Tweedle Dee Deluxe B+ questions

[Slim66]

Folks,

I have built the Tweedle Dee Deluxe, which sounds great. However, I notice one of the tubes red plating. Wall voltage is 123V. If I use a Variac to bring things to @ 110V, I have no issues. at 115V, I start to have a tiny bit of red plating. I tried 3 different pairs of tubes, and swapping positions (between V3 & 4). It's always the "weaker" tube that red plates. In other words, it does not seem to be an internal wiring issue.

Also, if I us a 5Y3, no red plating.

So, how can I lower B+ without a Variac, or a new PT. BTW the PT is: (see attachment).

PAM-PT-22-13R_Spec_Sheet_48ff9275-acc2-43d2-817a-35deee3f5bde.pdf

I chose the PT for the option of building a Pro-luxe down the line.

Any help for this newbie would be greatly appreciated.

Thanks,

Slim

[martin manning]

I'n guessing lowering the voltage with a 5Y3 isn't to your liking? The next easiest way would be with a voltage drop in the PT CT, a Zener or a MOSFET. MOSFET type is described here: https://ampgarage.com/forum/viewtopic.php?f=6&t=27035

[nuke]

I assembled a "voltage bucker" into an old surge suppressor metal box. It's just a 12V filament transformer with the secondary in series with the opposite phase of the line.

It drops line by about 12vac, which is about perfect. I run my tweeds on it. I take it with me out to play if I take one of the old amps in my collection out.

[martin manning]

Nuke suggests a good strategy for vintage amps where the PT was designed for 115VAC. In this case reducing the line voltage may result in low heater voltage. What do you get on the heaters when you reduce the line to the point where the red plating disappears? Is it within 10% of the nominal 6.3 VAC?

Another option would be to increase the power stage cathode resistor and run the tubes a little cooler.

[nuke]

Yeah, it is worth checking.

My usual process in determining what a vintage amp expects as mains voltage, is to measure the filament supply under normal load and adjust the mains with a variac until it hits 6.3 VAC RMS (or whatever the filament supply is supposed to be). Generally, they were pretty careful back in the day to keep the heaters spot on.

There are exceptions, some Dynaco phono pre-amps ran the heaters at 11.0vdc to reduce noise.

If you're within +/- 5% of the target heater value, you're probably OK.

The bucker is simple, doesn't even get warm and I can safely run my vintage amps on it when the wall voltage is hitting 123VAC, as it is today.