newbie question: is the connection to the output xformer somewhat odd?

[jarthel]

here's the schematic: http://www.simpletube.com/databank/S...a_Ciuffoli.jpg

also can I replace the 12ax7 with 5842 (or 5687)? what resistor/cap values need to be used?

thank you.

[Tweeker]

The connection provides cathode feedback to the KT88s.

[Audio_idiot]

Originally posted by jarthel
here's the schematic: http://www.simpletube.com/databank/S...a_Ciuffoli.jpg

also can I replace the 12ax7 with 5842 (or 5687)? what resistor/cap values need to be used?

thank you.

The front stage is SRPP, of course you can use 5842 or 5687 for the job, but I think the B+ of about 430V is a tat too high for 5842 and 5687, may have to drop the B+ to about 300~350 if you don't wish to exhause the tbes too quickly.

I recon the Rk should be depend on which operating point you wish to operate the tubes.

Hope this help...

Cheers

[Kent Smith]

They double up on the 12AX7's because of their low current, high plate resistance. I would think it would be better to use a single 5687 or Russian 6N1P (which I prefer).

Also, you will have a small DC voltage present on the speaker output.

[Tweeker]

The 5687s gain is too low, though running 210V across it is no problem for it. Rating is 330V.

The 5842 rating is 200V, and it prefers less volts with more current.

If using a single tube, one will have to be careful of heater to cathode considerations, as designed the amp avoids the problem by giving the top pair of triodes their own elevated heater winding.

[Tweeker]

Unless your using a preamp, I have doubts as to 5842 or 6N1P giving enough gain. 12AT7 may work.

[Audio_idiot]

Hi,
12AX7/Ecc83 mu=100, very high rp, very low current normally about 1ma or less... normally a bad choice to drive anything at all, but very common as voltage gain stage.

12AT7 mu=60
5842 mu=45
5687 mu=15
6n1p mu=35


The othe tube are just not direct substitution in this circuit. You must do some adaptation.

Why not look at 12BZ7 high gain higher current, but its uses octal socket. 6SL7 is another possibility.

Cheers

[Tweeker]

12AT7, sections parallel: resistor 75ohm/envelope. Bypass capacitor: 2,200uF- 16V. Panasonic FM or other low impedance type, next higher or lower size ok.

Im leary of the 2.2ohm cathode feedback resistors, wouldnt this lead to excessive current through the KT88s as cathode current through the secondary? Misprint of 2.2K? Whatever the value is, if 12AT7 does not quite produce enough gain, one could increase the value of these resistors and reduce the fixed bias voltage to compensate.

[Tweeker]

Adjusted 12AT7 for 2V input: resistor 150R, capacitor 680uF-1,000uF. ~7ma section.