Re: The man wants to make a 212 SET amp.

"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote

> ...Rgin of 1k is easily overcome by using a CF tube with Ro = 100
> ohms.

Of course, I was commenting on the design posted by Oskari. Have you
discussed this CF idea with your customer?

> Even when gI isn't used, a CF is supposed to improve the sound

Some say, others disagree.

> as borne out with the Ongaku amp, which has a CF drive to the 211
> grid
> anyway.

>> Also wondering what the cross-hatching on many of the cap symbols
>> means.
>>
>> Is it permissible to use a distributed load with these things?
>> Not
>> much gain to play with but DF could be significantly improved all
>> the same with perhaps 30% of the winding on the cathode. Need
>> plenty
>> drive volts though.
>
> As I explained in several posts, the last of which was last night,

I don't generally read your posts.

> the use of a high Ea,
> RL of 15k, and almost all A1 op

I was talking about the circuit posted by Oskari

> will give a DF of 6 without CFB.

Par for a SET.

> 6dB of CFB will double the drive voltage from around 70vrms to
> 140vrms,

Correct. And halve the effective input impedance. You have 1000V HT,
what's the problem?

> and make matters worse,

What matters? How?

> with very little reduction of Ro and thd if any.

Rubbish. In proportion to the amount of feedback. As I said, not
much gain to play with, but the improvement in DF without the use of
global feedback could be worthwhile. Do you know what speakers your
target final user has?

My question was whether a distributed load was allowable in the
tradition of the genre.

As for thd, much would depend on how well the voltage amp and output
stages suit each other. Feedback round the output stage will reduce
the distortion produced by that stage in proportion. The problem of
designing a voltage amp for best fit, distortion-wise, would be the
same.

> If FB is to be used, it should be global

Says who?

> and around a tube drive stage
> with no
> ISTs, and with an OPT with about 50kHz open loop bw to start with.

Have you discussed this global fb idea with your customer?

And why ask a question if you already know all the answers?

Still wondering what the crosshatching means...

cheers, Ian


.

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