concertina phase inverter not balanced-Sano 500r

[alchemy]

Starting a new thread for the Sano 500r as I think i've narrowed down the problem.

The amp looks to use a concertina phase inverter direct coupled to the previous gain stage. Plate and cathode resistors are 50kohm. The cathode output looks good on the scope, 8.5vpp sine wave going into grid and just slighlty under that coming out. The plate output on the other hand only has about .3vpp and the bottom half of the wave is clipped off. I replaced both resistors with a matched set. The plate is at 250v, the cathode at 214v (which is also the plate voltage of the previous stage and hence the grid voltage of the PI). Also,there is no grid resistor on the input. I've not seen this type of arrangment before and am not really sure where to begin. Seems like the 218v on the grid and lack of grid resistor would throw things off but I'm not sure how this design works.

Any help?

[soundguruman]

Starting a new thread for the Sano 500r as I think i've narrowed down the problem.

The amp looks to use a concertina phase inverter direct coupled to the previous gain stage. Plate and cathode resistors are 50kohm. The cathode output looks good on the scope, 8.5vpp sine wave going into grid and just slighlty under that coming out. The plate output on the other hand only has about .3vpp and the bottom half of the wave is clipped off. I replaced both resistors with a matched set. The plate is at 250v, the cathode at 214v (which is also the plate voltage of the previous stage and hence the grid voltage of the PI). Also,there is no grid resistor on the input. I've not seen this type of arrangment before and am not really sure where to begin. Seems like the 218v on the grid and lack of grid resistor would throw things off but I'm not sure how this design works.

Any help?

Kind of interesting, but you never said what the problem "was."
Have you tried changing the tubes?

[Enzo]

He said the problem was that one side of his phase inverter was making 8.5v of signal but the other was making 0.3v. Half his phase inverter is not working.

[soundguruman]

Well then I guess he has a bad plate or cathode resistor.

[alchemy]

Solved the PI problem. It came with a 12ax7 and needed a 12au7. Didn't catch it as there are no schematics available and I've never seen one of these amps! Nice and balanced now. The overall volume is still pretty low, at least for what I'd expect and what I've heard of these amps. I'm not too good with load lines and such so I'm not quite sure if the 8417's are getting the full signal they need. The preamp gain from input to phase inverter output is 40. What kind of voltage should I be seeing at the PI output to drive the 8417's sufficiently?

[Bruce / Mission Amps]

Solved the PI problem. It came with a 12ax7 and needed a 12au7. Didn't catch it as there are no schematics available and I've never seen one of these amps! Nice and balanced now. The overall volume is still pretty low, at least for what I'd expect and what I've heard of these amps. I'm not too good with load lines and such so I'm not quite sure if the 8417's are getting the full signal they need. The preamp gain from input to phase inverter output is 40. What kind of voltage should I be seeing at the PI output to drive the 8417's sufficiently?

40?
What is the clean PP vac output of the phase inverter, across the two grids of the 8417s? The 8417s are pretty sensitive tubes and might not require as much drive as you think.

[alchemy]

Can you tell me how to measure the "clean PPvac"?
The way I did it just now: I fed a .5vpp sine wave into the #2 input (slightly higher gain than input #1) and have 30vpp at the grid of the PI at full volume. The output tube grid to grid vpp is 150vac (but this is clipped). Not sure of standard procedures for measuring as you have described. Enlighten me?! Thanks for the help....