power transformer bad?

[Chuck H]

I've implemented the changes to my bias, thanks to Chuck.Though I can only get -56 to -42 for bias.

Well that's 3 more negative volts on both ends of the adjustment than I thought you would get. Not bad. I really doubt you'll need less than -42V with what will be your plate voltage, but if you do you can change the 4.7k resistor to 6.8k and that'll put your range about -37V to -52V.

[TarheelTechinTraining]

Thanks again. I'm still concerned about the 545 b+. Anyway to bring it down or just go with it.

[g1]

EDIT: Looking at the JTM600 circuit again, it doesn't look like a voltage doubler to me.?. It looks rather like a complicated cousin of the one I posted.

Ooops! You are correct. I saw caps and an oddly drawn bridge and was also thinking a doubler may be required. It's not a doubler in the jtm600 bias circuit, just a cap coupled bridge. They needed to isolate it via caps so they could use the same winding to make the low voltage supply for the preamp (+/-26V).

[Tom Phillips]

Thanks again. I'm still concerned about the 545 b+.

Since you are still working out your bias supply I assume that you have not been able to measure the B+ under working load conditions. Correct? We need to know the loaded voltage before we start to worry about it.

[TarheelTechinTraining]

Yes I have not installed the tubes. Just want it right.

[Tom Phillips]

OK. When you install the tubes and set the bias the B+ voltage will drop to a lower value. Are you aware of that?

[Chuck H]

Ooops! You are correct. I saw caps and an oddly drawn bridge and was also thinking a doubler may be required. It's not a doubler in the jtm600 bias circuit, just a cap coupled bridge. They needed to isolate it via caps so they could use the same winding to make the low voltage supply for the preamp (+/-26V).

I figured the caps were there for isolation. But I've never seen a cap coupled bridge before. Now I know why they did that

[Chuck H]

What Tom said. Voltages for the amp that PT came out of linger around 480Vp. You'll be using a very similar setup as far as the transformer knows. I'll mention again that I had a project where I blew up a few EL34's at 470Vp before trying the Ruby EL34B. The Ruby's are still going strong and accept occasional trips into heavy overdrive with no problems.

[TarheelTechinTraining]

Thanks for the tip. I'll look into them.

[TarheelTechinTraining]

I'm gonna be using the 16ohm tap off the OT and wondered if the NFB should be hooked to the 8ohm or 16ohm tap?

[Chuck H]

I'm gonna be using the 16ohm tap off the OT and wondered if the NFB should be hooked to the 8ohm or 16ohm tap?

Which tap is used for the circuit you plan to build? Use that one. The circuit will be similar enough and the tap is chosen for it's voltage characteristics. So whatever circuit you emulate will be suited to those voltages.

[TarheelTechinTraining]

OK so I have it up and running. Using hi input and everything is well. Move over to Lo input and one of the power tubes starts red plating. How can I delete the Lo input? I never plan on using it anyway.

[Tom Phillips]

OK so I have it up and running. Using hi input and everything is well. Move over to Lo input and one of the power tubes starts red plating. How can I delete the Lo input? I never plan on using it anyway.

Switching inputs as you describe would not cause power tube red plating. More diagnostics are required to find the cause such as.
1) Verified the bias voltage at pin 5 of each poser tube. Please measure and post. Monitor that voltage when red plating starts if possible and report.
2) Move the position of the power tubes and determine if the red plating follows the tube or stays with the socket.

What was the final loaded B+ voltage?
What is your bias setting? (Current draw at idle)

[Chuck H]

OK so I have it up and running. Using hi input and everything is well. Move over to Lo input and one of the power tubes starts red plating. How can I delete the Lo input? I never plan on using it anyway.

If the amp is wired correctly there is nothing about using the low input that should affect the operating conditions of the power tubes. It is a coincidence. I promise.

Don't start the amp again with tubes plugged in.

The initial test for an amp should include firing it up with no tubes in and checking voltages on ALL the socket pins and power supply nodes from the rectifier on. If you don't know how to interpret the results, post them here.

Possibilities are numerous with a new build like this so I'll reserve speculating while we wait for more information.

[TarheelTechinTraining]

No power tubes or no tubes?