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Old 17th October 2005, 06:27 AM   #1
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Default Ditching some High Voltage

I've built a pair of Menno Vandervenne VDV100 monoblocks using the Plitron PAT4006 Toroidal Output trannies and "companion" Plitron 754709 Toroidal Power trannies. After messing about and setting to work I settled on 4 x JJ EL34 in Triode Mode - No negative feedback. I also ditched the 12AU7 and substituted a JJ ECC99.

Because Oz line voltage is 250V standard I'm getting high voltage rails of 480V istead of the 450V "design centre". High Voltage is from a full wave bridge off the 340V (nominal) @ 700mA winding. Before proceeding with a full front end redesign I'd like to get rid of that extra 30V of High Voltage. I don't want to go to a choke input power supply.

Does anyone have any experience / educated guesses as to the best place to add dropping resistors.

That is should I use resistors as per A or B below:
A/ In each AC HV feed before the rectifier
B/ In the DC HV feed between the rectifier and the filter caps

or perhaps some other scheme (MOSFET based simulated inductor for example).

Any ideas /advice?
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Old 17th October 2005, 06:47 AM   #2
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H Ian,

is that 480v with no load?? I'd be sure that would drop with the standing current of the el34 tubes.

From lv hi current designs in series with the diodes is not the place as there will be some high peak currents which will make regulation poor(er).

If you just want to drop the supply for the pre-amp section I'd go the R>C filter also isolates/decouples the preamp supply and wont you want that less than 300v?.

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Old 17th October 2005, 06:53 AM   #3
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Nope - thats 480 volts with all 4 of the EL34s set for 48.0 mA idle.
That is with 192mA + approx 5 mA for front end. Actually it drifts with mains fluctuations from about 475 to 485 volts. I want to get this down to at least 450V before I start work on ditching the CC/Concertina ECC99 wimpy front end and fit a Hedge / Sonic Frontiers / Allen Wright ish Differential Cascode front end.
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Old 17th October 2005, 06:56 AM   #4
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ps I guess thats c/ none of the above.

and as for the ECC99 ok 400v plate max mmm

but thats my 2 bobs worth.
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Old 17th October 2005, 07:02 AM   #5
AndrewT is offline AndrewT  Scotland
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I would go A since it makes no difference whether the resistance is before or after the rectifier. It could just as easily be a thinner winding on the secondary, the amp won't know.
I would also add a second stage of smoothing giving an RCRC type supply. Try the first C about half the value of the second C. Some extra C will compensate for the loss in transformer regulation.
Use http://www.duncanamps.com/psud2/index.html to check your numbers before starting the mod.
regards Andrew T.
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Old 17th October 2005, 07:06 AM   #6
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I see your problem, I remember some old equipment use to have a low r hi wattage r in series with the primary for this very reason.

without trying it out there are a few issues that would need to be sorted out first, but this will also result in poorer regulation perhapes not as bad.

looks like you need to drop about 15 volts on the primary! which brings it close to 235ish, is always about now I ask myself why is it never simple,.... supose you dont feel like over winding several more turns on the primary ;o(

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Old 17th October 2005, 11:37 AM   #7
SY is offline SY  United States
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Presumably, the heaters are running high, too? That's bad.

I'd attack it at the line end with either an autoformer or using a filament transformer as a bucking element.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 17th October 2005, 12:52 PM   #8
anatech is offline anatech  Canada
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I was about to suggest bucking the primary AC as SY has done.

Although, resistance in the HV in series with the diodes would reduce the peak currents and switching noise from the rectifiers. In the heater circuit, resistance would also limit the cold inrush current. Win - win if you ask me.

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Old 17th October 2005, 05:34 PM   #9
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I had a similar problem with a toriodial. I ended up running a hybrid bridge with 2 silicon diodes and a dual vacuum diode. That dropped the B+ down to an acceptable level. The tranny did not have 5v tap, but it did have 2 6.3v taps, so I used small resistors to drop the voltage down around 5v on the second unused 6.3v tap. Since then I have found a couple 6.3v rectifier tubes that did the job better.

A question. Perhaps running a high wattage zener on the grounded junction of the diode bridge drop you down? I have used that trick as well as mosfet boosted zener strings, to drop doen B+ on Center tapped tansformers, but I don't know if it will work properly for a bridge.
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Old 17th October 2005, 11:41 PM   #10
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I wouldn't worry about it. VT Design Center values have a +/-10% tolerance. Your 480V is within that. Just add some low resistance, high wattage resistors at the tube sockets to drop the heater voltage, if that's too high.

Back in the bad ol' days, they didn't have precision components. No 0.1% ultraprecision metal film resistors, etc. Most components were good to a tolerance of 20%, and 10% was considered "precision".
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