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Old 26th October 2007, 01:38 AM   #1
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Default Using Horizontal Output Tubes - Power Supply

There has been some chat about using Horizontal Output Tubes for Audio Output. Here for example:
A Tee-Vee T00b Amp
These tubes invariably require a screen supply of around half the voltage of the anodes.

See the schematic posted on this thread:
EV4417 6CM5 (6x) P-P Amp Schematic

Checkout the Power Supply. I thought that was a very neat trick for generating a screen supply of exactly half the main B+. I had not seen this method before so I thought it worth bringing to the attention of those of you who want to mess about with 6CM5 (EL36), 6DQ6, 6BQ6 etc.

Cheers,
Ian
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Old 26th October 2007, 02:00 AM   #2
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Default Re: Using Horizontal Output Tubes - Power Supply

Quote:
Originally posted by gingertube
Checkout the Power Supply. I thought that was a very neat trick for generating a screen supply of exactly half the main B+. I had not seen this method before so I thought it worth bringing to the attention of those of you who want to mess about with 6CM5 (EL36), 6DQ6, 6BQ6 etc.

Cheers,
Ian
You don't often see that in audio circuits, but it's quite common with ham rigs. The ARRL Handbook discusses it, and calls it an "Economy" PS.
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Old 26th October 2007, 02:16 AM   #3
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The power supply circuit isn't new or unique. It is the same circuit that is used in almost every solid state audio amp. The unique aspect is the placement of the ground symbol. Place the ground symbol at the negative end of the bridge, and you have the dual positive voltage version shown here. Place the ground symbol at the transformer CT and you have the typical bipolar supply seen in SS amplifiers. Both versions have been around for years. As Miles pointed out this circuit was common in Ham and CB linear amplifiers that used sweep tubes.
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Old 27th September 2008, 10:57 AM   #4
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Ressurecting an old thread to ask something about the mentioned power supply.

In another thread I saw a reference to a similar PS used by Pete Millet in a SRPP with EL34. The schematic can be seen here:

http://www.pmillett.com/images/SRPP_EL34.PDF

While the schematic for the EL36 amp connects the center tap straight to the elco's positive pole, Pete puts an extra rectifier in series with the center tap and the positive pole. Will there be a difference in performance? Better regulation with Pete's schematic?

And a second question. Does this trick only work for 'perfect' center tapped transformers? I scored some nice power tranny's with a single 510V secondary with several taps (at 200, 220, 250, 300, etc). Lets say I need about 700VDC for the output tubes and 250VDC for the driver valve: for the 700VDC I will connect a bridge + capacitor across the 510V winding. And for the 250VDC, can I take the 200V output and connect it (through a diode) to the positive pole of an additional capacitor (with its negative pole grounded) to have about 250VDC?

Thanks for your attention!

Erik
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Old 27th September 2008, 11:17 AM   #5
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The additionnal diode in the Pete's schemo is superfluous, unless you need some delay on the half voltage cos the two 1N4007 (wich are effectively the rectifier) have not.

Another intersting variation is to move the choke and the associated caps in the negative wire (as pointed by Tubelab, this is no more than moving the ground symbol) so it become active for the mid voltage output also.

And you may even pick some negative voltage accros the choke !
Just filter it a bit.
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Ham rigs routinely do that.

Yves.
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Old 27th September 2008, 12:38 PM   #6
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hey-Hey!!!,
I'd go a bit further and say that reasonable g2 voltage for the audio app of a sweep is more like 1/4 of the B+. Take one of the good ones, perhaps a 6KD6 and B+ for an AB1 amp would be perhaps 450V. Into a 3k2 a-a output TX, maximum current at full power through the single conducting tube will be ~1/2 A. That corresponds to a g2 voltage of 100V for a g1=0V upper envelope. Now if you want to go Class A into a larger load( numeric ), g2 voltage drops. Stuff like CFB output circuits would require this to be higher, since minimum plate voltage also corresponds to minimum g2-k voltage...you have to start higher, or use less CFB percentage. Nice thing about sweeps is they've got plate resistance on par with a 211.
cheers,
Douglas
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Old 29th September 2008, 05:41 PM   #7
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Hi Yves, hi Douglas

Thanks for your input, the very smart tricks on PS (I needed a negative supply for a CCS) and suggestions on sweep tubes are most appreciated!

One question, however, remained unanswered. If I have a secondary with several taps and connect the 'extremes' across a bridge + capacitor, can I take the output of any of the taps, connect it to the positive pole of an extra capacitor to have a second B+?

Many thanks, Erik
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Old 29th September 2008, 06:19 PM   #8
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Quote:
Originally posted by ErikdeBest
Hi Yves, hi Douglas

Thanks for your input, the very smart tricks on PS (I needed a negative supply for a CCS) and suggestions on sweep tubes are most appreciated!

One question, however, remained unanswered. If I have a secondary with several taps and connect the 'extremes' across a bridge + capacitor, can I take the output of any of the taps, connect it to the positive pole of an extra capacitor to have a second B+?

Many thanks, Erik
Mmmh, no !

Just figure a classic full wave rectifier: one center tapped wiring, two diodes and a cap.
Each diode conducts and charges the cap near the peak AC value.
But what happens if the AC value for each diode is different than the other one ?
Probably you no longer have a real full wave rectifier but something with a large 50 (or 60) hz component.
Pushing at the limit, if one half winding delivers zero volts, you obtain a single wave rectifier.

A bridge can be seen as two full wave rectifiers, one with positive output, and one for the negative, both sharing the same winding.

Yves.
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Old 29th September 2008, 06:50 PM   #9
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Hi Yves

Thanks for the explanation, and well, now it somehow makes complete sense that it can't be done. a pitty...but, on the other hand I learned how to extract half the B+, and some negative voltage!

Many thanks, Erik
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