I recently purchased a Sony TC-500 for the purposes of disassembly. I don't usually sacrifice working gear; hence this was purchased as a unit that was in non-working condition (which also got me a good price on it). Shipping was quite a bit, as this unit is HEAVY.
I carefully took it all apart and kept everything I thought might have some future use, for myself or anyone else. What I was most interested in, however is seen below.
I have heard through the grapevine that the iron in this unit is something special. The PT and OPTs are apparently made for Sony by Tamradio (Tamura?). The PT features an adjustable power input from 100V to 240V by removing and replacing a jumper, and 300-0-300 secondaries, in addition to the necessary 6.3 VAC. I am also told that the OPTs are 5K input and have both 600 ohm and 8 ohm outputs. I noticed that they are quite large and heavy in comparison to most output transformers I come across in phonos and R2R tape machines. The PT is quite the monster.
The speakers I did not think would be very interesting; very small full-range units that have more bass, even freestanding, then I would have suspected.
The tube complement is interesting. All NEC. I have a single 6AU6A, a 12BH7A, and a pair of 12AD7 and 6ABQ5 tubes. There is also a 6CA4 rectifier.
So I'm interested in putting these parts to work in a simple stereo SE unit, and I'm casting about for a nice schematic.
Given the parts I've got here, can anyone offer any suggestions? I'm rather not wanting to do PP, just a simple low-power SE with the tubes that came with the unit if I can. Thanks!
I carefully took it all apart and kept everything I thought might have some future use, for myself or anyone else. What I was most interested in, however is seen below.
An externally hosted image should be here but it was not working when we last tested it.
I have heard through the grapevine that the iron in this unit is something special. The PT and OPTs are apparently made for Sony by Tamradio (Tamura?). The PT features an adjustable power input from 100V to 240V by removing and replacing a jumper, and 300-0-300 secondaries, in addition to the necessary 6.3 VAC. I am also told that the OPTs are 5K input and have both 600 ohm and 8 ohm outputs. I noticed that they are quite large and heavy in comparison to most output transformers I come across in phonos and R2R tape machines. The PT is quite the monster.
The speakers I did not think would be very interesting; very small full-range units that have more bass, even freestanding, then I would have suspected.
The tube complement is interesting. All NEC. I have a single 6AU6A, a 12BH7A, and a pair of 12AD7 and 6ABQ5 tubes. There is also a 6CA4 rectifier.
So I'm interested in putting these parts to work in a simple stereo SE unit, and I'm casting about for a nice schematic.
Given the parts I've got here, can anyone offer any suggestions? I'm rather not wanting to do PP, just a simple low-power SE with the tubes that came with the unit if I can. Thanks!
The comments about the iron are right on, Tam Radio is indeed related to Tamura.
The speakers are the sought after holey basket Corals amoungst the very best of the "FE103A" genre -- dozens of variations across many vendors. Not to be scoffed at. They do often end up suffering from stiff surround syndrome which reduces their bass capability.
dave
The 12AD7 is a low hum version of the venerable 12AX7.
For your low power stereoblock, all you need is the 6CA4/EZ81 rectifier, a 12AD7, and the 2X 6BQ5/EL84 power pentodes. Sony seems to have taken liberties, as many other manufacturers did. 300-0-300 strikes me as being too "tall" for Class "A" 6BQ5 "finals", working into a cap. I/P PSU filter. If you want to CLC filter the B+, save those Japanese 6BQ5s for another job and buy 2X Russian 6П14П-EB (6p14p-ev), AKA EL84M, tubes. The 6П14П-EB is a tough 7189 equivalent that sounds good.
Topology is easy enough. A 12AD7 section voltage amplifier drives a pentode mode "final". Global NFB goes from the O/P trafo secondary back to the voltage amplifier's cathode. It's a very good idea to regulate O/P tube g2 B+. If you buy 6П14П-EB O/P tubes, you could use a stack of 2X 0A2 gas discharge tubes to regulate the g2 B+. A very honest 5 WPC is what you'll wind up with.
Protect the O/P "iron" cores against saturation by rolling infrasonic noise off at the unit's I/Ps. A 100 Kohm grid to ground resistance and a 0.068 μF. cap. will do that for you.
