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
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
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.
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.
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.
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.
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.
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.
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.
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.
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
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
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.
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.
OK a long shot as you posted your thread on the Trix T621 2x EL37 PP audio amp 12 years ago!
I've had one of these for years - I rescued mine from the skip in Wales UK at the end of the 70s!
I've traced out the schematic some time ago in view of rebuilding the components on a new and more attractive chassis, and currently it serves as a sub-woofer amp in my setup, but running 2x EL34 instead of the EL37s. I still have the EL37s, so it will be interesting to see if their emission is still good enough to run the amp, providing the surrouding driver circuitry is rebuilt with good condition parts. I'll enter the pencil drawn circuit into my CAD program so I can share it with you.
Cheers for now,
Peter
I've had one of these for years - I rescued mine from the skip in Wales UK at the end of the 70s!
I've traced out the schematic some time ago in view of rebuilding the components on a new and more attractive chassis, and currently it serves as a sub-woofer amp in my setup, but running 2x EL34 instead of the EL37s. I still have the EL37s, so it will be interesting to see if their emission is still good enough to run the amp, providing the surrouding driver circuitry is rebuilt with good condition parts. I'll enter the pencil drawn circuit into my CAD program so I can share it with you.
Cheers for now,
Peter
Kind of you to draw out the schematic and post it here. Thank you for that.
I think there's a little mistake in it though. The plate current of the EL37's is indicated as 24 mA each. But looking at the datasheet for the EL37, I think that 24 mA is very low (but the value of the shared cathode resistor is rather high, since the datasheets show a value of 130 Ohm). When looking at the cathode voltage of approximately 24 V like indicated in the schematic, the total current of the two EL37's combined would have to be I = V / R = 24 / 220 = approximately 109 mA which conflicts with the 2 x 24 mA.
On radiomuseum.org this amplifier is said to have 2 x KT66 as the output tubes. That would fit better with the value of the shared cathode resistor of 220 Ohm as indicated in your schematic.
Trix T621 - Radiomuseum.org
KT66 GEC
EL37 Mullard
I think there's a little mistake in it though. The plate current of the EL37's is indicated as 24 mA each. But looking at the datasheet for the EL37, I think that 24 mA is very low (but the value of the shared cathode resistor is rather high, since the datasheets show a value of 130 Ohm). When looking at the cathode voltage of approximately 24 V like indicated in the schematic, the total current of the two EL37's combined would have to be I = V / R = 24 / 220 = approximately 109 mA which conflicts with the 2 x 24 mA.
On radiomuseum.org this amplifier is said to have 2 x KT66 as the output tubes. That would fit better with the value of the shared cathode resistor of 220 Ohm as indicated in your schematic.
Trix T621 - Radiomuseum.org
KT66 GEC
EL37 Mullard
Hi Kwadjo,
Many thanks for spotting that error for the plate current - it was a cut & paste error left over from another similar PP circuit I modified to generate the T621, and as you say, the plate current should be about double that, as I measured the cathode bias at 24v.
The output tube type is a bit of a mystery, as when I acquired this amp it was fitted with 2x EL37. I've run it for quite some time (as a sub-woofer amp!) with a small mod for the screen grids with 2x EL34s but have kept the original EL37s. I am planning to rebuild it on a new chassis, the original being rather rusty and not very attractive, so might try the EL37s again if they're not too low emmission.
If you look at Rotaspec's first comments above, you see his example had "EL37" penned in on the chassis. I can't find any original specifications on the web about this Trix Electrical amp from 1954, so maybe they started with the rather short lived EL37, and moved onto KT66s later ??
I did find a reference to this amp with the date in a WW July 1954 new products info article, but it doesn't give the valve types, just the power o/p at 25W.
https://worldradiohistory.com/hd2/I.../50s/Wireless-World-1954-07-OCR-Page-0009.pdf
The mains transformer in the amp appears to have a very generous size (11.5x9x14cm) and weight (3.8KG) for a 25W amp - I'd love to have a copy of the original rating specs for this component!
So, I'm curious to see what performance the rebuild will give, and I'll check it with EL37s and KT66s when I can get some!
cheers
Peter
Many thanks for spotting that error for the plate current - it was a cut & paste error left over from another similar PP circuit I modified to generate the T621, and as you say, the plate current should be about double that, as I measured the cathode bias at 24v.
The output tube type is a bit of a mystery, as when I acquired this amp it was fitted with 2x EL37. I've run it for quite some time (as a sub-woofer amp!) with a small mod for the screen grids with 2x EL34s but have kept the original EL37s. I am planning to rebuild it on a new chassis, the original being rather rusty and not very attractive, so might try the EL37s again if they're not too low emmission.
If you look at Rotaspec's first comments above, you see his example had "EL37" penned in on the chassis. I can't find any original specifications on the web about this Trix Electrical amp from 1954, so maybe they started with the rather short lived EL37, and moved onto KT66s later ??
I did find a reference to this amp with the date in a WW July 1954 new products info article, but it doesn't give the valve types, just the power o/p at 25W.
https://worldradiohistory.com/hd2/I.../50s/Wireless-World-1954-07-OCR-Page-0009.pdf
The mains transformer in the amp appears to have a very generous size (11.5x9x14cm) and weight (3.8KG) for a 25W amp - I'd love to have a copy of the original rating specs for this component!
So, I'm curious to see what performance the rebuild will give, and I'll check it with EL37s and KT66s when I can get some!
cheers
Peter
A couple of extra points about this Trix Electrics T621 PA Amp.
1/ I can verify Gary's open load measurements on the mains power transformer match what I see on the example I have : 335-0-335v for the HT.
2/ In addtion to the shared PP output pair cathode resistor of 220 Ohm, the output transformer has the screen taps at 20% which reinforces the argument that the circuit intended to use KT66 beam tetrodes rather than EL37s. (see the GEC KT66 data sheet)
Peter
1/ I can verify Gary's open load measurements on the mains power transformer match what I see on the example I have : 335-0-335v for the HT.
2/ In addtion to the shared PP output pair cathode resistor of 220 Ohm, the output transformer has the screen taps at 20% which reinforces the argument that the circuit intended to use KT66 beam tetrodes rather than EL37s. (see the GEC KT66 data sheet)
Peter