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Swap EL34 for 6L6 in SE amp
#1
Hey all,

I finished building a SE EL34 amp and love how it sounds. I'm interested in trying the preamp with a 6L6 to see what kind of sound it produces but cant find a clear answer on how to do this just by swapping tubes. The output tube is cathode biased to about 25 Vdc at idle and pins 1 and 8 are electrically connected on the tube socket.

Would I need to rebias if I swapped in a 6L6? If so where can i look to find what value i should rebias to?

I've read that screen resistors may need to be replaced. Can anyone confirm this and explain the basis for it?

Thank you!
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#2
Do you have a schematic so we can make direct comments on what you have?

6L6 typically needs a more negative Vgk than EL34, so for something cathode biased you'd need to increase Rk. There is a range of acceptable bias points though, and an overlap between the two tube types so what you have already may be acceptable if not optimal. 

EL34 has higher quiescent Ig2, being a true pentode as opposed to beam tetrode, and the screens conduct more heavily at Vgk=0V (clipping), so are typically fitted with larger Rg2 resistors than 6L6 to limit G2 dissipation. These higher value resistors (most commonly 1k vs 6L6 470R) will marginally increase screen grid compression and limit max power output for a 6L6, but the difference is small enough to be difficult to measure, let alone hear. YMMV.
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#3
Thank you very much for the info Greg. Here's a link to the output section schematic:

http://imgur.com/a/N2erQm1

Disregard the S3 switch and "feedback" line to the 16 ohm OT tap. That was removed. I also left out the 10uf cap between screen resistors.

Also, the EL34 draws about 72mA cathode current on idle, which gets me just over 25 Vdc cathode bias.
So from what I understand based on your advice and other information, the 356 ohm Rk should be increased to maybe like 600 ohms? (600 ohms × 0.072 A = 43.2 Vdc cathode bias). Would this be a sufficient bias for a 6L6?
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#4
I'd also need to know B+1 and B+2 voltages to make an Rk suggestion. 

Why did you remove the B+2 decoupling cap? That would normally result in extra noise and will also limit power and increase screen compression, as Rg2 is now effectively 3k (1k+2k).

Vgk (25V/43.2V) by itself says very little in isolation. Also need to know those other voltages to be able to calculate what the tube is actually dissipating, so I'm not sure where you've picked those other numbers from (72mA cathode current and 600ohms).
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#5
To be honest...I didn't have any appropriate capacitors on hand. Let me draw out an actual schematic with measured values so this is more clear. The 72 mA was measured cathode current through that tube.
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#6
(06-12-2020, 12:33 PM)Longhorn450 Wrote: Hey all,

I finished building a SE EL34 amp and love how it sounds. I'm interested in trying the preamp with a 6L6 to see what kind of sound it produces but cant find a clear answer on how to do this just by swapping tubes. The output tube is cathode biased to about 25 Vdc at idle and pins 1 and 8 are electrically connected on the tube socket.

Would I need to rebias if I swapped in a 6L6? If so where can i look to find what value i should rebias to?

I've read that screen resistors may need to be replaced. Can anyone confirm this and explain the basis for it?

Thank you!


Just use the cathode bias option per the SEL ...(PaMax * 0.9) / Va
Sel Options
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