Comments for page: HF-60 with 6550s

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Posted by Dave April 02, 2016 - 02:27 am
Hi Nick -- Yes it's possible, although I'm not sure what you'd gain, other than -- if correctly done -- slightly increasing the input sensitivity of the design. This would come about from changing the input stage from a cathode bias to fixed bias operation. Such a move would eliminate the normal degeneration occurring across the cathode resistor network, providing the slight additional gain mentioned. But for it to be actually realized, the NFB network would need to be adjusted to account for the increased Open Loop Gain (OLG) of the new bias arrangement, so that the original NFB factor could be maintained in spite of the new OLG level. With that adjustment, the input sensitivity would then increase.

But the increase would be insignificant, as because the 6CG7 is a medium mu tube, it's gain level is relatively low to begin with, meaning that there is little degeneration occurring to begin with. Also, most of the existing cathode bias resistance is used in the bottom leg of the NFB voltage divider network, meaning that the degeneration you could eliminate is only that which occurs across the 270 ohm resistor -- or not very much at all.

It would be an awful lot of work for precious little return on that work -- meaning that it would be more of an exercise in showing it could be done, rather than accomplishing any significant result because of it.

If you want to try it -- go for it, and let us know how it turns out. I'm just saying that from this vantage point, any sonic improvement will likely be all due to expectation bias, rather than improvement that could pass any measure of scientific evaluation.

Thanks for the interest!


Posted by Nick April 01, 2016 - 01:22 am
Hi! Dave, is it possible to use battery bias for 6CG7? how to do it? Thanks.

Posted by Dave April 13, 2015 - 08:38 am
Hi Zekk -- Thanks for the kind words, and your interest in my HF-60 modification!

If you applied to modification to an actual HF-60 with the Acrosound TO-330 output transformer, then the response of the modified amplifier itself is quite flat from easily below 20 Hz, to well over 20 kHz. And, because of the NFB level employed, the damping factor is also quite high for a vacuum tube amplifier (nearly 18 by classic calculations). Therefore, if you are finding that bass frequencies are excessive, you might also check the damping of your speakers, their location, or the LF response of your preamp before altering the components of the NFB loop. Besides the characteristics of the amplifier, all of these things can individually or collectively have a huge impact on the level of bass frequencies produced.

Thanks again for your comments, and good luck with your projects!


Posted by Zekk April 12, 2015 - 09:50 pm
Hi Dave,
I find no words to describe adequately how delighted I'm, after following your modded schematic. Have gone thru 4 builts with the same massive potted A-431 but none came close!
However, the bass was a bit heavy. Wired the feedback to a switch and have found no effect. I believes with minor tweating, it'll sounds fabulous!

I'm using a #26 linestage .

Nevertheless, I'm very, very happy with it .

Dave, thank you very much for contributing this HF-60 MOD to the communityof diyers.

With Best Regards.


Posted by Dave December 05, 2013 - 04:42 pm
Hi Al -- There is no doubt that the EF86 could be triode connected, the feedback reduced, and a reasonable input sensitivity then still maintained.

As for the feedback level though, it is always a trade off. If you reduce it to target certain performance aspects, then others will be degraded. If you increase it to address degraded aspects, then others will be compromised, and so on.

I believe that a definite point of feedback level can be reached where upon any further feedback provides little if any useful return, and can act to degrade sound quality. But I also believe that too little feedback is harmful to performance and sound quality as well. The correct level, applied correctly, will always yield a maximum of benefits, with a minimum of collateral damage in the process.

Regarding the production of TIM, I think any effort spent trying to isolate any real effects from this form a distortion in classic vacuum tube amplifier designs would be all but impossible -- if it is even produced at all. My experience is that it takes feedback levels far in excess of the typically maximum 30 db or so used in classic designs for TIM to become of any significant concern.

Ultimately, the intent of this article never was to address the HF-60 as a complete redesign, but simply to address properly operating the amplifier with 6550 tubes, rather than the original EL34 tubes. Simply changing the tubes and modify the bias appropriately will actually degrade performance, where as done properly, performance is superior to that of the original design.

Thanks for your interest!


Posted by Al December 05, 2013 - 09:20 am
Do you have some comment about my post dated December 4, 2013 ?


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