fixed bias ?

[chuckb]

Hey guys I have a 15w amp that is cathode biased and I want to change it ti fixed bias. I went by the information given on the Hoffman website. The amp doesn't have a bias supply so I ran a 470ohm resistor from pin 4 of the rectifier tube into a 1N4007 diode to a 100uf/100v cap with positive going to ground.I have a 25ohm bias pot with the center lug going to the neg side of the 100uf/100v cap and a line going from lug 3 to the two 220ohm resistors that are tied together going to a 2k2 resistor to each of the power tubes. I have a 20k resistor on the remaining lug of the pot going to ground. I powered up the ap and the 470 ohm resistor got really hot. Is there something I'm doing wrong here?

Chuck.

[Bruce / Mission Amps]

Hey guys I have a 15w amp that is cathode biased and I want to change it ti fixed bias. I went by the information given on the Hoffman website. The amp doesn't have a bias supply so I ran a 470ohm resistor from pin 4 of the rectifier tube into a 1N4007 diode to a 100uf/100v cap with positive going to ground.I have a 25ohm bias pot with the center lug going to the neg side of the 100uf/100v cap and a line going from lug 3 to the two 220ohm resistors that are tied together going to a 2k2 resistor to each of the power tubes. I have a 20k resistor on the remaining lug of the pot going to ground. I powered up the ap and the 470 ohm resistor got really hot. Is there something I'm doing wrong here?


Chuck.

YIKES! You need something like a 100,000 to even as high as a 200,00 ohm resistor with some power trannys.
You made a voltage divider with low DC resistance that sent a lot of the raw high voltage DC to ground.
Look at the bis supply for a brown face 6G3 Deluxe... do something like that one including lifting the 220K grid load resistors on the power tubes and inserting the bias voltage there.
As far as making it adjustable, look at the way the black face Deluxe Reverb bias supply is made up but use your 100K resistor off the high voltage secondary (lug 4 or 6 of the rectifier tube socket) instead of a singular, lower AC tap from the PT, as the Deluxe Reverb has.

[chuckb]

Hey Bruce, I was looking at the schematic for the Princeton ,where there is no bias tap needed and pin 6 was used for power. It goes from pin6 to a 100k/3w -1N4007-225KL intensity pot-47uf/100v cap wit a 27k resistor tied to ground.One lug of the intensity pot goes to the point between the 1n4007 and the 47/100v meet. The middle lug goes to the middle of the 220k resisitors join together and the other lug isn't used.I used a 100uf/100v cap instead of the 47uf and a 22k resistor in place of the 27k. Also for the 100k /3w resistor I used a 470ohm/10w.

Chuck.

[chuckb]

Hey Bruce ,when you say to lift the 220k grid load resistors and inserting the bias voltage there ,do you mean remove the two 220k resistors altogether and the bias voltage then goes to the two power tubes?

Chuck.

[MWJB]

What Bruce is saying is because you are drawing your bias voltage from your high voltage rectifier winding (likea Princeton), rather than your dedicated, low voltage bias winding (like on a Deluxe), that the 470ohm dropping resistor is way too small, you need to use at least 100K in place of your 470ohm.

Bias voltage is applied at the junction of the 220K - DO NOT REMOVE THEM, they are also the load resistors for the power tube grids.

While you are experimenting, keep the power tubes out of the amp. You can set ball park bias without the the tubes installed, thus saving them from getting burnt up. When happy that bias voltage is in the safe range, install power tubes & fine tune plate current.

A 25ohm bias pot isn't going to do anything, 20K-25K might be more useful. You said in the first post that the bias circuit load resistor was 2K2, later you say it was 22k, please confirm.

I assuming that the amp is 6V6 powered? If your amp is EL84 powered the voltage divider will need to be different (you just say "15W amp"). What -dc voltage sweep are you getting at power tube pin 5 (no tubes installed)? What kind of plate voltage are you expecting under load?

[chuckb]

Hey MJWB, Thanks for your reply. I have a 22k as the bias load resistor,sorry about that.I do have a 100k/2w ,will that be alright for the dropping resistor?Yes, the amp is 6v6 powered. I haven't checked the -DC voltage sweep @ pin 5 yet but I will post it later.I have a 10KL,25KL,100KL,250KL pots. The deluxe reverb circuit calls for a 10K pot. Which will I use?


Chuck.

[MWJB]

I'd use the 25K pot, in series with your 22k to ground, but wired as a variable resistor - you want the resistance to decrease (& current to therefore rise) as you turn the pot clockwise. Bias voltage comes off the wiper, end of pot connects to 22K to ground.

Set negative voltage, to say -40vdc, if your B+ is around 400vdc, maybe -30 if 350vdc (somewhere in between if ...er, somewhere in between), so it will be cold when the tubes go in, fine tune current (10W idle dissipation would be a good starting point) with all tubes installed.

[chuckb]

Hey thanks, I appreciate all your help. I'll let you know how it works out when I get it done.

Chuck.

[MWJB]

"Bias voltage comes off the wiper, end of pot connects to 22K to ground." Actually it might be safer to tie the wiper to the 22K & have the bias voltage at the end of the pot, still want resistance to drop as turned clockwise.

[chuckb]

I have everything hooked up as you say with a 22k from one lug of the pot to ground,center lug going to - side of 100uf/100v cap and the last lug going to the junction of the two 22K resistors going to the power tubes.I have 34vdc coming off pin 6. I turn the pot clockwise and I don't get any readings from either pin 5,only every now and then it shows -1 to -3 sometimes when I turn the pot it goes up to -7or-8 and dissappears.I tried taking a reading on pins 5 with the tubes in and still the same. I thought I might have a leaky cap so I changed the 100uf/100v cap and still nothing.

Chuck.

[chuckb]

I have a reading now of -50 on first tube and -36 on the second tub(closest to rectifier). I can get very little or no change when turning the pot. How do I go about getting the bias as close to each other as possible. Can the power tubes have a individual bias pots so I can bias them individually?

Chuck.

[Bruce / Mission Amps]

I have a reading now of -50 on first tube and -36 on the second tub(closest to rectifier). I can get very little or no change when turning the pot. How do I go about getting the bias as close to each other as possible. Can the power tubes have a individual bias pots so I can bias them individually?

Chuck.

If you built this bias supply correctly and measured this bias voltage with the tubes installed, it sounds like you have a bad power tube.
There should be exactly the same -vdc on the grids.... so, something isn't right.

The stock amp has it's 220K grid load resistors grounded... lifting them means lift them from ground and insert the negative bias voltage to the junction of the two 220K resistors.
The amp does not need more then about -35 to -40vdc so I'd yank the 22k fixed resistor an use a 6k8 to 10k first.
Just add that up.... it is holding your new bias supply from ground by 6.8K + 22K = <-30K
the 330vac is across a 100K resistor, then a diode... reverse biased of course... and then on a filter cap which charges up to a peak voltage of what is left over.
If the bottom half of that resistance (the pot), {100K/29K} is variable then you have a variable low current bias supply.

[GrungeMan]

I try to make sure the power tubes a matched set, are they?
Did you measure the 220K resistors to see if they are of equal value?

[chuckb]

Hey Bruce,I have it sorted out now. I removed the two 220k grid load resistors from ground and I didn't have any 22k resistors left so I used a 20k+1.5k connected in series across the 100ohm/100v cap. I have a 250kL bias pot.
The reading I'm getting is -29vdc on both tubes now.I can adjust it down some also if I like. Thanks again guys.

Chuck.

[GrungeMan]

You can adjust it to taste from there, my preference is 26-27v.

Did you measure the two 220k resistors to see if they are a matched pair?