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Old 25th November 2006, 11:57 AM   #21
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Quote:
Originally posted by ray_moth

Afterthought: if you have a CCS in the tail of an LTP, there is no need to worry about the mu in each half being different. AC balance is assured, as long as the plate loads are identical. All you have to do is make sure the quiescent current through each triode is the same, which you can correct with resistors in the cathodes.

There is a downside to this. Broskie of Tubecad fame pointed out that with a fixed grid voltage the current will differ between the two halves in a differential circuit. He had a special on Blumlein's current balancing in differential output stages (21 May 2005). Also this might, I think, easily lead to drift.
The classical method is indeed, as you pointed out, to have a potmeter in the anode circuit (which of course in an ideal world is non-inductive and doesn't have any temparature effects...). But I suspect that this can result some distortion-spectrum differences.
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Old 26th November 2006, 01:01 AM   #22
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I didn't say anodes, I said cathodes and I was talking about fixed resistors, not a pot. It doesn't need to be a pot, just having resistors there will improve the DC balance, just in case the two triodes arfe not perfectly mathced.

If you're using a CCS in the tail, balancing the loads with a pot is only necessary if the fixed resistors used for the plate loads are not well matched. If they are, then the CCS will ensure that the AC signal balance is perfect.
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Old 26th November 2006, 05:03 AM   #23
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So if ">=" reads not worse than, then
(1) LTP+CCS >= (2) split-load (cathodyne) >= (3) LTP(passive)? So looks like (1) will be the next thing for me to play with. Thank you all.
When browsing Altec schematics posted on the web, found this asymmetric treatment (from 1568A, attached) of the coupling/blocking capactors following a 6cg7 split-load. What is the advantage of doing this?
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Old 26th November 2006, 05:39 AM   #24
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Quote:
Originally posted by 2004ex
When browsing Altec schematics posted on the web, found this asymmetric treatment (from 1568A, attached) of the coupling/blocking capactors following a 6cg7 split-load. What is the advantage of doing this?
Probably phase compensation for stability with gNFB.
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Old 26th November 2006, 01:06 PM   #25
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Quote:
Originally posted by 2004ex
When browsing Altec schematics posted on the web, found this asymmetric treatment (from 1568A, attached) of the coupling/blocking capactors following a 6cg7 split-load. What is the advantage of doing this?
Notice that the bottom section of the PP 6CG7 is direct coupled to the cathode of the split-load tube through R8. C4 is not a coupling cap; as Miles said it appears to work in combination with R8 as a phase compensation network. The 1M R7 provides grid bias for the top section of the PP 6CG7 which is cap coupled to the plate of the split-load. Perhaps Altec decided that the compensation network is necessary because only one of the two sections is direct coupled. WTFDIK.

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Old 26th November 2006, 01:25 PM   #26
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I should add, I'm not completely opposed to asymmetric PP drive just because it doesn't look good on paper.

I haven't heard one, but many people report that the Seth amp sounds very good. I do know (from conversations with someone that has probed a similar circuit) that the signals at the two PP grids of the Seth are FAR from symmetric. The top grid - which is really just cap coupled with a grid choke - looks very 'good' while the bottom grid - which is really just fed from an inverting interstage - shows all the expected frequency anomalies. Still, people say the amps sound fantastic.

Returning to the subject of this thread, I have built very good sounding PP amps with isodyne phase splitters (as found in the simple Magnavox amps.) This was some years ago when I was experimenting with PP EL84 amps. I found that the isodyne seemed to have more of the realism present in SE amps than the much more symmetric concertina and LTP.

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Old 27th November 2006, 01:56 PM   #27
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Split-load p/s.... despite slight unsymmetrical o/p's it 's the simplest and best sounding configuration I've come across... of the of all the decades I've been fiddling with tube amps.

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Old 29th November 2006, 12:50 AM   #28
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Hi RayMoth ,

Sorry for the delaied reply , about your post # 18 ,
I don’t know how to calculate the gain of Scoyoc
phase inverter , too .

The only information I could get from the bibliography
is that the total gain of Scoyoc original design ( 6SN7
and 6SL7 ) is ~ 60 times . It’s very close to the “mu”
of a 6SL7 tube . May be the answer ??

Best Regards ,

Carlos
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Old 29th November 2006, 01:34 AM   #29
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Hi Carlos,

Thanks for getting back. That figure of 60 looks about right for the gain of both sides of the splitter with unbypassed cathodes, which is what I thought.

Assuming for 6SL7:
Mu = 70
Internal plate resistance Rp = 45k
Plate load Rl = 120k
Cathode resistor Rk (unbypassed) = 1k

We would get, from each half of the 6SL7, a gain
= Rl * Mu / ((Rk * (Mu+1) + Rp + Rl)
= (120,000 * 70)/((1000*(70+1) + 45,000 + 120,000)
=35.59

Together with the cathode follower, we get a net gain
= 35.59 * 0.9
= 32

Since there are two halves to the splitter, both with roughly the same gain, the total gain = 64

Best regards,

Ray

P.S. Does anyone know of a phase splitter (or invertor, if you like) using tubes, where you don't lose at least half the gain?
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Old 30th November 2006, 11:10 PM   #30
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Quote:
Originally posted by refference
Many applications
can easy tolerate the wide variation in output impedance
between the two signals”
Just a correction here (and this was not said by Refference; he quoted somebody else), if needed:

Contrary to superficial appearance, the output impedances of the two halves of the cathodyne are the same if the external load impedances are the same. Kirchoff's laws forces that - same current through both halves, etc.

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