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Old 29th January 2007, 09:39 PM   #1
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Question Capacitor choice for attached schematic

Im changeing some old elektrolytic caps in an Tim de Paravicini design, the Michaelson & Austin TVA-10.

Does anyone know what function the crossed 22uF caps over the powertubes have? Is the capacitance important or would there be any advantage/disadvantage adding larger ones there? (30-50-80uF) which I have on hand.

Tva-10
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Old 29th January 2007, 10:30 PM   #2
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The caps are there to provide cross-coupling between the output tubes. This design looks like it has some simularities to both a circlotron and a unity coupled output stage. I would not change the value of those caps, it might adversely affect LF stability, however you might want to try some good quality films in place of the electrolytics. Solen or similar if there is enough room.
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Old 13th February 2007, 06:59 PM   #3
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Thanks for clearing up the issue.

So i guess the 30uF JJs I have dont fit the bill.


There are som space issues so I will have to go with electrolytics
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Old 13th February 2007, 07:47 PM   #4
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If there is any way to do it make sure the electrolytics you choose are relatively well matched capacitance wise - otherwise don't worry about it..
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Old 13th February 2007, 08:00 PM   #5
EC8010 is offline EC8010  United Kingdom
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Many years ago, I substituted 470nF film for those 22uF. Didn't make it any less stable. Mind you, it was hard to tell. The thing would burst into violent oscillation at the slightest opportunity. I know more now, and I'd stick with some nice electrolytics intended for switch-mode power supplies. My TVA10 had some very cheap and nasty parts in it.

You might want to have a look at that bias supply. If you substituted a bridge rectifier you would have full wave rectification (which would halve the ripple). The bias voltage would rise, so you could drop it back down again with an additional RC filter section before the bias adjustment and lower ripple even further. Another useful thing to do might be to fit individual bias adjustments (one for each output valve).

Another useful tweak might be to put a cheap 10nF capacitor from each heater pin of each driver valve directly down to the chassis (as short a path as possible).
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Old 13th February 2007, 11:27 PM   #6
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Thanks for the help Kevin & EC8010

I will make sure the caps, whatever i decide on will be matched.

I have thought about the bias supply and will definetly consider the fullwave there +RC there, just have to measure upp the current draw first.

As you might have noticed my V-BE in my schematic has an additional RC which didnt exist in the original schematics but added in mine by fabric (ive added this myself to above schematic). Also some values caps& resistors seems to have been tweaked by factory compared to the above schematic, which I havent changed in the schem.

Even if the tubes are tightly matched will i still gain by adding separate adjustments for each tube?

The heater tweak adding would be very simple since there is terminal strip right beside each socket with the chassis-earth-pin unused. If i understood correctly I should add one on each heater pin, eg. 2x10nf on each el34?


Other changes ongoing:
*For the main caps I have got a pair of BHC 1000uF to substitute
*The other axial caps are undergoing changes, I will probably fit in Rifa PEG124 since their easily available here in sweden.
*The rectifiers are so far changed to UF4007 and BYV96e for bias.

*Another thing done is that I removed the 100k potentiometer and input capacitor.

Thanks for the tweaks & assistance!
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Old 14th February 2007, 07:23 AM   #7
EC8010 is offline EC8010  United Kingdom
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2 x 10nF for each ECC83 and ECC81. The EL34 aren't as significant.

I've never really trusted "matching" - even when I've done it myself...
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Old 14th February 2007, 12:44 PM   #8
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Personally, I wouldnt 'waste' money buying expensive audio-caps.....

Just use good-quality poly-props and low ESR electrolytics, especially if the amp is still in 'pre-bread-board/not completed' stage.

Get the thing built with standard parts then tinker if you feel the need, changing coupling caps etc....

(Personally, I have yet to 'hear' a real difference between a std good poly-prop low ESR coupling-cap and an expensive esoteric mega-costly what-have-you. make cap)--Then again maybe Im deaf!

All my bread-boarding is done with either std off the shelf or second-hand parts at very little cost..It works for me!

Pointless wasting dosh if you find the design isnt quite what you were after, and decide to change to something a bit different.
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Old 17th February 2007, 04:48 PM   #9
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So, finally the amp is up and running!
With Ipod as source and dynaudio confidence 5 the amp seems to work without any problems @-30V bias

The Voltages over the 22uF crossed caps measured some 570 at startup and evened out around 470-480.

I stand for three alternatives for substitutes:

1. Fischer & Tausche 550V seems like average elektrlyte

2. Rifa PEG124 450v, I know for fact they are good for the nominal voltage in my amp, but will they withstand the startup peak?
otherwise very good specs, and easily available

3. Fischer & Tausche polyprops these will actually physically fit, will cost 2x the rifas.

Any suggestions?
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Old 17th February 2007, 04:49 PM   #10
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A picture of the unit up and running
Attached Images
File Type: jpg tvapod.jpg (87.0 KB, 190 views)
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