Mercury Power Supply Design - Checking for Efficacy

[Paully]

Excuse the cross-post from Tube DIY, and hopefully Doc won't ban me for mentioning a tube rectified power supply, but here it goes.   Grainger and I are going to build an 866jr full-wave (two tubes) rectified stereo amplifier. Why?  Because they glow!  45 outputs with a 76 driver. For the power supply according to the tube manual (link below) if our max input voltage is 500V we should choose a choke input to follow the rectifiers with a minimum value of 2.1 Henries. That is followed by a capacitor with a maximum rating of 4.2uf. The manual says we can increase the inductance and then increase the capacitor in proportion. We are planning on using an LCLC setup of a 5H 400V choke / 10uf cap / 5H choke / 10uf cap being fed by a 375-0-375 power transformer. It is going to be a stereo amp, not monoblocks, with four of these rails, one for each driver and output. Sound good?

Last question, I obviously have the filament chokes in my Paramounts.  They seem like a very good option.  Any reason not to use them with 45's in the new amp?

http://www.tubebbs.com/tubedata/sheets/084/8/866Jr.pdf

[ALEXZ]

Are you really planning to use mercury vipor in your living room?
Wikipedia:
Mercury compounds are toxic, highly persistent in the environment, and present a danger to humans and the environment. The use of large quantities of mercury in fragile glass envelopes presents a hazard of potential release of mercury to the environment should the glass bulb be broken. Some HVDC static inverter stations have required expensive clean-up to eliminate traces of mercury emitted from the station over its service life. Steel tank rectifiers frequently required vacuum pumps which continually emitted small amounts of mercury vapor.

Also, just curiosity, what are the objectives of this project ?  Are you after the best SET 45 sound or there is something else ?
Thank you,
Alex

[Paully]

From what I have read, and its worth what its worth, there is less Mercury in one of these vacuum tubes than in a typical CFL light bulb.  Maybe, maybe not but I have plenty of CFLs in the house with plenty of mercury and don't worry about accidentally breaking one when I have them throughout the house and I am more likely to break one of those than one of my precious vacuum tubes.  Not worried about continual release during operation, its not like the vacuum tube is going to work if the vacuum seal is broken.

The point is to build a vacuum tube rectified SET amp and not much more, whether it will sound better than solid state is open to question.  I wouldn't want to state that it would when I have no idea and the guys around here who design the amps think SS rectification works much better in the bass.  I want to play around and have some fun with it.  And the MV rectifiers look interesting.

[ALEXZ]

Personally, I'm using tube rectifier with my 300b set. It's Mullard cv378 (version or 5u4g). I tried number of SS rectifiers (soft, fast, whatever) and ended with tube rectifier with CLC (20mkf/30H/660mkf) filter.
I did not calculate any values for the design you've described, but looks like with the values you have, there will be 120hz buzz and LF roll off , due to very low value of capacitors, and these values are direct result of rectifier tube selection ( lets not to worry about mercury for a moment).
By looking tube data, seems like 5y3, 5u4 etc. are better candidates. Also you can easy compare tube with SS.
Also there are a lot of other things to consider and compare - driver tube/topology, autobios vs. fixed one, direct coupling vs. cap/transformer, output transformer/topology.  All this will probably change sound more than type of the rectifier. Just built oversized power supply and worry about other things.  That is my 2c ...
Google please, Flesh and Blood by Reichert - he has pretty good advice on PS.
Good luck with project - 45 is fun!
 

[Grainger49]

Paul owns the 866JR tubes already.  The last C in the LCLC filter should be increased to give a larger storage capacity as well as better filtering.  The choke input for the filter is required for the 866JR.  

After the tubes for rectifiers there will be separate LCLC filter circuits for each 76 and each 45.

Both the driver and output tubes will be cathode biased, much like my Paramours.  

Paul has also run his Paramounts with 45 outputs and this is why he wants to scratch build this amp.

[ALEXZ]

That is why I asked what are the objectives.  Ok, so the objectives are to utilize what is avalability and built paramount with the glow. Correct?

[Grainger49]

The circuit will resemble the Paramour and Paramount.  A driver capacitor coupled (possibly not) to the output.  Parafeed output, the iron is already bought.  It is the power supply that is still up in the air.

The post above is modified with a little more information.

[ALEXZ]

Again, it's just an idea, nothing more: why not to built on the sheet of plywood outboard ps and try different tubes, capacitors etc. with the paramount. That will give you very good reference point - I guess you know sound of you amp. well enough.

[Paul Joppa]

The inductance from the data sheet only ensures that the peak rectifier current is not exceeded. To get good regulation, the first inductor should be at least equal to the kOhms of the load. For a 375v transformer you will see about 335v, and if the whole amp draws 80mA, the equivalent resistance is 4.2K, so you need at least 4.2 henries. If on the other hand there is a separate LCLC feeding one driver at 5mA the the equivalent resistance is 67K and the choke must be 67 henries. The voltage will still rise to over 500v at startup, until the tubes draw full current. Traditional practice was to use a bleeder resistance to set a minimum current (usually 10% to 20% of the total) and size the inductor for that current, so that the voltage won't rise that high even if a tube goes out.

[Paully]

Sounds like a mess.  So by having four filtering circuits, one for each major tube, I drop the current for that section from the total for the amplifier and therefore have to raise the inductance of the inductors in that section.  I am just starting to look up equivalent restance, but say for a single 45 drawing around 30 ma, we are talking about a choke somewhere around 30H, give or take, if I am starting to get the gist of this plus it needs to be around 600V And that doesn't even begin to show what size caps we need which will be large.  Ugh.  Anyway, I will look up the equations you used to figure out the Henries and decide if this is still worth the playing around given the cost.

[Paul Joppa]

Remember, for small-current stages (drivers) you can use small-current chokes (small and cheap).

Power supply filter theory is fairly simple and widely documented; google "choke input filter" for instance.

[Paully]

I appreciate the input, thanks.  Time to do some more research.  Thanks again!

[2wo]

With just 2, 45's and 2, 76's. your not drawing a lot of current. Maybe try 2 power supply's, one for each side, or one for drivers, one for outputs...John 

[chard]

 I read on a DIY site that 83 mercury vapor tubes which are similar to 866's glow in proportion to their current draw. Someone who had an amp that drew 30ma was complaining that the 83 rectifier didn't light up much. He was referred to a youtube video (Bach Toccta D-moll on a 1000w audio tube), which showes an impressive array of 83's with a bright blue modulating glow. More current more glow less current less glow. 

[Grainger49]

A dummy resistor could be used to increase the load current.  Maybe after the initial LC filter?