of that kink in my sine wave?

    That does not happen on the normal channel.

    This is probably not related to my brittle noise, but it caught my attention anyway.

    1 Not allowed! Not allowed!

  32. #32
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    6,459
    Thumbs Up/Down
    Received: 1,308/1
    Given: 1,007/1
    Rep Power
    16
    I see what appears to be high frequency noise/oscillation riding on the sine wave. Some generators are not that "pure". You might scope the generator output and make sure it's clean. That said, I've got a '68 Custom Deluxe Reverb Reissue in the shop now that's doing the same thing. I'll be following along and when I get to it, I'll post anything helpful I come up with.

    1 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  33. #33
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by The Dude View Post
    I see what appears to be high frequency noise/oscillation riding on the sine wave. Some generators are not that "pure". You might scope the generator output and make sure it's clean. That said, I've got a '68 Custom Deluxe Reverb Reissue in the shop now that's doing the same thing. I'll be following along and when I get to it, I'll post anything helpful I come up with.
    Thanks, that would be awesome.

    My "generator" is actually an app on my phone. I adjust the volume and measure the output before I put it into the amp. I don't get that kink on the normal channel though. The signal through the normal channel stays pretty pure throughout the gain stages. I only get that kink on the vibrato's first gain stages and it's only halfway through the volume sweep. At low volume, the signal stays intact. And at max volume the signal is again a smooth sine wave. It's just in the middle it gets goofy.

    0 Not allowed! Not allowed!

  34. #34
    Member
    Join Date
    Aug 2015
    Location
    Edmonton, AB
    Posts
    259
    Thumbs Up/Down
    Received: 232/3
    Given: 120/3
    Rep Power
    5
    Quote Originally Posted by Greg_L View Post
    Thanks, that would be awesome.

    My "generator" is actually an app on my phone. I adjust the volume and measure the output before I put it into the amp. I don't get that kink on the normal channel though. The signal through the normal channel stays pretty pure throughout the gain stages. I only get that kink on the vibrato's first gain stages and it's only halfway through the volume sweep. At low volume, the signal stays intact. And at max volume the signal is again a smooth sine wave. It's just in the middle it gets goofy.
    I'm just spitballin' here...

    I have a 1970 Traynor YGM III that had similar looking sine wave in the middle of the vol pot sweep.
    Turned out to be the reverb circuit.
    If I disconnected the tank leads it cleaned up.
    I don't remember from earlier in the thread if you tried that?

    Edit; ahhh NVM, I see you pulled the reverb tube.

    In my case it was reverb circuit oscillating.

    2 Not allowed! Not allowed!
    The world is full of people that are right.

  35. #35
    Member
    Join Date
    Jun 2018
    Posts
    175
    Thumbs Up/Down
    Received: 96/0
    Given: 156/2
    Rep Power
    2
    Quote Originally Posted by galaxiex View Post
    I'm just spitballin' here...

    I have a 1970 Traynor YGM III that had similar looking sine wave in the middle of the vol pot sweep.
    Turned out to be the reverb circuit.
    If I disconnected the tank leads it cleaned up.
    I don't remember from earlier in the thread if you tried that?
    No I haven't, but I sure will. Thanks!

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Weber 6A20 - AB763 Deluxe Reverb build - completed.
    By Greg_L in forum Debugging Your Build
    Replies: 10
    Last Post: 07-02-2018, 01:28 PM
  2. Replies: 221
    Last Post: 12-25-2017, 10:03 PM
  3. Adding NFB control to AB763 Deluxe Reverb build
    By Stratz in forum Theory & Design
    Replies: 14
    Last Post: 05-03-2017, 04:54 AM
  4. AB763 Deluxe reverb clone - Ugly break up
    By el_fela in forum Build Your Amp
    Replies: 47
    Last Post: 10-08-2013, 09:37 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •