ChrisM wrote:Just thinking here if I want to use a pentode in V1 and that will push the PI and output tubes.
OR
Use a triode/pentode in V1 and have the triode pushing the pentode which would push the PI and power amp.
Some data sheets suggest the pentode drive the triode, but I think this is more for a direct coupled circuit where the triode is a split-load (cathodyne) PI.
The Sunn Solarus, etc use this arrangment.
I've succesfully added a 500k grid stopper before the triode's grid to such a circuit.
If you put the pentode in your first gain stage, you might be back to the same old mircophonics issue.
I have had great results (read: built/sold five of them) building amps that use the 5879 as V1. From there on, think Plexi cathode-driven tone stack.
In other words: 5879 > gain control > V2a > V2b cathode follower > tone stack > LTP, etc.
The trick to taming microphonics (in my experience) is to run the 5879 at a very low plate voltage, around 100 VDC or even less. Since the pentode makes loads of gain, a lower voltage still puts its effective gain in single triode territory, but with a very smooth top end and great overdrive. Since the small signal pentodes have higher input sensitivity than a 12AX7, you get enormous touch out of such a beast.
The other thing is that the 5879, unlike a 12AX7, won't push DC up the input jack when run at very low plate voltages, like 85 VDC.
