70s Ampeg V4B Blows Fuses

[g1]

Any idea why 4th power tube tested, caused R28 to burn?

You had replaced it earlier, now it burnt again? That would indicate a problem either with V4 or it's associated circuit.

[vintagekiki]

Why are you chasing power tube quality, it is not important now.

Power tube quality, it's a long story ...

Checked power tubes in dark, had one(1) power tube not lighting or warming up

Three(3) other power tubes I had with me also tested bad. I'm curious what could have caused them to be bad now?

Repair amp always starts from output to input.
From his posts the amp is in a completely unexamined state.
The rule of any healthy repair is to use new tubes as a tool and not "as-as" or "as new" tubes.

Here is 210 post ca7922303 still at the beginning, he accepted a job that he can't do it, upload some measurements (# 193, # 195) that are only clear to him ... ...
We try to help him, he simply has some vision of his own, the only thing that is certain is that he is having a crazy time here.
One is desires and the other is possibilities.
At least he learned something.
At least now he knows what he doesn't know.

[ca7922303]

You had replaced it earlier, now it burnt again? That would indicate a problem either with V4 or it's associated circuit.

So I have replaced R28 a and R203 again and I'm noticing that V201(6ac10) is microphonic. Would it be safe to run amp with everything but V201 installed to see if all the hum and static changes? Have a 6ac10 on order. Thanks.

[g1]

If you can hear that V201 is microphonic, that indicates most of the stuff after it should be ok. So that is a good sign. Did you replace R203 again because you got the proper part, or did it burn?
You can safely run the amp without V201, but you won't get sound from the input or anything before it.

You should monitor the DC voltage at R28 (pin 3 or 8 of V4). Shut the amp off if it rises much higher than 10V (like 15V). It will just burn R28 again.
If the voltage does go high there, V4 is probably bad.

[ca7922303]

If you can hear that V201 is microphonic, that indicates most of the stuff after it should be ok. So that is a good sign. Did you replace R203 again because you got the proper part, or did it burn?
You can safely run the amp without V201, but you won't get sound from the input or anything before it.

You should monitor the DC voltage at R28 (pin 3 or 8 of V4). Shut the amp off if it rises much higher than 10V (like 15V). It will just burn R28 again.
If the voltage does go high there, V4 is probably bad.

Apparently R203 burnt same time as R38, so I replaced it with a 10k 1/2 that I had bought from radio shack earlier( have 3 left now. AlApprecia the help.

[ca7922303]

R28*

[vintagekiki]

About 6AC10
Compactron tubes 6K11 and 6AC10 are not interchangeable, as they are not designed for the same purpose.
Microphonic at 6AC10 appears because the operating mode is not the same as for 6K11.

http://tdsl.duncanamps.com/show.php?des=6K11
6K11 containing two high-mu triodes and one medium-mu triodes.

http://tdsl.duncanamps.com/show.php?des=6AC10
6AC10 containing three high-mu triodes use primarily in color TV

[ca7922303]

About 6AC10
Compactron tubes 6K11 and 6AC10 are not interchangeable, as they are not designed for the same purpose.
Microphonic at 6AC10 appears because the operating mode is not the same as for 6K11.

http://tdsl.duncanamps.com/show.php?des=6K11
6K11 containing two high-mu triodes and one medium-mu triodes.

http://tdsl.duncanamps.com/show.php?des=6AC10
6AC10 containing three high-mu triodes use primarily in color TV

6ac10 is what is in there and that's what I ordered from reading online that they were a good replacement. Ate you saying that the 6ac10 could be causing the preamp issues?

[g1]

6AC10 will work. The gain structure will be changed but it won't prevent the preamp from working.
6K11 are hard to get but fliptops has some: https://www.fliptops.net/catalog/p-1...reamp-tube-nos

[Enzo]

One thing about those triple triode tubes. WHen they were invented it seemed like a god idea. At this point they have all become fairly rare, and are well picked over. They tend to be microphonic and even NOS you may find can be marginal.

[ca7922303]

6AC10 will work. The gain structure will be changed but it won't prevent the preamp from working.
6K11 are hard to get but fliptops has some: https://www.fliptops.net/catalog/p-1...reamp-tube-nos

Do you think that a bad V201(6ac10 ), could be causing the preamp issues I'm having, or any ideas on where to go next? Thanks.

[vintagekiki]

6AC10 will work.

Works but how?
See details #217
Average characteristic each section
6K11 plate current 10.5mA (S1), 1.2mA (S2, S3)
6AC10 plate current 9 mA (S1, S2, S3)

[g1]

Do you think that a bad V201(6ac10 ), could be causing the preamp issues I'm having, or any ideas on where to go next? Thanks.

Did you find out why R28 is burning? (see second part of post #214). Did you get all the power tube heaters running yet?
There are several problems with your amp and they need to be dealt with, if you have done so, let us know. If not, tackle them before moving on to other issues.

[The Dude]

Oh, come on g1, we're only at 224 posts on this one amp. There's plenty of space on the server.

[vintagekiki]

Do you think that a bad V201(6ac10 ), could be causing the preamp issues I'm having, or any ideas on where to go next? Thanks.

No bad tube, no "as new" tube, no 6AC10 tube, no, no ...
Only new tube 6K11
And stop imagining yourself, how you understand amplifiers, because after 220 posts you haven't moved from "do you think", "any idea why" and "thanks"