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In Reply to: RE: HK Citation 2 rebuild questions posted by Triode_Kingdom on April 19, 2015 at 12:06:39
How about a clean factory built Citation 2? I don't think you would butcher that either.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
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I've seen some that the only thing worth salvaging on them is the output iron. The power transformers on the Citation II are not the best to begin with. Many of those amps ran so hot the potting material literally melted out.
I kind of wished the Citation II was allowed to be designed with a pentode output. I think that wold have made a very good amp even better.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
" The power transformers on the Citation II are not the best to begin with."
I don't agree at all, IMHO those transformers were top shelf stuff. What about them was substandard?
"Many of those amps ran so hot the potting material literally melted out."
Often the heating was due to the trafo being on an amp that needed work. And early on H-K got crazy with the bias spec of 100 ma. per tube. So the transformers were worked VERY hard in many cases. But even many of those trafos that were not well treated are still running just fine today, 50+ years later.
IMHO those trafos may have been assassinated by other amp issues but they very rarely failed on their own. We've seen some issues with the bias winding in the last few years, but that's it.
If I were to get another HK Citation II, I would have to seriously consider a newly made power transformer with more reserve (overkill) to help out performance. I'm sure the original quality of the power transformer is fine. It's more the fact that it was run HARD near its limits for a lot of years, so many of them are stressed.I also wonder if a pentode output would have been better in the long run.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Edits: 04/19/15
and they don't generate a lot of heat unless people leave old leaky caps in the unit that stress the power trafo..The Heath W6m also uses a doubler and as far as the Citation 2 running hot,that was true back in the day when they were biased at a 100ma per tube..With the McShane mods and the reduced bias on the tubes,they run much cooler than before.
I wasn't a fan of doublers either until I realized their wonderful attributes..Do you realize the power transformer in a doubler has fewer turns of wire that is thicker and heavier and therefore has a lower impedance secondary.
The biggest reason for doublers back in the day was the price and availability of higher voltage and higher current silicon diodes.I wish they would use them again.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Edits: 04/22/15 04/22/15
Always good when you provide some good insight, Mikey. Valid points to consider.
Having said that, I still prefer not to use a voltage doubler. I'm also not a fan of cathode bias. To me, fixed bias is a better technical solution, especially in the DIY world.
I never cared much for the sound of the Mac's you mentioned. Always thought the Citation II sounded better. I have always wondered what the Citation II would sound like as a all pentode design.
Give me fixed bias pentode output any day.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Interesting: Marantz 8 and 9's also used voltage doublers. Doublers were quite common in the old days. For modern designs, Quicksilver has almost totally ( i haven't see all his designs) gone over to a doubler system.What is interesting is Mike has used the doubler for B+ supply for the front end tubes ( 600 volt B+ = 300 volt at the half way point) or for the screen 2 taps. This, in effect, enables him to run a non ultralinear output and also enables his amps to be stable running anything from a 6L6 to a KT 120
Edits: 04/19/15
All the smaller HKs I'm rebuilding have doublers. They work very well, and I wouldn't hesitate to design one into new gear. That's a useful trick regarding the 1/2 B+ tap. Of course, it can also be done with a FWB if the PT has a center tap.
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