Bias supply filtering

[wizard333]

Is there any reason not to up the filtering or add another stage of filtering on a bias supply?

[Enzo]

There is virtually no current running through a bias supply, mostly just whatever the voltage divider uses. SO there is very little ripple to hold up. That is why you see half-wave rectification all the time, and simple filters.

On a lot of amps, particularly Fenders, I see caps with lower than proper voltage rating. Don;t know quite why. When Fender installs and calls for in the schematic, a 50v cap with 60v across it, I'd call that wrong. I usually put in 100v caps.

And if you want to increase from 50uf to 100uf, you get no argument from me.

But I think adding a couple more filter stages is a waste of time and parts. Certainly won;t hurnt, but if the bias DC is already smooth, more filters won't make it smoother. From a factory point of view, adding the extra diodes to make full wave rectification or adding more cap stages, is just adding to the cost of making the amp with nothing to show for it. And even if the cost is ultimately 20 cents, over tens of thousands of amplifiers, that adds up.

[ThermionicScott]

There is a good argument for keeping the RC time constant of the bias supply low, too -- it can recover from bias excursion faster. Remember that even if there is a tiny bit of ripple from the bias supply, it should be cancelled by the push-pull output stage that uses it.

- Scott

[booj]

The higher the cap values in the bias supply, the longer it takes the bias supply to come up to voltage, or down to in this case. It usually doesn't make much difference unless it takes longer for the bias supply to get to its proper voltage than it takes for the tubes to start conducting. Depends on the circuit architechure. 6V6,s and EL84's warm up pretty quick. I wouldn't want the tubes to be conducting with inadequate biasing.

[wizard333]

Well since I had some time over the holidays, I decided to mess with this Marshall 4210 combo I've had forever.

Among other things I did: The stock bias supply was 2 stages of 10uf, which left about 9mv of ac in the bias supply. I went to 2 stages of 100uf which dropped that to .3mv of ac; i.e. really quiet.

The thing still has a bit too much power section hum; thats with volumes all the way down on both channels. I twisted the heck out of the heater wires, something Marshall failed to do at all. That and the bias change improved it some, but it still hums too much for my taste. The tubes are matched really well (JJ KT66) so thats not the source either.

The PT has a center tap on for the heaters, so I was pondering elevating them, but the stock channel switching circuit derives its power supply from the heaters, and a lead boost/switchable midrange pot thing I added for solos also pulls the power supply from the heaters. I asked about that elsewhere but I'm not sure that, given that, I can elevate the heaters, or if I may be able to do it if I put blocking caps before the rectifiers on the channel switching/lead boost so that those still only see the AC portion and not the elevated DC.

[Gibsonman63]

Same amp, same problem

I am using my 4210 for gigs. The clean side isn't too hummy, but the gain channel can definitely get there. Other than that, I think the amp sounds incredible. I'll try twisting the heater wires this weekend to see if it helps. I don't know about Marshall, but Fender was notorious for swapping the polarity of the heater wires between the power tubes, so I will be checking that as well while I am in there.

[defaced]

Are the OT and PT coupling? For instance, if I sit my soldering station near my OT, and the amp is off, I can hear hum out of the speakers. The same thing can happen in your amp. Pull the power tubes, and if you still hear humming when the amp is on, you have a layout issue, not a circuit issue.

[wizard333]

Marshall is good about not reversing the polarity like Fender does.

I've never heard about that OT/PT coupling thing; I'll check that out.

Still interested to know if I can elevate my heaters without screwing up the power supplies drawn off them, no one seems to have a clue about that.

[defaced]

Elevated heaters just use a reference voltage, there's little current being pulled with that setup, so I wouldn't think twice about trying it out from current loading standpoint. There was a pretty recent thread about it if you're interested in how to do it and such.

[wizard333]

Yeah I realize that, but the problem is there are two dc power supplies being pulled off the heater supply. If you just DC elevate that, its going to mess that up right? There are 2 supplies, one stock one I added, that rectify the heater supply and use it for some switching options.

