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Old 4th November 2010, 09:24 PM   #101
djn is offline djn  United States
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OK, I have another question. In going through all my junk, I found a can with two 10mh chokes in the same can. I thought that I could use it on this amp the remembered that the PS on the TM OTL does not have chokes. I am use to building SET amps and correct me if I am wrong, but this amp is a PP design is it not? So the ripple does not have to be treated so carefully as a SET amp. Is my thinking correct?
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Old 5th November 2010, 02:00 AM   #102
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Yes.
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Old 5th November 2010, 03:52 AM   #103
djn is offline djn  United States
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Thanks A, maybe I can put that choke on the bay to pay for some of these parts.
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Old 6th November 2010, 01:22 PM   #104
nafunga is offline nafunga  United Kingdom
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Default OTL by TIM MELLOW

Hi All,
I've attached my quick re-draw of a previous re-draw (!) of the Tim Mellow OTL circuit. Just that I find the 'crossing over' bit (between ECC83's and EF86's) a bit confusing when it comes to construction (just me?) Might help someone? I've not labelled all components as they're on the first re-draw.

Regards,

David.
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Old 6th November 2010, 09:11 PM   #105
djn is offline djn  United States
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Good deal Nafunga.
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Old 8th November 2010, 10:12 AM   #106
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Ok I am a week in and say a total of 25 hours of listening time. The amp has opened up quit a bit. I now have it back on my DIY Tannoy mono vinyl system with a custom tube preamp and here are my thoughts overall between the DIY Tannoy and the Quad's.

Quads

Clean crisp sound but not to sterile. In general a bit bright certainly when you first turn it in and for several minutes (10 to 20) before she warms up but even then stays that way after it drops a few notches in the bright catagory.

DIY Tannoy

Deeper bass but that is mostly a product of the speaker (front loaded horn) but what she really does is control the bass very well. Less bright but still a bit higher then I would like.

On both the mids are reasonable and stand up well to my NYAL OTL3's but do not surpass or tie them. I think the "Bright" issue is a product of the 12AX7. I will update again over the next few weeks as she burns in more and as I get more time to listen directly.

Be well all.
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Old 8th November 2010, 11:41 AM   #107
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Quote:
Originally Posted by desperateaudio View Post
..... In general a bit bright certainly when you first turn it in and for several minutes (10 to 20) before she warms up but even then stays that way after it drops a few notches in the bright catagory............Less bright but still a bit higher then I would like....... I think the "Bright" issue is a product of the 12AX7.
I agree, and it's also been my experience that that type of sound is a 12AX7 trait, revealed when the rest of the circuit is up to it. A 6SL7 may be a much better choice.

Stuart
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Old 8th November 2010, 03:05 PM   #108
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Originally Posted by sepolansky View Post
I agree, and it's also been my experience that that type of sound is a 12AX7 trait, revealed when the rest of the circuit is up to it. A 6SL7 may be a much better choice.

Stuart
I love the 6SL7 great idea. A little lower on the amplification factor scale (70) but may work. What do see the changes being? I might bite and build it.

Thx
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Old 8th November 2010, 04:47 PM   #109
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Quote:
Originally Posted by desperateaudio View Post
I love the 6SL7 great idea. A little lower on the amplification factor scale (70) but may work. What do see the changes being? I might bite and build it.

Thx
Hey desperateaudio,

The 6SL7 sounds really good at 150V on the plate and ~1.3-1.4mA. Other O/P may sound still better, but I know firsthand that the stated point sounds excellent. Now, within the limits of your build, that may not be possible. The 12AX7, operating at 0.46mA per plate is pretty typical of old school (Marantz 7c, PAS 3x, et. al) thought. Is higher current in the tube better? I don't know. Trouble is, with 146V to play with and a big B- to cover, not many other choices work. Tube CAD says the 1st stage will have a gain of 59.26, but that's for a differential input. SE input gets you half of that or ~29.6.
HF -3dB point is 176kHz, output impedance is 64k.

Simply plugging in the 6SL7, no operating points or other elements changed, reduces stage gain to 47.17/23.6, with a -3dB high cut of ~87kHz and an output impedance of 60.3k. You could just rig up an octal socket and try it out.

If you are willing to build a different 1st stage power supply, you could increase B+ to 280V, change plate resistors to 200k, use a cascode DN2540 or a 10M45S current source in the tail in place of the 470k connected to something a little more manageable like a -12V supply, and change the series input resistor to 5.6k and end up with a gain of 55.4/27.7, -3dB of 177kHz and an output impedance of 101k. I don't think the increased output impedance is an issue since the driven load is over 1M.

I don't know if the feedback resistor needs to be changed.

A few points confuse me:

1) Is not the feedback dependent on the volume pot setting? The 34k seems to be in series with the shunt portion of the volume pot from the perspective of the series feedback resistor.

2) Why is it necessary to have the 1uF and 34k resistor series connected then paralleled with the 1M resistor in the unused grid circuit? If differential input is never to be used, cannot that grid simply be shorted to ground?

3) In the original article, Mr. Mellow referred to an alternate input connection which would negate the input coupling caps. That to me is a very good thing. Has anyone worked out that configuration? Mellow mentioned that differential balance would be thrown off, unless a CCS was used in the differential tail. That wouldn't be a problem for me, since that's how I'd approach it anyway, choosing to forgo a -430V supply and 470k for the -12V supply and a CCS as mentioned above, regardless of input tube.

Stuart
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Old 9th November 2010, 08:00 PM   #110
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Man do I know how to shut down a thread or what?
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