6sj7 grid leak bias- lower gain

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PRR

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Opinions vary (obviously).

If this is your "only" amp, the popular path would be a hi-Mu triode in front, and maybe some loss between this and the hi-Mu driver.

As an "alternate flavor" amp, about anything goes, if it makes you happy. I hate to see fine old amps hacked-over, but as this is "only" a PA amp and long obsolete for that use, why not? _I_ would try to preserve (don't smoke) the PT+OT set with the 6K6es; if they fail, 6V6 was the historic successor to 6K6, and will drop-in with small change of bias. (But "NOS" 6K6 still sell for less than 6V6.)
 
I'm going to try triode strapping the 6sj7. I think this is the best option.
If you do so, you will loose the the beautiful chimeyness of a pentode front end, and the tone while still decent, will be flat and dull by comparison. You can adjust the gain of the preamp stage to suit your tastes by simply altering a few resistor values. No need to spoil one of the best assets this amplifier has in terms of being a guitar amp.


Sent from my phone. Please excuse any typpos.
 
Convert to cathode bias as was said above, don't triode strap. Just type it in google jonesoctalone or ocktalmadness.

Jones' web page appears to be down. I don't know if there are other copies around on the net of his various 6SJ7 guitar amp designs. However, Doug Hammond's Rocktal Fatness amp uses the same 6SJ7 front end -
http://www.sewatt.com/files/sewatt/Rokfat.JPG So you can use that as the basis for how to modify your first stage. FYI Doug Hammond is an experienced designer/builder who is well respected in the guitar amp designer community.
 
First of all let's keep it clear that distortion and gain are not the same thing. Hiwatt amplifiers, for example have a lot of gain their preamps, but hardly ever break up. OTOH the Marshall 18W has very little actual gain, but distorts amazingly.

The actual gain level is primarily set by the anode load resistor value. Here is Valve Wizard's page on pentode preamp stage design - http://www.valvewizard.co.uk/pentode.html With cathode bias the first stage is less likely to have nasty sounding distortion on signal peaks. So the transition into distortion will be more under control, when overdriven.

Sent from my phone. Please excuse any typpos.
 
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If you do so, you will loose the the beautiful chimeyness of a pentode front end, and the tone while still decent, will be flat and dull by comparison.
Based on my limited experience, I agree with AquaTarkus. I too find that a small-signal pentode in the preamp creates much more attractive clean tones than you normally get with a small-signal triode.

The difficulty, if any, comes from two problems that small-signal pentodes seem to be susceptible to: microphony, and a tendency towards blocking distortion if overdriven.

I find that keeping voltage gain down to about the same as a typical 12AX7 stage (say 50 - 60 times) works well for me. I usually just run the thing at about 1 mA anode current, adjust the screen grid voltage until the grid-cathode voltage is about -1.5V or -2V, and then lower the anode resistor value till I get about the voltage gain I want.

This ensures enough Vgs so that powerful guitar pickups don't overload the input, and a low enough gain to avoid the worst microphony issues.

The final step is then to tweak the power supply voltage until I get about the right headroom. Typically Ra ends up between 47k and 68k, and B+ around 150V.

I have also had better luck with the pentode in a position other than the input stage. Using it as the second gain stage in the preamp reduces microphony problems, as there is less gain after it.

-Gnobuddy
 
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