Would parallel output tubes need both separate cathode resistors and capacitors and separate coupling caps or can i tie the grids together? Would save me cost on coupling caps. Have to add this would be cathode bias. My understanding is that i can just use 1 coupling cap and 1 grid leak cap.
In triode connection looks like there are those additional 9 pF of g1-to-g2 to form Cgp and be multiplied by A+1? However the actual gain will be less, typically some 70% with 5K load. So it's about 87pf per tube....
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Why make your life so complicated with 4 tubes in parallel? 2 in parallel are ok in many cases. I often go for the PSE but 4 tubes? You could get 10W, 15W and 20W respectively in triode, UL and pentode with a single KT88. To get 20W in pentode mode, I use 410-420V anode voltage, 250V REGULATED on g2 and 5K load. Then fixed bias adjusted for an anode current of about 80-85 mA.
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I have heard on the sound qualities of the 6V6 and the relatively low grid swing needed for full power.
If I were to go with the KT88 with the full pentode, UL, triode modes, what would be a good operating point? i..e all that would need to be changed is a switch and not the cathode bias .
Another tube I was looking at was the 7591a, but I cannot get more than a couple of watts out of it for triode it seems.
With a massive input capacitance of 4 6V6's in triode mode you might not get anything good. Multiplying the output tubes beyond 2 is not that straightforward.
For KT88 use 410-420V anode voltage for triode, UL and pentode with 80-85 mA. Then only need an additional 250V regulated supply when switch to pentode.
So it's not really a switch&play amp as you will need to readjust the bias when changing from pentode to UL or triode. But you could spend some time to find a good solution for that if want to switch easy from one to another.
For KT88 use 410-420V anode voltage for triode, UL and pentode with 80-85 mA. Then only need an additional 250V regulated supply when switch to pentode.
So it's not really a switch&play amp as you will need to readjust the bias when changing from pentode to UL or triode. But you could spend some time to find a good solution for that if want to switch easy from one to another.
i will need to revisit this idea. Using a single output tube seems way easier than trying to parallel... especially as it seems you cannot get much power out of a triode connected 6V6. plus it would make the drive requirements potentially easier.
Thank you for all the help so far.
Thank you for all the help so far.
Hi @AudioGeek , wondering if you pressed on with this idea, or if you went with a different solution. I am also considering a quadruple tube SE so was interested how the drive requirements were going to be met.
I did not press on with this... it is still in the back of my mind though. It seems easier to narrow it down to one output tube.
I'm just finishing off a PSE 4P1L amp with 27 mesh driver. That's a level up from PSE 6V6. Barely 18W dissipation but 4P1Ls are cheap and can be run hot. 40mA each at 240V a-k is the sweet spot as recorded recently in the SE 4P1L thread.