Is this PT too big?

[MickeyB]

Finished building a Vibroverb 6G16. Having trouble with too much voltage on preamp tubes, mainly reverb. I used a PT that I got cheap 380-0-380. 470v on the plate. I see the PT for it should be 340-0-340 with 440v. My question is, is that 30v enough to make the 12AT7 reverb tube to replate?

[52 Bill]

A tube redplates because it is drawing too much current. Check the grid and cathode voltages in relationship to the plate voltage. What is the voltage on the 12AT7 plates?

If you need to lower the voltages to the pre-amp tubes do it with the dropping resistor that's already there or add another.

[MickeyB]

I checked it and its almost the same as the power tubes, reading 465v.

[52 Bill]

I think design maximum plate voltage for a 12AT7 is something like 350 volts.

I need to look up the schematic for that amp.
Other than the transformer, did you build it exaclty to the schematic?

[MickeyB]

Used Ted Weber's schem & layout for it. Exact for everything but PT & OT.
https://taweber.powweb.com/store/6g16_schem.jpg
https://taweber.powweb.com/store/6g16_layout.jpg
Thanks for looking.

[52 Bill]

What is the voltage on the cathodes?
Did you use a Weber choke and reverb transformer?

[MickeyB]

I did use a weber choke, but the reverb trans is out of another amp. I dont have the cathode readings with me right now. I did measure voltage across the reverb transformer (red wire/blue wire) and it only dropped 5v (470 in, 465 out). Is that right or should it drop more?

[52 Bill]

the reverb trans is out of another amp.
What amp?

it only dropped 5v (470 in, 465 out). Is that right or should it drop more?
The voltage drop is based on the resistance of the winding and the current draw of the tube...so maybe.

When you get to the amp, check the voltage on the cathode connection. Check the cathode resistor (1K 1W) and the cathode cap (25uF@25V).

[MickeyB]

Checking the cathode resistor, looked over and saw one of the screen resistors loose. Sure enough. Took care of that.
Thanks for the help.
Mick