Improving a Fame VS5012

[niektb]

Hi All!

A friend asked if I could take a look at his Tube amp because it is quite noisy...
I said OK and screwed the thing open but couldn't find any signs of broken components... So it can either be by design or the tubes but I thought I would run by you guys first.
The Tube Amp in question is a Fame VS5012, which is an exact copy of the Randall RG50TC. Schematic here: http://schems.com/bmampscom/randall/Randall%20rg50tc.pdf

First of all, it hums. Not too much at the clean channel but definitely at the gain channel. It's totally independent of the EQ and GAIN/LEVEL knob. The master knob does affect the hum, crackle appears around halfway but disappears above 3/4th. Moving tubes around does change it slightly but not a very great deal...



But it is especially sensitive when you connect a guitar... Might the internal shield connection be poor?
I noticed that the hum (without guitar) increases greatly when moving my finger close to the connector/resistors in the first image and close to the cap in the second image. What might cause this? Or is that expected?



Finally, it suffers from the ice-pick highs... Are there any good mods to soften the treble a tad? 
Thanks in advance!

[11-90-an]

do the tubes all light up? seems to be a good way of knowing of they're working or not... i don't know much(or anything, at all!), as I've never worked with tubes! 

[willienillie]

Please be careful.  Sticking your bare fingers in to a running tube amp isn't a recipe for long life.  Use a chopstick (or similar) for pokin' and wigglin'.

Hum can have many different causes.  Was it always that way since new?  Did it just appear suddenly one day?  Did it slowly get more hummy over a period of time?

Do you have a tube tester?  Do you have other known-good tubes to try?

[Marcos - Munky]

If the hum disappears with the master volume, then it's somewhere before the master volume. To try to isolate the stage where it happens, remove V1 and turn on the amp with no guitar. Still hums? Turn it off, wait a bit for the heaters to cool down, then remove V2. Still hums? Do the same with V3.

If you remove one of them and the hum disappears, then the cause of the hum is related to that part of the circuit.

[PRR]

How is the reverb working?

C60 filters the driver, C46 the stage before that. I can't suss out where your problem lies, but they are both obvious targets for renewal.

[niektb]

do the tubes all light up? seems to be a good way of knowing of they're working or not... i don't know much(or anything, at all!), as I've never worked with tubes! 

hehe yes they all light up! I do get a proper guitar sound also (although it's not my taste)!

If the hum disappears with the master volume, then it's somewhere before the master volume. To try to isolate the stage where it happens, remove V1 and turn on the amp with no guitar. Still hums? Turn it off, wait a bit for the heaters to cool down, then remove V2. Still hums? Do the same with V3.

If you remove one of them and the hum disappears, then the cause of the hum is related to that part of the circuit.

Very good tip! I'll try that out! But the 50Hz hum is more or less constant and independent of any controls. The 'crackle' hum I tried to describe earlier ís affected but not linearly... It increases till the master volume is around 7~8 and then fades out with the master at 10... (which is very weird IMO)

How is the reverb working?

C60 filters the driver, C46 the stage before that. I can't suss out where your problem lies, but they are both obvious targets for renewal.

I've pulled the RCA plugs that go to the reverb tank so it's totally disabled, but inserting it and turning up the reverb doesn't really seem to affect anything...

One thing I could think of is that the heaters of the V3, V4, V5 and V6 are powered with directly from AC... Would that cause the 50Hz hum?

Can biasing the EL34 tubes to a lower value (I believe the 50mV is pretty high) improve anything or would that just be tonally different?

Btw, there is a difference in hum between Gain1 and Gain2!

[willienillie]

One thing I could think of is that the heaters of the V3, V4, V5 and V6 are powered with directly from AC... Would that cause the 50Hz hum?

If it is a full clone of the Randall, the bias supply is full-wave rectified, so that only leaves the heater supply, power transformer, and AC in wiring.  V1 and V2 have DC heaters, which normally reduces hum, but that circuitry is a very common failure point in amps that have it.  Since all tubes are lighting up and working, the bridge rectifier is good, but a filter cap could be bad.

Unmatched power tubes could increase heater hum somewhat, but that would be constant for both channels.  You could also have some coupling between the PT and OT, or PT and tubes, or PT and internal circuitry...a layout problem.  Or a wire routing problem.

Can biasing the EL34 tubes to a lower value (I believe the 50mV is pretty high) improve anything or would that just be tonally different?

50mA cathode current does seem a little bit high, but I don't see a plate voltage.  Either way, it's not related to your hum issue.

[niektb]

Okay, I think the biggest issue is the shielding. When I plug in the guitar and roll the guitar volume back it's very noisy and hummy badly...
If I touch the chassis it is as quiet as if I unplugged the guitar...
Is there something I can do to fix that?

[anotherjim]

The input jack switch contacts getting weak/dirty is a common problem, especially if it tends to get left with cables plugged in for long periods. All jacks with switched contacts should be suspected including any normally unused FX loop etc ones. As per the Randall schematic, the input jack sleeve contacts should connect direct to chassis.