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6as7 pre-amp questions.

hey folks,

got some questions for ya:

going to try a 6as7 loaded with a step-up transformer for voltage gain connected to a 6as7 cathode follower. looking for overall gain of 6-8

A) for the step-up transformer.. what impedance ratios and primary inductance do I need?
B) for the cathode follower output... should I use a choke and cap output or just a 1:1 tranny? again what impdance ratios?

C) where is a good operating point for 6as7's ?
D) should I build a choke or 1:1 tranny with the exact DCR required to bias the cathode follower 6AS7 or add some fixed bias on the secondary of the step-up tranny?

Thanks!!!!!
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> where is a good operating point for 6as7s?

Fat and HOT. 100V 100mA.

> for the step-up transformer.. what impedance ratios

Throw darts. The idea is so whacky that you can do it many ways. Which way is best is not clear at a glance.

Try loading the first tube's plate with 5X its plate resistance. That will give most of the Mu=2.7, and reasonably low THD. Say 1,250 ohms... oh, round to 1K ohms. Gain from grid to plate, cathode bypassed, is about 2.

You want gain of 7, you already have gain of 2, the output CF won't be perfect.... wind for step-up of 1:4. Secondary impedance is 1K*4^2= 16K. This is high enough that you need to rough-up a transformer design (don't forget the huge DC current), estimate the leakage inductance, and compare that to the input capacitance of the CF and your desired high frequency goal. This may be a very tough transformer.

> primary inductance do I need?

What bass response do you demand??? Take the plate as about 250 ohms (near 100V 100mA) and the bass-limit, use a reactance chart or slip-stick or fingers: 1 Henry times 20CPS times 2pies is 125Ω, so 2 Henries will do you.

As a quick check of the idea: get a 230V:60V 100VA power transformer, use it backward. Response will be like 50Hz-5KHz, but in the 100Hz-400Hz range the gain and THD will be within a whisker of a custom-design transformer, and the bandwidth is enough to listen to and judge the possible worth of the concept.

Or did you mean to say you want the first tube cathode-loaded, not plate-loaded? Sure, now you need a 1:10 transformer. Since secondary impedance is already uncomfortably high for a wide-band single-ended heavy-DC transformer, you still want to be around 15K secondary, so you get (scribble....) about 150Ω primary. Ooops, that's awful low for the 100Ω cathode resistance of the tube. Once again we see why CFs are often poor choices.

I hate to sound excessive, but some of the transformer issues will be much easier in push-pull. And the house will be warmer.