Gibson Lancer GA 35 RVT

[plexi50]

I have a Gibson GA 35 RVT amplifier. After warmup i switch off standby and the volume is strong for 10 seconds and then dies out real weak. I at first thought this to possably be a preamp tube. All the tubes including the 7591 power tubes are excellent in there readings

I clipped a lead to the OT CT and then measured the OT primary leads one at a time

I get 113.4 ohms on one primary lead and 131.5 ohms on the other. The OT still supplies 400VDC to each OT primary lead upon amp fully on

Does this sound like the OT is bad? I have checked all cathodes and preamp voltages and every tube is getting the voltages they need to operate

I believe it is the OT. What do you think?

[Dave]

I have seen 2 cases where a bad OT produced a very week signal. One was a Twin Reverb and the other was a Soldano something-or-other. In both cases, the sound was week 100 percent of the time and never rang true.
I have seen amps, however, that behaved similarly to yours where it turned out to be a tube or a bad solder joint.
Just because those tubes test good doesn't necesarily mean they are good.

Dave

[HotBluePlates]

Does this sound like the OT is bad? I have checked all cathodes and preamp voltages and every tube is getting the voltages they need to operate

I believe it is the OT. What do you think?

I wouldn't assume it's the OT.

If you want a real test, here it is, straight from Hoffman:
You have other amps around; take a known good transformer and wire it in place of the current one. You don't have to mount it to the chassis, just disconnect the existing wires and add you new transformer. If the power returns, it was the OT.

All the tubes including the 7591 power tubes are excellent in there readings

What about the rectifier tube (if any)? If you have a solid-state rectifier, is it selenium? Selenium rectifiers will almost always fail over time, and tend to fail as a short.

Are you certain that filter caps are good? What about anything else that could be affected by heat?

I haven't looked up a schematic, but knowing Gibson there are probably 50 completely different circuits with the same model number. Does yours have cathode bypass caps anywhere? Are they all new? If you unhook each of them, gain will drop but does the output now stay constant? My thinking behind that is wondering if a bypass cap is failing as a short and zapping output.

OT's are rarely bad, and often obviously damaged when they are. Or, a winding burns open and gives a different obvious clue.

If you had a signal generator, you could inject a guitar-sized signalinto the input jack and measure through each stage and see where there is no/lo-gain or no output. That would be the fastest way to track this down...

[tubesornothing]

I have a Gibson GA 35 RVT amplifier. After warmup i switch off standby and the volume is strong for 10 seconds and then dies out real weak. I at first thought this to possably be a preamp tube. All the tubes including the 7591 power tubes are excellent in there readings
I believe it is the OT. What do you think?

Bad tube rectifiers can cause this.  Does it have a tube rectifier? Does it still have 400V when it dies?  If yes, it is not the rectifier.

I clipped a lead to the OT CT and then measured the OT primary leads one at a time
I get 113.4 ohms on one primary lead and 131.5 ohms on the other. The OT still supplies 400VDC to each OT primary lead upon amp fully on

Thats to be expected.  Transformers are about number of turns of windings, not about the resistance.  The second half of the winding is longer, so more resistance.

[PRR]

> I get 113.4 ohms on one primary lead and 131.5 ohms on the other

{As ToN says} That's fine, utterly normal. Transformers are wound in layers. Two windings, same number of turns, will have different resistance because one layer is further-out and therefore longer per turn. This can be "fixed" with winding techniques, but it does no harm for guitar-amp type work.

> volume is strong for 10 seconds and then dies out real weak.

Open grid resistor. Measure grid to ground, every socket. Most will be 100K-1Meg. The one which is "infinite" is your problem.

No? Power-up and measure DC voltage at each grid. Most will be zero. (Longtails and cathodynes are different, non-zero; power tubes may be biased negative of course.) If you find one which is way-negative -OR- if the amp starts working while you have the meter on that grid, you found it. If one is way-positive, the coupling cap is leaking.

The other tips are also possible.

