Trainwreck express big troubles! Red plating

[colinaae]

Hello everyone!

I have built this thing like 4 different time and tried different caps, different wire ect... I keep coming back to the same problem. I'm red plating on my power tubes. I'm starting to test and I supplied my rectifier with 300-0-300. After the rectifier I am drawing 424v before turning the standby on. I flicked the standby switch to play mode and here's where my problems start.

Off the standby switch I'm getting 417v before the 1k 25w resistor. After that resistor im getting 398v.
Now this is my big problem that I need help with. After the 18.2k resistor to the B+2 I'm getting 54v. Then B+3 im getting 53v and then B+4 I'm getting 40v and declining. My two 9.1k resistors are getting so hot that it's melting the solder that is connecting them. I checked each resistor and they are all dead on. I think I'm using the wrong type resistor in that spot if it's heating up that much. After seeing these readings I turned the amp off and looked at all connections. Tried to rule anything out. After I let the amp cool down I was curious if I could get the bias to idle at around 30mv or so with 6v6 tubes. I did my test at the bias cathode resistor which is a 1 ohm 2 watt. I'm getting 282mv at the lowest point of my bias pot. I switched the 47k resistor before the bias pot to a 68k and that seemed like it did nothing.

Please help. I'll try anything at this point because this amp is frustrating me beyond belief.

Thanks
Colin

[BLT]

I can't see what's going on under your power supply board, but on the face of it, it looks like your bias pot isn't hooked up, and there's a possibility of some grounding problems in that same area.

[colinaae]

Hey BLT.
I have it wired under the board along with the ground from my bias caps. I am just wondering if I need a bigger ground wire to chassis? Or should it be at its own ground point? Right now it's grounded at the same point as my supply ground, rectifier center tap and filament center tap.
Also should my ground from my filter cap be bigger as well?

What should my voltage be after that 47k resistor into my bias pot?

I'll keep trying things but I appreciate the info on where to start.

Colin

[lumox0013]

I use those jjs capcans often and every once in a while you will get a bad section you should not be dropping that much voltage. I think that section of the can may be feeding directly to ground. that being said It looks like your power supply wiring could use some clean up (pretty sketchy) these amps like a bottom sheild and I would not be having a large metal plate over that many flyaway wires with high voltage running through them.you may consider eliminating the power board and puting a bias board up on the sidewall ala dumble good luck, these amps are super picky on the layout side . cheers lumox

[colinaae]

I am going to do exactly what you suggested. I am going to make my bias board smaller and mount on the side wall. I understand the voltages underneath and everytime I have wired this amp I've had the power supply wires in the same spot. I'll try that and get back to you with results. I also just ordered a "matched pair" of both el34s and 6v6s. I like the 6v6 sound so I really hope it works. I checked the tubes I've been using in the tube tester and they came out good but I seem to think they are too hot possibly for this amp?

Thanks for the suggestions.

Colin

[Tonegeek]

Unplug amp and Discharge the caps!!!!
If you are also having bias issues (red plating), I would remove the output tubes.

Then check the cap can and make sure it is not hooked up backwards.
If that checks out, unhook one end of the wire feeding the phase inverter plates and see if the 18k2 pair of resistors stopped overheating. Don't leave the amp on any longer than necessary to verify (This would be a good time for a light bulb limiter if you have one). If it still overheats but the other 9k1 resistors are OK, then you have a shorted or miswired cap can. If it checks out ok then the problem is on your main board somewhere. Look under the board around the PI and and see if anything is shorted or miswired.

[Firestorm]

You have a short after the 18K2. Figure out where. It also appears the output tubes are not getting negative bias voltage. Take the tubes out and measure Pin 5 to ground for DC. I can't make out the wiring at the master, but that may be where the bias problem is.

[colinaae]

So what I did was I disconnected the PPIMV and put the two 220k resistors back on the board to rule that out.
I also lifted my bias wire off the board and did a floating connection. Under the board I had a wire going from my connection point on the board to the 100k and 82k plate resistors. Meanwhile I had a jumper wire on top of the board to the 220k resistors. So that was my short!
I have my bias dialed in real good now so I appreciate all the help from you guys who replied.

Thanks a lot and keep on rocking!

Colin