My friend and I are in the starting phase of building a 100-Watt Vacuum Tube Amplifier. So far, we have the Power Transformer (400-0-400 @ 300ma), two 10-Henry 300 ma, and an output transformer to handle the 6550 tubes we are going to use.

However, since the full-wave rectification of 400 VAC will yield 565 volts DC after filtering, I cannot find any 600-volt capacitors of the value I need (about 80 Microfarads).

I have located some 440-volt RMS 75 Microfarad non-polarized motor run capacitors I might be able to use. Upon full wave rectification, the maximum value the capacitor would be capable of handling is about 622 volts filtered. This would allow about 57 Volts of headroom.

I have never used large non-polarized electrolytics for a DC filtering application. I have been told by many it should work.

Question:

Has anyone tried it, and will it work without employing the "Big Bang" theory?

(That would be one big helluva big bang. The capacitors are 2.5 inches in diameter, and 5 inches tall, oil filled.)

Rule of thumb: you can operate polarized lytics 10% over marked voltage.

BTW: Boy the 6550 is getting to be an EXPENSIVE tube. Hopefully you've lined up some spares. I've got an old McIntosh amp that uses
them - they get HOT even when idling! Best, Jeff

Also on another note you can build a solid state single ended amp that will surpass the performance of many high end tube amps. Its quite a simple circuit to build. They are here...... http://www.passlabs.com/aleph.htm

Finally there is a great DIY audio site for home built audio gear. This is the film-tech equivelent site for audio people. There are many engineers lurking there that are very helpful when it comes to answering questions. www.diyaudio.com
Mark @ GTS

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Jeff, I thought of stacking them, but the standard canned electrolytics can pose a serious shock hazard. Space was a limitation, too.

I was also going to use a choke input power supply, but forgot I needed a swinging choke instead of a smoothing choke on the input to make it regulate better. Swinging chokes are quite pricey, and getting one that will have a worthwhile swing in a wide current range is a problem.

Oh, well....now I have a spare 10 Henry choke. So, capacitive input it is going to be.

There is a store in Phoenix/Scottsdale that is a major supplier.....called Antique Radio Supply. There is also a distributor (I forgot the name offhand) that distributes high end components and kits...

I have seen ultra high end amps list for $100,000...each channel along w/the power supply having its own granite chassis. All silver wiring and gold plated contacts....hand wound solid silver wire transformers, porcelain tube sockets etc,etc...

For years Conrad Johnson has used LEDs across the tubes so that you do not need an oscilloscope, but I have heard mixed reviews about this....

Vacume Tube Logic has also published some interesting books but are in large advertisements...but they have partial schematics.

Being short of time, I sent the apprentice and the manager to get a replacement cap, (40 uf/ 350 volt), while I worked on an automation problem. I neglected to specify D.C. ratings for the capacitor.

The manager took the tech to an electrical supply outlet. They returned with this huge "bathtub" cap. As the capacitance and voltage ratings equaled (actually slightly exceeded) the original D.C. cap specs., I hooked it to the amp.

Several tie-wraps were required to secure this to the amplifier.

The alternative suggested by Jeff is a good one, as there are many types available that have a full plastic covering which protects probing fingers from those nasty little electrons.

------------------
Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

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The parts supplier that I was thinking of was THE PARTS CONNECTION, which was a part of Sonic Frontiers Intl. Their web address is WWW. The Parts Connection. com. (space between words). I just looked at their web site, and unfortunealtely SFI has decided, after 14 years, to close them down ! This was truly a popular company that supplied many high end parts and kits for both vacume tube and solid state systems (including transformers) Their catalogue is still on line. It would be wise of you to download, as it will give you a good general idea of what parts are available so that you can contact the manufacturer direct.

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