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Thread: Mod'ing a tube circuit (from early 60's stereo turntable)

  1. #71
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    Got you on the Mid-control; that's going in the soup!

    I have only observed things visually; I have yet to pull the whole PT section out. But i am sure that the black side of the AC is going to a large 'ground - looking' lug on the top of the xformer. That's all i'm saying. It's weird.

    I will try to get some photos together and also do some VM tests.

  2. #72
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    Here's a quick photo of what I'm talking about...

    pt-ground.jpg

  3. #73
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    The balance control is likely a log pot on one channel and a reverse log pot on the other channel, so this will need to be replaced. Unless: If you cascade the channels, the balance pot would be a distortion control with attenuation to keep the output level "kind of" the same at any distortion amount. You would also have pre-distortion and post-distortion EQ.

  4. #74
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    The balance control is likely a log pot on one channel and a reverse log pot on the other channel, so this will need to be replaced. Unless: If you cascade the channels, the balance pot would be a distortion control with attenuation to keep the output level "kind of" the same at any distortion amount. You would also have pre-distortion and post-distortion EQ.

  5. #75
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    Hi TimmyP1955 - i am so tempted to keep adding mods but i think i need to draw the line; this thing is getting crazy!! :-)

    Going to try to pull the PT section out today and have a closer look. Still bewildered by this black wire going to that lug on the top of the PT. Was hoping to start off with grounding the unit with the new power cable before going further.

  6. #76
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    Ok, seems to be that the thing on the top and side of the PT that the black AC is connected to is a 'spring-loaded rose metal fuse'.

    Couldn't find any info on them in a few initial searches. No biggy, if it ain't broke i won't fix it.

    So, as for where to attach the ground of the new power cable, since this is a wood cab the only really obvious grounding spot i'm seeing available is where the speaker jacks are. Is this a good spot?

  7. #77
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    I seem to remember grounding my PS stuff to the cathode ground of my 6V6 on my (single-ended) 5F2A Fender Princeton build.

    Found a different service manual for this model, by the way:
    http://www.kevinchant.com/uploads/7/...231/ag9115.pdf

  8. #78
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    Wired in the new power cable with plenty of slack on the ground wire to go wherever it needs to. Suggestions?

    Put in the 10k 1 watt in the power supply instead of the 18k, per Chuck-H!

    Most of the time went to trying to screw the PT back into the cab; tight fit!

  9. #79
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    Hmm, so the 80ma fuse blew.

    I have only changed out the power cable and the 10k resistor in the PS per Chuck.

    Pilot light and tube heaters are still powering up. No sound/report on the speakers at all.

    It's a SS rectifier so maybe its the PS caps?

    Will pull tubes and power up with new fuse to see if its the power tubes.

    As always, open to input...
    Chuck H likes this.

  10. #80
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    The fuse that was in there was rated for .5amps, not 80ma. Oye. It was vaporized inside and the end cap came off when i pulled it out.

  11. #81
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    Changed out the fuses - back in business!!! I guess they were just too old.

    @ChuckH: Already a giant difference just from the change to the input section!
    I A/B'd the unaltered channel with it at full volume; no contest - clearly louder with much more body.

    (I used a 39k instead of 33k since that's what i had on hand).

    I am noticing a bit more life to both channels since changing that PS resistor, too.

    Yahooo!!!
    Last edited by TubeNoob; 12-23-2016 at 01:15 PM.
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  12. #82
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    Ok, emergency post!

    I'm poised with soldering iron in hand, but....

    On the schemo C8 should be "330mmf" (330pf).

    The actual cap that i just pulled out is 3300pf.

    Should i still go with the 500pf cap here or keep the old one or go for another value?

    Wouldn't i want to go for a value higher than 3300pf to add more 'upper-mids'?

  13. #83
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    Here is a little recording of the amp so you can hear where things are at.

    The first 20 seconds is the unmodified channel. From about 22 seconds on is the channel with the input mod done.

    (you can hear (and see from their waveform) that the mod'd channel has more articulation and starts to break up sooner yet with less compression)

    Anyway, regarding my previous emergency post, check out the 2nd one to get an idea of the 'brightness' of the amp at this stage, this link should take you straight to this 2nd take:

    https://soundcloud.com/user-34254870...nputmod#t=0:21

  14. #84
    Supporting Member Chuck H's Avatar
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    The treble circuit cap should be 500pf. Be sure you are targeting the right cap as per the schematic. There are a couple of 3300p caps in the stock circuit that are NOT the treble cap. It's possible a 3300p cap was mistakenly installed in the treble circuit too though. But it would definitely have been a mistake in the original construction if that's the case.

