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Thread: Gibson Ranger GA20-T Rebuild

  1. #36
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    Here are a couple pics of the amp I overhauled for my uncle. Sorry for the poor picture quality...I took the pics 10 years ago with an early digital camera that had trouble with close up pics. The amp didn't work well when he gave it to me to fix...a very loud hum at all volumes and lots of crackles and pops. Two power supply electrolytics had leaked physically in the amp chassis, and every single cap except the domino mica were bad. This one is a 1958 GA-20T btw with original Jensen P12R speaker.

    Greg
    Attached Thumbnails Attached Thumbnails ga-20t-1.jpg   ga-20t-5.jpg   ga-20t-16.jpg   ga-20t-17.jpg  

  2. #37
    Supporting Member TomCarlos's Avatar
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    Thanks for the pics Greg....

    I finished installing the parts on this amp and did a slow power up. So far, the dc voltages look ok. Good thing I had an extra Rectifier tube! The original couldn't produce the expected dc on the Cathode (350vdc).

    Of the two channels, Channel 1 (Mic or Instrument) is working the best. But I don't think I'm getting the output that I should. I'm working with old pre-amp tubes and need to convince the owner we should swap those out.

    Channel 2 has some issues... there is some oscillating and other noise when I turn up the volume. I'll deal with that later.

    For now, I want to work on Channel 1 - replace 12AY7, 12Ax7, and drop in some new 6V6 tubes.

    As for the output transformer, I have read in other threads that the OTs on the older Gibson's are problematic. I'm wondering if I could use something like a Classictone 40-18045. It has an 8 ohm tap rated at 15 watts.

    Ok.. it will be a stage by stage analysis!

  3. #38
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by TomCarlos View Post
    ...As for the output transformer, I have read in other threads that the OTs on the older Gibson's are problematic. I'm wondering if I could use something like a Classictone 40-18045. It has an 8 ohm tap rated at 15 watts...
    My 2 is that I have not found that to be true. Certainly not to the extent that I would proactively replace the OT if it appears to be working now. I recommend that you just leave it be unless there is a symptom that you have not mentioned that indicates a problem with the OT.
    Last edited by Tom Phillips; 05-09-2015 at 11:33 PM. Reason: Clarified

  4. #39
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by TomCarlos View Post
    Good thing I had an extra Rectifier tube! The original couldn't produce the expected dc on the Cathode (350vdc).
    You bet, that original rectifier wore itself out trying to charge up those duff old filter caps.

    I'm with Tom on the OT, if it's working let it work, no need to replace.

  5. #40
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    The original Gibson transformers were maybe a little undersized for the power of their amps, but I'm with the other guys...don't replace it unless it needs it. Your problems are likely resistors and caps. Check all the caps in the amp for leaking DC...you will likely find that all of the coupling caps are bad and leak enough DC to shift bias points on stages after them. Making sure the tubes are good is also a good idea so you know you are starting with a good base. For testing you can put a known good set of tubes in there if you have them. The EH12AY7 is a nice sounding tube and all there is for modern replacements. Vintage ones are better but are hard to find and expensive.

    Greg

  6. #41
    Supporting Member TomCarlos's Avatar
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    Good-day Mates ...

    I'm hoping I can have a check-in point and get a "Go No/Go" poll before I continue working on this amp. I'm wanting to break this down in stages and learn as I go along.

    For now, I am working on the Mic/Instrument channel only (the top part of the circuit). I already know there is an issue with the other channel so I have removed the 5879 and 6SQ7 tubes.

    Things I checked....

    12AY7 preamp => Overall gain in that stage is around 14.

    12AX7 Phase Inverter => Voltage across the 680 Cathode resistor is .79 volts, so that is apprx 1.16ma cathode current to each side.

    6V6 Output Tubes => 279vdc on the screen grid (with no ac signal), 13.8vdc across the cathode resistor (20ma).

    With the Tone control set to 12 O'Clock high, I am seeing close to 8vrm before clipping. As you can see in the photo, the bottom side of the waveform looks "tighter" when compared to the top. I realize this is not Stereo Hi-Fi, but I'm curious to know if you all have seen this type of waveform.

