Driving Parallel Tubes

[CV6045]

I have a question that I think I know the answer to but just wanted to check with you Guys....I have just finished a SE Amp using a 6SN7 (with both sections strapped in Parallel)....this drives a KT88. What I want to do now is to strap 4 X KT88 in Parallel SE mode and use the same 6SN7 driver. My question is ....... will one 6SN7 be sufficient to drive all 4 KT88's ?

 

[DF96]

In pentode mode, possibly. In triode or UL, probably not - because of Miller capacitance.

 

[CV6045]

Driving parallel Tubes

Thank you for your reply, The 4 X KT88's will be used in Triode Mode....so I am a little confused...are you saying that I would need to use more than One Driver (6SN7) ????

 

[DF96]

4 KT88 running in parallel will have a quarter of the input impedance of one KT88. For a triode connected output valve a major part of the input impedance will be Miller capacitance. You need to estimate what this will be, and estimate what you will need to drive it. Numbers will tell you whether you can use paralleled 6SN7 triodes or if you need something different.

 

[CV6045]

Parallel Tubes

Hi and thank you for your reply. I did forget to mention that the 4X KT88's will be driven by an EL34 Triode connected Cathode Follower... So the actual Valve lineup would be....6J5 Cathode Follower Input...into..... A Parallel connected 6SN7 (driver voltage amp)...into ...an EL34 Cathode follower...into...4 X Parallel Triode connected KT88's

 

[DF96]

OK. Please ignore my reply. I will ignore your original question, as it doesn't relate to what you actually want to do.

 

[CV6045]

Many thanks....

 

[funk1980]

Why the paralleled 6SN7's?? Like DF96 said, the 6SN7's or their configuration is irrelevant since there's another cathode follower in between.

 

[CV6045]

Sorry, its a typo....its not a 6SN7 but 6N7

 

[DF96]

So wrong circuit and wrong valve. Are the outputs actually KT88, or was that a typo too? 😎

 

[funk1980]

Sorry, its a typo....its not a 6SN7 but 6N7

Same difference. Still doesn't matter since there is a cathode follower in between de 6N7 and power valves. Any decent CF should be able to drive 4 KT88's. Provided there's enough signal swing from the preceding stage.

But again, why the paralleled 6N7? The added current output/ lower output impedance of the paralleled tubes is wasted on the CF. Why not get rid of the EL34 (a pentode as CF? CF's is general already have massive feedback. You wont benefit from useful linearity) and use the second half of the 6N7 as a CF? Less tubes, simpler design, less heater current, fewer poles...