6100 : no sound out of preamp

[Thames]

Well... I was at a gig 2 weeks ago, all fine during the soundcheck. Went 2-3 hours later, no more sound of the amp.

The problem seems to be located near the input/preamp, as I get sound from the FX loop and the Line Input.

No sound from all 3 channels.

Anybody had the same issue on this amp or any other amp ?

 

[Mike_LA]

Well... I was at a gig 2 weeks ago, all fine during the soundcheck. Went 2-3 hours later, no more sound of the amp.

The problem seems to be located near the input/preamp, as I get sound from the FX loop and the Line Input.

No sound from all 3 channels.

Anybody had the same issue on this amp or any other amp ?

Did you check all of the valves?

First place to check

 

[tazzboy]

Check the Tubes first and then the Fuses.

 

[Thames]

Well, I dont think it could be the tubes.
V1 is only used by Channel 1.
V2 is were channel 2 starts.
All 3 channels dont work. I dont think the 2 tubes could fail together.

 

[chadjwil]

V7 for sure and V6 if I recall are also used by all channels. Cathode follower and phase inverter, although its less likely to be V6 if you are getting sound out of your FX loop. There are a lot of possibilities in that amp for your problem, but the tubes are by far the easiest to check...its good advice. And after we find out it is or isn't them we can put that behind us and start looking at other possibilities.

 

[chadjwil]

Are your power tubes lit?

 

[Mike_LA]

Well, I dont think it could be the tubes.

All 3 channels dont work. I dont think the 2 tubes could fail together.

Welll, the last time my 6101 was at the tech's it went dead the same way and it took him 3 hours to check the tubes and it was a bad 12AX7. He ate the extra time.
"I'm just sayin . . . "​

 

[Thames]

All tubes tested fine.
All fuses tested OK.

The power section seems OK as I get static noise when plugging a cable in the Line IN and FX Return. The Master Volume doesnt affect this noise so the problem is somewhere in the preamp. I can hear a hum/hiss change when switching channels.

 

[Thames]

I also plugged a loop cable in the Line Out Line In, and thru the FX loop, still the same problem.

I also tapped each preamp tube for microphonics and no noise came out.

 

[Thames]

I HAVE FOUND THE CULPRIT..

R58... on the preamp board.

 

[chadjwil]

Glad to hear you got it firgured out...but it seems strange that you would get sound out of the FX loop because that resistor feeds the plate voltage to all the preamp tubes. Hmmm, maybe I misunderstood what you were saying in your original post?

 

[mickeydg5]

But it does not feed the loop, line out/in nor the phase inverter circuitry.
So a signal injected into the loop return will be amplified and heard from the output.

 

[chadjwil]

Yeah, that's what I gather that he meant now. For some reason I had it in my head that he was getting sound from the FX send. I had it ass backwards from the start!

 

[Nilzon]

Hi Marshall Bros,

I´ve got the same problem as above, rehearsing yesterday and it just went dead without any hiss or fizz....just dead silence. At the workbench at home all ext accessible fuses are OK, loop checked and V7 changed. V7 is the only ECC83 thar lights up, all the other are cold.

So is this probably also a resistor problem as above, or are there internal fuses in the circuit board?

Any input would be great, before i dig in...

/Peter

 

[mickeydg5]

Hi Marshall Bros,

I´ve got the same problem as above, rehearsing yesterday and it just went dead without any hiss or fizz....just dead silence. At the workbench at home all ext accessible fuses are OK, loop checked and V7 changed. V7 is the only ECC83 thar lights up, all the other are cold.

So is this probably also a resistor problem as above, or are there internal fuses in the circuit board?

Any input would be great, before i dig in...

/Peter

That seems like a bad filament/heater wire connection after V7. Check wires and connections.

 

[Nilzon]

OK thanks, since V7 "is alive" I guess it aint the pow transformer then?