Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mats on July 24, 2016, 04:53:23 am

Title: Triode boost before 6SJ7
Post by: Mats on July 24, 2016, 04:53:23 am

Hi
I have an idea to set up an amp, like 6SJ7 and 6V6 SE build, and add one 6J5 or 6SF5 before the 6SJ7.
Like a 5C1, but 6SJ7 will be cathode biased.
I would like to make the boost, in a foot switch, if its possible.
So I can switch in and out the triode, before the 6SJ7.
So it will work like 1 Setup like the 5C1 and 2 Setup with the triode boost.
Anyone that can guide me with this, how to set up this ?
Perhaps a schematic.
/Mats

Title: Re: Triode boost before 6SJ7
Post by: PRR on July 24, 2016, 03:49:58 pm

6F5/6SF5 will be a HELL of a gain.

I'd try 6J5/6SJ5 first.

But changing tube and cathode resistor will get you from one to the other.

Title: Re: Triode boost before 6SJ7
Post by: kagliostro on July 25, 2016, 07:12:40 am

I'm not an expert and can only refer to schematics I've seen

here a pair of ideas that seems encounter your question (with the opportune arrangements on components values)

This is from Matchless
(http://i.imgur.com/f6bN0Rv.jpg)

This is from Selmer
(http://i.imgur.com/VqMytgU.jpg)

Franco

Title: Re: Triode boost before 6SJ7
Post by: Mats on July 25, 2016, 02:00:09 pm

Hi !
Thanks for answers.
I will still look for a way to have the 6J5 in/out before the 6SJ7.
Any advise on that ?
/Mats

Title: Re: Triode boost before 6SJ7
Post by: kagliostro on July 27, 2016, 03:05:10 pm

6J5
(http://www.mif.pg.gda.pl/homepages/frank/short/001/6/6J5.gif)

6SF5
(http://www.mif.pg.gda.pl/homepages/frank/short/001/6/6SF5.gif)

As you can on the above datasheet the 6SF5 has a gain of 100 (and the 6J5 20), so in the 6SF5 you have a single triode that has the same gain of the triodes of a 12AX7, on the schematic I linked V1 is exactly a 12AX7 (ECC83) one is arranged with both triodes paralleled and the other uses only one of the two triodes

In the Matchless schematic V1 uses a low value plate resistor (10k) that will result in a low gain also if the two triodes are paralleled (paralleled triodes will give around 15% more gain), in the Selmer V1 has a more "standard" plate resistor and gain is higher (at the level we can see in many amps)

You can give a try to this two different ways to arrange the circuit of V1 using the 6SF5 tube, if you decide that both this configurations give you an excess in signal level you can try the 6J5 tube that has a lower gain

The best way to achieve your purpose is to breadboard your 5C1 and experiment with the add on stage component values till you reach the best result

Franco