The thing I want to know is, is there a way to elevate the heaters without screwing that up. One way that might work is to use blocking caps before the rectifiers, but I'm not sure if that will work or not.

Thats the thing that no one seem to have a clue about.

[defaced]

Schematic? I'm going to guess that the devil will be in the details on this one. You might be able to get away with it depending on what the separate supplies are used for. Before you jump through a bunch of hoops for fun, how sure are you that the hum you're hearing is from the heaters?

[wizard333]

It might not be, but other supplies are very clean of AC ripple.

The supplies are used for 1) Stock channel switching and reverb switching, which uses an IC; 2) a lead boost/lead mid switch I added that adds a CK on the last stage before the PI for a lead boost and switches the mid control on the lead channel to a different pot. That is done with a relay and a transistor.

Both just rectify and filter the heater supply; the stock switching system uses discrete diodes; I used a self contained bridge rectifier.

The issue is, if the heaters are elevated to +40v ish, what you'd end up with, I think, without changing anything is -3v/+43v, instead of the un-elevated +3v/-3v, obviously not a good thing.

Unless I'm totally off-base there, it seems that blocking caps before the rectifiers might solve that, but I'm not sure if that will otherwise mess up that supply.

The stock pre-amp schematic with the stock switching is here, http://www.drtube.com/schematics/marshall/2205prem.gif

I dont have a way to post the schem for my boost mod but its nothing bizarre, just rectifying and filtering the heater supply. I attach the bridge rectifier right on the power tube heater tabs and the filters are right there to, so I'm not sending an AC heater supply across the amp.

[ThermionicScott]

wizard, do you actually have a serious hum problem, or are you just looking to do some engineering?

- Scott

[Bruce / Mission Amps]

Yeah I realize that, but the problem is there are two dc power supplies being pulled off the heater supply. If you just DC elevate that, its going to mess that up right? There are 2 supplies, one stock one I added, that rectify the heater supply and use it for some switching options.

The thing I want to know is, is there a way to elevate the heaters without screwing that up. One way that might work is to use blocking caps before the rectifiers, but I'm not sure if that will work or not.

Thats the thing that no one seem to have a clue about.

If you want to lift your filament supply from ground and you have a center tapped PT, it's really pretty simple to add three parts and a tiny bit of wire to your B+ rail and come up with a low voltage, low current DC bias voltage.
The exact value of resistors will determine how far you want to go but the ones I dragged into the resistor bodies are close to give you an idea of how to do it.
I'd use a 22uF to 100uF 100v cap for the filter.
Look at the attached 2205 schem and the upper right hand corner where I added a few parts, you'll get the idea.
If you don't have a heater center tap PT you have to make a virtual center tap with two additional resistors.... two 100 ohm 1/2 watters.

[wizard333]

Scott - Both. This amp has a hum problem,AND I'm curious how to pull this off in general without messing up the supplies that are drawn off the heaters, because there are many applications for this.


Bruce........I appreciate the response but you're not answering the question I asked. I'm not asking how to elevate the heaters, thats covered elsewhere on the forum. I know how to do that, at least when a heater center tap is present, which is the case on this amp. I am curious how to do it when no heater center tap is present, but thats a different issue and not the question I'm asking here.

Look again at the 2005 schem you attached. See the switching supply? See where it says "From fillament supply"? If you elevate the heaters and run it through that, you are going to end up not with +/-3V, but with +43v/-3v are you not? That will fry that switching chip in nothing flat. The only way I see around that is maybe adding blocking caps between the elevated heater supply and the switching rectification, but again, STILL, no one seems to have an answer as to whether that will work or not.

I'm *NOT* asking how to elevate heaters. I'm asking how to avoid kiling the switching system that is run off the heater supply while elevating the heaters. Seems no one is really clear on that point.