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In Reply to: RE: Post favorite vintage EL84 amps and why posted by Audio Mind on February 18, 2015 at 17:16:09
...it's about Keroes push-pull implementation of the Loftin-White circuit where the driver is direct coupled to the output tubes, IMHO.
Follow Ups:
"it's about Keroes push-pull implementation of the Loftin-White circuit where the driver is direct coupled to the output tubes"
The amp is direct coupled but has no relationaship at all with the Loftin White amp which has very specific design criteria. Many amps are direct coupled and most have no relationship with the LW.
It's fine to disagree, but without any supporting statement to justify your assertion, the disagreement is unfounded. The Loftin White circuit is much more than just direct coupling; there are specific technical requirement that should be implemented to be considered to be a circuit derived from the Loftin White:
1. direct coupled driver to output tube
2. the driver tube derives its voltage from the cathode resistor of the output tube.
3. the last capacitor in the power supply bypasses to the cathode of the output tube.
Several contemporary topologies meet the criteria. Both the 'monkey' and the 'DRD' amps come to mind and predictably they are single ended just like the original; they derive their driver stage voltage from the cathode of the output tubes, and the last cap in the PS is bypassed to the cathode of the output tube.
The Acrosound 20/20 does not meet the criteria and I can think of no PP amp that does.
and I would point out that your assertion likewise lacks citations.
I also disagree that there are lots of direct coupled amps out there, we are speaking of vintage in this thread. I would appreciate knowing which ones those are.
nt
I pulled up Acrosound 20/20, Leak 20 and Pilot 232 schematics.
Yes not only does the Acro have one less set of coupling caps it also has takes away a gain stage or two and has a choke in the power supply.
All of which should make for a smoother amp with a blacker background than the other two amps I would imagine. Nice!
IMHO, all things equal:
1) I'd rather have a choke input power supply with a tube rectifier.
2) I'd rather have a LTP splitter, then a split-load, and lastly a paraphase. If you can't tell from looking at a schematic it would help you to learn that skill.
3) Direct coupling, when done in a safe manner, is to be preferred.
4) For most output tubes, I'd lean towards ultra-linear or triode over pentode/tetrode.
5) I'm not a fan of high mu (gain), low current tubes like a 12AX7 (12au7/12bh7/12at7 aren't that much better). Much rather have an amp that used a 6sn7.
6) It all starts with great output transformers, then a good power transformer and choke. All the rest is easily/cheaply changed.
"If" you could find an Acrosound 20-20 for sale I'm not sure you could stomach the asking price. I couldn't and wouldn't. The pilot used to be a sleeper. Sherwoods have a cleaner/less tubey sound that I perfer but need a "lot" of part replacements and are hard to work on due to cramped chassis. Fisher typically has some of the best resistors and caps of all the vintage amps....nice euro (usually german) resistors/caps/tubes.
The biggest lesson I have learned in my journey! Great amp designs are usually great because of the nifty feedback circuits (and these circuit values are very dependent on parts, parts that can't be bought anymore, so are hard to duplicate without knowledge and test equipment). These feedback circuits reduce output impedance and increase damping factor. That allows the amps to cope with difficult speaker loads.
Moral of the story? Obtain speakers that have a constant impedance, are upwards/over 100dB per watt, and use an electronic crossover with solid state amps for the bass under 125 hertz. If you do that, then you don't need one of those great high power tube circuit amps. In fact you can get by very nicely with a zero feedback, simple, low powered amp. Those are really easy to build yourself. The approach also open the doors to otherwise unsuitable tubes, such as most of the directly heated triodes.
1.Wouldn't you want to use a choke with solid state rectifier also?
2.What about pentode wired in triode?
3.I am curious on which parts you are referring to that can't be bought any more besides the filter cap cans? (even those have people who gut them and stuff them with new caps.) Weird output transformers?
I also wouldn't spend the money on an Acrosound 20/20 at high retail but now that I know they are a good sounding amp I spend a few bucks to pick one up in a dusty corner where I may have passed on it before and that is the point of the thread finding the pecking order of vintage amps.
Yes 6sn7's are some of my favorite driver tubes but are pretty rare in comparison to the 12XXX family unfortunately.
1.Wouldn't you want to use a choke with solid state rectifier also?
Yes, I would. I just find I like the sound of tube rectifiers. Damper diodes are some of the best IMHO.
2.What about pentode wired in triode?
Those are fine. Often better than they are as pentodes if you don't need the power. Pentodes aren't evil though. They can even make wonderful driver tubes.
3.I am curious on which parts you are referring to that can't be bought any more besides the filter cap cans? (even those have people who gut them and stuff them with new caps.) Weird output transformers?
Yes, the output/interstage transformers. And to a certain extent chokes that are used to load tubes and/or split phases. It is these very parts that account for the high prices of some vintage amps. Check out prices on anything western electric, the old rca theatre amps, some of the altec lansing, brooks 12a, etc. Remember the feedback loops (global ones) have to be designed around these parts behavior. So cloning some of the better older amps isn't so simple without the original parts.
I also wouldn't spend the money on an Acrosound 20/20 at high retail but now that I know they are a good sounding amp I spend a few bucks to pick one up in a dusty corner where I may have passed on it before and that is the point of the thread finding the pecking order of vintage amps.
I wish you luck. Better chance with an old magnavox:)
Yes 6sn7's are some of my favorite driver tubes but are pretty rare in comparison to the 12XXX family unfortunately.
One of the advantages of building your own is using non-standard tubes. Often tubes were offered in a range of heater voltages, some of them in seemingly odd ball voltages like 8 volts (8SN7GTB). Then you have the fact that a 6cg7/6fq7 is basically a 6sn7 in a 9 pin mini tube. Or consider a 7n7 in the locktal base. So if you are flexible there are values to be had. And in some cases, new production tubes, or old eastern block tubes, can be outstanding.
It should go without saying that anything with a 12XXX tube dates itself past the best years (IMHO) of tube audio amps.
Interesting point on the unavailable parts versus price on the vintage tube gear. Altec 1520 comes to mind for me, they are about 3 times as much as a similar Altec 1568 model.
I did not make it clear but I do understand I can build a new amp that would smoke most vintage models, those are in the works but I do have several systems and also like to use some of these EL84 amps for guitar.
Never say never with finding vintage gear, I have found some of the most amazing tube gear in the oddest places. When you get known as the guy who likes that old tube stuff sometimes the gear finds you and then there is Craigslist...
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