My first build - AB763 Deluxe Reverb clone - opinions please

[Greg_L]

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.

[Steve A.]

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.

[tubeswell]

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?

[Greg_L]

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!

[mikepukmel]

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).

[Greg_L]

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.

[Steve A.]

Some mods for your amp...

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.

[Greg_L]

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.

[Steve A.]

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.

[g1]

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.

[Greg_L]

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.

[tubeswell]

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.

[Greg_L]

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..

[Greg_L]

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.

[tubeswell]

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.