Parallel Push Pull 6AS7 or 6080

[desperateaudio]

Hey guys,

A few months back I posted a thread about a push pull Rauland Borg PA amp which had an OPT with 6.6K primary and a 500 ohm secondary (As well as 250, 166, 80 ohms as well). It used a pair of 6L6's as the output. I am trying to convert it to a parallel push pull 6AS7/6080 amp. I have worked up the attached schematic and was looking for some thoughts. I plan to start working on it this weekend.

Sorry for the not s great pdf. Scanner is having issues but I think it is reasonablly readable.

Thanks in advance for any comments.

[HollowState]

As far as I know, none of that series (6AS7, 6080, 6082, 5998, 7236 etc.) likes that much voltage across them. Per the books, 250V is absolute maximum. They were designed to work with a 150-200 differential voltage as a series pass tube. With 370V you're asking for premature failure by fireworks.

[desperateaudio]

i believe the max on the 250 is the different between grid and cathode such as if there is 370 on the plate and 135 on the cathode you have not exceeded the 250 because the dif is only 235. could be wrong though. love to here.

past that what about the remainder of the schematic. all comments welcomed

thanks

[gianis]

Dont try 6as7 and the similar pass regulator tubes with fixed bias..

[desperateaudio]

i suspect that is because of the problems with matching sides of the same tube right? if so what about completely separate neg v lines fpr each half of each tube?

[Jeb-D.]

That plate voltage exceeds the maximum rating of the 6as7g. That high of a voltage really wouldn't be a good operating point for the 6as7g if it could handle it.

[desperateaudio]

unless i am wrong max plat is not calculated that way. it is the difference between the voltage of the cathode and the plate as i stated above can someone else comment here on that

[HollowState]

Your diagram shows the cathodes at ground. If they are not, then the schematic is poor enough to not show where they return to. And when they arc out, your driver transformer will go with them.

[desperateaudio]

they are shown at ground due to fixed bias. plan on a fuss at that location should a decide to build these. i have a version of this without fixed bias and similar set up but i would thing cathode at ground is fine with fixed bias?

[tubelab.com]

Trust me I have blown up a few 6AS7's. The data book says not to run them in fixed bias, but I didn't listen. I am still experimenting with fixed bias, but tube runaway is a very real possibility. The only reason that I am having some success with fixed bias is the very low plate voltage that I am running (50 to 75 volts).

370 volts will instantly zap the life out of a 6AS7 in a most spectacular way. There will be lots of sparks inside the tube, with pieces of grid and cathode flying all over. If your power supply can source the current the thin strap that connects up the cathode will blow in half rendering the tube dead! Ask me how I know this!

It is true that the voltage between the plate and cathode is what the tube sees. The cathode bias voltage can run up to 100 volts under some conditions, but this still leaves 270 across your tubes. Too much for most tubes.

A 6.6K ohm load is also rather high for a tube like the 6AS7, especially if you run the sections in parallel.