Ok this is what I did, as you can clearly recognize it's a two Fender Champ pre-amp stages "ensamblage" just for fun, replaced one of the triodes by a pentode obviously the gain is tremendous but indeed even at low volume it picks up some hum noise, so what about controlling a bit the gain from time to time, am I going somewhere by gradually converting the pentode into triode mode ? I suspect being grid-leak this might cause problems with the bias though.
I haven't tried anything yet but I'm asking because after reading some older threads I'm more confused.
I haven't tried anything yet but I'm asking because after reading some older threads I'm more confused.
The valvewizard has a schematic for a pentode to triode control in his chapter on small signal pentode design. I've built it with success as the 1st stage in a guitar preamp.
iff you use a ef86 pentode
you can use the morpf control
triode to pentode with potentiometer
perhaps it works with other tubes also
you can use the morpf control
triode to pentode with potentiometer
perhaps it works with other tubes also
Remove .05 cap, otherwise the 2M pot will act as a tone control, not as volume control, the frequency of the bypass on the screen depend of the resistor and such a capacitor‼
Do you understand, colifa?
Do you understand, colifa?
I will search for that, as I couldn't find the schematic for pentode to triode at Valvewizard, read some threads stating this requires some trial and error. :/
Yes, that's why the question mark on the cap. I mean this can't be that hard, I've seen old schematics which employ two 6j7g such as the Amplivox and I'm sure the gain was kept down somehow in the first stage
I had no trial or error in applying the valve wizard's morph control to a 6AK5 pentode. Gave variable gain between appro 20x and 100x.
The valve wizard has a great site for beginners but the real meat is in his books. Buy them if you are serious about valve instrument amps.
Cheers
JimG
The valve wizard has a great site for beginners but the real meat is in his books. Buy them if you are serious about valve instrument amps.
Cheers
JimG
Thank you again DJ and civic, I will try that on the 6sj7 and 6j7 and share the results. In the schematic above when the 56k resistor is grounded there's no hum at all, zero even at max volume, until you plug a guitar, sounds great but after some time the hum becomes tiresome.
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