Hi,
I have built some amps with TT power transformers. With all the transformers wired to 240EU, I have the following measures with diode rectifing:

TT Trainwreck  PT, specs 300-50V-0-300 @.3A --> +B 384V with el34 or 5881 biased at 60% + 3x12ax7
TT 36watt PT, specs 300-0-300 @.2A --> +B 422V with el34 biased at 60% + 3x12ax7
TT Fender 5e3 laydown PT, specs 320-0-320 @.085A --> +B 392 with 6v6 biased at 60% + 2x12ax7
TT toroidal 106VA PT, specs 300-0-300 @.126 --> +B 424V with 6v6 biased at 60% + 2x12ax7


How is it possible that the trainwreck transformer gives me less plate voltage than the 36 watt one?

Why does the Fender 5e3 transformer give so little voltage if it is specified that it is valid for a Princeton reverb, which has two more preamp tubes than my design, uses a rectifying valve instead of diodes and according to the diagram it gives more than 440v +B

My main problem is that I want to buy the toroidal transformer 139VA with double 0-300v @.15A terminals. If I put in serial I guess I will have 300-0-300 @.15A. What will be the plate voltage? about 420V or less than 400V with diode rectifiers and 3x12ax7.


Thanks in advance for the help.

Hi,

without checking if these voltages are plausible i would say this may be normal behavior.

A transformer does have series resistance which leads into loss of voltage when being loaded.
When considiering "Voltage AC * 1,414 = DCVoltage" then this is only in free wheeling state - meaning unloaded. Once you load this voltage it will go down depending on how high the load is and how much series resistance the over all constuct of Transformer Seconday and Rectification has.

There is of course a relation to the max current handling of a transformer but as far as i am concerend (and i dont have enough knowledge about transformers in general) 0.3 Amps on one manufacturer must not be 0.3 Amps on another Manufacturer.
The 0.3 Amps - are if i am not mistaken - are also calculated by the max temperature rise in the transformer core and one manufacturer may have other specifications here as another one does have.

Maybe this helps.

Cheers, Geronimo

Zitat von: Leadman am Mär 13, 2024, 12:36
How is it possible that the trainwreck transformer gives me less plate voltage than the 36 watt one?

A B+ of 422V with the 36W PT seems too high.
Two possible explanations:
1) Actual HT voltage per side is higher than 300VAC at idle.
2) B+ was measured in standby mode. So essentially unloaded.

Generally B+ should be measured at the first filter cap with the amp idling.
(Idling means amp running, no signal.)

Zitat von: Leadman am Mär 13, 2024, 12:36
My main problem is that I want to buy the toroidal transformer 139VA with double 0-300v @.15A terminals. If I put in serial I guess I will have 300-0-300 @.15A. What will be the plate voltage? about 420V or less than 400V with diode rectifiers and 3x12ax7.

The voltage of the toroidal transformer drops much more than the voltage of the EI PT so the idle volltage will be higher.
The ratings of the transformer is with a load of ca. 80%.
A schematic of the PSU would be helpful too. Maybe it is needed to make some adjustments.

Regards, Dirk