Power tranny output voltages.

[Jazz P Bass]

Oh, that explains it. It's just using your pin 4 as a bridge from +B to the OT CT. No screen resistors heh....unprotected tubes...that'd be a good modification, add 470 to 1k OHM 5W resistors to those pin 4's!

Please make sure that you disconnect pin 3 & 4 when you add the screen resistor to pin #4.
Pin #3 should only go to the output transformer.

[pinkphiloyd]

I've made a couple of posts in this thread that don't seem to be showing up. Screen resistors are in. Amp is breaking up later now. Now, how do I check the bias again? It looks to me like there are already resistors from pin 8 to ground. I put them in when I built it, like in the diagram. Sorry for the brevity, but I don't want to be long winded if this isn't going to show. Everyone's been a huge help.

[pinkphiloyd]

I've made a couple of posts in this thread that don't seem to be showing up. Screen resistors are in. Amp is breaking up later now. Now, how do I check the bias again? It looks to me like there are already resistors from pin 8 to ground. I put them in when I built it, like in the diagram. Sorry for the brevity, but I don't want to be long winded if this isn't going to show. Everyone's been a huge help.

Ha haaa, I was hitting the wrong button!

[Jazz P Bass]

What value resistor did you install?
If a one ohm, 1% was used just measure the voltage across it.
The one ohm reading (in millivolts) will convert directly to milliamps.

[pinkphiloyd]

Thats what I thought. I can get from about 50 to 102 mV. Not sure what the issue is.

[MWJB]

50mV per tube is too much, you want more like 30-40mV...erring closer to the mid/early 30's if plate voltage is 450vdc+

[pinkphiloyd]

It's not. Plate voltage at idle is 396. Between pin 3 and ground?

[MWJB]

Still rather on the high side at 50mV, that's still 20W dissipation, 15-17W would be more typical, you really shouldn't need more than 35mA for the amp to sound right.

[pinkphiloyd]

So, how do I get it lower? Suggestions?

[Bruce / Mission Amps]

At idle, 410vdc at the plates and 18-22w per tube is typical for real vintage tweed 5F4s.
You could either try using a 1k5 to 2k2 resistor in front of your bias rectifier diode, or change the 47K grounding resistor, after the bias pot, to a 56K.

[pinkphiloyd]

At idle, 410vdc at the plates and 18-22w per tube is typical for real vintage tweed 5F4s.
You could either try using a 1k5 to 2k2 resistor in front of your bias rectifier diode, or change the 47K grounding resistor, after the bias pot, to a 56K.

Okay, I'll give it a shot when I get home tomorrow. Thanks!

[pinkphiloyd]

Just a thought, and I'm at work and very tired and therefore even more fuzzy than usual, but as per the diagram, I'm using a 25K linear pot for the bias. I've seen other similar amps that employ a 50K. Could that help?

[jmaf]

Just a thought, and I'm at work and very tired and therefore even more fuzzy than usual, but as per the diagram, I'm using a 25K linear pot for the bias. I've seen other similar amps that employ a 50K. Could that help?

The bias pot forms a divider with that network. If you add a 50k, you'll increase the voltage drop across the bias pot, and the other resistors will drop less. Take your current voltage readings, write them down, without the tubes in test maximum, minimum bias, etc, and redraw the bias circuit changing the pot, then test maximum, minimum and see if the voltages still seem ok. Don't just change the bias circuit with the power tubes in, because if you happen to get bias too low you will damage the tubes or transformer.

[MWJB]

Pot size controls sweep of the bias voltage, as long as you can sweep the bias current to usable ranges, say from 20mV to 40mV per tube, I'd change the value of the 47K fixed resistor to tune that sweep. Larger values will sweep over a colder rang, lower values will give higher plate currents.

[pinkphiloyd]

Well, after putting in those screen resistors, and changing the resistors around the bias pot, I now have 412V on the plates, and it's biased right at 40mV. And it doesn't sound anywhere near as sweet as it did!