    With an 8 ohm load, 8 Vrms, that is 8 watts rms. Does that sound right for this amp?
    Attached Thumbnails Attached Thumbnails output-1.jpg  

  7. #42
    Senior Member guitician's Avatar
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    Ha! Looks like my avatar, but upside down. Compare your signal into the phase inverter to the output, and the signal from the inverter to see if the drive signal is normal first. 12-14 watts I think is "Normal" for these types of amps. And what is the plate voltage on the 6V6's? 13.8v vs. 15v on the schematic could mean weak tubes. What frequency is that? 1khz? Low frequencies will normally have less output. The transfer characteristic curve shows how that waveform occurs. I've seen it a lot from old tube amps, not "Clean" sounding ones, but what cranked old tube amp is "Clean".
    Last edited by guitician; 05-16-2015 at 06:16 PM.

  8. #43
    Supporting Member TomCarlos's Avatar
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    Thanks Jeff...

    Good hint from you!

    Yes, there appears to be an issue with the phase inverter. The incoming signal to the 12AX7 looks good. The output signals feeding the Output tubes "looks" good. The problem is that I am seeing only half the voltage swing coming out of the Phase Inverter that feeds into one of the Output tubes. So this probably explains the 8 watt output. I am going to check the circuitry around the 12AX7, including the tube socket, parts, and connections.

    I hate this two sided PC board!!!!

  9. #44
    Supporting Member TomCarlos's Avatar
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    Update... getting closer...

    I found a wiring error in the Phase Inverter circuit. So I fixed that. I have increased the output power but still have the problem of a mismatch of ac swing feeding the 6V6 tubes.

    See attachment for measurements-

    - 119 mVac coming from the Pre-Amp and feeding the P.I.
    - 7 Vdc on the 680 ohm Cathode resistor = ~ 1.03 ma

    On the top side of the 12AX7 (Pins 1, 2, 3)

    94 Vdc on the Plate
    8.33 Vac on the Plate
    119 mVac on the Grid

    On the bottom side of the 12AX7 (Pins 6, 7, 8)

    100 Vdc on the Plate
    10.7 Vac on the Plate
    184 mVac on the Grid

    I am wondering if the ratio of the 220K to 4.7K voltage divider (calculate during design) is off and that is the causing the imbalance.

    I have swapped out several 12AX7 tubes thinking it might be that one side had more gain than the other- same results.
    Attached Thumbnails Attached Thumbnails phase-inverter-measurements.jpg  

  10. #45
    Senior Member guitician's Avatar
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    So did you set the AC input to 119mV because that is the point the output load wave would start clipping? How balanced are the two 220k resistors? Yeah, you could trim it to balance the PP drive signals, but how balanced are the 6V6's? I've always thought exactly balanced tube output sections were Hi-Fi, and not the classic old tube guitar amp.

  11. #46
    Supporting Member TomCarlos's Avatar
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    Hi Jeff...

    Yes, I applied signal so that I reach an output just before clipping. Even at lower signal levels, I still see the imbalance.

    The 220Ks (everything you see in that diagram) and brand new, metal oxide, all within 5% of each other. The 6V6 are brand new JJs, matched. I put those in to see if there was any difference compared to the old RCAs- no difference at all.

    Tom

  12. #47
    Senior Member guitician's Avatar
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    I wouldn't worry too much about that imbalance. That output waveform is not a pure sinusoidal wave, and the power from it is complex, but the power equation simple. You may already be putting out 12 watts. Have you played it into a speaker?

  13. #48
    Supporting Member TomCarlos's Avatar
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    Yes... I am playing through a Celestion Seventy 80 for testing- Channel 1 sounds ok. I want to get this working properly before I test out Channel 2.

    Eventually, my friend has a new Celestion G12-M, 25 Watt that he will use.

    BTW- In another thread, I am dealing with a "squeal" issue.

  14. #49
    Supporting Member Jazz P Bass's Avatar
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    That PI 'imbalance' is to be expected of that type of inverter circuit.
    My 2 cents.
    g1 and Drewline like this.

