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Old 15th September 2013, 07:08 PM   #21
SY is offline SY  United States
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If they're direct coupled, it's important. If they're AC coupled, it isn't.
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Old 15th September 2013, 07:15 PM   #22
rongon is offline rongon  United States
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And that explains why Morgan Jones did what he did with the Crystal Palace (a light bulb just went on!). He took the input LTP and AC-coupled that to the driver stage diff amp. Then the driver stage was DC-coupled to cathode followers and the output stage. So the AC coupling was between the two diff pairs, where the DC balance is not important.

But why is it not a "fail" that the driver stage diff and the push-pull output stage are DC-coupled? What do the cathode followers do, or does the fixed bias do something that allows good balance from the output stage? Or is it that the output stage is transformer loaded?

Thanks for the info so far.
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And what happens if the LTP is DC coupled to a PP driver stage with a center tapped choke as plate load?

Last edited by rongon; 15th September 2013 at 07:18 PM. Reason: afterthought
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Old 15th September 2013, 07:55 PM   #23
SY is offline SY  United States
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Morgan incorporated a DC balance pot in the driver stage and used a lot of matching of single triodes. I'm rather down on the idea of running 10 mA through a pot wiper.
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Old 15th September 2013, 08:32 PM   #24
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Quote:
Originally Posted by SY View Post
Morgan incorporated a DC balance pot in the driver stage and used a lot of matching of single triodes. I'm rather down on the idea of running 10 mA through a pot wiper.
Look at the ETF lecture from Morgen Jones about transformers:
http://www.triodefestival.net/upload...inductance.pdf

Strange that so less transformer manufactures do give the information people like to have about there transformers....
Just Tango, Monolith Magnetics, Menno van der Veen give detailed information about there transformers.
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Old 15th September 2013, 08:44 PM   #25
rongon is offline rongon  United States
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Thanks for all this info, SY.

It's a bit daunting to find out how many compromises have to be made when designing something as simple as a push-pull triode power amp.

So from what I think you've been saying, you think a single LTP with a common-cathode stage driving it is a less error-prone design than an LTP DC-coupled to a PP diff driver stage?

Therefore, the worst of all worlds would be a 3-stage PP amp, with an input LTP DC-coupled to a PP diff driver stage, DC-coupled to the PP output stage?

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Old 15th September 2013, 08:50 PM   #26
SY is offline SY  United States
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Originally Posted by rongon View Post
Therefore, the worst of all worlds would be a 3-stage PP amp, with an input LTP DC-coupled to a PP diff driver stage, DC-coupled to the PP output stage?
That can be made to work, but it will be much more complicated and twitchy. Look at everything MJ had to do for things like regulation and thermal compensation, then double it!
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Old 15th September 2013, 09:05 PM   #27
Merlinb is offline Merlinb  United Kingdom
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But why is it not a "fail" that the driver stage diff and the push-pull output stage are DC-coupled?
Putting it crudely: It is not a fail because the cathode followers are independent of one another. Small differences in their grid voltages will not amount to much difference in bias between them, as the large cathode resistors make them largely self compensating.

But with and LTP the triodes share a cathode resistor, so they cannot both come to a happy optimum bias at the same time.
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Old 15th September 2013, 09:10 PM   #28
rongon is offline rongon  United States
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OK, I think I get it now. Thanks, this has been most instructive!

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Old 15th September 2013, 09:13 PM   #29
chrish is offline chrish  Australia
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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.
Attached Files
File Type: pdf Chrish 6L6GC PP-1 amp schematic from post 460.pdf (117.1 KB, 101 views)
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Old 15th September 2013, 09:28 PM   #30
SY is offline SY  United States
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Quote:
Originally Posted by Merlinb View Post
Putting it crudely: It is not a fail because the cathode followers are independent of one another. Small differences in their grid voltages will not amount to much difference in bias between them, as the large cathode resistors make them largely self compensating.
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.
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