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Re: [Amps] Transformer shop update

To: "Ian White, G3SEK" <g3sek@ifwtech.co.uk>,"amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] Transformer shop update
From: craxd <craxd1@ezwv.com>
Date: Wed, 06 Oct 2004 12:15:46 -0400
List-post: <mailto:amps@contesting.com>
Ian,

It's no problem adding more voltage but the core size will most likely go up and the price. If I recall when I worked this one out, the coil come awful close to the window width. That means I'd have to go maybe one to two sizes of core up to get the coil to fit the core. The voltages I shown for the screen was the voltage needed for DC plus 10%. Does your regulator require more than 10% extra? I want to make these so they will work with everything, including your boards. The screen voltages I gave were for regulated supplies of 300, 350, and 400 Vdc. The 90 Vac bias I used is actually 250 mA but I rated it for 125 mA. I won't use anything under say 28 gage wire because of breaking.I think these were wound with 26 gage. I used the same size wire for it as I did the screen voltage. I'll look into raising it to 105-110 vac. I used to use 120 vac on my old ones but figured that was a little too much for some. The heater is actually 6.3 Vac which is a shade over the value needed. The 12 Vac winding was made 2 amps so one could use a doubler and get 24 Vdc to control an 800 mA relay coil with. However, I can change that to a 24 vac C.T.. If you would, let me know what the specs on that old one was and I'll quote around it and possibly offer two transformers for different circuits.

Will Matney


Ian White, G3SEK wrote:


Hello Will


I will first be offering some screen/bias transformers with heater and control windings included. The screen will have taps for 315 Vac, 275 Vac, and 236 vac. That will give 445 Vdc, 389 Vdc, and 334 Vdc. They will have a 90 Vac bias winding, 12 vac control winding, and 6 Vac heater winding.


The screen windings were going to be rated at 250 mA, the bias at 125 mA, the 12 V at 2 amperes, and the 6 V winding at 10 amperes. Most all the supplies will have losses and actually require more current than the DC output. These will be able to run full wave, half wave, and doubler circuits with room to spare, running at CCS. I wanted to do this to keep the heat way down to where they could be mounted in chassis that wasn't ventilated well. I figured a rough price of $100.64 each with a watt rating of 205 watts to compare to Dahls pricing of similar. These use an EI 1-5/8" laminate 1-1/2" in depth. The physical size of the core is 5-1/4" wide x 4-3/8" high x 1-1/2" deep, with the steel weighing 8 pounds. The estimated finished weight will be about 12 pounds. You can look up a similar transformer at Dahls website weighing 12 pounds and 205 watts. Thanks for the interest, I should be up and running around December.

I'm also very pleased that you're going into the business of transformers for "everything else" that a tetrode amp needs, other than B+.

For my Tetrode Boards, I used to have a similar multi-winding transformer made in England, but eventually stopped doing that because of the cost of shipping to the US customers.

Also there are many more different tetrodes in use today, compared with a few years ago. They range from the tiddly little GS15 up to 2500W tubes like the GU84B (or two, or even bigger tubes). It's very difficult to specify a transformer that will be good for all possible tubes.

The current ratings look good, but maybe you should up all the voltages by say 10%. There are different reasons for wanting to do this on each of the three windings.

Heaters: people will want to use a dropper resistor to adjust the heater voltage to 6.0V right at the tube pins, so you need more than 6.0V at the transformer.

Screen: with bleeder resistors or a shunt regulator, the transformer will run at quite high loading, so the averaged DC output will be nowhere near 1.41 times the AC voltage. Also, the averaged DC output is the wrong value for this application. The important thing is that the voltage at >negative peaks< of the 120Hz ripple is still high enough to allow the regulator to operate correctly. If you don't provide enough voltage on negative peaks, the screen regulator will "drop out" on every peak, and the output will have little negative blips at 120Hz. When I used Duncan's PSU Designer to actually calculate the minimum unregulated voltage on negative ripple peaks, under load, I found I had to up the transformer AC voltage quite a lot! As a rule of thumb, I add 50V to the required screen voltage, and then divide by 1.2 to find the required AC voltage. But if it's at all critical, I run it through PSU Designer - and >always< if I'm specifying a transformer for a customer to order.

That's why I suggest you up all the voltage taps by 10%. If the AC voltage is too high, the user can always increase the dropper resistor - but if the voltage is too low, there's no answer. (He'll come right back to you and me for tech support, and we don't have an answer for him!)

Bias: 90V will probably be OK, but if people want to use grid-block keying, you my not have enough voltage to take the tube to cutoff. I specified 105V for the stock transformer. One thing the winding company told me - be sure to have then wind the bias winding on the inside, next after the primary. All the windings expand when the transformer warms up, and if the bias winding is on the outside, the thin wire may snap.

Control: many people use 24V relays, which require a 20V AC winding. My Tetrode Boards actually require a CT control winding.

Extra taps and higher voltages are more expensive, I know, but my experience with marketing the Triode Board and Tetrode Boards is that amplifier builders >are< prepared to pay extra for an off-the-shelf solution.

Just a few thoughts...

Like I said, if you really can make a "one size fits all" tetrode transformer that will work with my boards, I will be very happy to send some business your way!




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