100 watt super lead lost sound

SilkWilk

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Hey guys I had sound drop out the other day while playing. This is on a Rockit Retro 100 Watt 69 spec super lead. I was running pretty loud an everything sounded fine then sound just completely dropped out (nothing no hiss/nada). I shut it down and discharged the caps to have a look around.

out of the 4 power tubes my v6 JJ EL34 is considerably faded. I also noticed the tube socket has a burnt/soot mark tracing around pins 4 & 5. I also burnt the 1 ohm bias resistor.

I've checked my cab and speaker cable all ohms out and works with another amp.

Its safe to say I've got a problem with power section on this amp and V6 was seeing a ton of current. I suspect a possible arc, possible tube short and then the fuse let go.

but I didnt blow the HT fuse or mains fuse - so far i've checked through the board and all resistors and caps are within range.

I guess it seems like a dumb question but could that lone bias resistor being open be causing the no sound? I tried pulling the two inner tubes (v6 and v5) to see if taking them out of the circuit would give me sound again and still nothing but looking at the schematic i'm led to believe the open is creating the no sound in the power section.

backstory: I had a tube go in this same socket a few years ago- I replaced it along with the screen resistor and the 1 ohm resistor at that time because they were burnt. I DIDNT replace the tube socket and I should have now realizing it was probably wrecked.

I'm fearing I killed the OT - but hopeful that its a simple open due to the burnt resistor and a replacement of the socket, tube and resistor will put me back into action.

I can post pics of the board if that is helpful let me know

Thoughts?
 

SilkWilk

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Sorry when I said the fuse went I meant the 1 ohm bias resistor blew. The fuses are fine. The amp powers on but no sound
 

Dr.Twang!

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No difference. It means the fuses are intact and problems lay elsewhere. How’s your 1k/5w screen resistors? Can you test the EL34s? Possibly shorted the one with the signs of arcing.
oops just reread original complaint.
I would troubleshoot OT for primary to secondary shorts or open, secondary to ground fault, etc.
 
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SilkWilk

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No difference. It means the fuses are intact and problems lay elsewhere. How’s your 1k/5w screen resistors? Can you test the EL34s? Possibly shorted the one with the signs of arcing.
oops just reread original complaint.
I would troubleshoot OT for primary to secondary shorts or open, secondary to ground fault, etc.
Thanks! I did check the screen resistors and they were within spec. My next approach was to check the transformer resistance. Haven’t done that yet was hoping it was maybe just an open one ohm bias resistor but it doesn’t sound like that would cause a no sound issue - darn.
 

SilkWilk

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Could one bad tube or a short to ground cause no sound from the power section?
 

SilkWilk

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I haven’t had a chance to check OT Ive also considered a pi tube that went out possibly. I also have a ppimv that ive noticed for many years had a week spot in the wiper and would cause sound issues at low volume(volume dropout/scratchy). So many things to check I was just hoping maybe if I threw this out there it would a “duh” for you guys that are so knowledgeable. I really appreciate the comments so far!
 

Pete Farrington

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Ive also considered a pi tube that went out possibly
The circuit it’s in couldn’t draw a fraction of the current required to blow a HT fuse.
I also have a ppimv that ive noticed for many years had a week spot in the wiper and would cause sound issues at low volume(volume dropout/scratchy)
Has it got safety resistors fitted, eg 2M2?
 
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SilkWilk

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The circuit it’s in couldn’t draw a fraction of the current required to blow a HT fuse.

Has it got safety resistors fitted, eg 2M2?
Yes it does and they both homes out ok. I also tested the pot resistance while moving the wiper and my meter seemed to follow it fine showing adequate resistance.

I’ve got an old Mullard 12ax7 “Great Britain” in the PI it glows as do the other preamp tubes (and all 4 power tubes with no red plating).

But I am suspect and hopeful it could have went given its age and I used to play religiously at low volumes so the ppimv was really grinding on it. I don’t have any spare preamp tubes so need to get out and get one yet.

I ordered a new tube socket and 1 ohm resistor in the meantime to atleast replace the obvious and try to get back to square.
 

SilkWilk

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The circuit it’s in couldn’t draw a fraction of the current required to blow a HT fuse.

Has it got safety resistors fitted, eg 2M2?
Thanks for your help on this Pete but I didn’t blow the HT fuse so I am pondering if I just lost signal from the preamp all together with a pi tube going out. But I didn’t have any bad sound up to the point of silence the amp was singing. So I’m hesitant to believe that any preamp tube just let go but you never know.
 

PelliX

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Although I find it unlikely, you could of course move your V1 valve to the PI and see what happens (there should at least be a little hiss).
 

Matthews Guitars

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If one of the four tubes shorted and blew the bias sense resistor then it's out of circuit and the amp would still be making sound on 3 out of 4 power tubes.

I'm not positive there can't be other explanations but I'd be testing the output transformer.
 

SilkWilk

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Is it possible to have an open OT without blowing a fuse? I always thought the HT fuse specifically protected the B+ in the event it got to high to protect the OT?
 

PelliX

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Just a random stupid thought; you mentioned that the cabs check out resistance wise and work fine on a different amp. Have you checked the output jack (and/or selector switch?) in this one? Sometimes it can be the stupid little things that get you. Also of course, measure the OT. If there's literally zero sound coming out, you should get a reading that's waaay off.
 


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