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First stereo tube amp build advice
First stereo tube amp build advice
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Old 24th November 2020, 10:00 PM   #101
Tom Kamphuys is offline Tom Kamphuys  Netherlands
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You're right. Problem exists between chair and keyboard. Mea culpa.

It is Carbon Film. But then I don't understand it at all. I've read the film is 50-250nm thick. Which is 0.0002 of a 2mm diameter resistor. How can that film have any significant impact on the heat transfer? I would guess it would be negligible w.r.t. the ceramic core and even the protective coating thermal conductance.
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Old 24th November 2020, 10:07 PM   #102
rongon is offline rongon  United States
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First stereo tube amp build advice
The resistor type of choice for high power applications is wirewound.

Wirewound resistors do have higher inductance than film resistors.

However, in the 'tail' of a LTP (the cathodes) inductance won't matter. Actually, you could use an inductor there to act as a sort of current source. Of course a MOSFET can be made with much higher impedance to act as a *constant* current source, so all the better. But still, that goes to show you that inductance is not an issue there.

Have you read Morgan Jones "Valve Amplifiers"? These issues are covered in good detail there.

Valve Amplifiers - 4th Edition
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Old 24th November 2020, 10:14 PM   #103
kodabmx is online now kodabmx  Canada
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First stereo tube amp build advice
My choice for high power is metal oxide, but 5W is high power to me

CC resistors have been discussed many times including by the late DF96. There is NO need for CC resistors in a grid circuit save for RF in the 1960's.
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Old 25th November 2020, 06:47 PM   #104
painted is offline painted  Luxembourg
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Quote:
The 10 Kohm CF resistor between the LTP's tail and the CCS drops some volts and also blocks the conduction of heat from the tube (via the wiring) to the CCS. Unwanted heat is the mortal enemy of all things electronic, especially SS. I took no prisoners. BTW, that resistor also slightly increases the net load the LTP tail works into.
Putting a resistor between the LTP kathodes and the constant current sink gives a sub optimal circuit in my opinion. If Vak is too high better diminish HT.
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Old 25th November 2020, 06:50 PM   #105
rongon is offline rongon  United States
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First stereo tube amp build advice
Quote:
Originally Posted by painted View Post
Putting a resistor between the LTP kathodes and the constant current sink gives a sub optimal circuit in my opinion. If Vak is too high better diminish HT.
Why? It's not going to decrease impedance there, as long as the CCS has enough voltage to work with.
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Old 25th November 2020, 07:49 PM   #106
painted is offline painted  Luxembourg
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Well, my reasoning is that both active devices work opposed and the resistor takes a little of the voltage available from the CCS. The amount would equal R/CCS impedance. If the CCS impedance would be infinite there is no influence but if it would be say 200K, you loose 2,5% of its ability. Then there is valve capacitance to consider.

Last edited by painted; 25th November 2020 at 08:02 PM.
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Old 25th November 2020, 10:08 PM   #107
Ketje is offline Ketje  Belgium
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How about a European version with iron you can find more easy over here ?
The power transfo NTR110 goes for 40 and the OPT ATR15 63 for two.
Mona
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Old 26th November 2020, 06:18 AM   #108
Tom Kamphuys is offline Tom Kamphuys  Netherlands
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Very El Cheapo european solution. Nice.
Can you give any comment about the quality, especially of the OT? It seems good OT's are important and I'm willing to to spend some more where it matters.

Could you also elaborate a bit about the circuit changes you made? Why did you do them?
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Old 26th November 2020, 10:59 AM   #109
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Tom Kamphuys View Post
Very El Cheapo european solution. Nice.
Can you give any comment about the quality, especially of the OT? It seems good OT's are important and I'm willing to to spend some more where it matters.
If you want cheap the quality is not the top.They are German made, one can hope they didn't make a mess of it .
Quote:
Could you also elaborate a bit about the circuit changes you made? Why did you do them?
With another supply transformer the circuit had to be adapted.After the rectifier nothing changed.The negative supply can come from the same transformer now and a more simple current source, constant voltage over the emitter resistor gives a constant current through the transistor.
The ECL85 resembles very much the 6AQ5 and 12AT7, no change there exept the decoupling and value of the cathode resistors of the output stage.
The value change is for a little less Pa, perhaps not needed but why push it.
And I don't like the common cathode resistor.If one tube starts pulling more current the cathode voltage goes up resulting in more negative Vgk for the other tube.With the more negative grid the current goes down making the unbalance even greater.
For the rest I changed some values more to my taste and higher input resistance.
Mona

Last edited by Ketje; 26th November 2020 at 11:01 AM.
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Old 29th November 2020, 10:38 PM   #110
Tom Kamphuys is offline Tom Kamphuys  Netherlands
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Thanks Ketje for the explanation. I found more comments about the double cathode resistors.

In the mean time I've been researching, reading and simulating.
I think I need to switch to 6V6's, because they are much easier to come by, especially as matched pairs. I found in an old article that the UL tap should be at 20%. Any comment on that? Any transformers around that have that?

I also read part of this: Modern High-End Valve Amplifiers (E-book) - Elektor

His toroid transformers were mentioned before. Turns out his transformers were made a mile away from the highway I use to travel to my work (in the pre-corona age). They've moved a bit further away (20km) in the mean time. See attachment for one of their OT's.

The 35% tap is optimised for EL84. How important is the place of the tap? 40% seems quite common. Is this a reason, or a consideration, to switch to the Baby Huey?
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