Finally: I had a tube with a cracked heater. It would work for about 10 seconds and then the crack opened-up from the heat. On glass tubes you can see the lack of glow. (Sadly this was a metal tube, and an odd type, so I spent too much time tracking it down.)

[plexi50]

Thanks for all the input here. I lost my motherboard and have been offline for 2 days. I will go through the cathode caps and other tips here. I tested all the voltages to the cathodes and they are all good. But i did not lift a leg on any of the caps to test for bad cathode caps or put a know good one across any suspected bad ones. All the grid resistors are near perfect in there value readings. More later:

[plexi50]

Well i overlooked something as usual. But at least i think i am in the right area. V5 pin #6 has no voltage to it. That is supplied from Z1 and the terminal lead to Z1 is broken off from Z1 and not connected. Silicon is all that was keeping it together. What is Z1 and where can i get one?

[sluckey]

Z1 is an incandescant optocoupler used as the trem roach. The Fender neon bulb roaches will not work. You may have to roll your own.

[PRR]

If this plan: http://www.gibson.com/Files/schematics/GA-35RVT.jpg

Z1 does not explain signal then fade. It is just the trem. If the lamp is burned open (very likely) and the photo-resistor is DARK, then signal passes full-strength. They are not using Z1 as DC grid resistor neither.

[plexi50]

I have a 30 watt hammond OT that i will tack in the Gibson chassis in a day or so. Just to busy at the moment to do it. It is already mounted in another chassis but not wired up yet so it wont be too much of a hassle. Just 4 bolts /

[PRR]

Why are you blaming the OT?

Do you have a signal injector?

[plexi50]

I was thinking OT because of the ohms difference in windings at first

I have checked al the cathode bypass caps and no changes in signal strength

No i dont have a signal generator now / I need to get one

[Maschinenmann]

Has this amp ever been recapped (power supply filters)? 

[plexi50]

Yes i just recapped it. But the recap made no changed in signal fade out /

[PRR]

> because of the ohms difference in windings

Measure all the OTs at hand. You don't have to disconnect the plates, as long as the amp is OFF.

I'd be surprised if any show "exact same DCR". Maybe some fluffy HI-FI OT, but not general audio OTs.

[tubeswell]

Could also be a leaky coupling cap affecting the bias of one of the stages (by putting DC onto the grid), causing it to lose gain. Test the coupling caps for DC leakage.

[plexi50]

I found the problem yesterday. It turned out to be the speaker out jack. It was soldered and looked good. But i touched it with an iron and it came up full volume and is plenty loud

I suppose this is a case of a cold solder joint. Really surprised me when the volume jumped. Once again i was thinking the worst case senario and it turned out to be a simple error. But it still took a week to find it

[simonallaway]

I found the problem yesterday. It turned out to be the speaker out jack. It was soldered and looked good. But i touched it with an iron and it came up full volume and is plenty loud

I suppose this is a case of a cold solder joint. Really surprised me when the volume jumped. Once again i was thinking the worst case senario and it turned out to be a simple error. But it still took a week to find it

I can understand how useful it'd be to use this technique on a live amp (you immediately know where the problem was), but it sounds dangerous. Forgive me as I am a newbie, but isn't that asking for trouble? Or is it true that there are parts of the circuit where that's safe (for the operator and the amp). i.e. ok to do this to everything but the powersupply side?

Simon

[plexi50]

I wouldnt recommend soldering anything in an amp that is live and operating. But i do it at times and chopstick and poke around when looking for a bad connection. As far as the speaker jack goes i took a chance under working conditions to touch it up. But no i wouldnt say that anyone should solder when an amp is on. I take chances at times and it's amatter of knowing where you hands and wrists are. Dont ever rest your palm or wrist on the chassis of a live amp and solder.

This was not a high voltage area or near one on this amp / Just the same dont do as i do

[HotBluePlates]

Not to toot my own horn, but...

