Fender Bassman 50 bias high

[KCman]

First, the amp came to me blowing fuses. Bad filter cap. I put a temporary cap in to get it running and check it. I found a few resistors way out and another bad cap.

But I also found the bias was around 146mah with a bias probe. I shut it down super quick.

I retested the power tubes (I tested all before anything). They were good on my tester. He said they were original (mid 70s). I put another set of tubes in just to check it. 145mah

bias voltage is -48v. Plates are about 496v. At the center tap of the output transformer, it’s not even a volt different. (Not sure if that’s right? The schematic shows a bigger difference)

i checked the coupling caps between PI and outputs and they were good. I even checked bias with them disconnected, same.

where should I look next? It seems too high to just need a resistance change. Especially since it’s got the USA tubes that he hasn’t changed (or if he did, it was long ago) and it was working.

i don’t have the schematic with me now. I can post it if needed.

[KCman]

I forgot to mention, I replaced filter caps and bias caps after finding a couple bad. Also, changed some resistors. No change after.

[The Dude]

Is this the correct schematic for your amp?

https://www.thetubestore.com/lib/the...-Schematic.pdf

I'm assuming the 146mA is for both tubes? What is current for each tube separately? I'm curious if the current is shared or if one or the other tube is running away with most of the current..

[KCman]

Yes. Thanks

[KCman]

I used a bias probe on one tube.

[KCman]

I also tried both sockets and about the same

[The Dude]

So, you have 146mA for each tube or did you add them together?

[KCman]

It was each tube. It’s one of those adapters that goes between the tube and socket. I didn’t add them. I’ve used it many times, on different amps, and never had it go close to 100?? I’m just confused.

[The Dude]

That is ridiculously high- as if you didn't know that. Are you sure your meter battery isn't low or there isn't a measurement error? Did you measure bias voltage right on the grids to make sure it's getting all the way to the tubes?

[KCman]

I’ll check a little more. I just stopped and asked, basically. It kinda freaked me out. I didn’t check the bias voltage that far. I’ll do that.
The tubes weren’t popp’n red. But I didn’t leave them on long.

[The Dude]

You can pull the output tubes to check if bias V is getting all the way to the grid pins on the sockets. No need to risk damaging the tubes.

[pdf64]

What was your light bulb limiter indicating?

[KCman]

I will check bias V. With power tubes out. Good idea. I did check each side of the balance pot. About the same.

The bulb limiter showed fine (after I fixed the bad filter cap). would bias current be enough to change the bulb? Or, is there something it would point out that’s related to the high bias?

Thanks

[Leo_Gnardo]

Plates at 496V is screamin' high. -48V bias may be close to -47 usually spec'd in Fender schematics, but in your case it's not enough to control the bias current. Here's how to increase bias voltage - in a negative direction - which should calm the bias current to something reasonable. Take a look at your bias control. In this amp it's actually a balance control, and overall bias is set with that 15K resistor between the balance control's wiper and ground. Swap in a 22K or 27K resistor, see if the bias current doesn't settle down. Bias voltage will probably be in the mid -50's, don't sweat it. Give it a go & let us know how that works for you.

FWIW I often substitute a resistor + trim pot for the 15K, then I can fine tune the overall bias, and let the balance control do its thing.

[pdf64]

… The bulb limiter showed fine (after I fixed the bad filter cap). would bias current be enough to change the bulb? …

But what you’ve measured is not a reasonable idle condition; 300mA HT current draw at 500V is 150W at idle that the HT is supplying, so your LBL should be bright.
That it’s not may indicate that your probe is giving a nonsense reading.
I suggest to verify the probe by measuring the OT primary resistances, and the voltage drop across those resistances at idle. Then the anode currents can easily be calculated and compared to the probe readings.