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Tube Amp Mystery Failure

I don't either. Thats a grid stop for V2. R15 is marked as 1K though so not sure the number is correct there either.

If a resistor was going to open because of a fault with V5 or V7, I'd expect it to be R39 and/or R32 and R36. R39 carries full current flow through those tubes as part of the bias regulator system, the other two are the screen resistors.
 
According to the schematic you posted, R15 is 1Kohm and connected to the grid of V2. The resistor you posted a picture of is 5.11Kohm, and as pointed out by Dave, one half of the R13A+B shown on the schematic. All of the tubes should be tested on a good mutual conductance tester if only for some peace of mind. Also, given the age of the unit on the voltages involved I would also take Tinerbelle's advice and look at the possibility of a electrically leaky capacitor somewhere. But, since you are sending it to ARC anyway...........
 
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Sol -- I would politefully suggest in the strongest possible terms that the resistor is not R15, but R13. From my chair there is no evidence I can see to support the fact that it is R15, while there is plenty of evidence to support the fact that it is R13 - be it:

1. The physical layout of the board,
2. The fact that the value ARC sent is one of the two resistors used as shown on the schematic to make up the total value of R13, and
3. The fact that the resistor that ARC sent is the one they say is used in the location in question.

I also agree that the issue with the output tubes is almost certainly unrelated to the issue concerning the resistor. It's hard to imagine any kind of circuit action that could connect the two events. The two output tubes may in fact be bad, but to have them both react the same way at the same time more points to some condition in the in the amplifier causing both tubes to act the same way in unison since they both reside on the same side of the push-pull connection in the same channel.

Beyond all that, whatever the solutions turn out to be, I truly hope you get them all resolved properly to your satisfaction. Good luck with it!

Dave
 
If it is in fact R13, and it fried from arcing to the B+ trace on the board, I can imagine that making some ugly signal to pass down the line. Possibly enough signal that the screens in the output tubes could flash because of it. It would be common mode noise too, so it would mostly cancel rather than actually blasting through the speakers.

but admittedly that is stretching things a bit. its just the only link I can possibly come up with.
 
The idea of the audible impact being muffled due to common mode injection is exactly what I have been thinking as well, so I don't think it's too much of a stretch at all. The real stretch would be trying to connect the failed resistor events with the glowing inside of just two of the output tubes -- imho of course........ and particularly so since the resistor and tube events didn't happen at the same time........

Dave
 
gadget73 said:
doesn't this require matched quads per side ?

if this is indeed half of R13, I don't see how a V5/V7 failure could be related. Not saying they aren't bad, but gut feeling is the two problems are more coincidence than anything.
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Gadget;

I agree- the murky explanation i got from the ex ARC guy was an issue with negative voltage to a tube- he indicated absent negative voltage the tube would go "wide open throttle".

Here's my uneducated observation: Tap P2 feeds the heaters on V5 and V7. There are no components between P2 and V5 and V7's connection. The heaters were pulsating slowly and irregularly. Would this not be a power transformer issue?
 
It also feeds V4 and V6 though, so I don't see how it would appear in 2 tubes only if it was a transformer problem.

Transformers rarely fail in a way that makes their output voltage unstable. Possible a bad trace on the board or something that caused the heater supply to make and break could do it. Not sure the physical layout of all this.

If there was some issue in V3 it could affect bias. Thats direct coupled to the grids of the output tubes.
 
Solheim said:
... the murky explanation i got from the ex ARC guy was an issue with negative voltage to a tube- he indicated absent negative voltage the tube would go "wide open throttle".
Click to expand...

I see a number of potentially troubling areas in this design. If V1 stops conducting for any reason (open filament, dirty socket, etc), V2 will see +320V on its grid(s). Personally, I wouldn't own a piece for gear from this company. Engineering is supposed to solve problems, not create them. Every design I've seen by ARC appears to be highly complex, and that's not beneficial for either reliability or sonics.

Jack
 
I agree with TriodeLuvr. If V1 was not conducting (possibly someone powered the amp without this tube socketed) R13 will have over 300 volts across it and surely will blow. I also agree with the comments on the amp design. The V5 V7 issue is secondary to this resistor failure. Possibly related, but haven't studied enough to make a call.

Make sure V1 is biased properly (voltages close to schematic values) b4 replacing the resistor. If it checks out good, then I feel good that you can replace it without blowing it again.
 
I overstated the voltage seen on R13 with non conducting V1. The voltage will still get too high and lead to R13 failure.
Too early in my postings history to start exaggerating !
 
I wonder if your Opto Coupler has gone bad? I sent my Classic 120's in to ARC because I was having trouble with the bias. They replaced the optocouplers ( PHoto Coupler, Vtl5C4 ) in both amps as well as all the electrolytic capacitors. The photocopiers were cheap...the capacitors not so much! The good news is they sound absolutely fantastic now! I disagree with Triodeluvr in that Audio Research has always pushed tube technology to the limit as far as perfecting a solid and powerful power supply so that tubes could play at their absolute best sonic ability. I do agree that they are complex. Without Audio Research tubes would probably have faded away by now.
 
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