Power transformer 2ndary voltage

[JoshBernstein]

I previously started a thread about my interest in kt66's. Ive built most of the amp i was planning to build, with a matchless ish ef86 preamp and a cathode biased pair of kt66's. All thats really left to buy is a power transformer. My question is, what should the 2ndary voltage be? Im looking to end up with between 400 and 425 volts on the plates using a 5ar4 rectifier. Thanks!

[Phil_S]

There are a variety of things that might affect loaded secondary voltage, so I suggest going with the rule of thumb for the 5AR4, which is 1.3x secondary voltage. To backsolve the problem:
400/1.3 = 308
425/1.3 = 327
I'd allow 4-5% for the loading factor (loaded vs. unloaded).
308*.95 = 292
327*.95 = 311
If this was mine, I'd go for something close to 300-0-300, maybe a bit higher, but not as high as 325-0-325.

[JoshBernstein]

I was thinking around 300-0-300, but wasnt sure if that would be too low, but ill give it a go and see what happens. Thank you, it's greatly appreciated

[ToneMerc]

I was thinking around 300-0-300, but wasnt sure if that would be too low, but ill give it a go and see what happens. Thank you, it's greatly appreciated

IHMO; 300-0-300 would be too low for your target range, even in fixed bias. I would start in the 335-350 range, dependent upon the chosen secondary HT rating and typical mains line voltage.

TM

[Phil_S]

Shows that rule of thumb needs adjustment: http://www.mif.pg.gda.pl/homepages/fran ... 5/5AR4.pdf
I should have looked before I leapt. Sorry for the bad advice.

[Stevem]

I have just recently used the Mojotone JTM high power PT in a 66 Bassman, you might check that one out?

[JoshBernstein]

Well the jtm45 transformers ive found are typically 345-0-345. Will this be okay? Im not too worried about going over my range, just that area would be ideal for me. Thanks again

[tubeswell]

A 5F6A PT - around 325-0-325 (loaded) @200mA, with a good 4A on the heater winding (Each KT66 filament uses 1.3A), with 3A for the rectifier winding should be fine for a pair of KT66 in fixed bias

If you're going to run KT66s in cathode bias, you might want to go higher VAC on the HT winding (say 350-0-350) if you're aiming for 400 on the plates.

[ToneMerc]

Well the jtm45 transformers ive found are typically 345-0-345. Will this be okay? Im not too worried about going over my range, just that area would be ideal for me. Thanks again

I would use it for cathode bias, in addition you could always drop voltage even further if you wished by using a 5U4G. Whereas, if you choose too low of a secondary voltage you can't increase it.

TM

[Firestorm]

This is the problem with almost everything said about tube rectifiers: that they will drop such and such a voltage. Under what conditions? If you look hard enough, you will find voltage data for half load versus full load and have to do the math in between. Amplifiers are dynamic.

[JMFahey]

Do what Jim did and you won´t be wrong.