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Old 14th November 2005, 10:26 AM   #1
MlinarS is offline MlinarS  Canada
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Default What did I do wrong with LEGACY????

Hello
I fineshed building prototype of Thorsten's Legacy amp. The amp performs well but there is a problem in the PSU.

I can't adjust HT pass 400V. According to schematic the voltage after the rectification (before regulation) should be 540V. I'm measuring 455V. Acording to Verdier's article and Thorsten's design the voltage neaded for proper regulation should be around 550V before regulation.

Here is the link to the schematic.

http://www.fortunecity.com/rivendell...cy/legacy.html

Can any of you please help me solve the problem. Thank you.
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Old 14th November 2005, 10:33 AM   #2
dhaen is offline dhaen  Europe
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I've fixed the link in the original. Shall I delete the second post?
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Old 14th November 2005, 10:36 AM   #3
MlinarS is offline MlinarS  Canada
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Yes please do. Thank you.
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Old 16th November 2005, 06:04 PM   #4
kevinkr is offline kevinkr  United States
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I would take a look at that power transformer. I use a very similar circuit in my own designs and a 800VCT/250 - 300mA transformer with a pair 5AR4 should provide at least 540V at the plates of the 6AS7 pass elements under load. This regulator should work very well.

Check the transformer secondary voltage under load - if it is much less than about 360Vrms - 0 - 360Vrms it is likely this is the culprit. It could be due to an undersized, or incorrectly specified transformer or possibly the 300B's are incorrectly biased and drawing a lot more current than expected.

Kevin
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Old 17th November 2005, 04:56 AM   #5
MlinarS is offline MlinarS  Canada
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Kevin thank you so much for your help.

I to suspected trafo but I was hoping that I'm wrong since I paid so much for the thing. The trafos I'm using are Lundahl 1650. On the Lundahl's WEB site they are specified as 350V, 0.630A.
Anyway the measurements I made are as follows:

AC voltage before rect. is 328-0-328VAC
DC voltage after rect. is 425VDC
300B plate voltage 400V
300B cathode voltage 64V
300B current 85mA

All the measurements are done with my Fluke 8062A True RMS Multimeter.

After your post I simulate behaviour of the PS on PSUD2 (I should do this before I ordered trafos. Expensive lesson.) and the results are exactly as my measurements.
Acording to PSUD2 to get 540V after rectification one should use trafo specified at 420-0-420V.

One thing I noticed during the measurement is that regulator is not regulating. When I vary voltage at primary with my variac, HT goes up and down as I change the voltage. I don't know if this is because the voltage in front of the regulator is wrong or because something is wrong with the regulator cct.

Is there anything I can do to solve this problem without changing the trafos?

Any toughts?

Thank you again for your help.
Bob
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Old 17th November 2005, 02:13 PM   #6
kevinkr is offline kevinkr  United States
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Hi Bob,
The voltage regulator is not regulating because the input voltage is too low. The behavior you describe is exactly what you would expect under the circumstances. Converting to solid state rectification might just barely given you the margin you need, but you would have to wait for the 6AS7's to warm up before applying the B+ to them otherwise cathode stripping will result. I would recommend replacing the power transformer as the only sure road. You can probably sell it on eBay for a substantial portion of what you originally paid for it - or try hawking it to people on this forum..

I have found that current 300B run above 75mA can be unreliable despite their plate ratings. This was particularly the case with all valve *rts types I tried, and sv*tlana types as well. I run JJ at 75 - 80mA without trouble.

Kevin
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Old 18th November 2005, 02:23 AM   #7
MlinarS is offline MlinarS  Canada
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Hi Kevin,

I think I'll go with your sugestion and replace the transformer.

In your first reply you mention using 800VCT trafo. Is this spec with no load or under the load?

Based on experiance with Lundahl trafo, being specified 350V and delivering only 328V under the load, I'm thinking now to order trafo 460-0-460V specified with no load. Under the load it will probably deliver voltage I need.

The 300B in my cct is TJ MeshPlate. It is rated at 40W. I'll play with cathode resistor to see if there is any diference in performance. I don't think there will be any except probably the prolonged life of the tube with lower current.

Thanx. Bob.
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Old 18th November 2005, 09:37 PM   #8
kevinkr is offline kevinkr  United States
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Hi Bob,
Certainly any transformer over 800VCT will fill the fill if the winding resistance is reasonable, (say 175 ohms or less total) and going with even higher voltages within reason may actually result in lower output impedance from the supply, which is a good thing - just make sure that the capacitors have sufficient voltage margin for at least 600V operation.

Generally the number I specify is open circuit or no load - of course this isn't the whole story as you have discovered because of winding resistance. (I usually specify that as well in my designs.) Still a good transformer's output regulation should vary less than 20% from no load to full load, most actually are worse at 30%. Don't foget the rectifier's voltage drop varies significantly under load, even the 5AR4 drops 25V or more at 200mA load current.

Incidentally to make the current crop of 5AR4 live a long time stick a 1N4007 diode in series with the plate leads of each one. Virtually eliminates flash over (tube destroying arc from plate to cathode = short in many cases) when the cathodes are partially warm and the power cycles for whatever reason.

I think your choice of replacement transformer is reasonable, just make sure your filter caps can handle a lot more voltage. I use up to 1500V film caps in this application or two 450V or 500V electrolytics in series with parallel resistors to swamp the leakage current of the electrolytics. Panasonic snap ins work well.

Reducing the cathode current as you proposed in the output a little bit will buy you longer tube life at the expense of a very small reduction in output power.

Regards, Kevin
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Old 19th November 2005, 01:08 AM   #9
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Sorry to butt in,Kind of OT,But..

Quote:
Incidentally to make the current crop of 5AR4 live a long time stick a 1N4007 diode in series with the plate leads of each one. Virtually eliminates flash over
Does this trick really work?
I have a Dyna ST-70 that seems to like to make the 5AR4 rectifier tubes arc..Atfirst I thought it might be leakage in the original can-cap,and checked it,it was fine,almost like new!
So I replaced the cap anyways,with the SDS labs Cap-board,and I still have arcing rectifiers..Both arcing rects. were Sovtek's..

Maybe i'll have to find some 1N4007/5408's and give it a try.
Might help protect the precious Mulard 5AR4,once I find one at a sane price..I knew I should have kept the one I had!
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Old 19th November 2005, 01:24 AM   #10
MlinarS is offline MlinarS  Canada
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Hi Kevin,

I use a motor run caps ASC oil filled polypropilene. they are rated 440VAC. I was discusing this with some people on this bord few months ago and they ensure me that this caps are good for at list 600VDC. This same capacitors are rated at Angela Instruments site as 600VDC. So i think I'll be OK.
Thanks for the tip with diodes. I should try this.

Thank you very much for your help.

Bob
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