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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 16th October 2017, 07:16 PM   #1
bigbargain is offline bigbargain  United States
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Default PS tranny alternative...

I am building a TSE 45 with vintage ST and Globe tubes (have several of each). Iím trying to retain/create the vintage look along with what is reputed to be a very fine sounding amp (I have high hopes!).

The TSE board assumes a center taped 6.3v supply for the 45ís. Which is then HALF WAVE rectified (jumpering pins 1 and 4) for ~3.15->4.4v via the C1 smoothing cap, then regulated to 2.5.

I have a couple vintage PS transformers from some very old radios that ran multiple 2.5v filaments Ė plenty of current for 2x 45s.

Looking at the TSE schematic, it looks like I could jumper pins 3 and 4 to FULL WAVE rectify the 2.5 v (2.5v * 1.41 = 3.5v via C1 smoothing cap), and leave pins 5 and 6 open so that the regulator still creates a 2.5v output. The dropout voltage of the regulator is 1/2 volt so it should work and dissipate even less heat. Right?

Does anyone see a problem with this approach?
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Old 16th October 2017, 07:55 PM   #2
woody is offline woody
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Yes I see a little problem. That 2.5 X 1.41 doesn't include the voltage drop across the rectifier or in the case of a bridge rectifiers. In the case of a bridge rectifier you could lose 2v or more .
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Old 16th October 2017, 08:34 PM   #3
bigbargain is offline bigbargain  United States
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Hmm. Good thought. Thanks!!

However if it's actually 2v, the original design would not have worked as 4.4-2=2.4 minus .5 dropout voltage means the regulator would not work. Or is it 1/2 because its a 1/2 wave rectifier in the original design?

Per the datasheet, it looks like 3 amps drops .35v. (cold. less when hot). 3.5 - .35 - .5 = 2.65. That is pretty close!

Or is that really 2x.35=.7 because there are 2 devices active in a full wave rectifier?

Is my above analysis not considering something or is this one of those "try it and see" things. (I am not a practitioner here. This is a hobby, which is why I need to ask.)

BTW. because line voltage is now higher, it the 2.5v is actually higher, but I suppose I can't cut it too close...

Last edited by bigbargain; 16th October 2017 at 08:37 PM.
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Old 19th October 2017, 03:03 PM   #4
Tubelab_com is online now Tubelab_com  United States
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Quote:
Which is then HALF WAVE rectified (jumpering pins 1 and 4)
With pins 1 and 4 connected the system operates as a FULL WAVE CT rectifier in a manner similar to the high voltage winding. Diodes D1a and D1b are high current Schottky barrier diodes with a 0.3 volt typical drop. This gives the voltage regulator about 4 volts to work with. At the same time diodes D4 and D5, which are conventional silicon diodes with 0.7 volts drop, produce a negative voltage of about -3.5 volts. The 5842 tube heaters are wired between the negative and positive supplies through an appropriate dropping resistor to give them about 6.3 volts.


Quote:
I could jumper pins 3 and 4 to FULL WAVE rectify the 2.5 v (2.5v * 1.41 = 3.5v
with pins 3 and 4 connected and a 2.5 volt transformer used, there is no way to generate 6.3 volts for the 5842's.

I suppose that it might be possible to wire the "multiple 2.5 volt" windings up in such a way to generate proper voltages for the two different heater sources, but it would require some careful thought, knowledge and testing before plugging rare vintage tubes into it. It would probably be easier to hide a small 6.3 VCT transformer under the deck.

Trust me.....it sucks to blow a pair of vintage 45's because you forgot that the last time you used the amp, the jumpers were set to 5 volts. That's why I recommend hard wiring them.
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Old 19th October 2017, 09:10 PM   #5
bigbargain is offline bigbargain  United States
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THANKS George!! I am always impressed with your support of the DIY community and your products. Thanks so much!!

I will reiterate that I am a novice when compared to so many folks on this forum.

I should have mentioned that I was not concerned about the 6.3v needed by the 5842 as I have a separate transformer for that. But it does not provide enough current.

So more than anything my question is around re-jiggering the board so I can have a cool vintage PS transformer on it. (out of a 1929 Atwater Kent Model 55, so very vintage and would be sharp next to a few globe tops.)

So maybe i need to recast my question. Pins 1+4 and 5+6 open, and 3+4 jumpered. Will that get convince the regulator to provide 2.5v to the 45's filaments?
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