Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bruno on March 07, 2016, 05:34:41 am

Title: AC30, bringing voltages down
Post by: bruno on March 07, 2016, 05:34:41 am

Hi guys, as I'm currently embarking on a KT88 build, I remembered I have an old ac30 chassis laying around without a PT.

In the same way I'm going to go with a lower voltage for the kt88 project I was wondering if I manage to bring voltages down on the ac30's El84 plates to around 270V and raise the current on the tubes, would I need to change the output transformer to get a more fitting primary impedance?

Let's say 270V on the plates, and 35ma per tube, that would mean a 2k primary OT would be more appropriate than the more common 4k one, right?

Title: Re: AC30, bringing voltages down
Post by: PRR on March 07, 2016, 03:36:41 pm

Class A, 270V, 0.140A total, looks to me like 4K plate-to-plate happy load. (Yes, SE would be 270V/0.14A= 2K happy load, but push-pull is different.)

We could also use a sloppy argument. EL84 is a european hi-gain skinny-bottle 6V6. 6V6 push-pull at around 270V supply can eat 35mA per tube and a pair is loaded with 8K for 14 Watts out. So a quad would be loaded in 4KCT for 28W out (rounding to "30 Watts" in guitar-amp sales).

Or find the EL84 suggestions (http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL84.pdf). Page 4, "class AB", you can find 250V 31mA or 300V 36mA, both with 8K load per pair. Logically 270V would be 33mA, and not terribly critical. Load per quad would be 4KCT.

Title: Re: AC30, bringing voltages down
Post by: bruno on March 08, 2016, 04:23:31 am

As usual PRR thank you so much for the comprehensive reply.

It's very good news, as I get to keep the existing output transformer.

Thanks