Homebrew Amp Bias Tap Lacking

[Ed_Chambley]

I just retubed an amp this morning.  Guy said his amps have been eating tubes.  Amp is basically a Marshall 50 watter with EL34, but with a fixed bias section like a fender.  Problem, the Bias Tap reads 20 vAc and I cannot get enough negative voltage which is why it is eating tubes.  I have no idea what the PT is, who made it or what it was intended use is.  Plates are seeing 470 vDc.

Anyone ever modified a fender bias with a voltage doubler?  Would it be best to just use an HT tap?  I could just put a half wave rectifier, but wanted to find out if anyone had a design where the doubler was incorporated into the bias supply?

[clyde]

Or make a tap from one leg of the PT secondary?

[JPK]

I recently had this problem on a new build last winter (Marshall Plexi 20 watt). I'm not sure if my bias circuit is identical to yours but all I did was swap out the 33k tail resistor with a 47k. Now I can get the bias current where it should be with a 6V6GT (28ma max if I remember correctly). You can experiment with resistors in series with the present one until it gets to the bias current you need. In general you need to raise the tail resistor to get higher negative voltage, which will allow you to bias colder. I have an adjustable bias on mine. It would bias too hot at its lowest setting until I raised the tail resistor.

[sluckey]

Problem, the Bias Tap reads 20 vAc

Yes, that's too low.

Would it be best to just use an HT tap?

That's what I'd do to my amp.

[stratele52]

Or make a tap from one leg of the PT secondary?

+1

[eleventeen]

You could do either the doubler or a HV tap...the doubler requires a few more parts...not a big deal either way. Inevitably you'll have to experiment with your dropping/dividing resistors, but it's going to be much the same process regardless of the circuit choice.

I'm a little confused when you say there is cathode bias (implying no need for a separate tap, backwards diode and simple little filter...like what is lacking on a 5E3 Deluxe) versus "tap". Is it a hybrid where some of the bias is derived from a cathode bias structure and some derived from a rectifier-filter? Such a thing is certainly possible but what, if anything, might be gained over either choice (fixed/cathode or variable) I couldn't say.

[stratele52]

Homebrew amp ??? Is this a homemade amps , homemade desing ? No schematic avalaible ?

[Ed_Chambley]

You could do either the doubler or a HV tap...the doubler requires a few more parts...not a big deal either way. Inevitably you'll have to experiment with your dropping/dividing resistors, but it's going to be much the same process regardless of the circuit choice.

I'm a little confused when you say there is cathode bias (implying no need for a separate tap, backwards diode and simple little filter...like what is lacking on a 5E3 Deluxe) versus "tap". Is it a hybrid where some of the bias is derived from a cathode bias structure and some derived from a rectifier-filter? Such a thing is certainly possible but what, if anything, might be gained over either choice (fixed/cathode or variable) I couldn't say.

Fixed is not cathode.  Self bias is.  Fixed adjustable like a Blackface, not self bias cathode like a Valco or 5E3.

[Ed_Chambley]

Homebrew amp ??? Is this a homemade amps , homemade desing ? No schematic avalaible ?

You are correct.  No schematic.  Looks like a parts amp.

[stratele52]

Homebrew amp ??? Is this a homemade amps , homemade desing ? No schematic avalaible ?

You are correct.  No schematic.  Looks like a parts amp.

So you may have check everything.....

[Willabe]

Problem, the Bias Tap reads 20 vAc and I cannot get enough negative voltage. 

Is that reading with the bias tap alone with nothing hooked up to it?


                   Brad     

[Ed_Chambley]

Problem, the Bias Tap reads 20 vAc and I cannot get enough negative voltage. 

Is that reading with the bias tap alone with nothing hooked up to it?


                   Brad     

It is.

[tubeswell]

What Steve said. Run the bias supply from one side of the HT winding with a voltage divider (decoupled at the knee with something like 10-47uF), like a BF Princeton.

[drgonzonm]

look at plexi_superlead_reissue 1987 in this forum's Marshall schematics.  to Provide Bias Tap.