  15. #50
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    Thanks John!

  16. #51
    Supporting Member TomCarlos's Avatar
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    Ok...

    If the PI is ok, let me attack what can be the last issue... the "high pitch squeal."

    In reading other posts, the place to start is the Power Supply.

    With no input and with the output tubes removed, I am seeing .258 volts of ripple, measured coming right off the transformer, between the 10K and 20uf cap on the right, the 350v point.

    If I insert the output tubes, I am measuring 4.8 volts of ripple at that same point. That seems excessively high.

    When I measure between the 10K resistors (at the 275v point), the ripple is negligible, maybe 30mv.

    The last 10K and 10uf cap have maybe 3mv ripple.

    So, my question is.. the first point off the transformer.. 4.8vac ripple, does that seem too high?
    Attached Thumbnails Attached Thumbnails power-supply.jpg   img_1445.jpg  
    Last edited by TomCarlos; 05-17-2015 at 09:31 PM.

  17. #52
    Senior Member guitician's Avatar
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    No, and a well made Push-Pull transformer balanced winding cancels the hum because its 180 degrees out of phase on each half of the primary.

  18. #53
    Supporting Member TomCarlos's Avatar
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    Ok.... Let's call it good then for the ripple.

    So, more testing and here is what I observed. The "Squeal" kicks in the minute I put the Tone Control at 12 O'Clock high (or higher). It seems like the amp does not like the high ends coming through. The .01uf feeding the 1 Meg Volume pot is new. The .01uf that goes to the 250K Tone pot is new. So this is telling me the problem is "frequency" related.

    The research continues.

  19. #54
    g1
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    Can you give a rundown of which electrolytics have been replaced?
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  20. #55
    Supporting Member TomCarlos's Avatar
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    As part of this rebuild, I replaced ALL electrolytic caps.

    In the Power Supply, I used F&T caps - the two 22uf, 500v and one 10uf, 500 volt.
    For the 6V6 cathodes, I have a 22uf, 100v, Non-Polarized cap.
    In the 5879 circuit, I have a 25uf, 63v cap.

    I think I may have discovered my culprit... the 5879 tube. Even though I was running a guitar into Channel 1, I guess Channel 2 can affect what is going on.

    When testing Channel 2, I noticed the Tremolo had an excessive "breathing" sound (sorry, not sure how to describe it). I replaced the 6SQ7, no difference. But when I replaced the 5879, the Tremolo became more quiet. In fact, that Channel cranked up and the Tone cranked up sounded great.

    So I am back testing again, on Channel 1. So far, seems ok. Maybe it was the 5879 from the other Channel that was causing the issue?

  21. #56
    Senior Member guitician's Avatar
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    Is this an acoustic, or electrical "Squeal"? Does it do it without anything plugged into the input? Can we rule out tube microphonics and pickup squeal?

  22. #57
    Supporting Member TomCarlos's Avatar
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    Shoot... swapping the 5879 did nothing... Squeal is still there.

    I disconnected the guitar. With controls at high noon, I can still hear the high pitched squeal - definitely electric. Gonna look closely at the input jacks.

    And still trying to figure out why this amp gets quiet (hum disappears) when I put a finger on the chassis?!?!?!?
    Last edited by TomCarlos; 05-17-2015 at 11:03 PM.

  23. #58
    Senior Member guitician's Avatar
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    How is that 1st tube wired? Like the schematic? I noticed that both halfs use the same plate resistor, that could be an issue. And that the grid resistors are 1M ohm for grid leak bias those are small, but may need to be that way for this circuit.

  24. #59
    Supporting Member TomCarlos's Avatar
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    Yes, the wiring matches the schematic.

    There is a new 100K metal oxide that feeds both plates. I am measuring 45vdc on the plates.

    The 1 Megs actually measure to 923K and 928K - close enough.

    The input caps are new - .02uf (I measured and verified the values).

