No bias windings on pwr tranny

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aerojazz
Senior tube assistant
Posts: 145
(2/29/04 3:20 pm)
 No bias windings on pwr tranny
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 I bought a power tranny from MM about a year or so ago. Just now got into this 100 watt plexi project. Well I have no bias windings. Is this a mistake or is there something I'm missing?

Would there be any drawback to doing it as on the 50 watt, into the 220K resistor? What other changes would be required?
Thanks

Edited by: aerojazz at: 2/29/04 4:05 pm
 
j allen shaw
Junior tube assistant
Posts: 9
(2/29/04 6:05 pm)
 Re: No bias windings on pwr tranny
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 Here's something I ran across the other day that may or may not help. I know this is from Torres, but I think this what Hammond suggests for their trannys.
Maybe someone else could confirm if this is legit info:
bias taps
 
GroundhogKen
Forum Moderator
Posts: 2304
(2/29/04 6:15 pm)
 Re: No bias windings on pwr tranny
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 Same idea right here in Hoffman Land:

www.hoffmanamps.com/images/Plexi50.gif


Ken

EDIT: With a bridge rectifier, you must capacitor couple this circuit. You can't follow it directly. Tiny Daddy has the right idea. With a bridge rectifier, the HV secondary never goes negative, so you must use AC ( capacitor ) coupling.

You should also put the standby switch on the B+ side of the rectifier. Otherwise the tranformer has no ground reference ( so no bias voltage will be generated ) when it is disconnected from the bridge.

I should have paid closer attention to this. Sorry.

Edited by: GroundhogKen at: 3/1/04 10:45 am
 
jbrew73
Junior tube assistant
Posts: 102
(2/29/04 8:50 pm)
 bias
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 will these work with a bridge rectifier?
 
Tiny Daddy
I will work on all amps
Posts: 521
(2/29/04 9:54 pm)
 Re: bias
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 If you are using a bridge rectifier you will need to capacitor-couple off the HV winding using .022uf@630V and rectify that.
 
aerojazz
Senior tube assistant
Posts: 146
(3/1/04 6:44 am)
 bias
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I just looked at a model Marhall 1987. It shows two .022 caps just before the diodes. I do have the bridge rec. Are these what TinyDaddy was refering to? Are there any other concernes I should be aware of?

Thanks guys, I wouldn't get this thing done if it were'nt for yawl.

Edited by: aerojazz at: 3/1/04 7:36 am
 
Tiny Daddy
I will work on all amps
Posts: 522
(3/1/04 8:29 am)
 Re: bias
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 The .022 I was referring to comes off one end of the hv winding, connects to the cathode of a 1N4004, then a resistor to ground to adjust the voltage. I have seen this used in newer amps such as Reverend. I'm not saying this is the best way to get bias voltage but it is in production.
 
6bq5
Junior tube assistant
Posts: 10
(3/1/04 10:28 am)
 Re: bias
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 hello there, this method was used on the sundown amps and also on many ampeg amps as well. you can look at examples of this in the schematics available at schematic heaven. good luck. rh
 
GroundhogKen
Forum Moderator
Posts: 2304
(3/1/04 10:46 am)
 Re: bias
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 Read the edit of my post above. I want to make sure I don't steer you wrong.


Ken
 
aerojazz
Senior tube assistant
Posts: 148
(3/1/04 1:43 pm)
 Re: bias
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 I think I have a handle on it now
I'm using the Ampeg V4 as my guide.

Q.
Does the capacitor comming of one side of th HV secondary have to be a polarity concious can type or will one of my zircons do the trick?
 
6bq5
Junior tube assistant
Posts: 11
(3/1/04 3:57 pm)
 Re: bias
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 hello there, i would use a poly type cap between values of .022 - .1 and voltage rating of at least 630v--if you have a 1000v cap that's even better. i think the old v-4 i used to own had a .047/1000v cap in it and i have worked on couple of sundowns that had something similar (250v ac rating). good luck. rh
 
aerojazz
Senior tube assistant
Posts: 149
(3/1/04 4:12 pm)
 Re: bias
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 Thanks for you help on this, I spoke with Paul at MM and he has offered to let me return it, even though I did order it.
Knowing this would you proceed with the mod or would you return it. I guess what I'm getting at is what would be the drawback as far a performance/tone/whatever? Marshall must have had a reason for this.

 
6bq5
Junior tube assistant
Posts: 12
(3/1/04 7:06 pm)
 Re: bias
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 hello there, i'd keep it and use it---it will be a good learing experience making the bias supply. where/how you get your bias supply voltage should make no difference in how your amp sounds. look at the "post 70's amps" section in schematic heaven and look at the 3rd sundown amp--i'd do it exactly like that if your marshall clone trannie is one that will have plate voltage in the 500v range. if not you will probably have to dink with the 2 resistor values a little but like i said it's cool to try something new and learn from it. i've made bias supplies like this several times in amps i've built from p.t.'s that had a bridge rectified h.v. sec. good luck. rh
 
aerojazz
Senior tube assistant
Posts: 153
(3/4/04 5:13 pm)
 Re: bias
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 Thanks 6bq5
Been studying for a couple days now, I see Sundown's example using a capacitor off of the hv secondary,
and then there's ThunderTweak using a 220K resistor right out the B+ end of the bridge rect.

Can anyone describe the advantage to going one way over the other?
Thanks again
 
6bq5
Junior tube assistant
Posts: 13
(3/4/04 5:27 pm)
 Re: bias
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 hello there, i saw the the thundertweak diagram you spoke of, but there the h.v. sec. is not the bridge type, it has a center tap (think 50w marshalls with a solid state rectifier string and you get the idea). all of the diagrams i've ever seen involving manufacturing a bias supply from a bridge rectified h.v. sec. use a .022-.1/630-1000v poly cap to connect voltage to rest of the bias supply parts. i would stick to the sundown/ampeg way to do it. good luck
 
 
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