help with bias voltage doubler for peavey butcher?

[Chuck H]

Ive had to do this before with a Sound City 50 just to run modern el34's safely. That was easy but this power board has a ground to the heater and a reference off the b+ into a transistor or fet maybe to regulate it. I dont know anything about design just repair

I can't be sure what the CT circuit on that bias wind is doing. It looks like it may be the power indicator and possibly a bias fault indicator, but I don't think it's an amp function circuit. You may be able to bypass it, lifting the CT, implement a different power indicator circuit and use the whole winding to implement the voltage doubler.

Can someone ID what that bias wind CT transistor circuit is doing?

In every example I can find -55V should be fine for 6550's at 500V with any reasonable load. Try the bias rite with only one tube to avoid any confusion and retest. Do this with two tubes in two separate tests. What current reading do you get?

EDIT: Looks like the circuit on the CT of the bias winding is just the power and standby indicator lamps. Looks like a PITA to retrofit all that windings circuitry. Please report on the bias current when you can.

[g1]

It wont. 470 ohm is stock. It would probably get it to -60 but I doubt it. Its gonna have a doubler

Nooooooooooooo! I hate doublers .
Why not via a high value resistor off the HV winding, or even cap. coupled (charge pump) off the HV winding?
See Marshall JTM45 for resistive type, JCM900 for capacitive type.

EDIT: Looks like the circuit on the CT of the bias winding is just the power and standby indicator lamps. Looks like a PITA to retrofit all that windings circuitry.

The +26V line (via R55) is also being used to elevate the heater supply above ground.

[xjbear]

Ooh now we gettin somewhere! I gotta a ton of leftover current from my freed up heater winding

[Chuck H]

Er, ah... Why does available current at 6.3V help?

I'm going with g-one here. Maybe build another peripheral bias supply and ignore modification to the complication of circuits on that winding.

[nickb]

Ooh now we gettin somewhere! I gotta a ton of leftover current from my freed up heater winding

The bias circuit uses a tiny current so that doesn't matter.

If you do choose to pull off the HV AC winding you cannot use a resistor as the voltage swings from 0 to 500V and never goes negative due to the action of the full wave rectifier. The JTM45 or JCM900 use a center-tapped winding where the swing is +/-500V. You'll have to use a 600V capacitor.

[xjbear]

Well your signature will pretty much sum this up after I figure this one out

[g1]

to pull off the HV AC winding you cannot use a resistor as the voltage swings from 0 to 500V and never goes negative due to the action of the full wave rectifier. The JTM45 or JCM900 use a center-tapped winding where the swing is +/-500V. You'll have to use a 600V capacitor.

Whoops! Sorry about that, thanks nickb, I didn't look at the butcher HV supply close enough.
So something like this should work:

[loudthud]

The polarity of C1 in the schematic of post #15 is wrong. The plus side should go to the input.

[xjbear]

Would two watt resistors work?

[Enzo]

Whatr is wrong with doublers? The very similar PV 5150/6505 combo uswes one, as does their Triple XXX amp. There are no reliability issues. Also, this bias supply is always on, while the B+ derived ones are sometimes missing in standby.


Stock R52 is only 470 ohms, on top of 47k in a voltage divider. By the schematic only 2ma is flowing through that 470 ohms, so I donl;t thinm loading is an issue. However, getting voltage division sure is. A nominal 1v drops across that 470 ohms according to the schematic, so there is very little you can do to raise the voltage there. A doubler sits in the space easily and requires no other support circuitry.

The bias winding is center tapped, yes, but it is one end that is grounded, so ther is no point in trying to rewire the thing, it is already taking the highest voltage the winding can offer.

Have you verified that 85ma PER TUBE current by measuring by other methods?

[xjbear]

No I just through a 4 unit bias rite on there and that gives me plate and bias readings on all four tubes. 3 were 84-87ma and one was 97ma ranging from 489-497v plates. If you have any suggestions on multiplying off the taps that are there I would be in your debt. I can even make up a daughter board on vero and stick it in there. I got tons of room

[Chuck H]

I would have expected a more stable bias condition with -55V @ 500Vp for 6550's. But then I haven't been there that often. Only once really.

I was just waiting for someone to chime in with caveats regarding the center tap circuits. The voltage doublers I'm familiar with float the normally grounded side of the winding, so I thought it might not be possible to implement a doubler without buggering the circuits on the CT. With a single rail doubler (like the circuit shown in post #15, with LT's correction) it should be possible.

[xjbear]

I have not played with grid leak/bias resistors either it still has 220k. Ive seen that jcm800's have from 82k or 150k with 6550. Will swapping those 220k down to 82k make a difference?

[Enzo]

So look at the circuits I mentioned in those other PV amps.

[xjbear]

Will do. Ill try 82k here in a bit on bias feed resistors, drop grids to 5.6k and look at my 5150 schems