Peavey MX VTX Series Amp - Output Section

[TomCarlos]

MEF Members:

A friend has an old Peavey MX VTX Series Amp. These amps are a hybrid design with four 6L6 output tubes. The amp is setup for a 4 Ω load (using the internal speaker).

As of a few years ago, the amp was working, with the exception of the blown speaker. It has since been tossed.

My friend has a spare Black Widow 8 Ω speaker in his arsenal. He asked if it was "ok" to use that speaker. Before giving an answer, I was thinking about impedance matching. My first thought was that you disconnect two of the four output tubes (one of the pairs) and it should be ok. You double the speaker load value from 4 Ω to 8 Ω and remove one set of output tubes - so the output transformer will be happy. However, in looking at the MX VTX schematic, I see a resistor hanging off Pin 4 of the screen grid of V1 but not V2, the same for V4 and V3. Unlike other schematics I have seen that have four 6L6 tubes (which appear to be wired in parallel), this is new to me. So is it possible to remove V2 and V3 and the amp will be happy?

Thanks for the advice.

Tom

[Old Tele man]

Short answer: probably.

Schematic indicates OT 4-ohm (minimum) load with ONE speaker, which is "switched" to OT 2-ohm when second 4-ohm speaker is plugged in.

The 'rule-of-thumb' for vacuum tube OT output loading is that loads between HALF (Z/2) and TWICE (2Z) are OK, with only slight loss of power. But, since schematic says "minimum 4 ohms" I wouldn't use anything below that. A single 8-ohm should be OK, but should be plugged into the master output only...NOT into the auxiliary/external speaker jack.

[Enzo]

My concern is not for the impedance. We all know we can get away with being off one notch. My concern lies in the tube bias. The transistors biasing the cathodes might get confused. Think cathode biased regular amps (not cathode driven) Pull half the power tubes and the current through the cathode resistor drops, meaning so does the bias. SO maybe?

[glebert]

If there is a concern about the bias if tubes are pulled, a 2x impedance from spec is very typical with guitar amps, and the only spec on the schematic is for a min load, it seems like it would be better to keep 4 tubes and use the 8 ohm speaker. Maybe keep an eye open for a 4 ohm speaker to put in there.

[uneumann]

My concern is not for the impedance. We all know we can get away with being off one notch. My concern lies in the tube bias. The transistors biasing the cathodes might get confused. Think cathode biased regular amps (not cathode driven) Pull half the power tubes and the current through the cathode resistor drops, meaning so does the bias. SO maybe?

Enzo points rightly to the bias issue. However, as I see it, Q5 and Q7 are current sources that bias and drive the tubes (in common grid mode). If you pull one set of tubes, the same current that two tubes shared now goes through the one remaining tube. It's going to see twice the idle current and drive current it normally sees. I don't think that's a good idea. I don't think the idea of pulling 1/2 the tubes is a good idea on this amp.

[TomCarlos]

Gents....

You bring up good points and I am now taking a closer look at the schematic and thinking about bias. I think I will recommend to my friend that he should get himself a new 4 Ω speaker. Otherwise, he would install the 8 Ω that he has and he'd need to use an 8 Ω extension cab. I will also help him check the bias with the existing tubes.

Thank you once again.

[Helmholtz]

so the output transformer will be happy.

As others already said, there shouldn't be a problem with connecting an 8 Ohm speaker to the 4 Ohm min output. It is certainly no OT issue. In fact the 8 Ohm load means lower primary and secondary currents and thus relieves the OT. With some amps having high screen voltage, doubling the load can cause excessive power tube screen currents/dissipations. But this is a UL (ultralinear) circuit with reduced large signal screen dissipation.