Sovtek 100u Diode questions

[Mr_bibbles]

Sovtek 100u on the bench:

http://music-electronics-forum.com/a...ek-mig-100.jpg

This things been a pain in my butt, blowing out the cathode resistor from V2A (1k from pin 3). Replaced a couple failing filter caps and restored a lot of mods to stock, but it still seems to be pulling a lot of current. Bias diode does not measure perfectly...

My question is how would I go about identifying the Bias Diode in this circuit? It is an unlabeled Zener. Could I use 1n4007 in this application?

Could I also use 1n4007 for the unlabeled rectifier diodes? Although I don't think they are the problem, I'd like to expand my knowledge in this area and am under the impression that 1n4007 will work as a rectifier in almost any tube amp...

[Enzo]

I don't see any zeners in there at all, the rectifier in the bias supply is just a plain old rectifier. Current it very low and voltage isn't high, so yes, 1N4007 would work fine.

But you skipped over important stuff. Is there nice clean DC of roughly -50v at point F? and at pin 5 of each power tube socket? If so, the diode is working.

What is a lot of current to you and where are you measuring it?

[Mr_bibbles]

at pin 5 and point F DC is -45 with no lamp. To be more specific about the issue, when the master is up and a signal (guitar) is applied the 1k (910 on schematic) cathode resistor of v2a (*edited*) has smoked twice so far. I'm using a current limiter now and when the master is up and signal is applied the lamp becomes uncharacteristically bright, leading me to believe if I removed the lamp I'd smoke it for the third time.

When measuring DC at point F the DC changed from -35 (lamp connected) to -30 once signal is applied, and creeps back down to -35 after a few seconds when I stop playing. There is no AC present on any socket's pin 5.

[g1]

To be more specific about the issue, when the master is up and a signal (guitar) is applied the 1k (910 on schematic) cathode resistor of v2b has smoked twice so far.

There is a 910 ohm cathode resistor shown for V2A and a 100K shown for V2B, which one are you talking about?
In either case, have you tried a different tube for V2 ?

[Mr_bibbles]

My apologies, it's the 910 (1k was originally on the board so i replaced it with the same) V2A not V2B. I will edit the first post. I have tried a different tube, the first time it smoked it took the tube out with it, which is why I just replaced the resistor and fired it up again... D'oh!

[g1]

I take that to mean the tube has been replaced.
Check for DC on the grid which could indicate the .002 from the previous plate stage is leaky.
The bright bulb issue is probably just from running signal while on the lamp.
What was the original fault and history of the repair?

[Mr_bibbles]

On this amp grid would be Pin 5 of the (el34) output tubes yes? The DC is -45 there. Is there a better place to measure it?

This amp came in heavily modded and blowing fuses. Had to disassemble the preamp to even find the smoked resistor, covered in rubber cement and incorrect capacitors . Amp sounds good now, I was pretty scrupulous about rebuilding it from the schematic, but my first thought was to double check my work. I also replaced several supply caps back up to 100uf from 47uf. Rectifier diodes look OK, plate resistors are cement 1k 5w screens are 1.5k.

[The Dude]

......plate resistors are cement 1k 5w screens are 1.5k.

You mean screen grid and control grid I assume.

[g1]

The V2a cathode resistor and the power tube circuits are completely unrelated.
You said the resistor had burnt twice, is this still a problem?
If so, my post #6 was in response to that issue.

[Mr_bibbles]

Dude: I probably do! Going to revisit Rob Robinnette's site and read up....

G1: Ah ok! I haven't taken it off the limiter to find out if it will smoke again, as the only thing I've changed is a couple 47uf caps with stock 100uf which I don't think would account for the failure. I will measure DC on V2a's grid tomorrow and check the coupling caps from the preceding stage, thanks as always for your valuable time.

[eschertron]

The V2a cathode resistor and the power tube circuits are completely unrelated.
You said the resistor had burnt twice, is this still a problem?
If so, my post #6 was in response to that issue.

G1: Ah ok! I haven't taken it off the limiter to find out if it will smoke again, as the only thing I've changed is a couple 47uf caps with stock 100uf which I don't think would account for the failure. I will measure DC on V2a's grid tomorrow and check the coupling caps from the preceding stage, thanks as always for your valuable time.

I think you can measure for DC on V2's grid pin without V2 in circuit. The plate of the previous tube will still pull that point up if the coupling cap is leaking. With the preamp tube a safe distance away, it should be safe from harm

[Mr_bibbles]

Everything is looking pretty good now, replaced a couple coupling caps before some DC that was present and so far it hasn't blown up. Going to follow up with the dude who owns it and hopefully it's all good! Thanks guys, as always this is my favorite forum where I can simultaneously prove and chip away at my own ignorance.

_Mr. Bibbles, dingus extraordinaire.