If a signal source was available, applied to the input and you measured signal at each stage, you would have found quickly that everything from the input jack to the output tubes seemed good. You could even use the estimated amp wattage and the speaker impedance to guesstimate a proper speaker-level voltage, and measuring the OT secondary for that voltage would have confirmed that the OT was working correctly.

With input jack to OT secondary ruled out as the problem, the only thing left would be the jack and the speaker itself.

It been a long time since I repaired amps. I should sit down and think up a logical way to run through checking out a malfunctioning amp in the least amount of time. There are people out there who already know how to do this, but it's never written down anywhere, and their personal experience with certain models and certain failure modes can lead them to the problem with very little time spent troubleshooting.

If you don't already do it, I'd recommend keeping a logbook of amps that you've worked on. Document the observed malfunction/symptom(s). Also document the model, year, modifications, circuit type, etc. Keep notes stating what you tried with the amp to fix it (espeically the stuff that didin't work). And of course, record what the final found problem was. If you do this, you might notice either a pattern of problems with a given model, circuit, or construction type that could lead you to a more efficient way of testing that type of amp. It might also point out where your troubleshooting methods need to be revised to save time.

[plexi50]

I need a signal generator no doubt:

I have been keeping pic's,before,after and during the repair process. I have a 2 Gig USB stick that is 3 MB from being full

That USB stick is my life. I need to get another one with a larger data storage pronto for other work that has already been done to other amps as well

Sometimes i look at my amplifier Cataloge and it's hard to believe i have worked on so many amplifiers since i became a member of the Hoffman Forum

I will count them all sometime. It's well over 75 amps. Thats a lot of learning /

[mojo]

Great job Plexi, keep up the good work. You got er' done again.
Regards,
Al

[plexi50]

Shear luck i say! It's a first for me. Usually i would just replace the jack. And thats what i was in the process of doing when i hit it with a bit of solder. This was one of those fluke last ditch attempts to figure the problem out. I know that a bad connection can cause this sort of problem throughout an amp circuit board. And with the OT still having some volume to it i thought again about the jack connections. Shear luck /

[imaradiostar]

Not to toot my own horn, but...

If a signal source was available, applied to the input and you measured signal at each stage, you would have found quickly that everything from the input jack to the output tubes seemed good. You could even use the estimated amp wattage and the speaker impedance to guesstimate a proper speaker-level voltage, and measuring the OT secondary for that voltage would have confirmed that the OT was working correctly.

With input jack to OT secondary ruled out as the problem, the only thing left would be the jack and the speaker itself.

It been a long time since I repaired amps. I should sit down and think up a logical way to run through checking out a malfunctioning amp in the least amount of time. There are people out there who already know how to do this, but it's never written down anywhere, and their personal experience with certain models and certain failure modes can lead them to the problem with very little time spent troubleshooting.

If you don't already do it, I'd recommend keeping a logbook of amps that you've worked on. Document the observed malfunction/symptom(s). Also document the model, year, modifications, circuit type, etc. Keep notes stating what you tried with the amp to fix it (espeically the stuff that didin't work). And of course, record what the final found problem was. If you do this, you might notice either a pattern of problems with a given model, circuit, or construction type that could lead you to a more efficient way of testing that type of amp. It might also point out where your troubleshooting methods need to be revised to save time.

I keep a "Moleskin" notebook to document all of my amp stuff- ideas to try, schematics, characteristics of transformers, test circuits, formulas and so on. It's become my tube amp brain. There are charts with voltage output from various transformers I've acquired over the years- tested into a load so I can estimate how they would behave in an amplifier. Any time I acquire an output transformer I calculate all the turns ratios, resistances and inductances and document them. I have charts with voltage in, cathode current, plate and screen voltages, and output voltage for various output sections I've breadboarded and tested with a scope, oscillator and a dummy load. The pages are 5 square/inch graph paper so there are quite a few amp layouts in there too.

Sorry for the distraction, just wanted to say I liked your idea and wanted to take it a step further.