    I did find another schematic for a Gibson 20T. There are some differences in how the Tone control is wired up. I guess there is more than 1 way to skin a cat, eh?
    Attached Thumbnails Attached Thumbnails channel-1-input.jpg  

  25. #60
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    Something else I just noticed... and going back to using a signal generator and speaker...

    If I run a signal into Channel 2, Volume 2 turned up, the Voicing (Tone Control) half way, I get a nice signal. If I then turn up the Channel 1 volume (with nothing plugged into Channel 1), say to 7, 8 , or 9, the volume drops from Channel 2 (significantly) and the wave begins to oscillate.

    I am going to test for a couple days, jot down results, and report back.

    Grrrrrrr!!

  26. #61
    Senior Member guitician's Avatar
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    OK then, it seems that both inputs are coupling signals. How well do those shorting jacks work? Check to see if they are really shorting to near zero ohms. Another thing you can check is the lead dress of the wires for the separate channels and keep them apart. With that type of circuit there will be interaction between volume controls since they a tied to the same point.
    TomCarlos likes this.

  27. #62
    Supporting Member TomCarlos's Avatar
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    My plan for the next week or so is to test each channel separately. I am going to remove tubes not needed, remove connections to the Phase Inverter for the unused channel, and look at wiring and lead dress. I then reverse the process for the other channel. Once i know that each channel is ok, i will then reconnect both channels and see where I stand.

    And while doing this, I'm going to look at other versions of the Gibson GA-20 and 20T. I see differences in how the channels are combined and differences in the Voicing control. I wonder if Gibson realized some difficiencies in the first design and then made some changes. I believe it is the Ranger model that has the changes. I think our goal is to keep the amp as original as possible. But if this problem persists, maybe that will be a deciding factor.

    Thanks for the input!

  28. #63
    Supporting Member TomCarlos's Avatar
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    A quick note on the inputs....

    Channel 1 jacks seem to be ok, near "0" ohms.

    Channel 2 does not use jacks that short to ground. There is no cap on the inputs. Each input has an attenuator of some sort - a 220K to ground and a 100K to the grid. So when I measure at the grid, I get 168K ohms (two 320K ohms in parallel). I guess that is the design for this channel? Seems odd you wouldn't be grounding the inputs somehow.

    I found a couple interesting articles on this amp and the Fender 5E3. This one comes from "The Gear Page.net." Besides grounding issues, the post also has some other tips. Anyway, the issue is Grounding. I asked about this before but our friends here on MEF advised against changing any grounding connections. But look closely, you will see a grounding issue with the 6SQ7- It has two paths to ground. The same can be said for the 12AX7, 5879, and 12Ay7 tubes.

    The other flaw is that the Power Transformer Center taps, the Power Supply Caps, the 6V6 Cathode cap, all these are connected at the 6V6 Pin 1 terminal, then go through a very thin wire to the preamp section, and you have daisy chaining to multiple ground connections. I'd think you would want the "hi side ground" going to the chassis on it's own.

    Now then, this amp might be working to some degree but I bet you all wouldn't design it like this if you had the chance!!
    Attached Thumbnails Attached Thumbnails ground-diagram.jpg   inputs.jpg   img_1446.jpg   img_1447.jpg  

  29. #64
    Senior Member guitician's Avatar
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    Having the power ground all on one point is good, but using the chassis as a signal ground is bad. A thin wire to the preamp section is not bad, but having another path could cause problems. It's best to just use the wire as the ground, maybe thicker would help. That lead dress doesn't look too good, were they all neatly tucked along the chassis at one point?
    The 5879's 168K impedance will make that tube more sensitive to picking up stray signals.

  30. #65
    Senior Member guitician's Avatar
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    Having the power ground all on one point is good, but using the chassis as a signal ground is bad. A thin wire to the preamp section is not bad, but having another path could cause problems. It's best to just use the wire as the ground, maybe thicker would help. That lead dress doesn't look too good, were they all neatly tucked along the chassis at one point?
    The 5879's 168K impedance will make that tube more sensitive to picking up stray signals.

  31. #66
    Supporting Member TomCarlos's Avatar
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    I will clean up the lead dress. Some of that is temp wiring so that I do not lose track of connections.

