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I see a lot of talk about how great the Dynaco ST-35 / SCA-35 Z-565 transformers are. I never see any discussions on how good the Scott transformers are. I have a fair condition Scott 222c with a bad power supply transformer and was considering a DIY project with the output trannies.Can anyone compare the quality of the Scott and Dynaco Z-565 transformers.
thanks
Follow Ups:
I would rather replace power transformer in the Scott. 222C is a sweeeet amp, and I doubt you will make anything better if you build from scratch with salvaged OPTs. 222C power transformer has pretty standard specs: 700 VCT @150 mA, 5 V @2 A, 6.3 V @ 3.5 A. The only problem is filament/bias winding, 70 V @ 150 mA, which you won't find in common tube power transformers. However, you can use a separate small transformer for this purpose. Cannibalize a 10 VA wall wart and rewind its transformer's secondary, which is a relatively simple task.I should correct myself re. Dyna ST35 transformer. Like Scott, original ST35 transformer also belongs to Group A. I an not sure though about Dynaclones...
I think you have miscalculated the 6.3 VAC requirement. A pair of triode/pentode (7199/6GH8/6U8) voltage gain/phase splitters draw 0.9 A. 4X EL84 "finals" draw 3.04 A. So, the total 6.3 VAC draw is 3.94 A. A 4 A. or better heater winding is needed.Electrically, the Hammond 373BX is "up to snuff". The rectifier winding is 350-0-350/175 mA. The 5 VAC winding is 3A. The 6.3 VAC winding is 5A. The 50 VAC bias tap is fine used ONLY for bias. A piece of sheet metal, screws, lock washers, and nuts will close the hole removing the OEM drop thru power trafo leaves. All that remains is to provide DC heater power to the 'X7s.
The switched accessory outlet can support a "wall wart". Mouser stock # 418-TR1518 is a $12.99 18 VDC/830 mA. switching unit. Pass the O/P of the WW thru a choke to kill switching noise and feed 2X 7812 3 terminal regulators. A 100 nF. film cap. goes across the I/P of each 7812. 4.7 muF. 'lytics go across the 7812 O/Ps. A 4.7 muF. 'lytic paralleled by a 100 nF. ceramic goes across pins 4 and 5 of each 12AX7 socket.
Eli D.
The problem in replacing the power trafo is mechanical. I believe the unit uses a drop thru mount power trafo. Sourcing a replacement that will fit is going to be DIFFICULT.If we were dealing with a LK72/299, a DynaClone ST70 replacement trafo can be made to work. Advantage 7591, which draws less heater current than the EL34. One of the 2X 6.3 VAC windings on the ST70 trafo can supply heater current to the 2X 6U8s and 4X 7591s. The 2nd 6.3 VAC winding can feed a Schottky diode voltage doubler and 2X 7812 3 terminal regulators to provide DC for the 12AX7 heaters. The ST70 power trafo's bias tap IS adequate, if used for bias only.
Eli D.
Hey-Hey!!!,
I would question the 'bias only' status you confer upon the St.70's PA060. The bias tap can supply current just like the main HV winding it is part of. I've sourced common cathode stages/LTP loads from that negative voltage.I wouldn't run heaters from it...:)
I would caution against running a SS-rectified, cap-input filter on the same coil you're sourcing B+ from...but if B+ is cap input there should be little penalty.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
It would be hard to source replacement that has the same size as original transformer. But in most cases, these amps are used with open chassis, so a more common upright over-the-chassis unit would just increase height somewat and change the look. It is not Harley Davidson, anyways. 70 V transformer will be needed though because 222C uses string connection of its 4 12AX7 filaments for bias source. Another simple solution would be to get 12 VDC wall wart that has 24 VCT winding, and get 48 V through voltage doubler.
I bet they are at least as good as the current production Triode Electronics Z565. It is possible they might be better, but I doubt it. It's all economies of scale. When there are enough OEMs demanding that the winders produce output transformers with a threshold passing set of specifications, the winders find a way to produce them in volume at an affordable price.Those transformers are perfect for an inexpensive and rewarding PP Triode project. PP 6V6 would be very easy to pull off. You'll need
to choose a B+ transformer. With the Edcor plate transformers, a 6BY5 rectifer will do the trick. You'd need one more small transformer for output and driver filaments. An option would be to buy a 12VAC transformer from a surplus site and then use 12V tubes (i.e. 12V6, 12W6, 12BH7, etc).Check out the Poinz Musical Machine or the Boozhound 6v6.
Rock on
-- Jim
Mr. Doyle is correct. The Scott O/P "iron" is good enough for use in Poindexter's "Musical Machine".Another design you can put that "iron" into is the "El Cheapo" I've been involved with. If you must have tube rectification, the replacement ST35/SCA35 power trafo that Triode Electronics carries can be used to power everything, except the rectifier filament. A 5R4 would be used, as its LARGE forward voltage drop is favorable under these circumstances. "Planet10" and "Chrisby" have recycled Scott stuff into an "El Cheapo" variant.
