Univox U-1101

[Gaz]

Hi I was replacing a 2-pong power cord on a buddy's vintage Univox and noticed that it has to 100R resistors blown, which are connected to the plates of just 2 output tubes. Here's the schematic, so you can see just how it's wired:

What is the purpose of those resistors? I'm guessing that the tubes on the other side are probably toast. Funny is that the owner did not even notice a change!

It seems to me that with the plates voltage cutoff the screen would be come the new plates

Should I replace the resistors with the same 2W ones, bigger ones, or get rid of them altogether perhaps?

Any advice on this circuit would be helpful.

Thanks!

[teemuk]

What is the purpose of those resistors?

Preventing oscillation.

[g1]

If those resistors are blown, then you will only be running on 2 output tubes, so power output will be reduced. No big surprise the owner didn't notice unless he runs it at very high volumes.
They probably acted as fuses for some bad power tubes. The 2 watt type will probably be fine if used with good power tubes.

[Gaz]

If those resistors are blown, then you will only be running on 2 output tubes, so power output will be reduced. No big surprise the owner didn't notice unless he runs it at very high volumes.
They probably acted as fuses for some bad power tubes. The 2 watt type will probably be fine if used with good power tubes.

Thanks. I tacked in a couple spare 3W 100R resistors, and at full blast they were dissipating 2.8W! It's either that this amp was never meant to be turned all the way up, or because of the fact that at some point someone changed the 6L6s for 6550s, which draw more plate current. I'm guessing the latter, since both resistors were blown, but I only found one of the tubes to be bad.

My other theory is that the tube actually shorted after the resistor blew because although the 100R/2W blew, the screen supply was still connected with no screen resistor to boot! I think I will change out the resistors for at least 3W units, and add some screen resistors for good measure.

[Jazz P Bass]

I would not go too crazy on the screen resistor value.
Remember that the screens are at roughly half of the plate voltage.
Here is a link to a newer SVT.
The screen resistors are 22 ohms.
Link: http://music-electronics-forum.com/a...1941h3_sch.pdf

[TimmyP1955]

The 22 Ohm resistors were later changed to 220 Ohms.

The U-1011 is a weird design, as there are grid stoppers on only two of the four tubes, and the plate resistors on the other two tubes. I'd be tempted to make it 'normal'.

[Gaz]

The 22 Ohm resistors were later changed to 220 Ohms.

The U-1011 is a weird design, as there are grid stoppers on only two of the four tubes, and the plate resistors on the other two tubes. I'd be tempted to make it 'normal'.

I agree it's weird. I installed 1K screen resistors last time I had the amp. Not gonna play around here for a few extra watts! Actually the tubes that have the plate stops also have the grid stops. The person that ha the amp before me added two 500pf caps from grid to ground at the power tubes, which was probably a smart move for stability. The amp does have a lot of NFB as well.

In my other recent thread I found the OT was arcing and one of those damned plate resistors blew again. Do you think I can just remove them altogether? I don't like the fact that if they open up the screen becomes the plate. No good.