Tremolux bias mod

[nosaj]

If there's 484V there I'm going to want to know what the main caps are (voltage rating).

That's what i wanted to know in post #5.

nosaj

[pdf64]

Bear in mind that sticking a probe and associated cabling onto a power tube plate terminal invites a significant increase in coupling to earlier circuits, greatly reducing the amp’s margin of stability.
It’s regrettable that it’s become standard practice to take the plate voltage reading, rather than the HT, as the latter is far less problematic.
So 484V may be a nonsense reading due to oscillation.
When possible, it would be beneficial to take the HT Vdc, and mains and heater winding Vac readings.

[nosaj]

Bear in mind that sticking a probe and associated cabling onto a power tube plate terminal invites a significant increase in coupling to earlier circuits, greatly reducing the amp’s margin of stability.
It’s regrettable that it’s become standard practice to take the plate voltage reading, rather than the HT, as the latter is far less problematic.
So 484V may be a nonsense reading due to oscillation.
When possible, it would be beneficial to take the HT Vdc, and mains and heater winding Vac readings.

By this you mean a bias probe?

nosaj

[Chuck H]

I'm thinking Enzo has it right. I ran a simulation on PSUD2 taking into account the bump for modern wall voltages and the unloaded voltage was...

482

BING BING BING BING BING BING!!!!

I think it's too close to be a coincidence. I also think that since that model amp runs the EL84 screens at the same high voltage as the plates that if it was a loaded voltage the owner would be asking Randall why his amp blew up rather than requesting a voltage reduction.

[Helmholtz]

Bear in mind that sticking a probe and associated cabling onto a power tube plate terminal invites a significant increase in coupling to earlier circuits, greatly reducing the amp’s margin of stability.
It’s regrettable that it’s become standard practice to take the plate voltage reading, rather than the HT, as the latter is far less problematic.

I agree that in most cases it is completely sufficient and more reliable to take the HT reading instead of the plate voltages. But it would be extremely unlikeky if connecting the meter probe would cause oscillation at both plates. So significantly different readings at both plates are an indication of instability (or a defective OT).

In the rare cases where knowing the plate voltages really matters I recommend to pull the PI tube to interrupt the signal and global feedback paths. This will reliably prevent oscillation and allow for correct plate voltage readings.

[pdf64]

By this you mean a bias probe?...

No, I was thinking of an open amp chassis and a regular probe, meter etc.
Sorry, I should’ve made that clear.
With a bias probe the issue I raised is much less significant, as the extra stuff is outside the chassis, so the likelihood and degree of coupling to earlier circuits is much reduced.

[Randall]

OK, so to follow up, there was indeed 484v on the plates with modern wall AC, but the reason was not the rectifier or the PT, it was because as Chuck H. said, too much bias voltage. The tubes were icy cold. This was with GZ34. Then he tells me he doesn't run it straight off the wall, he always uses a variac at 115v. Adjusting for this, the tubes went down to like 2 -3 watts! We did extensive measurements before and after making any changes (forgive me I already can't remember exact figures). There was a 33K bias resistor where the schematic calls for a 56K, it was a carbon comp, I don't know if it came like that or not. Using the 27K and 50K pot, I found the magic number to be 30K ohms. This put the tubes at about 8 watts, and dropped the plate voltage down to under 10% over 398V on the schematic.

No wonder he wasn't satisfied with the amp, too high plate voltage and barely conducting. Now it sounds really great, and very rich. He is one happy camper now!

[nosaj]

OK, so to follow up, there was indeed 489v on the plates with modern wall AC, but the reason was not the rectifier or the PT, it was because as Chuck H. said, too much bias voltage. The tubes were icy cold. This was with GZ34. Then he tells me he doesn't run it straight off the wall, he always uses a variac at 115v. Adjusting for this, the tubes went down to like 2 -3 watts! We did extensive measurements before and after making any changes (forgive me I already can't remember exact figures). There was a 33K bias resistor where the schematic calls for a 56K, it was a carbon comp, I don't know if it came like that or not. Using the 27K and 50K pot, I found the magic number to be 30K ohms. This put the tubes at about 8 watts, and dropped the plate voltage down to under 10% over 398V on the schematic.

No wonder he wasn't satisfied with the amp, too high plate voltage and barely conducting. Now it sounds really great, and very rich. He is one happy camper now!

So how long was it run at 489v with 450v caps?

nosaj

[Chuck H]

Wow! I was sort of expecting the usual time lag between when you first report and when the customer picks it up. I wanted to suggest adding 1k screen grid resistors. And yes, I know they're not OEM in that classic amp (air quotes) but then Fender didn't think that guys would be clipping them hard for two hour gigs either! At something like 420+ volts on the plates, and more tragically the screens, any el84's that survive won't last more than a few hundred hours a pair before becoming all rattly and microphonic. A little less in my experience.

EDIT: Not to be entirely negative about it, so... No, the tubes won't last very long, but IMHE el84's run like that DO sound huge and amazing. You can hardly believe your ears when you look at how little they are So like I've said here before, smoke 'em if you got 'em.

[Chuck H]

So how long was it run at 489v with 450v caps?

nosaj

That ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^too

[Randall]

It wasn't running at 489 volts with 450v caps, at least since I re-capped a while back. And, as I said he runs it with a variac dialed to 115v, so the plates were at or around 455v I think, with 475V F&T caps.

He has vintage 6BQ5 in it that have been in it for quite a spell I believe, modern bottles won't fit. They've lasted this long, why wouldn't they be better off with lower plate voltages now?

[g1]

Not sure if I'm misreading something but going from 33K to 30K on the bias resistor is barely any difference. Should not change idle diss. from 3W to 8W.
Were there other changes? Or the 33K was bad?

[nosaj]

It wasn't running at 489 volts with 450v at least since I re-capped a while back. And, as I said he runs it with a variac dialed to 115v, so the plates were at or around 455v I think, with 475V F&T caps.

He has vintage 6BQ5 in it that have been in it for quite a spell I believe, modern bottles won't fit. They've lasted this long, why wouldn't they be better off with lower plate voltages now?

So did he not like the amp since you recapped it?
Your very confusing when you state "OK, so to follow up, there was indeed 489v on the plates with modern wall AC," and now you say "It wasn't running at 489 volts with 450v "
You had to run it that way to know that it was indeed there.
nosaj

[Randall]

There was 489v on the plates when measured plugged into the wall. There was significantly less than that when powered by a variac at 115v, which is how uses it, and was not clear at the beginning. I proceeded with it on the variac once that was clear.

I made a typo by omitting "...450v caps, sorry if that was confusing.

"Not sure if I'm misreading something but going from 33K to 30K on the bias resistor is barely any difference. Should not change idle diss. from 3W to 8W.
Were there other changes? Or the 33K was bad? "

The 33K was not bad. And I know, it wouldn't seem like 3K would make a big difference, but it did. At first I clipped another 33K across the original to see what it would do, and the tubes pulled like 42mA straight away for a few seconds. I thought maybe a 1/2 reduction in that resistor would produce something close to double the plate current, but it didn't seem linear like that at all.

The bias resistor was the only thing that was changed today.

I think he has always had this complaint with this amp, I don't think the re-cap has anything to do with it, except to fix some hum and weakish bass response.

[Chuck H]

Just taking all I've read here into a bigger picture... The voltage is too damn high on that amp. My PSUD2 simulation was done taking modern wall voltage into account and I came up with 482V COMPLETELY UNLOADED.

I think this guys coveted vintage amp may have a replacement power transformer or something.