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Trix T621 PA amplifier (British)
Trix T621 PA amplifier (British)
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Old 7th March 2010, 10:11 PM   #1
rotaspec is offline rotaspec  New Zealand
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Default Trix T621 PA amplifier (British)

A long shot I know. I'm not trying to restore it, just use some bits.
I picked up an old Trix T621 public address amp without valves. It looks to have run two octals like EL34 (someone has penned in EL37) in the output in UL PP, so I can probably figure out what the OPT figures are, but it's the PSU components I need to identify. The other tubes were octal rectifier, and two nine pin small tubes like ECC81. There is a small can-shaped mic transformer as well.
The mains transformer has secondary windings which measure off load: 340-0-340, 5, and 3.15-0-3.15. Only labeling is T36/24 Batch 86/7. It's 140x115x90mm with a 33mm thick lamination.
The filter choke is labeled T36/25 86/8. It's 80x65x70mm with a 26mm thick lamination.
It's mainly what the current ratings of them both are likely to be, and the value of the choke. Choke was in a tube rectified CLC filter with a 50uF each end.

Any help (even a diagram ) would be great.

Gary
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Old 7th March 2010, 11:24 PM   #2
Rod Coleman is offline Rod Coleman  United Kingdom
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Trix T621 PA amplifier (British)
340-0-340V suggests a 50W PA with EL34 PP.

If it meets that spec, the dc supply would need to deliver 250..280mA dc. The rectifier was most probably GZ34, so the low end of that would need to apply.

The choke rating depends on whether the anode current ran through it or not. It went both ways - sometimes the choke filtered only the preamp supply.
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Old 8th March 2010, 03:24 AM   #3
rotaspec is offline rotaspec  New Zealand
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Thanks Rod.
The PT might yet be enough to do a 5B254M (small bottle 807) PP in stereo if I run at minimum acceptable quiescent current and use some big filter C's with SS rectification?
25wpc would suit me fine.
After I get through with this little ECL86 PP I'm building, I'll mock up one channel at about 100mA quiescent to see how much the secondary voltage sags and how hot the PT gets.
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Old 8th March 2010, 03:49 AM   #4
Eli Duttman is offline Eli Duttman  United States
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Gary,

The way to squeeze extra mA. out of a power trafo is with SS rectification and a SMALL 1st filter capacitor. You use just enough capacitance to keep the rail voltage up. Follow the 1st filter cap. with a "meaty" choke and pile the energy storage up in the 2nd (reservoir) position.

A low value in the 1st cap. position implies a comparatively large conduction angle. A large conduction angle reduces the amount of I2R heating in the rectifier winding. Less heat from charging pulse action provides a thermal reserve for an increased draw from the PSU.

The small forward drop in SS diodes compensates for the Volts lost in holding the value of the 1st filter cap. down.
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Old 8th March 2010, 10:59 AM   #5
Rod Coleman is offline Rod Coleman  United Kingdom
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With SS rectifiers, it should be possible with 20..25mA through the 254Ms.

Eli's advice is sound. I choose the first cap C1 by starting PSUDii and varying the C1 value while watching trafo rms current and the dc output voltage.

If you model the original first, you can adjust the circuit values (including dc output voltage) till the trafo rms idle current is the same as before, allowing for at least 15VA gained by adopting SS rectifiers.

To ride first class, C1 can be an audio MKP for better sound. But if you use an electrolytic, use PSUDii to evaluate the rms ripple current through it - low values of C in 'lytic often can't handle high ripple, and suffer short life.

After the choke, have at least 100uF in place, for pentode mode, with more filtering/regulation for the screens. If you're in triode, more will probably be needed. Setting the dc voltage to about 420V allows 470uF/250V x2 in series (with 150K 1W resistors across each for balance). These will have Low Rdc and high current handling.

A choke for each side is worth the trouble.
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Old 8th March 2010, 08:38 PM   #6
rotaspec is offline rotaspec  New Zealand
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Rod was right, it looks like the choke filter was just used for the driver and preamp. No matter, it will do nicely for other projects once I determine its value.

How about using those big paper capacitors (like used in UPSs and electric fences) as filter C's. I have some 40uF 380vac and can probably track down smaller values.

Time to get intimate with PSUD
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Old 9th March 2010, 08:23 AM   #7
Rod Coleman is offline Rod Coleman  United Kingdom
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Paper Caps worth a try - two in series is 20uF, which is about where I'd start looking in a C1 search.

I found that the sound changes hugely according to the construction of C1. Be sure to compare the final design against something like Ansar CPA series, LCR AG or other audio-wound MKP. I found that the ordinary motor-run MKPs sound much worse than a decent electrolytic!

If no MKPs fall to hand, the Samwha HJ series 'lytic 68uF 400V is cheap and good, and can take 560mA ripple, enough for 807 PP. 3 or 4 in series works well to get low value C1. Need 220K 1W across each to balance. I have used this kind of C1 with vintage guitar amps, and pleased the owner - although I think that even there the Ansars sounded better.
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