power transformer bad?

[Chuck H]

Well... Since you already fired it up and it pretty much worked (if this is your first build that's a miracle BTW ) go ahead and leave the preamp and PI tubes in. It'll get us through some readings a little faster. So... With only the power tubes out of the amp read and record AC volts to ground from all tube pins. Post results as:

V1-1-200, V1-2-.04, V1-3-1.8, etc...

Where V1 would be the input tube and V2 the next tube in the signal chain, etc.

[TarheelTechinTraining]

Gotcha.

[TarheelTechinTraining]

here are the voltages. No power tubes,no load

[Chuck H]

Those look right enough. It occurs to me that with your HV you may have higher than optimal preamp volts for the amp you're building. If you like the tone it's fine. If you think it feels too hard and bright, strident and harsh, peaky and, oh whatever, If the amp needs to be "brown"er you can lower the preamp voltages. Your cathode follower hates you BTW (more on tube selection later ).

With -46 at the grids I'll guess that you're just a little hot for the tubes at your HV. You'll need to increase your bias voltage for the next step. If you still have range take it up to (down to?) -51V at the grids. If you don't have range to -53 change the 4.7k resistor in the bias supply to 3.3k. Do you have a way to measure current? Like a 1 ohm resistor at the cathode ground connection or a bias test socket? If not then what is the smallest value resistor you have? You'll need a way to measure current through the tubes.

[TarheelTechinTraining]

At the grids(5) I have a swing of -41 to -59. Yes I have a 1 ohm resistor at the cathode ground.

[Chuck H]

Sweet. Measure the actual value of that cathode resistor for accuracy. Turn all the controls to 0, turn the amp on and Bump the bias to -51. Plug the tubes back in and warm it up for several minutes in play mode (unless it starts red plating again). Measure the plate voltage.
Now measure the voltage across the power tube cathode resistor. Divide that voltage by the measured cathode resistance. Now multiply that figure with the plate voltage. This is your bias watts. You want about 16 or 17 watts (per tube). If you have more, increase the bias voltage. If you still get red plating with the bias set like this your tubes may not be up to the high Vp.

[TarheelTechinTraining]

Alright Chuck. I've got it biased at 16 watts, 518 Vdc and 31mA. No sign of redplating. What do you think? BTW could you suggest some "brown" sound improvents?

[Chuck H]

What do you think?

Great! How does it sound? Loud enough? Noise level? Pops or intermittent static sounds? All controls functioning?

[g1]

Switching inputs as you describe would not cause power tube red plating.

Agree. Possibly there is some ultra-sonic oscillation happening with the low input. Triple check jack wiring and grounding.

[TarheelTechinTraining]

All controls work, though I didn't try Lo-input. Kinda sterile, decent OD pre dimes master 3/4. No red plating after an hour. On oddity there is a him in speakers w/ sb off power on.

[TarheelTechinTraining]

here is a drawing of my input. Any advice?

[The Dude]

I don't think it's an input issue assuming that you mean the hum is there when the amp is in standby mode- no B+ to the OT. Is that what you mean and is that your configuration? Is the hum there when the amp is running (out of standby mode) and with the volumes down?

If this is correct and I read you correctly, I'm wondering about your filament supply. Can you draw up how your filament supply is wired? Is there a center tap on the filament windings, virtual ground, etc. ?

[TarheelTechinTraining]

Yes you got it. No power to OT, hum in speaker. Power to OT, no hum.The heater is center tapped

[The Dude]

Is the hum from the amp chassis or the speaker?

This may help. Great info from R.G.

http://www.geofex.com/ampdbug/ampdebug.htm

Go through the section "Excessive Hum" and follow the suggestions.

[TarheelTechinTraining]

Definitely not the amp, only the speaker. I check it out the read. Thanks so much,Greg