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Old 21st January 2011, 09:24 PM   #11
dcgillespie is offline dcgillespie  United States
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Join Date: May 2010
Location: Ball Ground, GA
Determining what kind of effort and resources to put into a piece of equipment starts from accurately defining what it is. This amplifier was designed to be a "PA" type of record player amplifier. That is hardly to diminish what it is, as no doubt it did/does it's intended job quite well. But it is also to say that it was not particularly designed to be a high fidelity piece of equipment either, as that term has been established to mean.

As designed, this unit has very minimal tone control facilities, was designed for use with an economic crystal type pickup, has remote volume control capabilities, and has distortion, frequency response, and power bandwidth capabilities and a damping factor in keeping with PA type equipment.

In looking at it from a modification standpoint, you can look at two basic building blocks of the design: The AF amp/tone control section (1st 6SN7), and the power amp section (2nd 6SN7 and 6L6s).

You can certainly place a simple volume control at the input to control high level inputs into the unit. It's value is best determined by what source will be driving it. Use a 100K control for SS sources, and 500K for VT sources. However, a much better approach would be to bypass the AF amp/tone control section all together, use a good outboard mono preamp, and inject it's signal straight into the power amplifier section of this unit. This would bypass all the existing tone switches and crystal pickup equalization built into the unit now, allow for much more versatile use from a more flexible and capable preamplifier, and allow the unit to basically operate as a mono block power amplifier.

In doing that job, one of the biggest differences (i.e. most noticeable differences) between PA type power amplifiers and high fidelity power amplifiers, is that PA amplifiers offer very little electrical damping to the speaker. Therefore, PA type amplifiers often tend to have a larger bass sound than high fidelity amplifiers do. It may be enjoyable, but it is not accurate. However, used with a more flexible preamplifier, this could be addressed to some degree with the bass and/or loudness controls.

Finally, one of the greatest things about this unit is it's size, design, and construction. There's lots of room to work with, it's a straight forward, well proven circuit format, and is a great piece of equipment for you to learn and cut your teeth on. Basically, the sky's the limit as to how much you might want to modify it, from a simple restoration of the existing unit, to gutting the chassis for an entirely new design.

I offer all of this simply to help you channel your efforts with it. Good luck!

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Old 22nd January 2011, 08:27 PM   #12
John Bessa is offline John Bessa  Canada
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Join Date: Jan 2011
Location: New Brunswick
Thanks DCG, I will use this to initiate the engineering section. For the time being, I will be obtaining parts and cultivating the local contact.

I am unapologetic when I say I believe that proper bass response has been critically missing since the migration to solid-state; this I get from my Sherwood and high-end jazz recordings from that last stages of the tube era.

The parts quandary is that these units are all worth restoring, and, hence it makes no sense to "cannibalize" any of them (unless one intends to swap parts between them like race mechanics swap around engines and tires, and such).

Expansion into, say, tube-based reel-to-reel tape recording, will also be difficult as these units are museum pieces--and priced accordingly.
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Old 1st February 2011, 06:51 PM   #13
John Bessa is offline John Bessa  Canada
diyAudio Member
Join Date: Jan 2011
Location: New Brunswick
What kind of volume control will I need?

I am going to ebay for the new valves, and a volume control (and hence the above question.

I contacted the senior go-to only to find that he ditched his valves with the onset of solid-state, and went straight for computers. He is a retired math teacher, with a PhD, and still has active interested in tech education.

When I told him that I feel that tubes sound better, he immediately confirmed this. His own oscilloscope testing showed that transistors add distortion.

I asked him about the theory of tubes, such as why tubes require much simpler schematics, and his response when right over my head. I guess I should work on the physical side for a while, and then attack the theory.

(I am quietly hoping that this guy can help me form a computer club to look at L4, the most advanced, but ignored, microkernel)
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