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Old 15th September 2013, 10:00 PM   #31
rongon is offline rongon  United States
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Quote:
Originally Posted by chrish View Post
Tubelab helped me develop an amp that has a very similar front end to the amp in discussion here. A dual diff amp DC coupled driving MOSFET source followers to PP 6L6. As Sy mentioned, DC balance was an issue identified by Tubelab in the design, and he recommended a small compensation circuit to adjust for it. The whole discussion is in this thread 6L6GC AB2 Amp The circuit in discussion is here 6L6GC AB2 Amp
Here is the of the schematic of the complete amp. You can see the circuit described.
I remember that thread, and read it with much interest. Is there no gNFB loop around that amp? If so, I'm surprised that PP UL 6L6GC's would sound acceptable run that way.

My last stab at a PP amp had 12AT7 LTP DC-coupled > 12BH7A diff AC-coupled > 6L6GC-triodes. It needed some gNFB to get rid of 'nasties.' Might have been the Dyna ST70 OPT's. They're designed to have NFB around them.

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Old 16th September 2013, 12:58 AM   #32
chrish is offline chrish  Australia
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No, there was no GNF in the amp. Reasonably easy to implement if required, as there was plenty of gain. On listening and measuring, distortion was remarkably low, so I did not bother. Output transformers are Tamura - good quality. That amp is my daily driver.
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Old 16th September 2013, 02:47 AM   #33
rongon is offline rongon  United States
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Ah, interesting, and good to hear. Something else to try. It's pretty amazing (and encouraging) that you got a clean sound from UL 6L6's. Thanks.
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Old 16th September 2013, 07:49 AM   #34
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by SY View Post
Remember that the diff amp is direct coupled to the cathode followers- that's the primary source of DC imbalance unless there's a balance pot in the diff amp cathode circuit.
Yes, admittedly any DC imbalance will be passed on to the power stage, but the cathode followers themselves will be happy.

Last edited by Merlinb; 16th September 2013 at 07:58 AM.
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Old 16th September 2013, 11:42 PM   #35
multi is offline multi  Australia
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Default noisy circuit

The problem with the original Mullard circuit using EF86 and 12AX7 is for sensitive speakers it has far too much gain and is noisy. To reduce the gain one way is to change the EF86 to a triode, but maybe best to change the 12Ax7 for the very linear 6CG7.
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Old 17th September 2013, 01:09 PM   #36
DF96 is offline DF96  England
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12AX7 is very linear too, in the right circuit.

6CG7 LTP phase splitter has low mu so will need a CCS tail to preserve balance.
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Old 17th September 2013, 07:43 PM   #37
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Just a few aside comments (since SY has scooped all the essential details! )

Folks seem to get over-absorbed with changes to the front end EF86 in comparison with rather greater gains (as in advantages, not !) regarding a phase splitter change.

Yes, the gain is high, and pentodes make more noise than triodes. But that is relative. One really has to stick one's ears into the loudspeaker to hear the difference in noise, other things being in order. To me the advantage lies in the almost 4x lower distortion for pentodes than triodes at low signal. As said low, but one must also consider the effect of mixing NFB signal on a cathode with signal on G1. Any non-linearity there is not cancelled by global NFB (but that is another subject).

A greater advantage of pentodes for me is in the lower influence of any Miller capacitance because of vartying input impedance of the feeding source. With triode input of the 12AX7 kind this can influence GNFB h.f. stability. Further, the lower anode operating voltage in the tube's linear region possible with pentodes than with triodes (and to boot, the gain step and phase functions possible with g2 bypass constants, should l.f. stability enter the fray.)

Going on to the phase splitter, Miller effect from high triodes on the first stage again comes into play. For these reasons I also prefer a lower lower Rp tube for the phase splitter (in fact, if folks promise not to pick up stones, I do not like 12AX7s anywhere in a power amplifier ). My choice has changed from 12AT7s to ECC88s, which I found somewhat more linear, but that will depend on other circuit values.

Thus I myself found the most satisfactory application, bandwidth and distortion-wise (open loop naturally), with an EF86 and ECC88. I myself have not indulged in using CCs', finding the old cathode resistor satisfactory, also over time (perhaps I was just fortunate in my double triodes aging in equal fashion ...) But the possibilities are multiple, even allowing for some taste.
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Old 17th September 2013, 09:20 PM   #38
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Quote:
Originally Posted by SY View Post
Morgan incorporated a DC balance pot in the driver stage.
http://img153.imageshack.us/img153/3...00508at348.png

That's the way to go, I think. A pair of LEDs with a pot across them will give enough bias to balance the DC.

I could add a 100ohm resistor in series with C1 and ground and feed the GNFB there.

Last edited by payitforwardeddie; 17th September 2013 at 09:35 PM.
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Old 17th September 2013, 10:12 PM   #39
multi is offline multi  Australia
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DF96
where can I find this linear circuit using 12AX7's.
Do you have one?
Phil
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Old 17th September 2013, 10:14 PM   #40
SY is offline SY  United States
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High plate load (a CCS works well for this), as little cathode degeneration as possible (an LED works well for this). Keep the load on the output as high impedance as possible. The linearity is startlingly good.

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