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In Reply to: RE: Harman Kardon TA5000X output transformers posted by Tubecase on May 05, 2016 at 06:21:21
The part numbers would be different regardless of impedance; A500 transformers are horizontal mount. In any event, TA5000 OPTs are almost certainly 4K P-P or less. I've measured the A500 transformers, and they're 3.6K P-P. [EDIT: I remeasured these same transformers years later using different test gear and a more refined procedure. The examples I have are in fact about 5K.] HK's TA-260 also used 7355s, and Sams shows the transformers to be 4K P-P. Unfortunately, these low Z OPTs aren't optimum for use with most triodes.I experimented with an A500 last year, substituting 6L6-family tubes for the 7355s. Results were very good, and I'm planning to eventually use the A500 iron (including the power transformer) to build a new amp. My suggestion would be for you to consider doing the same. I'm a fan of big triodes, but good pentode amps (non-UL) can also be a real joy to hear.
Ken-Rad 6L6 Specs:
Edits: 02/25/24Follow Ups:
Hello Triode Kingdom,
Thanks for your reply, now I have at least a better idea what the primary impedance of these OPT's is.
And you are right, good penthode amplifiers can sound very nice too.
Thanks also for the operating points of the 6L6's.
Best regards, Tubecase.
Full pentode mode "finals" are fine, when g2 B+ is regulated at a considerable fraction of anode B+. Max. open loop linearity is obtained.
GNFB, to linearize the O/P "iron" and achieve a satisfactory damping factor, is (IMO) in order.
Eli D.
Hello Eli,
How do you regulate the g2 B+ at a fraction of the anode B+?
When I look for instance to the operating points of the 6L6 which Triode Kingdom gave, then the g2 B+ is at 300V and the anode B+ at 400V.
Do you perhaps use zener diodes to regulate the 100V difference between them?
Best regards, Tubecase.
Before, SS, tube regulators were often used. They work great but require a little more power to work than a modern day regulator. If i am not mistaken the RCA Tube Manual shows one with there hi power 7027 amp. cheers, Dak
A pair of 0D3s on the chassis won't hurt a thing!
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Buy Chinese. Bury freedom.
"How do you regulate the g2 B+ at a fraction of the anode B+?"
Use a Maida style regulator. I suggest you buy Tom's PCB, in its fully populated version. Pay the man his money and leave SMDs to an expert.
Tom's PCB is highly competent. You might have enough current for the screen grids and the small signal circuitry. ;> )
Eli D.
I really dislike the jerks running that board. I'd design my own rather than join again.
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Buy Chinese. Bury freedom.
To each his own.
A direct link to Tom's PCB is provided below.
Eli D.
There's no real documentation at that site. Do you have a schematic to share?
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Buy Chinese. Bury freedom.
You can read the thread, without signing up. Unfortunately, access to the uploaded stuff is unavailable.
Perhaps Mike Maida's seminal paper (linked below) will prove beneficial.
Eli D.
!
The Mind has No Firewall~ U.S. Army War College.
Whenever semiconductors are used to monitor and directly adjust a DC voltage, and that voltage powers vacuum tubes, there's the risk of degrading the sound qualities for which the tubes themselves have been employed. We've been creating superb sound for decades with vacuum tubes, and with no need to resort to circuitry of this type.
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Buy Chinese. Bury freedom.
There is also the possibility of better results than either tubes or SS, standing alone, can deliver.
Be pragmatic! Results, not preconceived notions, are what matter.
Eli D.
I don't disagree, although there's something to be said for the legacy of all-tube circuitry. My point though was that one must be selective and not assume that the advantages of SS won't bring degradation to the system. The specific application and implementation are all-important.
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Buy Chinese. Bury freedom.
Most tube audio amplifiers don't regulate the screens. However, it's necessary to do so if lowest distortion is to be assured. I'm planning to add a string of VR tubes for this purpose when I rebuild the A500. The rebuilt amplifier will not include the original preamp/tone sections, so there should be a little extra HV current available for the regulators.
Eli is also correct about the need for NFB with pentodes. I found that the original HK configuration and values worked well when I substituted 6L6s. I should probably mention that this wasn't a simple "drop-in" replacement. The 6L6s exhibit considerably less gain than the original 7355s, so they won't provide the original factory output power (25W/ch) as-is. I raised B+ in my prototype by using a bench bias supply. That allowed me to ground the output cathodes and recapture the voltage previously lost in the bias resistors. The subsequent increase in cathode-anode voltage was just enough to allow driving the 6L6s to 25W output. The final design will include a fixed bias supply using a separate bias transformer and filter.
I'll just add that the HK transformers that were paired with 7355s back in the day are really very good. I wouldn't hesitate to use any of them as a basis for a high-end pentode amplfier.
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Buy Chinese. Bury freedom.
Time honored and very well proven are the Williamson and Mullard styles. As Mullard style is more forgiving of O/P "iron" weaknesses, I lean in that direction.
An updated version of the H/K Cit 5. should work out very nicely. The "5" used 7581 "finals".
Eli D.
Most HK amplifiers (other than Cit II and V) use a different topology than the designs you've mentioned. They employ a Cathodyne phase splitter attached directly to the output stage. In this regard, they're similar to Dynaco. Even then, they differ in that the power amp block uses a triode, rather than a pentode, ahead of the splitter. I much prefer a Cathodyne to be followed with a buffer, but I'm not going to change this when I rebuild the A500. The feedback network is more complex than many, and IME it works extremely well with the HK iron. With NOS 6L6s, I measured 0.4% THD at 20W and less than 0.1% THD at 5W. Those numbers indicate to me the 6L6s are at home in this design, and I'm hoping to do a little better in the final build. My intent is to create an all-octal amp, so the 12AX7 and 12AU7 will be replaced with a 6SL7 and 6SN7 respectively.
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Buy Chinese. Bury freedom.
Go for it!
I made the suggestions, thinking more voltage gain than OEM would be needed.
Just in case, I'm uploading the Cit. 5 schematic and an alternative voltage gain setup. The 12BY7 is going scarce and I think the available stock should be saved for already existing units.
Eli D.
"I made the suggestions, thinking more voltage gain than OEM would be needed."
Funny you should say that, because I thought the same thing. However, with 6L6s in place, I measured 27W output with slightly less than 1/4V RMS input. It was this result that made me realize a 6SL7 could be subbed for the 12AX7 and still have sufficient gain.
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Buy Chinese. Bury freedom.
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