I have a pair of the smallest Electra Print OPT's in my 45 amp. They were custom wound with only an 8 ohm secondary. They were $100 each at the time (a while ago). I think that they are the best sounding transformers that I have used.
Back when I built the amp I did a listening test using 5 "expert listeners" who all owned tube amps. There were 3 different OPT's tested in the same TubelabSE 300B amp. The Electra Print, the One Electron and the Transcendar. It must be noted that the One Electron is a 5K ohm transformer while the other two are 3K ohm. In blind testing everyone could hear a difference between the One Electron and the other two. Only one person could repeatably tell the difference between the Transcendar and the One Electron. It is also interesting that one person really did not like the sound of the Electra Print. He found it "irritating". The others liked it best.
The testing is on my web site here:
http://www.tubelab.com/BudgetOPT.htm
So thats where that transformer went. I was watching it, but I was preoccupied with phone calls, and missed the auction end (it was my birthday). Ordinarily I wouldn't use a One Electron transformer for experimenting, but for $25, why not.
The 6EM7 is the same tube in an octal package. May make finding sockets easier. I am using the 6EM7 as the driver in my cathode follower amplifier experiments. The drive requirements are actually worse than the 845. The 6EM7 (or 6DF7) can live with B+ voltages in the 350 volt range, and can pull its plate down to about 50 volts. It can put out some serious drive voltage at low distortion. Really low distortion if CCS loaded.
I will need some serious 600 ohm SE OPT's once I get the amp all scienced out. For now I am using the Eastern Audio transformers, but they can't cope with what this amp puts out. I have a pair of P-P OPT's made by the previously mentioned con artist. I have no problem taking them apart for experimentation. As I stated earlier a 600 ohm transformer is a lot easier to make than a 6.5K or 10K transformer.
I have a mess of 6GF7s and 6GF7As and enough of the sockets to play. I was thinking of using them in a teeny single ended amp, but I'm going instead with a triode connected 6LR8, so I have a lot of 6GF7s looking for gainful employment. The big triode in these has the lowest Rp I've seen in a common tube, along with fairly high gm. It would be a natural as a follower for light duty class A2 diver as long as you have a respectable negative bias supply - not hard with a switching supply. The 6GF7A would be a nice complement for the GM70s as far as looks are concerned.
I was planning to use the One Electron 1600 ohmtransformer for a big sweep-tube enhanced triode amp.
I was planning to use the One Electron 1600 ohmtransformer for a big sweep-tube enhanced triode amp.
Cool, I have only a few, and no sockets. But I do have a bunch of 6EA7's, so I experiment with them.
Been there, built that. It worked pretty good too. I used a similar design for the driver in my cathode follower experiments, except I changed to fixed bias in the output stage to gain more output voltage. I need about 400 volts P-P.
I have built that too, except that I used a 6LU8 because I have 12 pin sockets. I remember liking the sound of the pentode in UL with cathode feedback and the $19 Edcor transformer. It was only a temporary experiment which no longer exists.
Another one of my favorite subjects.
Ever get anywhere with the 6HV5's? I never could get that one to sound good. The plate resistance is just too high.
George - You know how initial ideas morph when you finally begin working on them - this thread is proof of that. The Shrine amp (another thread) is the first in a series of amps with switching power supplies. The second will be a 2-3W amp using a 6LR8, triode strapped. I eventually want to use it as a tweeter amp in a biamped system. The next big amp for woofer duty will probably be a pair of 6550s in triode or ultra linear mode. I was thinking of matching oak cubes for woofer and tweeter amps, small and large, with works and transformers inside, tubes on top.
This particular project will poke along slowly. I expect to get my hands on the GM 70s next week - then I'll try to scare up some sockets. The power supplies will take a long while to sort out, as well as the make/buy decision for the output iron. I need to also spend some time with tube curves to see whether I can get away with a simple voltage amp-follower scheme using the 6GF7 or whether I will need to use the second big triode for more gain, driving a source follower.