[plexi50]

I removed the tremelo photo cell as it was no good. I have recapped the am and have a slight hum that increases with the second channels volume. Do i have to un connect something in the tremelo circuit as this may be the cause of my hum? If i pull V5 of course no signal at all. Do i need to jump a wire or ground some thing in the trem path to completly remove it from the signal path?  Reverb works fine. /

[HotBluePlates]

I removed the tremelo photo cell as it was no good. I have recapped the am and have a slight hum that increases with the second channels volume. Do i have to un connect something in the tremelo circuit as this may be the cause of my hum?

Does the schematic that PRR linked match your amp?

If it does, the thing that sticks out is C32 (0.022uF) which connects to the photo cell. If you remove the photo cell and not this cap as well, it is basically an antenna to pick up noise.

But it is after the volume pot. The only thing that would explain this cap injecting noise that increases with volume is that when the volume pot is turned down, it shunts some of the noise away to ground. That would be more likely to happen if you had also removed the T-filter tone network between the volume pot and the next tube stage.

[plexi50]

Yes PRR's schematic is the amp i have and is much clearer than the PDF i have. I will get back on it later tonight and remove C32 and report back / Have not messed with the T filter tonestack or the volume pot and the next tube stage

[plexi50]

I removed C32 cap and no change in increasing hum as volume is turned up on the second channel

[HotBluePlates]

Hmmm...

Do you have a big cathode bypass cap, like 50-100uF, handy? If so, tack it across V1B's 5uF cathode bypass cap. I'm wondering if the hum (which is likely occurring ahead of the volume control, since it's affected by it) is due to heater-to-cathode leakage in the first stage.

If the cap cures it, your only likely choices are to keep swapping input tubes until you find one with low leakage that doesn't hum, use a massive bypass cap to bypass the hum, or neuter the low end of that channel to roll off anything near 60Hz.

[plexi50]

I tacked a 100uf bypass cap across V1B and all other bypass caps with no change in hum

Reverb is labled on the inputs jacks of the second channel

The reverb sounds good but for the hum in amp

With no reverb the hum is still present and increases with the volume being turned up

I started to think of putting a pair of 100 ohm atificial CT resistors on the power tubes but then the first channel would be noisy as well if this were the case

I did remove the old dead photocell for the tremelo and was thinking that there may be some part of that circuit that i need to connect with the photocell removed. The red X in the pic shows where the photocell was connected from V4B

Im guessing at this point. The circuit board is a jam packed mess

[sluckey]

If you want to rule out filaments as a cause of the hum, just do the 6 volt lantern battery trick.

[plexi50]

Thanks Steve i will try that. I have just discovered that the (2) channels seem to be linked together at some point. I dont know where but when i plug in a guitar cord lets say into the Normal channel and i touch the tip of the cord the second channels volume is interactive as i turn up that volume pot. Same goes if i plug into the reverb channel (Second Channel) the Normal channels tone stack is active

This a weird one. There a bad part in here some where

[plexi50]

I changed some of the preamp tubes in the Gibson for some of my NOS tubes and the amp has dramatically less hum

Some of the tubes that were in the preamp were EH 12AU7 & Sovtek 12AU7

Changing these 2 tubes has made a world of difference / Any input about this?

[HotBluePlates]

Is the gain of the amp still strong?

The only thing that swapping tubes can do to eliminate hum is either the new tubes have reduced leakage (what we were trying to find with the 100uF bypass cap) or the new tubes have less gain thus amplifying the hum less.

If the new tubes seem to perform well in the amp, you might just have to use the new tubes. It doesn't necessarily mean the old ones were bad. Try them in another amp.

[plexi50]

The amp still has plenty of loud gain. What i dont understand is why the bypass cap made no difference in reducing the hum. It just popped into my head to try some different tubes for GP. I did try a .022 snubbing cap on this 100K resistor in the pic but the volume got muddy and the amp lost all definition. All hum did go away. I should have tried a 250pf but i just grabbed what i had at the time on the bench. I also did remove that 100K resistor and tack a new one in thinking it was bad and the cause of the hum. It wasnt /