Eli D.
Forget the 5V filament... The ST35 transformer gives you two windings
each for 6.3V 2.5A. Use the 6BY5 full-wave damper diode instead.
If not that, then two more octal sockets and use a pair of 6AU4/6AX4 to build a full-wave rectifier. They are cheap, available and very good. Remember you cannot share the 6.3V winding used to light rectifiers with anything else...The second 6.3V winding needs to be budgeted carefully. Another
option is to add a 12.6VCT transformer. 12V trannies are far easier
to find and cheaper than 6.3V trannies - especially among the surplus vendors. If using a 12V filament transformer under the chassis, then
your world opens up to 12V tubes (12SN7, 12V6, 12BH7, etc) - which are often cheaper NOS than their 6V counterparts.-- Jim
Jim,In "El Cheapo", we want a B+ rail in the 350-370 V. range, as that puts an "ideal" 200-220 V. on the 12AT7 plates. The large forward drop in a 5R4 is advantagous, as the amount of waste heat from dropping resistors is kept small. The Hammond 266M5, will do NICELY to power the 5R4 filament.
The paired 6.3 VAC windings on the ST35 power trafo don't get wasted. One of those windings is quite capable of supporting 2X 12AT7s and 4X 6V6s. The 2nd 6.3 VAC winding would power the B- rail using a 1/2 wave parallel voltage multiplier. A stack of 7X 100 PIV/1 A. Schottky diodes should be quite sufficient to overcome the losses introduced by a pair of RC filter sections following the multipler. Caps. and diodes cost less than chokes. The B- draw in "El Cheapo" is only 12 mA. RC, as opposed to LC, filter sections should be OK.
Eli D.
I once saw a web page describing measured characteristics of output transformers from virtually every significant piece of vintage gear. All measurements were performed under similar conditions. Measurements included frequency range, distortion-free power at 30 Hz, and shape of 10 kHz square wave. With regard to the last characteristic, all transformers were divided into five groups. Group A was flat plateau; B, single overshot w/o ringing; C single undershot w/o ringing; D, uner-or overshot with fast decay ringing, and E, bad complex ringing. Transformers from Group A were few, all from very respected gear. Names included Peerless, Marantz, Macintosh, and few others. Tango transformers also do not ring in the 10 K square wave test (from a different study). One Scott model that he measured (as I remember, 299C) was there too. Because all Scott iron came from the same manufacturer, I guess all these transformers have very similar characteristics. I have 222C and 233 (233 uses the same iron as 299C) and their transformers have the same size (though different numbers). Scott transformer also measured -3 dB at 43 kHz and 3 W at 30 Hz.Bottom line: Scott transformers are as good as they get, among the best of the best. Based on what I know about Dyna, I am not sure it is anywhere close. It is more likely the level of modern Hammond transformers that are all in the Group E.
I could not find that web page. If anybody knows, please post.
mostly group A and B, although somewhat less high frequency extension than the Scott transformers tested. I would say they are roughly comparable.
You talking about John Atwood's transformer tests?http://www.one-electron.com/Trans_Tests.html
Here's the result that I got a couple years ago with my 211 prototype using a Magnequest FS-100. Some of you may have seen this before. The test was performed at 1kHz, rather than 10kHz, so it's difficult to compare with John's tests directly. Still, it looks pretty darn good. Mike really knows his stuff!
Hey-Hey!!!,
Seems to me that if you cut the period to 1/10-th that you're going to have that ringing through most of wave-form. Now if it looked that good with a 10k signal, you'd have something...:)Now with the Atwood testing, the Z565 got an 'A' even though the slope of the leading edge of the square wave was not as steep as a 'B' output. It is just a test...if you want to try to get more out of it, examine the raw data yourelf.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
> if you cut the period to 1/10-th that you're going to have
> that ringing through most of wave-form.The frequency and amplitude of the undesired energy render it inaudible. This is excellent performance for a transformer of this type.
> Now with the Atwood testing, the Z565 got an 'A' even though
> the slope of the leading edge of the square wave was not as
> steep as a 'B' output.Perhaps Atwood was acknowledging that a 10kHz square wave isn't entirely relevant.
TK
> > > The frequency and amplitude of the undesired energy render it inaudible.That is a matter of opinion. Atwood certainly thought testing at 10k with square waves was useful. It is certainly easy enough to claim that testing at 1k is just as good as testing at 10k.
That same argument can be extended to the LF performance...can't hear it, speakers won't reproduce it...so why bother?
Better parts make better amps. I don't quite believe the arguement that just because I shouldn't be able to hear it, that I won't hear it.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
nt
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