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

[Kagliostro]

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

[roberto]

The easiest way is to add a IRF820 as a cathode follower.
No extra tube, easy to find spare parts, cheap too.

[10thTx]

I would parallel the triodes PRIOR to the CF

with respect, 10thtx

[Kagliostro]

Ciao Robi grazie per l'info, qui sto valutando una soluzione diversa da quella dello SS, un FX Loop SS comunque mi interessa anche per altri circuiti

Hi Jeff

there is something I'm missing in the schematic you attached



it will be better that I post a link to a reference schematic

http://www.diyitalia.eu/forum/download/ ... &mode=view

Do you mean it will be better to parallel the V3b triode with one of the triodes of the added CF ?

Thanks

K

[Andy Le Blanc]

what do you intend to drive with it?
there are other followers to explore

[Kagliostro]

Here is the schematic of the preamp I want to build as standalone

and

Here is the schematic of the FX Loop from which I want to use the send as my CF

I want to use the preamp as a stand alone preamp to be connected directly to Power Amps or to pedals than to Power amps

K

[tubeswell]

Yes you could parallel 2 triodes in a CF stage, but what a waste of triodes! A single CF triode typically has more than enough current to drive the heaviest of loads you're likely to encounter in a guitar amp.

Much better to do what Jeff said and use that spare triode for another application, or use an IRF820 source follower and have two triodes for somethin' else.

JM2CW

Pete

[Kagliostro]

Hi Tubeswell

reading your post one other idea come to my mind ...........

what about the use of the second unused triode as a return circuit but configured as a second CF ?

this way I can connect pedal effects input and output direct to the preamp

(there is confusion on my mind at the moment)

K

[tubeswell]

Better still, you could use the spare triode as an inverting stage return for an FX loop, and take the FX send from one of your other cathodes like this:

[img:881:492]http://farm8.staticflickr.com/7115/7657 ... 44b1_b.jpg[/img]

in which 5k 'send level' pot also doubles as a variable gain boost for the inverting stage when the FX loop is not plugged in

[Kagliostro]

Tubeswell do you mean to build a complete FX loop for the preamp ?

if so the preamp has no spare triodes, the added tube for the CF will have a spare triode and I can build a complete FX Loop

But as I want a standalone preamp, can be the output of the return of the FX Loop configured as a second CF ?

I hope you can understand what I mean

K

[tubeswell]

I thought you meant that you had a dual triode which you were using one side of for a CF and were looking for a way to use the other side?

My suggestion was to use the other side for a FX loop recovery (inverting) stage, and to take the FX send from one of the other stages in the pre-amp using the above example.

So are you saying you are looking at building a stand alone pre-amp with a CF buffer stage output? (and that you have a spare triode left over?) Or...?

[Kagliostro]

Well, the preamp is a mix of two Marshall preamps

that uses 3 x 12ax7 tubes

as to have an adequate output signal I must add a CF

if I use a 6av6 tube for the CF there are no problems (no unused triodes)

but 6av6 tubes are difficult to be find quickly (if you are on stage as example), 12a*7 tubes can be find much more easily

on the other hand if I use a 12a*7 tube for the CF exit one triode will remain unused

May be I can use this second triode as a return, giving to the preamp an FX Loop send + return

but in this case the exit will be inadequate to be used as to drive a line

if the return is configured as a second CF I solve the problem and I can drive a line with his exit

but in this case the gain of the CF return will be less than 1 and I'm not convinced this is convenient to manage signals coming from pedals

K

[tubeswell]

Have you got a schematic of this idea you can post?

[Kagliostro]

If you mean the idea to have an FX Loop on a standalone preamp, no, sorry, I haven't a schematic, is one my own idea

the referring schematic is those of the FX Loop I posted previously on this tread, this one

I consider the recovery circuit output inadequate to drive a line

K

[thejaf]

Forgive my ignorance here, but you already have a CF output in the form of the tone stack. I recently built a 2-tube "shiva" dirty channel preamp and my output taken from the wiper of master pot. Signal goes into a Dynaco Mark III power amp modified with an input volume (50K pot).

It works and sounds fine to me.