[AMRadio] More On Supply for PP 304TLs

Fri Nov 1 15:28:14 EST 2002

"Screen" is what the original builder's notes show instead of "grid" or "bias". So I was propagating the mis-wording when I used "screen." The 304TL is of course a triode. Otherwise the audiophools could not love it.

It looks like he was running the grid bias at about 80 volts. He had the primary of the HV transformer connected to a motorized variac that slowly brought up the HV. This sits behind a mechanical timer assembly of WWII surplus vintage that only starts the variac moving after the filaments have been on for a while. He's got the relay sequence set up so that you can't get HV if you don't have bias--er, I mean "screen" voltage. 

It's all a bit overly-clever if you ask me. But W9TB ran it for almost 40 years from the late 40's to the mid-80's. And he's got a pretty impressive log here to show for it. I've got all his old parts and his original diagrams so I'll probably duplicate the whole thing--except for that doggone HV supply. That is going too far for me. 

The thing with the HV supply is that he converted to solid state diodes at some point. But the lower voltage drop of the new diodes left his HV and "screen" supply too high. But instead of building a new supply, he took transformer filament windings and hooked them up backwards to buck down some of the voltage. When that wasn't enough, he inserted these huge ceramic resistors in the line in banks of 3 or 6 in a haywire parallel/series lash up to lower the voltage to where he wanted it. 

The rest of the rig is built like a fine Swiss watch. But I guess he sure didn't want to buy a new power transformer. So he made what he had work. But I'm not going to heat the house with all these resistors or fool around with bucking transformer windings. And the pole pig is just too damn big.

I have some perfectly good HV supplies here that are not in use--sitting in my garage. I want to adapt one of those somehow. That's my goal. 

73, Don Merz, N3RHT

-----Original Message-----
From: George Pritchard [mailto:gpritchard at comtechpst.com]
Sent: Friday, November 01, 2002 2:52 PM
To: amradio at mailman.qth.net; glowbugs at piobaire.mines.uidaho.edu
Subject: RE: [AMRadio] Re: Supply for PP 304TLs ??

Use 3000 Volts max. I use a pair in AB1 Audio. No... I did not pay $85,000
for it! I would not sell it for that either!!! Having to much fun. Anyway,
No screen supply needed since they are triodes. Neutralization is a big
must. For class C, I would use 304THs since the bias requirement is higher,
and will give better efficiency and class C linearity when plate modulated.
They do require plenty of drive however. The TLs versions are more linear
for audio. Use a pair of THs for the class C, and a pair of TLs for the
audio... if you have them. Audio phools are driving the price up on these.
Glad I bought spares years ago! Have fun,
George AB2KC

-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of Brian Carling
Sent: Friday, November 01, 2002 1:33 PM
To: amradio at mailman.qth.net; glowbugs at piobaire.mines.uidaho.edu
Subject: [AMRadio] Re: Supply for PP 304TLs ??

Here you go DOn.

This outfit sells transformers for building a 304TL amplifier.

You can even buy the complete amplifier kit for $85,000

Hehe - yes guys, that IS $85,000

http://www.alumrocktech.com/prices.shtml

Of course, it's a stereo audiophool amplifier.

Looks like you could get a set of transformers for a mere $14,000 or so.

On 1 Nov 2002 at 11:59, Merz Donald S wrote:

> By any chance is anyone running an HF amp or transmitter with
> push-pull 304TL tubes in the final? I have this homebrew here that
> uses this configuration. But the power supply it came with is in
> pieces and it is a mess. It was modified many times over the years by
> the builder and has been patched more than a Microsoft software
> product.
>
> So this thing needs a filament, screen and HV supplies. I can use some
> of the original parts. But for the HV, I don't want to use the
> original design because it is built around a 230V pole pig transformer
> and has lots of quirky design anomalies that I can live without
> (Anbody need a 125 pound pole pig? It'll give you 1700 to 2200 volts
> all day. The only drawbacks are that it is huge and weighs a ton).
>
> So I'd like to cheat and see how someone else is powering their rig to
> get some ideas.  References to complete designs in any of the popular
> literature would be useful too. Can anyone help?
>
> 73, Don Merz, N3RHT
> _______________________________________________
> AMRadio mailing list
> AMRadio at mailman.qth.net
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