Changing input tube from 12ax7 to 6AU6A on OTL project

[Hotze]

Dear All,

Having built / bought the OTL project once published in Glass Audio and built by Bruce Rozenblit. I have too less gain in order to get maximum power and 'easiest ' way to improve gain is to change input tube (12AX7)
to the 6AU6A pentode/triode tube also opted by Bruce himself.
I cannot get better gain or operational voltage by changing the 12AX7 tube by a high gain version or the odd 12BZ7 super gain 12AX7 due to
lay-out and feedback in the circuit.

See for schematic folllowing link: 6AS7GA OTL

According Bruce DC potential should be 200V DC after the 6AU6A.
Want to wire the 6AU6A triode connectod in order to get a fine linearity.

Has anyone of you an idea or even schematic to use so the 6AU6A
instead of the 12AX7?

Would appreciate very much!!!

Best regards from Holland,

Hotze

 

[DF96]

If you wire a pentode in triode-mode then you get much less gain. You choose: gain or linearity? Perhaps the best compromise of the two is a good 12AX7/ECC83 circuit.

 

[the_manta]

Gain of ECC83: 100
Gain of EF94 in triode: 36

Source: http://frank.pocnet.net/sheets/020/6/6AU6.pdf Page 2, near bottom

There is such a HUGE amount of feedback in that amp, that it absolutely doesn't matter in any way what tube you use. ECC83, EF94, 2SK170...Just use anything.

 

[Hotze]

Thanks! Feedback is enormous indeed, so what would you propose, anyway can a 6AU6(A) be used as pentode (lineary as pentode also good) instead of the 12AX7 tube
despite the huge amount of gain and can I get a much better operational gain, or
should I just add before the actual 12ax7 another tube ( e.g. also same type) and use
the plate voltage from the first tube? The transformer can deliver enough current and
I can even use one channel in just one half of this tube keeping same quantity of tubes. I am having yet a nice and simple 12AX7 preamp schematic.
I do prefer use of the 6AU6 ( nicer pentode than the ordinary EF86 ) but a bit uncommon and NOS prices are very low...

Hotze

 

[the_manta]

I think you should try just a simple 6AU6 Pentode instead of the 12AX7.
The GE datasheet has a nice circuit and table for the part specs which you need:
http://frank.pocnet.net/sheets/093/6/6AU6A.pdf

Take that circuit and look how it works.

Additional stagess may be possible, but this could lead to stability issues due to another coupling cap. Furthermore you can't use the usual feedback arragement because the additional tube will add a 180° phase shift.

 

[DF96]

If the amp has a large amount of feedback then you don't want to fiddle with it by changing the forward gain unless you are competent to redesign the feedback network and re-check the stability criteria. On the other hand, if there is already plenty of spare loop gain then you don't need to change the forward gain.

 

[kevinkr]

I would just change the closed loop gain to what you need, fiddling with the input tube without doing this is not going to get you any more gain anyway.

 

[Hotze]

Thanks again for your great contributions.
I was being advised by a tube guru not to touch it as making calculations for an OTL is a business apart. Have some understandings of regular tube amplifier with an OPT. I am using this otherwise great sounding OTL ( much more fluid and transparant than previous 211 mono blocks) with a Taiwanese MHDT
Steeplechase DAC ( GE5670 output tube (maybe yet to replace by another but for the time being combinations sounds fine)).

The actual words from the designer of this schematic (transcendentsound founder Bruce Rizenblit) wrote this about the alternate 6AU6A tube:
" There is one avenue that I did not explore. The input triode could be replaced
by a high gain pentode, which should add another 10dB of voltage gain ( I used the term operational gain..earlier in thread). Pentodes are not as lineair as triodes and are a little harder to work with. I would be interested in hearing from anyone who tried using a pentode first stage (like a 6AU6A) in this amp. If it doesn't degrade the sound it might be an improvement. This is one of those personal preference issues - pentode sound vs triode sound. The DC potential on the pentode's plate must be around 100V to bias the second stage properly" ( I also made a wrong phrase about this in my first post(written at the office...))
Hotze

 

[DF96]

Unclear, but he may be suggesting using a pentode (but not in triode mode) to get more loop gain and so reduce distortion created later in the circuit. The input stage has small signals so small distortion itself. Using a triode-wired pentode would increase distortion by reducing loop gain, while not creating any advantage from the reduced stage distortion. Amplifiers must always be designed as a whole, not in parts.

 

[stocktrader200]

raise 1k feedback resistor to 1.5k or 2.2k

 

[Miles Prower]

I wouldn't do it.

Dear All,

Having built / bought the OTL project once published in Glass Audio and built by Bruce Rozenblit. I have too less gain in order to get maximum power and 'easiest ' way to improve gain is to change input tube (12AX7)
to the 6AU6A pentode/triode tube also opted by Bruce himself.
I cannot get better gain or operational voltage by changing the 12AX7 tube by a high gain version or the odd 12BZ7 super gain 12AX7 due to
lay-out and feedback in the circuit.

It's not gonna make very much difference since it will be inside the gNFB loop. I'd be especially hesitant to try that since there are two RC couplings inside the feedback loop.

See for schematic folllowing link: 6AS7GA OTL

According Bruce DC potential should be 200V DC after the 6AU6A.
Want to wire the 6AU6A triode connectod in order to get a fine linearity.

A trioded 6AU6 will have even less gain than a 12AX7. If it's an input sensitivity problem, the best recourse would be a preamp. You might try reducing the gNFB to raise the closed loop gain, but you're still playing with the overall stability.