For your low power stereoblock, all you need is the 6CA4/EZ81 rectifier, a 12AD7, and the 2X 6BQ5/EL84 power pentodes. Sony seems to have taken liberties, as many other manufacturers did. 300-0-300 strikes me as being too "tall" for Class "A" 6BQ5 "finals", working into a cap. I/P PSU filter. If you want to CLC filter the B+, save those Japanese 6BQ5s for another job and buy 2X Russian 6П14П-EB (6p14p-ev), AKA EL84M, tubes. The 6П14П-EB is a tough 7189 equivalent that sounds good.
Topology is easy enough. A 12AD7 section voltage amplifier drives a pentode mode "final". Global NFB goes from the O/P trafo secondary back to the voltage amplifier's cathode. It's a very good idea to regulate O/P tube g2 B+. If you buy 6П14П-EB O/P tubes, you could use a stack of 2X 0A2 gas discharge tubes to regulate the g2 B+. A very honest 5 WPC is what you'll wind up with.
Protect the O/P "iron" cores against saturation by rolling infrasonic noise off at the unit's I/Ps. A 100 Kohm grid to ground resistance and a 0.068 μF. cap. will do that for you.
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Fascinating, thanks you! I think these surrounds are not too stiff. Cloth, things, somewhat supple yet.
The 6AQ5 is, for all practical purposes, a 6V6 in a 7 pin mini package. The key difference between the 2 is that the 6AQ5's g2 is more fragile. All the O/P tubes that have been mentioned are "12" W. types.
For the most part, my previous remarks still hold. You can use the 6AQ5s, but use an 0A2/0B2 stack for g2 B+ regulation. Definitely self (cathode) bias the O/P tubes, to consume "excess" B+ volts. Check the 6CA4 data sheet out. A 47 μF./400 WVDC part, as the 1st filter cap., will be fine. A Hammond 159Q is quite suitable as the CLC filter's choke.
Thank you! I have seen, as I recall, one example on the web of a pair of 0A2 tubes used for voltage regulation, which seemed astonishing. Could you elaborate on how they are used and what benefit they bring over traditional CLC type power stage filtering? I would appreciate being educated on this.
I humbly recommend those transformers. I used them in a RH84 amp. More than a few times I'd tried to sell the amp just to fund other projects and every time it wouldn't happen I was sort of relieved. I keep putting it back into use simply because it sounds great. Pictured here: Rotel RCD-955AX ---> RH84 ---> Yamaha NS344. In the end I'll probably just keep it for myself...
EZ81 rectifier, 6P14P-EV output tubes, Hammond 193C choke.
EZ81 rectifier, 6P14P-EV output tubes, Hammond 193C choke.
Attachments
Look here for some info. on gas discharge regulator tubes. The 7 pin mini types are numbered in descending voltage order, with 0A2 being 150 V. and 0B2 being 105 V. A Zener diode is the SS analog of a gas discharge regulator tube. Both device types can be used as voltage references in "high" current applications or directly for regulation, when current demands are small. As the screen grid current demands of 2X "12" W. power O/P tubes are modest, direct regulation by gas discharge tubes is feasible.
Thanks, Tom. Good to see you again.
Can you tell me what the value of the capacitor in the feedback circuit is? The one bypassing the 2.7K resistor at the top of the schematic? Looks like a very clean circuit, that's probably exactly what I'm looking for.
What would you think if I were to use the 12BH7 tube in place of the 12AX7? I realize the 12BH7 is more of a replacement for the much-lower gain 12AU7, but just wondered what your thoughts might be on using it.
😀
I never installed those caps - I know there is a peak in the gain around 40
KHz, but never got around to hooking up a scope to tweak it. Something in
the range of 220 pF, I would expect. But no big deal if you leave it out
entirely, since there isn't a lot of feedback. The feedback resistor is for
8 Ohms, around 2K for a 4 Ohm transformer. Power supply uses a voltage
doubler since that was the transformer I had. Voltage is a bit high, so
there's a dropping resistor. 12BH7 may not have enough gain for feedback,
but you could triode wire the 6AQ5 and do without any.
KHz, but never got around to hooking up a scope to tweak it. Something in
the range of 220 pF, I would expect. But no big deal if you leave it out
entirely, since there isn't a lot of feedback. The feedback resistor is for
8 Ohms, around 2K for a 4 Ohm transformer. Power supply uses a voltage
doubler since that was the transformer I had. Voltage is a bit high, so
there's a dropping resistor. 12BH7 may not have enough gain for feedback,
but you could triode wire the 6AQ5 and do without any.
Thanks, Tom! I truly appreciate it. A peak at around 40KHz, eh? That's dog-whistle territory anyway. I can't hear a thing over 12KHz these days. 😉
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