Gibson GA20RVT amplifier hum

[Palan]

Hello,

I had a 120hz hum in a Gibson GA20RVT amp and replaced a few capacitors, which were not original. That corrected the problem. While looking at another issue the chassis slipped and fell a few inches and the circuit side landed on the back of the speaker magnet. Now it has a worse 120hz hum. I have not seen any physical damage at this point. I have tested caps for capacitance and ESR and so far have not been able to find an issue. I just ordered a small scope to see if I can track it down. I would appreciate any suggestions from experience about how to proceed.

At his point I suspect the filter choke L1 but have dounts. The part number is TF-1003-C and I have been looking for the specs on it. Does anyone know who the manufacturer might be my this part number?

Thanks, Palan

[Palan]

doubts*

[R.G.]

You could use your voltmeter set to AC to measure the AC ripple voltage on each bypass capacitor. If the ripple voltage is the same at both ends of the choke, then it's not doing anything for you. You could also measure the AC voltage from end to end on the choke. If this is nearly zero, the choke isn't "choking" the ripple. Likewise, if the AC ripple voltage doesn't decrease with each step away from the first filter cap, then either a resistor is shorted or a cap is high-impedance.

Just by chance, have you checked the path from the transformer power transformer CT to the first filter cap negative? High impedance there will cause 120Hz hum.

[Stevem]

Try the amp with just the output tubes in to see if it hums, if not add the preamp tubes back in one at a time from the outputs on back to the first one.

In this way you may be able to narrow the issue down.

Does the amp work and now it just has a hum?



If I had to guess I would say that now you have something bent that has dc on it and it’s touching a input grid of a tube.

If it’s a hum due to a dc voltage ( 120 hz ) it will sound close to a low B on a guitar, if it’s a ac hum (60 hz) that will sound close to a octave lower then the low B.

[Palan]

R.G.
Thank you for these great test point suggestions. Here are the results:
RG: You could use your voltmeter set to AC to measure the AC ripple voltage on each bypass capacitor.
Palan: Measurement of AC voltage at each bypass capacitor.
1st bypass cap, input of L1: 8.2V with a scope: 20v p-p at 120Hz
2nd bypass cap, output of L1 0V; with a scope 150mV p-p at 120Hz
RG: You could also measure the AC voltage from end to end on the choke.
Palan: Across inductor measures 8.2Vac
RG: Just by chance, have you checked the path from the transformer power transformer CT to the first filter cap negative?
Palan: I had not checked that but it is 0 ohms DC.

So, it does appear that the choke is doing its job and I can forget about that thanks to your help. There is still the 150mV 120Hz ripple feeding the output which I assume should be 0v at that point.

Note; I added 40uF capacitance across the second 20uF cap and the ripple was reduced to 50mV but it is still there. I checked capacitance and ESR on both capacitors and they measure good.

Thanks again.

[Palan]

Stevem:

Thank you for these very helpful suggestions:
Stevem: Try the amp with just the output tubes in to see if it hums, if not add the preamp tubes back in one at a time from the outputs on back to the first one.
Palan: I remove all the tubes except for the output tubes and the hum is there but very quiet. When I put the fist preamp back (higher gain 12ax7) the hum got very loud again. That sounds as if the source could be before the preamp doesn't it? I continued and the hum did not change after that.

Stevem: Does the amp work and now it just has a hum?
Palan: Yes, the amp still works. I can plug a guitar in and it works ok. There are a couple other issues, tremolo, reverb and a noisy jack but the amp does work to amplify the guitar.

Stevem: If I had to guess I would say that now you have something bent that has dc on it and it’s touching a input grid of a tube.
Palan: I have been looking for that type of damage but haven't seen anything yet. I will pay particular attention to DC touching a tube input though.

Stevem: If it’s a hum due to a dc voltage ( 120 hz ) it will sound close to a low B on a guitar, if it’s a ac hum (60 hz) that will sound close to a octave lower then the low B.
Palan: Yes, I did check the hum against a low B (124Hz) of a guitar and it is about right on, maybe closer to a B flat.

