I bought a nice set of power and output transformers. The seller didn't know anything about them, except that they came from a closed shop making high end tube amplifiers. Each transformer weighs about 20 pounds 8.8 kg). I measured them and got the following:
The power transformer secondaries are: 360 VAC 14 Ohm (no center tap) , 100 VAC 35 Ohm (presumably for fixed bias), three 6.3 VAC, and one 10 VAC.
The output transformer is SE, with 7.8 K primary and secondary with usual 4, 8, and 16 Ohm taps.
Wondering what output tube this set was intended for?
The power transformer secondaries are: 360 VAC 14 Ohm (no center tap) , 100 VAC 35 Ohm (presumably for fixed bias), three 6.3 VAC, and one 10 VAC.
The output transformer is SE, with 7.8 K primary and secondary with usual 4, 8, and 16 Ohm taps.
Wondering what output tube this set was intended for?
Attachments
Voltage doubler for ~1000Vdc, perfect for GM70. The 7.4k primary output transformer should also work well for GM70.
Last edited:
Thanks for the link, Lampie. So, everything falls into place. 7.8 K looks like optimal for 1,000 V B+. The 10 V winding is probably for the 845 filament.
Type 211 also uses a 10V filament, but would really like about twice that output transformer primary Z. Two in parallel would allow the opposite-polarity filament trick to balance some AC filament modulation, but would the 10V winding cooperate at 6.5A? Tough to guess, tough to measure.
All good fortune,
Chris
All good fortune,
Chris
211 10V direct heated filament on AC has the following character:
A virtual center of the filament.
One end going up by 5V, while the other end goes down by 5V, then all that reversed on the other alternation polarity.
The transconductance of the filament end at 5V positive is less than the transconductance of the filament end at 5V negative.
And, the increase of transconductance at one end is not equal to the decrease of transconductance at the other end. The total transconductance changes at a 2X power mains rate. Intermodulation.
A study of the effect of intermodulation of 2 X power mains frequency, is on other threads on this forum.
The most recent thread uses 5 12AU7 tube triodes to find the effect. The 5 cathodes were subjected to AC supplied to cathodes that were separated by resistors. The center cathode is the virtual center, the output cathodes have the largest change, and the other cathodes are 1/2 way to the outside cathodes.
That simulates the effect of AC supplied to a direct heated filament (AC along the path of the filament).
No need to repeat this effectual difference between AC DHT and DC DHT.
One (AC) has 2X power mains intermods on the musical notes, the other (DC) does not.
Paralleling 2 filaments, using AC power, and wiring the 2 filaments in opposite phase does not reduce the 2X power mains intermodulation on each and every musical tone.
The amount of the intermodulation is different for different DHT tube types.
An easy test:
Power the DHT filament with DC.
Bias the tube, call that "B" Volts, measure the plate current "Ip1".
Calculate 1/2 of the filament voltage, call that 'A' Volts.
Increase the bias volts by "A" Volts + "B" Volts, measure the plate current Ip2, compare to "Ip1" above
Decrease the bias volts by (-)"A" + "B" Volts, measure the plate current Ip3, compare to "Ip1" above
Is the change (delta) of those two exactly equal? If not, that is the relevant factor of how much the AC filaments will intermodulate at 2X mains power, with each and every of the musical notes.
I do not know of an easier test, or simpler method to show the effect.
If you can read all of this, you are working real hard, even if you do not understand it. I am sorry for the long "speech".
Your Mileage May Vary
A virtual center of the filament.
One end going up by 5V, while the other end goes down by 5V, then all that reversed on the other alternation polarity.
The transconductance of the filament end at 5V positive is less than the transconductance of the filament end at 5V negative.
And, the increase of transconductance at one end is not equal to the decrease of transconductance at the other end. The total transconductance changes at a 2X power mains rate. Intermodulation.
A study of the effect of intermodulation of 2 X power mains frequency, is on other threads on this forum.
The most recent thread uses 5 12AU7 tube triodes to find the effect. The 5 cathodes were subjected to AC supplied to cathodes that were separated by resistors. The center cathode is the virtual center, the output cathodes have the largest change, and the other cathodes are 1/2 way to the outside cathodes.
That simulates the effect of AC supplied to a direct heated filament (AC along the path of the filament).
No need to repeat this effectual difference between AC DHT and DC DHT.
One (AC) has 2X power mains intermods on the musical notes, the other (DC) does not.
