Hello all,
I have a MM HD-130 chassis number 2475-130. The problem is that the amp is extremely quiet. The notes are very farty and very quiet when played. I have measured the plate voltages on all 4 power tubes and they measure at 420VDC, much lower than what they should be. From what I can tell the amp appears to be all original. The turret board has some brown discoloration (almost like a burn) around where the bias pot\zener diodes are. This amp is from the 12ax7 phase inverter years.
I have the schematic. I have some training in tube amp repair/building/diagnostics, however, I am new to this and schematics are still very difficult for me.
I am hoping that you good people can help me locate where I should be measuring voltages and what those voltages should be.. and then hopefully we can go from there and restore this amp to what it should be.
Thanks in advance,
-M
Well, right off the bat,
Well, right off the bat, has the amp been re-capped in the doghouse with the filter caps? With the voltages being THAT low, MAYBE a crapload is going south. To verify, you COULD pull the transformer leads loose and measure them un-loaded and you should get a boatload more but I'm guessing the filter caps have taken a dump. You SHOULD have around 675-700 volts on the plates with the proper caps working right. It KIND of sounds like your gozinta is going to ground. I'm kind of shocked it hasn't blown the mains fuse! Also, you PROBABLY need to check the voltage coming out of the bias board with the tubes out and see what the bias voltage is on pin 5 of the power tubes with the tubes out. I'm thinking a full re-cap, including the driver board and bias board MIGHT be required. Mike.
PS-I have one of those out and open that I keep around to take pics of because I'm too lazy to fix it and I SURE ass hell don't need a 130 watt amp anymore!
Help is appreciated and more is needed
First of all, thank you for your time and input..
All the caps appear to be original to the amp, that was an easy starting point for me but I just thought I would check here before I jumped right in and started. In fact, one of the electrolytics is signed by "Pat" as is the driver board! I've ordered all the necessary caps and plan to replace those immediately.
The bias voltage on pin 5 of all 4 power tube sockets (sans power tubes) is between -35.3VDC to -51.7VDC depending on where the bias pot is adjusted to.
The heater voltage measured on pin 7 is 8.1VAC, again with no tubes installed.
Also of note, one of the electrolytics (100mfd 450VDC) in the doghouse measures exactly the same (420VDC) as my plate voltage. The other cap of the same size measures nothing. Like you said, this is probably the majority of the problem. I'll let you know after the recap!
Could you clarify which transformer leads to isolate and measure? Sorry if this is rudimentary but again, I'm fairly new at this.
Thanks again!
Sorry I haven't responded to
Sorry I haven't responded to this but I've been out of the loop for a while. Let me look at a schem in a few days and I'll try to help if you haven't done it already! Mike.