Peavey Valveking II 100W - TSI circuit burnt

[Intripped]

Hello everyone,
I'm trying to repair this amp that stopped working producing smoke.
You can find the schematics in the attachment.

Inside, I found the TSI (Tube Status Indication) circuit badly damaged and partially carbonized. There's actually a hole in the PCB, due to excessive heat and probably flames; one smd resistor is destroyed and other components are blown.
I will later point out all the damaged components on the schematic, if necessary.

I also found another damage at the output stage:
Two of the four 1.5k grid resistors were burnt as well (R38, R39).
These resistors are receiving the signal from the plate of the phase inverter tube, so I suspect C7 has started leaking, compromising the output tubes BIAS, and causing all the issues to the TSI circuit as well (please tell me if I'm in the right direction, here).

I replaced this cap with a new, 630V rated Mallory capacitor; I don't know how to test the old one: it is not visually damaged, and measures the nominal capacitance on my capacimeter. I should test it under high voltage, but I don't know how to properly do it. (Do you have any suggestion?)

I've also replaced R38 and 39.

Now I would like to try the amp, but I definitely need some help.
I would like to bypass the TSI circuit, and I think I will remove some components from the PCB, in order to isolate the TSI circuit, but I'm not sure about the output stage.
Do I have to remove/bypass the polyswitches on the cathode of the output tubes? Or can I safely leave them there?

Out of curiosity, ...what do you think of all the (over)complicating features of this amp? Among others, there's an output power switch, a speaker enable/disable switch, and of course the fabulous TSI system.

Thanks a lot,
Cristiano

[xtian]

Sorry, no advice to offer, other than, scrap that piece of firewood and build a nice JCM 800 2203 in there.

[Intripped]

Haha!
Well, I will take it into consideration...

[alexradium]

Hello everyone,
I'm trying to repair this amp that stopped working producing smoke.
You can find the schematics in the attachment.

Inside, I found the TSI (Tube Status Indication) circuit badly damaged and partially carbonized. There's actually a hole in the PCB, due to excessive heat and probably flames; one smd resistor is destroyed and other components are blown.
I will later point out all the damaged components on the schematic, if necessary.

I also found another damage at the output stage:
Two of the four 1.5k grid resistors were burnt as well (R38, R39).
These resistors are receiving the signal from the plate of the phase inverter tube, so I suspect C7 has started leaking, compromising the output tubes BIAS, and causing all the issues to the TSI circuit as well (please tell me if I'm in the right direction, here).

I replaced this cap with a new, 630V rated Mallory capacitor; I don't know how to test the old one: it is not visually damaged, and measures the nominal capacitance on my capacimeter. I should test it under high voltage, but I don't know how to properly do it. (Do you have any suggestion?)

I've also replaced R38 and 39.

Now I would like to try the amp, but I definitely need some help.
I would like to bypass the TSI circuit, and I think I will remove some components from the PCB, in order to isolate the TSI circuit, but I'm not sure about the output stage.
Do I have to remove/bypass the polyswitches on the cathode of the output tubes? Or can I safely leave them there?

Out of curiosity, ...what do you think of all the (over)complicating features of this amp? Among others, there's an output power switch, a speaker enable/disable switch, and of course the fabulous TSI system.

Thanks a lot,
Cristiano

what i find very stupid in these low cost amps,but in general in all the modern production where engineers think they are going to simplify the whole "tubes" experience for everybody,by introducing very complex circuits to bias,monitor,power control,etc. in the end they lose the game.
The simpler you make the amp,the most it will last,what is not there can't break.
Anyway,i would remove the connections from the cathodes to the monitoring circuit,just take out the resistors connected to F4,5,6,7 points,maybe remove the opamps as well in case there is some short.
Polyswitch,if they are good you can leave them on,test for continuity.

