7199 to 6AN8 conversion

[Andy Le Blanc]

How successful are 7199 to 6AN8 conversions? Any experiences?

[LOUDthud]

I've never converted an amp, but built one from scratch with a 6AN8. Got a bunch of 6AN8s on ebay and they were all over the place as far as operating point. You'll need to tweek on the screen resistor or cathode resistor to get the plate voltage where it needs to be especially if you are direct coupled to the triode as a split load phase inverter. Ampeg capacitor coupled to the triode in a 7199 circuit that puts a fixed bias on the triode (B18 and B25). Not a bad way to go with a few extra parts.

[Andy Le Blanc]

Thanks, that exactly the application I'm looking at. The fellow tried to get a
conversion socket, but its not compatible with the tube shield. I'll mind the
operating point. The last time I saw a 6an8 was in an old Alamo. The pentode
side was driving the triode as a follower. that drove the verb tank. Haven't
played with the type much. If the sound is satisfactory in this guys Ampeg
I might play with some in a project.

[skyboltone]

Thanks, that exactly the application I'm looking at. The fellow tried to get a
conversion socket, but its not compatible with the tube shield. I'll mind the
operating point. The last time I saw a 6an8 was in an old Alamo. The pentode
side was driving the triode as a follower. that drove the verb tank. Haven't
played with the type much. If the sound is satisfactory in this guys Ampeg
I might play with some in a project.

There are a bunch of tubes of that ilk out there. Some are designed for higher (VHF) frequencies but what the hey, it's just a bottle of electrons. I've got a couple of sleaves of RCA 6EA8s I've been holding on to for such things. I was thinking more of a pentode input and a cathode follower setup though.

Dan

[Andy Le Blanc]

jeez..... the transconductance of the triode is 5x that of a 12AX7.
The plate resistance is less than a 12AU7. The pentode has a third the
plate resistance of others used for audio but looks promising.

[Andy Le Blanc]

The 6AN8 that I have turned out to be excessively microphonic. So I went
with a 6GH8A. The tone is a bit tight but the gain level seems more appropriate.
The 7199, 6AN8 and 6GH8A share their heaters and triode plate
in their pin outs, but everything else different for each type. I wouldn't do
the conversion without clear basing diagrams. Try to maintain the wire color code
and label the tube type inside with a sharpie for the next guy, it could be you.