Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JayCobie on October 04, 2019, 05:54:16 pm

Title: Interstage transformer for distortion?
Post by: JayCobie on October 04, 2019, 05:54:16 pm

Having read a number of posts from various audio geeks about the supposed "special something" that transformers can lend to an audio signal, distorted or no, a thought occurred to me:
I haven't played around with driving, say, a microphone input transformer to the point of saturation and distortion in any meaningful capacity.

I've (understandably?) yet to see an implementation of an amplifier that uses an interstage coupling transformer in between resistance coupled preamp stages, where the primary winding is fed by a coupling capacitor so it won't take the brunt of DC plate voltages.

Granted, my knowledge on the finer points of transformers is still in its infancy, but I've got a few lying around that hold no great value to me, so I want to run some tests on a 1:1 or 1:2 mic input transformer and see how it reacts to some healthy signal levels.

I found a mention of this setup (attached below) in this article by W.G. Morely from circa 1967, but it indicates a fixed bias triode on the secondary side
http://www.r-type.org/articles/art-129c.htm

So my question is:
Are there any pitfalls to using a coupling transformer in between two cathode biased triode stages, with the primary safely behind a suitable coupling capacitor?
Audio fidelity is absolutely not the goal here, so no worries there. I'd just like to avoid a premature meltdown when flipping the switch for the first time.

Title: Re: Interstage transformer for distortion?
Post by: PRR on October 04, 2019, 06:00:16 pm

Mike windings are aimed for 150-600 Ohm impedance.

Small tubes like 5k or more; else the tube will distort bad.

But just try it.

Title: Re: Interstage transformer for distortion?
Post by: shooter on October 04, 2019, 06:35:25 pm

I've fixed a few Gibson's with IS trannies, got inspired, built a plexi-ish EL34's and used a PI tranny, was well liked, the OD distortion doesn't look good on a scope but makes you smile on a guitar.  :icon_biggrin:

If you decide to try, find a simplish Push-pull well tested amp of old, Use the tranny Cap coupled otherwise you gotta be carful with your math n measuring, If they're direct coupled (think: plate Resistor comes out tranny goes in), just like the R is rated in watts, the tranny has a current limit ~~ 50mA typical.  Cap coupling WILL flavor the tone different than direct coupled

Title: Re: Interstage transformer for distortion?
Post by: OrganicEffects on October 04, 2019, 11:48:27 pm

Something along the lines of what you’re talking about, except it’s interestageness is more modular than built in to an existing amp.

http://solorb.com/elect/musiccirc/fuzznikator1/