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Ampeg Heater Circuit Questions

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4/19/2000 5:25 PM

Ampeg Heater Circuit Questions

I've noticed that some Ampeg's have a different way of referencing the heater voltage than the standard center-tap to ground or the use of 2 100 ohm resistors.

1. The center tap of the heater transformer is referenced to the power tube cathodes in a cathode biased amp.

2. A non-center tapped heater transformer is connected via a 100 ohm balance pot (hum control) to the bias supply in a fixed bias amp.

Are there any benefits to this type of connection?

Andrew.

4/20/2000 12:36 AM

Andrew,

Supposedly, biasing the AC heater voltage with a positive DC voltage (such as that available at the bypassed common cathode resistor of the output stage) reduces hum. I have experimented with this and not had too much luck with it, although it was common enough practice in the 50's and 60's to reduce hum levels in amps and preamps. A 100 ohm balance pot would perhaps be more effective at 'zeroing out' the hum, although in my book it's hard to beat a center-tap; and those 100 ohm pots are both hard-to-find and prone to burnout.

Regulated DC is the way to go, but it's a lot easier to design it in at the git-go than add it later.

Ray Ivers

4/20/2000 1:14 PM
andy fuchs	Keep in mind that the DC available on power tube cathodes (especially with the bypass cap in place) is very quiet and makes a good way to lift heaters above group without extra parts.

4/22/2000 2:34 PM

I remembered an old Scott audio trick: They actually used their 4 12AX7's in the preamps, to function as the cathode resistors for their output tubes. I think they used 7189 or EL84's. They put them in series with a cap across them, and they were very quiet. An old timer I worked with at the time said that, "this is the quietest DC filament supply you could get at the time". I assume this was because you didn't have DC regulators like we do today (practical ones at least). Cool concept.

4/22/2000 3:02 PM
Ray Ivers	Scott Heaters for PA Cathode Resistors Andy, VERY cool! The hi-fi guys make me feel very humble sometimes, and that's a good thing because a) I SHOULD feel humble and b) it makes me get off my ass and pick up a book. As soon as I think I'm getting this stuff down, I refer back to a) above. Ray

4/22/2000 7:47 PM

Ray: Not that I'm anyone to pass judgement, but from the looks of your postings, your not exactly without knowledge of the subjects discussed here ! I like this idea a lot. I am working on a prototype small amp in which I may try this. Heck, if you can handle the biasing right for the output tube you choose, you eliminate the bag-ass resistor (cost AND heat) on the cathodes, and get "free" DC filaments for your input and driver tubes ! Almost a free lunch.....af

4/22/2000 8:09 PM

Andy,

Thanks for the compliment! I think this is a cool idea - I guess the first gain stage's heater winding would be at the 'high end' of the string, at the power tube cathodes, and the ground end would be the last gain stage - does that sound right to you? I just hope it doesn't hum - maybe you've had the same experiences I've had with DC heater supplies (I've always gone back to balanced AC, too lazy to put a regulator together) but this approach is novel... it will be interesting to see the warm-up behavior, too.

Ray

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