Ampeg VT-120 Restoration / Repair - hum on channel C

[q9522678]

John

Apologies if I’ve done something wrong. Honestly not trying to be difficult or cause problems.

I suspect there’s two hums occurring. There is a persistent lower pitched background hum that doesn’t appear to be affected by any of the controls and also remains after V1 is removed. This is the hum I’m trying to eliminate.

There’s also a higher pitched, likely normal hum, that is introduced when the gain controls are increased. I hadn’t really noticed they were separate until I gave a close listen today when trying your suggestions and realised the deeper hum prettt much remains unchanged.

If anyone has any further suggestion - would be appreciated. Otherwise will keep digging in and report back if I figure it out. Have lots of time on my hands at present..

[Jon Snell]

Time to ask, if you increase the Gains and the hum increases, the hum is still there when V1 is removed ... why.
The hum will stop when V2 is removed.
Without working on it, I am sorry but we seem to be going around in circles.
Good luck with the repair.

[Helmholtz]

Hi Helmholtz,

Reading at pin 2 of V2 is 1.4vdc. The junction of f R51 and R52 measures 2.3vdv so perhaps the meter is loading down grid voltage noting there’s a 0.9v difference.

Supply voltage at node D is 293vdc.

Yes, you meter is loading down grid voltage. Seems it has an input resistance of only 1M. Also R56 seems to have drifted to a higher value. But this is not the cause of your hum problem.

[q9522678]

Hi Helmholtz,

Reading at pin 2 of V2 is 1.4vdc. The junction of f R51 and R52 measures 2.3vdv so perhaps the meter is loading down grid voltage noting there’s a 0.9v difference.

Supply voltage at node D is 293vdc.

[Helmholtz]

Voltage on pin 2 of V2 is around 1.4v so reasonably close?

Grid voltage at pin of V2 should be identical to the voltage between R50 and R51. If lower than that your meter loads down grid voltage. If identical it means a plate current of 0.64 mA. Consequently plate resistor R56 (100k) must drop around 64V. What is supply voltage at node D ?

[q9522678]

Hi Helmholtz - good to hear from you. I hope you’re looking after yourself through these crazy times.

Voltage on pin 2 of V2 is around 1.4v so reasonably close?

[q9522678]

Did a bit more investigating this morning.

I jumpered R33 and the hum remained unchanged.

I tried lifting C22 and the hum remained unchanged.

I linked pin 7 of V1 to ground - hum remained unchanged.

Checked continuity of ground pin on all gain controls and they all looked good.

Also checked continuity of input jack sockets to ground and also appears good. I’d already reflowed solder on all of these connections just to be sure and hit all jacks and sockets with deoxit.

So after some head scratching based on my earlier advice to you with V1 removed hum dissappeared. Decided to try and remove V1 again and hum remained! I could have sworn it stopped when I removed it the other night...perhaps was suffering tube disorientation!

So then went ahead and removed V2 and hum definitely stopped when V2 was removed. Amp was pretty much silent other than some minor hum that you’d expect in a tube amp of this era. So perhaps something in / around V2?

[Helmholtz]

Also pin 2 of V2... should there be any DC voltage here at all?

Should be around 2V. What's the input resistance of your meter?

[Jon Snell]

Link pin 7 of V1 to ground and the hum should stop.
If it does, there is a problem with the grounds on the input jack sockets.

[Enzo]

When he reported no hum on B and A it was because he had the masters for those at zero. When he advances the masters, THEN those other channels gains advance the hum.

[Jon Snell]

Keep logical and you will fix it.
Best regards from the UK and keep safe and well.

[q9522678]

Thanks John,

I’ll give these things a go and report back. I appreciate the thoughts and advice.

I’m always confusing myself! I’m more musician than electronics whiz but generally know enough to get by and not kill myself. I just hate seeing these old amps put out to pasture and have had pretty good success over the past few years in getting these old things back to full functionality and use.

A lot of the stuff I do is out of interest and curiosity regarding the inner functioning of the amps I restore as well. Makes me wish I studied electronics as a youngster! But enjoy the process nonetheless.

[Jon Snell]

I am surprised.
You say that if you remove V1 it stops and Gain C pot has no effect therefore either the hum is from a poor ground point on that pot or a leaky valve or a problem with the FET.
Link R33 and if the hum stays it is either a bad ground point , a faulty valve or the FET/driver.
Try lifting C22.
That should isolate a few issues.
Keep an organised mind and don't lose sight of the fault. Some people go around in circles and confuse themselves.
Take notes if you need to.
Good luck.

Don't forget Gain A & B & C come from the same point!

[q9522678]

Here’s a link to a very short video to demonstrate the hum. The amp starts in channel C, then I briefly switch to channel B and then back to C. You can hear the very low hum on channel C. Not present on Channel B in the video as the volume for channel B is at zero - just to show with and without hum.

[q9522678]

I’ll also add some key power supply voltages:

C7 - 380v
C8 - 298v

These seem to be in spec according to voltages marked on the schematic. Makes me wonder why for V1 and V2 pins 1/6 why the voltages are 30-40v down on what the schematic says?