7199 as Phase Inverter

[kurisurokku]

Hi everyone, sorry in advance if this question doesn't make sense, I'm pretty new to digging around amps. But I love it!

I have a question about soon-to-be-in-my-possession Ampeg Gemini VI amplifier. It has a 7199 tube used as its phase inverter, and since that tube has a pentode + a triode in it, I'm kind of wondering how that works.

The source of my confusion is the following: I have heard that for your normal 12AX7-type dual triode phase inverter, you want to use a tube that is well balanced, with the two halves of the tube as close as possible. Since the two halves of the 7199 are so different, how does that work? Looking at the schematic, it looks like part of the pentode does the PI work, then the triode does it's thing..... ?

Here is the schematic: http://members.aol.com/portaflex/schems/gs-15-r.gif

[loudthud]

The 7199 is typically used in what is called a split load or concertina configuration. The pentode acts as a voltage amplifier and the triode performs the phase inverter duty but does not provide voltage gain except that it has two outputs that are out of phase. The RCA Tube Manual (60's editions) has examples in the back. The Fender Deluxe 5E3 is an example of this configuration but it uses a 12AX7.

[jonnyeye]

The way the schematic has been drawn makes things a bit confusing. Here, the pentode is a recovery gain stage after the tonestack while the triode is being used as a cathodyne (split-load) phase inverter.

How can you tell? Look backwards from the output tubes, and find the coupling caps to the previous stage. Here they are attached to the plate and cathode of a single triode, which says cathodyne (to me, anyway).

[Corona Blue]

Thread from the dead... but...

I am working on an Ampeg Gemini II G-15, reading up and pondering whether it would be possible to convert this 7199 stage to a more typical Fender 12AT7/12AX7 long-tailed P.I. Reason being that this thing eats 7199's for lunch! And they are pricey! I have heard it is possible to convert from 7199 to 6U8A, but just wondering if anyone has any thoughts on trying the more typical Fender set up.

[jazbo8]

I don't think using 12AX7 will get you enough gain to replace the 7199 pentode section, so 6U8 seems to be a better alternative, but the socket needs to be re-wired.

[Corona Blue]

Thanks jazbo8-

Yes, I figured that would be the case on the gain. I just don't know if the 6U8A's will be any more reliable than the 7199's. But at least they are cheaper (for now).

[soundmasterg]

You could also try rewiring the socket to the 6AN8. These and 7199's were used in Sunns depending on the model and the tubes themselves are pretty close to each other. The 7199 was supposedly "optimized" for audio but it isn't really...without looking at the datasheet, I think it had slightly quieter heaters or something.

Greg

[Enzo]

SUnn used the 6AN8 for amps using KT88 power tubes. They used the 7199 in amps that used EL34 power tubes.