Very unbalanced plate voltage and bias @ power tubes - 50-watt Plexi Kit

[hourglass]

Hey guys,

Been awhile since I posted here, as I haven't had any issues at all with the kit I put together a long time ago. It's a "mostly" MetroAmp 50-watt Plexi kit in a Weber chassis. I kinda went about things the long way... anyhow it was working fine for a really long time, and then someone knocked it off my speaker cabinet at a rehearsal and it took a hard hit onto a concrete floor.

This cracked one of the power tubes, and I'm guessing it affected _something_ else, too. Ever since falling, it doesn't sound right - it breaks up way too early - like on 1 or 2 with the guitar plugged into any single input. the II inputs breakup later than the I inputs, but they still breakup way before they should.

I was just going through it and I don't see any obviously broken components or connections. After trying a bunch of different pre-amp tubes w/o much change to the early breakup, I decided to try another pair of power tubes. When I checked the bias on the tubes, one of them reads about half of the other one.



I'm reading the bias at the resistor mounted between pins 1&8 and ground. So if V5 measures -33ma, V4 reads -16. I put the Mullard Reissues back in that have been in there since I had to replace my Winged C tubes, and the readings were different, but followed the same pattern -- -24ma on V5, -11 on V4. I cranked it up to -33 on V5 and then V4 went up to -15.


So I guess the question is --- where do I start looking to find what's wrong with this crazy thing?


There is a master volume in this amp. I don't recall if it's a LAR/MAR or a PPIMV. It's whatever one was available first in the Metro store. All that wiring still seems solid and appears to be working fine, but I could be totally wrong about that, too.

Thanks for any help you guys can offer!

Ryan

[hourglass]

After typing all of that, I already have corrections/changes...

Just read thru the 50-watt instructions again and checked the bias voltage at pin 5 on both power tubes -- it reads the same there as on the leg of the last bias resistor, so both tubes are seeing the same bias voltage.

But V4 is running at 15ma (when checked at pin 1) while V5 is running at 33ma. What would cause such a mismatch? It happens on the Mullard Reissue tubes and a brand new set of EH.

Ryan

[hourglass]

Might be getting somewhere... but where I dunno.

Realized I was reading the wrong pin for plate voltage - I was reading about 440V DC on pin 4 of each tube.

Double checking, I see I should be reading pin 3 and on V5 I get 450VDC but on pin 3 of V4 I get just about nothing - like a wavering reading around 1.3V

Same thing without tubes in the sockets, but showing 460VDC on the working V5 socket.

[hourglass]

Maybe I figured it out, and if I did I guess I'm not happy about it.

When I test impedance between the brown wire of the OT and each of my OT primary leads going to pin 3 on V4 and V5 -- I see about 40ohms on V5 and an open circuit on V4.

So I guess I need a new output tranny.

Ryan

[Tazin]

Sounds like the OT's primary is toast. Just to double check I would unsolder the primary wires from the octal sockets and the Brown center-tap wire from its connection point and remeasure.

[hourglass]

Looks like I actually had a few things to iron out...

I replaced the OT with the 50-watt Plexi/800 one they sell at MojoTone. Looks exactly like my Metro 784-139 clone, just with different numbers on the sticker. I thought the old Metro ones were Heyboer and Mojo was Hammond, but whatever... it's back to sounding like it's supposed to.

When setting the bias after replacing the OT, I was still showing different values when measuring on the pin 7-8 side of the 1-ohm resistor. The V4 reading was only 15ma when the V5 was showing 38ma.... and then I thought - what if the big bang that took out the OT and the V4 power tube cooked the little bias check resistor enough to make it super-drift out of tolerance?

So I popped the tubes out and checked them out. The one on V5 measures 0.7 ohm. The one on V4 measures 0.3 ohm. So that certainly explains the discrepancy, and also tells me that I'm probably running too hot so I need to get new resistors to put in place of those guys. In the meantime, I'll have to do some math and figure out where it should be.

[Tazin]

I usually use 1% or 0.5% tolerance 1-ohm resistors.

[hourglass]

The ones in there now are whatever came in my Metroamp kit way back when. Not sure what tolerance they spec'd back then. I'll replace them with tight-tolerance guys.