Paralleling 2 filaments, using AC power, and wiring the 2 filaments in opposite phase does not reduce the 2X power mains intermodulation on each and every musical tone.
The amount of the intermodulation is different for different DHT tube types.
An easy test:
Power the DHT filament with DC.
Bias the tube, call that "B" Volts, measure the plate current "Ip1".
Calculate 1/2 of the filament voltage, call that 'A' Volts.
Increase the bias volts by "A" Volts + "B" Volts, measure the plate current Ip2, compare to "Ip1" above
Decrease the bias volts by (-)"A" + "B" Volts, measure the plate current Ip3, compare to "Ip1" above
Is the change (delta) of those two exactly equal? If not, that is the relevant factor of how much the AC filaments will intermodulate at 2X mains power, with each and every of the musical notes.
I do not know of an easier test, or simpler method to show the effect.
If you can read all of this, you are working real hard, even if you do not understand it. I am sorry for the long "speech".
Your Mileage May Vary
Last edited:
Depends on current rating of the heater windings.
When 3A, you can put them in series and create a nice current source filament supply à la Coleman.
Take a look at the thread "JC Morrison 300B", Post # 30.
You can see the simulation of AC filaments on a DHT.
Look at the simulation, see the results, and consider the cause of the results (different Delta transconductance, just as it occurs along the length of a real DHT).
Done?
Done.
You can see the simulation of AC filaments on a DHT.
Look at the simulation, see the results, and consider the cause of the results (different Delta transconductance, just as it occurs along the length of a real DHT).
Done?
Done.
daanve,
I think all the Coleman current source filament supplies are DC current. Correct?
I am talking about AC DHT filaments exhibiting 2X mains power frequency sidebands on each and every musical note.
AC is a baseball park, DC is a soccer stadium.
I really enjoyed the Munich team that came to Oregon a few years back.
A good day watching the game with my brother and my friend.
Sorry for the diversion.
I think all the Coleman current source filament supplies are DC current. Correct?
I am talking about AC DHT filaments exhibiting 2X mains power frequency sidebands on each and every musical note.
AC is a baseball park, DC is a soccer stadium.
I really enjoyed the Munich team that came to Oregon a few years back.
A good day watching the game with my brother and my friend.
Sorry for the diversion.
@Daanve,
Possibly, only not very likely. I don’t think that this has been the idea when purchasing these transformers, but then again it is possible for sure to use them this way, so you could even use a PL519 ( my personal favorite of all time ) with some adjustments ;-)
Best regards, Frank
Possibly, only not very likely. I don’t think that this has been the idea when purchasing these transformers, but then again it is possible for sure to use them this way, so you could even use a PL519 ( my personal favorite of all time ) with some adjustments ;-)
Best regards, Frank
Yes, the OP and I have discussed this recently, and IIRC, it was he who suggested that push-pull operation would cancel first-order effects. We then expanded this to include parallel operation of matched valves with filament polarities reversed.
It's something worth testing!
YOS,
Chris
Coleman supplies are indeed a very fine current source, Rod has put tremendous effort in making a very high impedance current source for tubes like the 211/845. He uses very low parasitic capacitance mosfets so any higher order mains noise does not cascade straight through the regulator.
I have a competing design, but that is an active circuit with an opamp that is bombproof, at the expense of worse noise and low Z at the lower end of the audio band. But this is over 100US in parts alone for a set.And i'm not going to order a new run of boards anytime soon.
One thing to note is: that if you go that way: you need TV damper tubes for the rectification, to give the current sources some time to heat up the fillament, so that you dont get a very high unloaded B+ supply before the outputs warm up.
@Chris
In a PP amp it is better to have the heater AC in phase as it will be cancelled in the OP transformer. If out of phase it will be enhanced by it. But i have not tested this.
Best regards, Frank
In a PP amp it is better to have the heater AC in phase as it will be cancelled in the OP transformer. If out of phase it will be enhanced by it. But i have not tested this.
Best regards, Frank
I have commercial 2A3 PPP amplifiers made by Rauland in 1930s. All filaments are powered from a single 2.5 VCT winding, all in the same polarity. CT is grounded. It is fixed bias.
In PP, there is no filament swing IM simply because composite plate volt-ampere characteristics are straight lines, not curves.
In PP, there is no filament swing IM simply because composite plate volt-ampere characteristics are straight lines, not curves.