Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shark38j on May 09, 2019, 09:55:34 am

Title: Transformer Recommendation
Post by: shark38j on May 09, 2019, 09:55:34 am

I am looking to build a small amp.  It will have the following:

2 - 12ax7 (1 in parallel and 1 cathode follower)
1 - EL84
SS rectifier

How would I go about finding  the correct transformers for this circuit?

Thanks,
Marc

Title: Re: Transformer Recommendation
Post by: shooter on May 09, 2019, 10:22:15 am

you need a little more info to go shopping;

but i'll get things started with "typical" on the OT side
look at the datasheet, the Ra value is your primary side impedance "answer", the Ia value gives you the other "answer".  Rk is important, but you'll want to have 4-5 resistors "around that value" for tweaking later.

You'll have to deside the speaker configuration for the secondary side of the OT, so if you want to drive a 4 X 12 cab at 2ohms, there must be a 2 ohm tap, or you can get creative and use a 4ohm tap, which with double you primary side (Ra) so you'll want a tranny that has 1/2 Ra into a 4ohm speaker to use a 2ohm cab

PS to follow.....

Title: Re: Transformer Recommendation
Post by: Leevi on May 09, 2019, 11:02:29 am

I would say 5K primary for the OT.
250-280V for the PT secondary, current ~70mA.
1.5A or bigger for heaters is fine.

/Leevi

Title: Re: Transformer Recommendation
Post by: SILVERGUN on May 09, 2019, 12:17:33 pm

Hammond 269 series is a nice little PT
I used 269JX recently and really liked the small size for my application
AX, BX, EX, GX, JX are all the same physical size with different secondaries

https://www.hammfg.com/electronics/transformers/classic/200

EDIT- corrected myself so 2deaf didn't have to - AX and BX are slightly smaller

Title: Re: Transformer Recommendation
Post by: PRR on May 09, 2019, 02:07:48 pm

This is "a Champ". The euro-tube and extra AX7 hardly matter. Champ Replacement iron is usually your Best Buy.