CF with 2 paralleled triodes (12a*7) - What do you think ?

[roberto]

The tonestack needs an high impedance output (more than 500k).
If you go directly from the wiper of the 1M volume pot to the 50k volume input pot of the power amp, your preamp won't work properly.

[Kagliostro]

Yes, the TS is preceded by a CF, but the TS itself introduces a great loss of signal

K

[thejaf]

I'll take a look at the preamp output and Mark III input again this weekend to be sure that's how I did it (getting older, and CRS is creeping in more often than not). I do know that plenty of signal is getting to the power amp, and the setup is more than than enough volume through a 4x12 to keep up with a loud drummer in a band context.

[tubeswell]

Whilst a discussion about the characteristics of various types of amplifier stages can be interesting, Kagliostro's original question about what to do with the extra unused triode is difficult to give a practical suggestion to without being able to see a schematic of the intended design, in order to assess the gain, loss, input and output impedance, AC and DC loads etc of the various stages and work out where the extra triode would be best applied.

[Kagliostro]

In a previous post of this tread you can see tha schematic of the preamp and the CF I want to use

K

[tubeswell]

When you combine these two schematics, with the FX send/return stages tacked onto the end of the other pre-amp, that will work fine, and you will have used all four dual triode stages. That way the CF in the FX send is driving/buffering the pre-amp output signal when the return stage is not in use.

[Kagliostro]

Hi Tubeswell

Those I consider a problem is that I want to build a standalone preamp

and the output of the return tube isn't a low impedance output so inadequate (to me) as driver for a line

Are you thinking to a particular arrangement of the return section ?

K

[tubeswell]

What about either:

a) Use the FX send as the output for the stand alone pre-amp. It has variable send level output, so just re-design it a bit to allow unity gain output when the send level is at maximum pot rotation. When you want FX loop use the other 'output' as the preamp output.

Or

b) Keep the whole FX loop as an active parallel FX loop with a passive (resistive) mixer to alternate between the normal preamp out and the FX loop output.

Or

c) a combination of a) and b) above?

[Kagliostro]

This I understand and follow you

a) Use the FX send as the output for the stand alone pre-amp. It has variable send level output, so just re-design it a bit to allow unity gain output when the send level is at maximum pot rotation.

Here I'm missing something and I'm not able to follow you (may be it's late and I'm tired)

When you want FX loop use the other 'output' as the preamp output.

b) Keep the whole FX loop as an active parallel FX loop with a passive (resistive) mixer to alternate between the normal preamp out and the FX loop output.

May be a simple block diagram (not necessarily a schematic) can clarify to me what you mean

Sorry, I go bed, tomorrow I must get up early, with a friend I go to an electronics fair & radio amateur flea market

Many thanks for your effort on giving me help

K

[renshen1957]

I want to build a stand alone preamp

the preamp uses 3 12ax7 tubes and I split the schematic from a complete amp

so I must add a CF to the circuit as to have a low output impedance

the problem is adding a 12a*7 tube to be used as CF, I've an unused triode

and I really don't like that, I know I can use a 6av6 tube as CF but this will give one other problem in case there is the need or a spare part, 12a*7 are much more easily available

So I was wondering if I can use the two triodes of a 12*7 tube in parallel as CF



What do you think about ?

Many thanks

K

Hi, If you are going to be driving long cables, low impedance circuits (solid state FXs most aren't sufficiently buffered), or import into a transistor sound board, then use a paralleled 12AU7 as the CF. If you are using the preamp to drive a tube amp, not so critical.

Best Regards,

Steve

[Kagliostro]

Hi Steve

If you are going to be driving long cables, low impedance circuits (solid state FXs most aren't sufficiently buffered), or import into a transistor sound board

The scenario is exactly that

so you think a paralleled tube can be used in this situation

Thanks

K