BTW - a few tidbits - The One Electron transformers are based on the EI112 lamination, while the Hammond single ended transformers are based on the EI150 laminations. The EI150s have a big center leg area, accounting for the high primary inductance and good bass one gets with them. The large core may compromise the sound a bit due to losses, or it may be that Hammond doesn't use as much interleaving as the tonier vendors in the interests of economy (labor costs). Any way, I'm not about to spend $100 on a piece of their iron just to hacksaw it apart to see what they did. One question, though, since you have a few - do they use thin laminations or the standard 14 mil size?
On the 6HV5 - I suspected that I would need to do a few things to get around the high plate resistance. Perhaps the way to go would be to feed back a fair portion of the 6HV5 gain (cathode feedback?) and drive it with a medium gain voltage amp/follower pair, perhaps using one of the better matched dissimilar triodes like the 6CM7 or 6CS7. Anyway, with the high plate resistance I would think you'd have to treat it like a sort of half-a**ed pentode to get decent output impedance. I was also thinking of driving paralleled sets of 6HV5s. Local feedback might eliminate the need for close tube matching.
This particular project will poke along slowly. I expect to get my hands on the GM 70s next week - then I'll try to scare up some sockets. The power supplies will take a long while to sort out, as well as the make/buy decision for the output iron. I need to also spend some time with tube curves to see whether I can get away with a simple voltage amp-follower scheme using the 6GF7 or whether I will need to use the second big triode for more gain, driving a source follower.
BTW - a few tidbits - The One Electron transformers are based on the EI112 lamination, while the Hammond single ended transformers are based on the EI150 laminations. The EI150s have a big center leg area, accounting for the high primary inductance and good bass one gets with them. The large core may compromise the sound a bit due to losses, or it may be that Hammond doesn't use as much interleaving as the tonier vendors in the interests of economy (labor costs). Any way, I'm not about to spend $100 on a piece of their iron just to hacksaw it apart to see what they did. One question, though, since you have a few - do they use thin laminations or the standard 14 mil size?
On the 6HV5 - I suspected that I would need to do a few things to get around the high plate resistance. Perhaps the way to go would be to feed back a fair portion of the 6HV5 gain (cathode feedback?) and drive it with a medium gain voltage amp/follower pair, perhaps using one of the better matched dissimilar triodes like the 6CM7 or 6CS7. Anyway, with the high plate resistance I would think you'd have to treat it like a sort of half-a**ed pentode to get decent output impedance. I was also thinking of driving paralleled sets of 6HV5s. Local feedback might eliminate the need for close tube matching.
Yes, none of my designs turned out like I first envisioned them. Some morph into a good working amp, and some morph into trash and wind up in the trash can!
I don't know much about the GM70. The 845 requires two gain stages in front of it to get to full power from a CD player. I am using a 5842 followed by a 45. The gains are similar to the two halves of a 6GF7.
The newer 1628SEA's are dipped in some kind of lacquer before the end bells are put on. One of them looks like it dove into the deep end, the wires are black for the first six inches, and there are runs of lacquer all down the side of the core. The laminations are hard to make out on the ones I have here, but they look to be about half as thick as the lams on a Hammond power transformer that I have.
My last 6HV5 experiment was the ultimate in low parts count amplifiers. It had three parts plus the power supply. Cathode grounded, resistor from grid to cathode (zero bias) and the OPT (Edcor). I had to crank the power supply up to max (550 volts) to get some current, and the sound was wimpy. I tried connecting the beam plates to the plate instead of the cathode. That got the current up, but it really wanted to oscillate. Never had the patience to make it play. I will get back to them some day.
I have some 6CM7's. I haven't played with them yet, but that 550 volt DC plate voltage spec (RCA, 500V GE) with a 2200 volt peak rating sounds interesting. I will have to find out what they can really do. Might make a good driver tube.
Some food for thought, re 211 loadline.
http://www.siteswithstyle.com/VoltSecond/211_Load_lines/211_LOAD_LINE.html
http://www.siteswithstyle.com/VoltSecond/211_Load_lines/211_LOAD_LINE.html
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