    Too early to listen to the clips in my home
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  15. #85
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    Ok, that's what i get for slingin' solder around late at night on xmas eve... I put that cap back, replaced the 'right' one this time!

    Also did the following:

    Changed out C9 (cut cap); Changed V1 cathode resistor; put V1 bypass cap on a switch; put neg feedback on a switch with a 2k instead of 1.5k.

    Here's a one minute recording i just did which demos each of the stages sequentially.

    1) Un-modded channel
    2) Modded channel with V1 bias resistor in play, the 500pf treble cap and the new cut cap to ground (what a HUGE difference already!)
    3) Now with the V1 bypass cap engaged
    4) Now with the negative feedback resistor engaged

    Its sounding pretty killer!

    https://soundcloud.com/user-342548702/demo2-2nd-wave

    yeah, i know, i'm probably the only guy on here today...

    happy holidays everybody!!!!
    Last edited by TubeNoob; 12-25-2016 at 02:54 PM.
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  16. #86
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    Happy New Year to everyone!!!

    So today I finished all the mods (to one channel) except for the 500pf cap on the volume pot. Changes include:

    -Escherton's idea to put a pot in the tone-stack to drive the mids
    -Chuck-H's volume pot mod and also changing that cap that was attenuating LF's to the power stage.

    Holy cow this thing is screamin'!
    WAY more volume now.
    The mid-control pot adds a ton of flexibility. (Used a 500k log)

    Between the mid-control and the various switches for the mods it is doing a great job of being able to sweep from clean and punchy to dirty and growling.
    Exactly what I was after! Thank you guys for the incredible help and suggestions on this!!!! Couldn't have done it without you and I learned a helluva lot in the process.

    Remaining stuff: doing the same mods to the other channel, cutting the aluminum face plate, soldering up the power supply for the effects circuits and the circuits themselves. Still a bit to go on this little beast but it'll be a cool secret weapon!
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  17. #87
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    Ok, all the tube circuit mods are done to both channels. Yahoooooooo


    One last request for info on a Mod!!!

    In order to use the tube amp as an 'effects' device without the speakers, what are the options? 8ohm 5watt resisitor? cathode follower? tapping it before the power tubes?

  18. #88
    Supporting Member Chuck H's Avatar
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    If you like the way the 'whole amp' sounds then you'll want to use the whole amp. Since it's stereo you'll need to disable one side. You can put it on the same switch that engages a split resistive load to the other side. That is, a fat DPDT toggle wired so one pole switches the output of one channel from the speaker out to the resistive load and "recording" jack and the other pole does something like lift the cathodes of the unused channel. The split resistive load should probably be a higher impedance than the 8 ohm specified load because an actual speakers "average" impedance would be higher than that. At least that's become the common MO for resistive attenuators. So maybe a total of fifteen ohms split to send the required -10dB and/or +4dB signal to any effects depending on your requirements. You could use a 15R rheostat too and just adjust it until the effects unit/s are happy and the noise floor is optimized. Speaker emulation would be more complicated and I personally haven't found much happiness in my attempts, but there are supposedly some good products that can be put in the loop for that too. If the circuit sounds too flat and brash (like you're not using a speaker ) something that might help would be to use a series cap and a rolloff cap from the CT of the rheostat or the divider circuits. Something like starting to roll off LF around 100Hz and HF after about 6kHz. If you had, say, a 10R series 5R load divider the series cap would be 100uf and the shunt cap would be 4.7uf. That would seem 'nominal'?
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  19. #89
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    Chuck, you are a true beast of the tube wilderness - thank you very much for that explanation. I dug in a bit to know more and i think it's best that i just build a power-soak as its own device later on!

    So, with humble heart, i must confess that my 2nd channel (which i just mod'd to be like the first channel) is not working. I didn't check my changes as I went along by turning on the amp after each mod - i will NEVER do that again

    Troubleshooting time!

    - I have done continuity checks on all the mods and they seem to be connecting ok. (Wouldn't call this conclusive, though).
    - Swapped tubes from 1st to 2nd channel, no difference.
    - Checked voltages and compared them to the working channel - they all seem to be similar.
    - When checking voltages, I'm getting a report on the speakers from C12, so problem comes from before that i presume.
    - V1 tube socket a little dirty, but once i got it into place, voltages were correct yet no sound.
    - Getting normal voltage on the V1a plate.
    - Input section seems to be ok (input making it to the grid of V1a in continuity test)
    - Not getting much of a report back when i touch C8 with the test leads as i do on the 1st channel.