    I guess the inputs to the 5879s stays as-is. I don't how much modification the amp owner will accept.

    As for the grounding, I guess that is trial and error. This amp isn't using any Star Grounding techniques. Looking at how then preamp section is grounded, and wiring on the tubes, i am hesitant to touch that. But I might try cutting off anything that is a second path to ground.

    Thanks!

  32. #67
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    The one that I fixed for my uncle is a GA20T, we figured it was 1958 or 1959. The point is, when I got the amp it didn't work as one of the electrolytics had physically leaked goo in the chassis. Once the electrolytics were replaced I was able to check the amp function out more fully and realized it needed an overhaul. Once I did the overhaul the amp worked flawlessly with the original circuit and grounding. This amp should also be able to work correctly once the likely out of spec parts are sorted.

    Did you replace the coupling caps in the amp? Or test them for leakage at least? I realize it is a pain because they are under the board, but check that before you check anything else. Having leaking coupling caps throws off the bias for the stages that follow them and can drive something that would normally be stable into oscillation. Every single coupling cap in my uncle's amp leaked too much DC and had to be replaced. Likely you will have the same condition in that amp. Those old Astron paper caps and the bumblebee paper and paper in oil caps that came in that amp are notorious for leaking DC.


    The 5879, being a pentode, can amplify at higher frequencies quite well, and as such is more susceptible to oscillation that a triode. Also, I haven't looked closely at the schematic in awhile and don't have time at the moment but the 5E3 has interactive channels with the same behavior that you describe about the volume going up and down depending on the position of the unused channel's volume control. On a 5E3, if you turn the unused channel up half way on the volume, and then adjust the channel in use to whatever volume desired, this results in the loudest volume, and of course having the other channel operating changes the tone slightly also. I would guess that the GA20T is much the same as far as having interactive channels. If you really need to know about it, I can have my brother check the amp as he has purchased it from my uncle.

    Greg

  33. #68
    Supporting Member TomCarlos's Avatar
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    Hi Greg....

    Yes, I replaced ALL caps. Once I had that PC board out, I wasn't going to put it back in and solder up connecting wires without replacing the caps. As I saw in a restoration video, just do it!

    Now that the board is back in place, I am going to shorten all connecting wires between the pots, tubes, and board. Hopefully, this will solve some of the problems. I agree with you on the grounding- the amp should work. But optimally, this isn't how we would ground various points in the circuit.

    Maybe the happy point here is to get the Tremolo channel working and we leave the 12AY7 out. I think the owner really cares about Channel 2. Beyond that, I can only see myself spending tons of time on this project.

    As an FYI, I found another blog for this amp and the problem. So let me post it here just in case anyone else comes upon this thread.

    The Gear Page.net - High Pitched Squeal

    I'll report back my Monday night.

    Thanks Greg and Jeff!

  34. #69
    Senior Member guitician's Avatar
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    Yeah, grounding is most likely the issue. I don't think grounding the speaker frame would fix it by itself, but if you have a clip lead, it can't hurt to try.

  35. #70
    Supporting Member TomCarlos's Avatar
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    So.... I cleaned up the leads, shorten wires, fresh solder, all that didn't solve the problem. But I found this link and suggestion of running a wire from the speaker negative terminal to the chassis. It's the equivalent to grounding the output jack.

    While I was at it, I used some Star Grounding hints as shown here on the Aiken Amps Web site.

    Well, for now, no squeal.

    Here was another post with a similar problem: The Gear Page - Ground Issues in GA-20T.

    I looked at everything else, not wanting to change the circuit components or add anything new. The same article suggests "grid stoppers." I'll see if I can understand what that does.

    So for now... I guess I can claim victory.

    And a footnote -the Celestion G12-M is too big for this cabinet (the magnet is too big). The chassis has no room to be mounted. I believe the original speaker for this amp is the Jensen P12R.

    Thanks everyone who helped me along the way!!
    Last edited by TomCarlos; 05-24-2015 at 07:45 AM.

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