Thanks,
Palan

[Stevem]

Your amount of ac ripple on your output tube plates is fine.

Here’s how for example my 1954 Gibson GA 77 checks out and it’s very quiet.

Here’s the filters used from the OT on back up to the preamp section

20uf
20 uf
10 uf
10 uf.

Here’s the level of ac ripple voltage in millvolts.

198
171
157
152

At idle with both volumes down and of course with the input jacks shorted, at the speaker I only have .017 volts of 60 hz hum showing up.

Part of this low hum level is due to well matched 6L6GC output tubes.

[Palan]

Thanks Stevem,

I must say up front that I am new to tubes so please forgive ignorant questions and statements.
This is the schematic I am working from :
https://www.prowessamplifiers.com/schem ... 20rvt.html

V5a (12AX7 normally 12AU7) is the preamp to the power tubes V6 and V7 (6BQ5). With a DVM I measure about 1VAC at pin 1 of V5a ref. to gnd. According to an Uncle Doug video on amp basics this should be very clean. I am without a scope right now so I will take a better look when I can and let you know.
Across the primary winding of T3 (fed by V5a) = 80mVAC
Across the secondary winding of T3 connected to pin 2 of both power tubes = 185mVAC
Across the primary winding of T2 connected to pin 7 of both power tubes = 34VAC (really high I know)
Across secondary of T2 (speaker) = 1VAC.
Again, this is with a meter, you can watch the speaker voltage climb as the tubes warm up as you can hear the hum volume increase with the speaker connected.

[Stevem]

34 vac on the primary of T2 tells me that D1 or D2 is leaky.

[Stevem]

34 vac on the primary of T2 tells me that D1 or D2 is leaky.

[Palan]

Thanks Stevem,

I am trying to find the specs for those diodes but haven't found them. They are labeled D-1 404. I will try a 1N5408 when I can get a couple.

Phil

[Stevem]

You don’t need spec’s there so cheap just replace them both , even two 1n1007 in series on each transformer leg is fine.

[R.G.]

You don’t need spec’s there so cheap just replace them both , even two 1n1007 in series on each transformer leg is fine.

Absolutely correct!!

[Palan]

Replaced both diodes and the problem stayed the same.
Measuring with a scope:
With both V6 and V7 installed there is a sawtooth waveform of 7v p-p at pin 7 of each tube, which gradually amplifies to 35v p-p as the tubes warm up.
Measuring V6 pin 2, with no input signal to the amp, the signal gradually increases from 0 to 750mv but much later than the output of V6 and V7 (pin 7) increases to 35v.
Pulled V6 and V7 and pin 7 shows a 7v p-p sawtooth that stays the same after warmup.
Plugged V6 and V7 in one at a time, each one plugged in without the other installed. The 7v p-p sawtooth gradually increases to 25v p-p. I see the same thing at D1 and D2 cathodes, L1 input.
The DC signal at the L1 output looks very clean.
Note: the sawtooth signal is very clean untill the input at pin 2 increases which then appears to distort the sawtooth.

Stevem, you suggested this earlier: Try the amp with ust the output tubes in to see if it hums, if not add the preamp tubes back in one at a time from the outputs on back to the first one.
When I tried this earlier the preamp tube did change the volume of the output. I did not do this while measuring with a scope. I will look at that next.
Stevem you also said early on: If I had to guess I would say that now you have something bent that has dc on it and it’s touching a input grid of a tube. These symptoms make that look like a good possibility. I do not see the sawtooth on the output of V5 but I will have look closer.


R.G. you had suggested test points at the filter caps/L1, which were very helpful. These measurements are a little cleaner I think.

Thank you

[Stevem]

Try this.

Leave all the tubes in and unsolder the down stream end of r37 which is the 1K 1 watt after the choke .

Do you still have a hum from the speaker?