    So, i'm guessing the prob is between the input and the volume pot (R2).

    As sucky as this is, i'm really hoping to learn some troubleshooting techniques from it, so please feel free to throw me suggestions!!!
    Last edited by TubeNoob; 01-12-2017 at 02:15 PM.

  20. #90
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    I have the most wonderful and embarrassing news all in one fell swoop! I wired the volume pot wrong - i thought it was cranked the whole time!

    Everything seems to be working good on both channels!!!

    I will try comparing the two soon and report back.

    Meanwhile, the dual 15v PS for the FX units is now wired up...
    Attached Thumbnails Attached Thumbnails power-supply-wired.jpg  

  21. #91
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    My main intention for this unit was for re-amping stuff that needed some pizzazz and was super glad to crank it up this weekend and do just that. I had a guitar line that even though it was tracked through an amazing 60's Bassman with killer mics and preamps in a great studio, it was lacking in upper frequencies. (crazy, i know) This box did just the trick! Was really cool to switch things in and out of the circuit and use that mid-boost pot to dial in the right sound. Yahoo!
    Chuck H and eschertron like this.

  22. #92
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    A strange thing happened on the way to jamming out... One channel stopped working. DRATS!

    When I remove the tubes and test voltages I am getting a report from the speaker when I check the plate on the EL95.

    The issues I have found:

    1) Was very hard to get a reading for the 2nd plate (pin 6) of V1b with my probe, which fits down into the pin hole.
    2) Pin 6 showed 280v when I finally got a reading. Pin 6 of the other channel shows 265v.
    3) C12 is not showing any voltage at all.

    I'm going to:

    A) Clean the pin hole
    B) Check solder connections for Pin 6 and C12

    Anything else I should be looking for?

  23. #93
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    You may have a leaky cap putting voltage on the chassis. That's all I can think of to cause a report from the speaker with no tubes in. Did you install a chassis safety ground?

    The difficulty getting a reading from pin 6 of the preamp tube could be a short, a fault or just old grimy flux preventing good contact. One tip I read here was to sharpen the meter probes for poking through that stuff. I haven't done it yet but it seems smart. Check voltage on the other side of the 220k resistor connected to pin 6 If it's still troublesome to read you may have a problem with the voltage rail.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  24. #94
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    Hi Chuck!

    Regarding the speaker report, it's when I test the plate voltage of the output tube. It does the same on the working channel, as well.

    I did check that 220k (R19) and it's putting out 280v. That's what's strange to me, the cap (C12) next to it is not seeing any voltage.

    Yes, i did put a grounded cable on it and tied it to the chassis.

  25. #95
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    Ugh, been out of the loop for a while due to my first cold in probably 5 yrs... But i'm back!

    So, i did check R19 as i said, and its getting 280v. C12 is not seeing any voltage.
    I checked continuity from the tube socket to R19 and C12 and its just fine.

    Really strange to me and I admit I'm not sure what to check next on this thing!

  26. #96
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    So there is 280V on one side of R19 and no voltage on the other side?
    Does it check out at 220K? Are both heaters lit at V1 ?
    Does V1 pin 8 measure about 3.4K to ground?
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  27. #97
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    Hi g1, thanks a lot for those ideas; I checked those and here's what I found:

    V1b on Good Channel - Pin 8 to ground: 3.5k
    V1b on Bad Channel - Pin 8 to ground: 3.5k

    R19 on GC: 200k
    R19 on BC: 220k


    So, then I re-flowed the solder over the pins and R19 and the dang thing is workin!!! Yahoo!!!! I guess something got jiggered loose during my mod work; though i did find a little component lead floating around beneath the circuit board which i removed, so it might have been that?

    Anyway, if my luck holds then there shouldn't be anything left to do on the PCB and i can go ahead and fix it in place.

    Just tried both channels with this knock-off Strat i just got for 30 clams, (my usual Strat-knockoff is a Samick that is really dark sounding, so it was nice to hear the amp with that crazy Strat chime). This amp kills it! Very different sound from my Princeton build, that's for sure. Apples and Oranges, though.

    Thanks again for everyone's time and expertise!!!!!

    Still working on the effects/mixer stuff that will go in place of the turntable. Having issues with the dual PS where it checks out fine (+15 / -15) when not under load, but the negative side is collapsing down to 2v under a test load. Tube stuff is so much easier to work on ;-)
    g1 likes this.

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