Blockhead Custom 50 - Redplating and Impedance Weirdness

[LesPaulMan]

Just picked up a second Blockhead Custom 50. I love the first one I have so much that I've been keeping my eye out for a second one.

Problem is that the newer one likes to redplate tubes at half volume (biased at 60% dissipation). Upon examination, I found the amp has no control grid resistors (swamp resistors. My first one does have them). The new one does have the standard 1k 5 watt screen grid resistors.

I add a pair of 5.6k swamp resistors thinking what I'm seeing is parasitic oscillation (I mounted it right on top of the socket. Overall lead dress looks pretty neat). Red plating still occurs but seems to be a little better... but not much. Ok, I now bump up the resistors to 10k. Seems to be about the same as the 5.6k's, meaning I'm still seeing red.

Finally, just for a test, I change my output impedance selector on the amp from 16 ohms to 8 ohms (still running in to the same 16 ohm 4-12 cab).
The amp now sounds like it has about 25 percent more power. A much fuller, punchier sound. Harmonics are everywhere and the sustain will last for days. The amp now sounds like my other one, which is killer (really does sound like early VH). The best part... NO RED PLATING what so ever (even with the amp dimed and all the tone controls at 10).

An impedance mismatch not only solved the red plating, but made the amp sound much better.

Can any experts out there tell me what the heck is going on???

[Mastervolume]

You have an issue with your OT. I would check to make sure that it is wired in properly.

How are you checking bias. My guess is that the tube that is redplating is seeing way more than the 60% bias that you think it is getting.

[LesPaulMan]

Thanks for the tip. I'm using a bias-rite.

I'm not a tech, but am learning (I'm very careful working around high voltages). Will look in to the OT's wiring (my first thought was that it was wired wrong, though find that hard to believe considering the build quality of this thing).

Any other opinions are welcomed.



Updated Info: I swapped the tubes from my other Blockhead to the one that was redplating. No Red plating with the different tubes (16 ohm head and cab). I then put in the tubes that did red plate in to the other amp, they redplated. So, the tubes that red plated in both amps were a pair of Mesa branded Sovteks (It does seem that one Sovtek was worse then the other when I did some swapping). The tubes that didn't red plate are SED winged C EL34's.

The Sovteks are now in the garbage.