[Intripped]

hi Alexradium,
and thanks

i've desoldered many components of the TSI circuit, in order to completely isolate it, and avoid any possible shorcut due to blown components.
i've also cleaned the PCB and scraped off all the carbonized parts.

the polyswitches test good for continuity, so i haven't desoldered them.
when the new tubes arrive, i'll try the amp.

i'd really like to know if the C7 capacitor is leaky or not; can anyone suggest a good and safe way for testing it?

[alexradium]

The only way is to apply a high dc voltage on one side,and measure on the other,it should be less than a volt.
You can just install it in place and measure in circuit with no power tubes,if on pin 5 you have much less voltage than the raw bias,its leaky.
I would recheck the bias circuit first,just in case anything is fine.

[Intripped]

i've done my homework,
firstly i've measured all the resistors in the bias circuit and they are ok.
then i've measured the raw BIAS voltage, at max trimmer setting, and it's almost -60V.

when i flip the standby switch there's no change, the bias voltage is steady and unchanged (on the other side of C7, the PI plate, there are around 330V)

...so this capacitor is not leaky, after all.
but these are not good news for me, since now i don't know what could have caused the issue.

[alexradium]

lets say that the 2 grid resistors burned out first,just because they are very low wattage and maybe there is some hot trace underneath or the screen resistors,if they break you dont have any negative voltage on pin 5,that means that current in those tubes goes skyhigh and maybe turned out burning also the other circuit.
The fact that just those 2 resistors burned could be because of that vari class control,who knows.
I would replace all grid resistors and the 220k bias res with 1Watt,carbon film or metal film,then try with good tubes.

[Dave H]

...so this capacitor is not leaky, after all.
but these are not good news for me, since now i don't know what could have caused the issue.

I don't think a shorted C7 could burn out the resistors because there would still be R111 100k in series to B++ Perhaps there was a short from grid to plate or screen in either V5 or V7 then the fault current would flow through R38, R39 and the grid of the other tube to ground?

[Intripped]

I don't think a shorted C7 could burn out the resistors because there would still be R111 100k in series to B++ Perhaps there was a short from grid to plate or screen in either V5 or V7 then the fault current would flow through R38, R39 and the grid of the other tube to ground?

this.
one of the output tubes, V7, looks overheated; brand is JJ, and the red logo stamped on the glass is now brownish.
so the tube fault is not a consequence, but the start of the issue. that makes sense.

@Alexradium:
screen resistors are ok. i've also checked C44 and C43 (from the cathode of the PI) and they also do not leak.
The vari-class potentiometer and the connected resistor R22 are ok on my multimeter.
definitely something has happened on the V7 and V5 side only.

[Intripped]

i'm here again just to say that the amp works.
new JJ-EL34 tubes are in (this amp can use 6L6 or EL34 tubes), and bias adjusted at 31-32 mA, with plate voltage at 465V.
yes, quite a cold bias, but i followed what Peavey recommends: -42.5V bias voltage, with el34s.
...well, i've set it to -41.5V actually, just to stay at ~60% of tubes' max power dissipation

thanks guys!

[fingerpicking]

hi Intripped, I am glad you fixed the amp, I am sure it's a brilliant tube amp.I want to buy one of these amps myself, seems like a great choice for my needs, for reasons too long to explain here. Can you tell me if this amp has a tube effects loop and not a solid state one? The reason is that I like to boost the power amp and don't want to boost a bunch of transistors before it. Thanks!

[alexradium]

hi Intripped, I am glad you fixed the amp, I am sure it's a brilliant tube amp.I want to buy one of these amps myself, seems like a great choice for my needs, for reasons too long to explain here. Can you tell me if this amp has a tube effects loop and not a solid state one? The reason is that I like to boost the power amp and don't want to boost a bunch of transistors before it. Thanks!

you gave me a thumb down for which reason?

[alexradium]

you gave me a thumb down for which reason?

bravo

[fingerpicking]

BTW, I gave you a few more. And to others as well. The reasons are none of your business. People don't have to give reasons why they thumb you down. Thumb me down too, I could care less.