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Old 2nd April 2009, 01:18 AM   #11
kevinkr is offline kevinkr  United States
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Default Re: Instability

Quote:
Originally posted by vaslejean
Thank you,
it just makes one wonder how could Keroes in the introduction of his afordmentioned article states his pretention as follow:

..." Here is a modern version of the Loftin-White amplifier that is free of distortion, is exeptionally stable, and as exellent frequency response..."

Commercial pitch of an over exited engineer ?

Now for all I know of Loftin-White amplifiers, is that when properly made, they do sound superior to quite a few , but then they may have more diy amateur swearing at them than swearing by them as it appears to be a difficult topology to masters.

For now I would be tempted to experiment on a mono layout, hoping to (Am I being naive ?) reduce the conditions that could lead to instability ?

As for the EL84 operating voltage, the only amplifiers respecting the tube manufaturer specified data that I know of seems to be the Leak TL12+ at around 290 or 300 V. Dc. Some other french made amplifier that I know of seems to do the same.

My mass producted Heathkit runs the output tube well over 360 V.dc actualy closer to 385 V. (and it never had problem with that) on a nice day. Dynaco SCA-35 are running them just a little lower.

Thanks for the input.

No, a bit of hyperbole perhaps, but I have heard the Acro 2020 which was the commercial version of this amplifier, (had a loaner for several months and have fixed a few over the years) as well as having heard several homebrews based on it. i have heard very few PP 6BQ5 amps that sounded better.
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Old 2nd April 2009, 04:10 AM   #12
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Thank you kevin ,

This is getting encouraging.

Do you have any recolection of the "homebrew" versions of this amplifier that you may have had in your hands ? I.E, were the transformers used in these versions acrosound units or just regular EL84 specified pp UL transformers. My comprehension when it comes to transformers is that a well designed can certainly work just as well as any "holy grail" flavoured one ...I would not replace a burnt down McIntosh output with a Hammond ...;-)

As I mentioned I would like to assemble a mono version following the information as per the Electronic Experimeter Handbook I have no data on the TO-370 OT that was used. Assuming it was anywhere close to what would be used as UL transformer for the EL84 , 7.6K to 9K primary, I am assuming I could use the Dynaco Z565 safely, to experiment at least.

thanks again
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Old 2nd April 2009, 07:41 AM   #13
Jaap is offline Jaap  Netherlands
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Default Re: Re: Instability

Quote:
Originally posted by kevinkr


i have heard very few PP 6BQ5 amps that sounded better.

If you heard some designs that were better, than we better built that/those one(s)
You make us curious
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Old 9th May 2010, 05:56 PM   #14
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Hi vaslejean. Have you built finally the Keroes PP EL84 amp?
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Old 9th May 2010, 08:17 PM   #15
kevinkr is offline kevinkr  United States
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Note that the power supply in this thing is a half wave voltage doubler, better performance can be had with a full wave doubler or better still a conventional psu with a bridge and the appropriate secondary winding voltage.

Old reply to my comments about sound, the wording I used is generally intended to imply that I have not heard something better.. Whether or not I in fact have is debatable, it's a good sounding design. (And I can' remember anything that stood out as sounding better than this design.)
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Old 9th May 2010, 08:48 PM   #16
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Quote:
Originally Posted by Andrewbee View Post
Try this now: http://www.wimdehaan.nl/downloads/dccoupledamp.pdf

dave
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Old 12th May 2010, 12:08 PM   #17
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Thanks for the info. I have made several versions of that amplifier, also with my own power supplies, and with current sources for the output stages. The thing is that all of that was ... 30+ years ago. Now, I'm thinking into re-make a new version, because I remember that the sound was very good. It is only a romantic thing, because nowadays, I use a "powerful" amp with two KR300Bs in push pull (30+ watts each monoblock)
It was very pleasant to see the article sent by planet10 (Thanks dave!!) and it's fig 3. I have done the same configuration with the same valves (ECC88), but 30 years ago. At that time, to use that valve (ECC88) in audio, was an heresy.
Surely, I will make again some of them.
Thanks again.
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Old 12th May 2010, 01:09 PM   #18
SY is offline SY  United States
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Quote:
Originally Posted by finisvox View Post
I have done the same configuration with the same valves (ECC88), but 30 years ago. At that time, to use that valve (ECC88) in audio, was an heresy.
Marantz 9 (that was even longer ago). Modjeski/Beveridge preamp. The Zeligman preamp featured in Audio Update. Nope, the ECC88 has had a long and honorable history in audio and rightfully so.
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Old 12th May 2010, 02:30 PM   #19
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There is no reason that the Acrosound DC amp should not work well. As for instability, it would be helpful to know from those who encountered instability, just what kind of instability they encountered. Low frequency? The power supply is well bypassed, and the output tube cathode connection provides DC NFB to stabilize the output stage operating point, so that should not be an issue. High frequency? When using a different output transformer, the HF stabilization circuits need adjusting more often than not. In this design, that is accomplished by the cap in the GNFB circuit. The Dyna transformer provides about the best match for 6BQ5 tubes out there. It is capable of very stable operation, and should be able to operate in this circuit with minimal adjustments. Note however that in the Acro design the feedback is coming from a 32 ohm winding, while the original Dyna transformers only have a 16 ohm winding.

Dave
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