Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on April 20, 2023, 09:01:42 am

Title: 4k vs 5k transformer impedance
Post by: fossilshark on April 20, 2023, 09:01:42 am

Hi all, my friend wants a 5w jcm800 so im thinking im going to do an AX84 high octane build.

The closest output transformer I can find has 5k primary impedance instead of 4k @5 watts. Will there be any other issues besides a loss of output power?

Title: Re: 4k vs 5k transformer impedance
Post by: bmccowan on April 20, 2023, 09:37:15 am

I think you would be fine with the 5K - but Heyboer makes a 4k to 4/8 ohms I believe. I think I bought one from Carl's custom Amps on ebay.

Title: Re: 4k vs 5k transformer impedance
Post by: tubeswell on April 20, 2023, 09:44:32 am

5k reflected load would be better than 4K for a SE EL84 in center-bias Class A.


Zout = Va/(Pa/Va)


250/(12/250) = 5k2

Title: Re: 4k vs 5k transformer impedance
Post by: shooter on April 20, 2023, 10:57:35 am

^^^ agree
I would suggest reading Merlin's stuff on DCCF
also expect with big gain feeding small PA you might wind up with the sags like I did

Title: Re: 4k vs 5k transformer impedance
Post by: jeff on April 25, 2023, 09:15:04 am

Quote from: tubeswell on April 20, 2023, 09:44:32 am

5k reflected load would be better than 4K for a SE EL84 in center-bias Class A.


Zout = Va/(Pa/Va)


250/(12/250) = 5k2

Hello,
Would this mean that a 6V6 running at 360V be better suited to a 10K OT than a 7K?
I built a champ style amp using a 7K but have an extra 10K OT. Worth it/OK to swap out to experiment with the differences?

Title: Re: 4k vs 5k transformer impedance
Post by: jordan86 on April 25, 2023, 12:47:46 pm

I know you did not ask this, but I have built and tweaked an AX 84 SEL. I would highly recommend a conjunctive filter on the output transformer. I have a few threats about it on here.