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Dynaco VTA-inspired KT88 PP AMP project
Dynaco VTA-inspired KT88 PP AMP project
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Old 18th February 2019, 09:19 PM   #21
peteki is offline peteki  Australia
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To 6A3sUMMER: The matched plate loads will probably go close to balancing the drive but does not allow for differences in each side of the tube. Also there will be a drift in characteristics over time. Using a CRO to balance the drives overcomes drift etc. The balance network in the cathodes of the output tubes eliminates residual magnetism in the core of the output transformer. Again the characteristics of the output tubes drift over time.
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Old 20th February 2019, 08:01 AM   #22
6A3sUMMER is offline 6A3sUMMER  United States
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peteki,

Consider a cathode coupled phase splitter, with the grid of one side driven, and zero signal volts to the other grid. Then use matched plate loads; and matched coupling caps and matched grid resistors in the next stage:

Then with the cathodes directly connected together, and a perfect CCS (nearly perfect with extremely high impedance versus the cathode impedance), When the cathode current of one side changes, the other cathode current has to change exactly as much, but in the opposite direction (as long as either side does not have grid current). If the change in cathode currents is exactly the same except for one going up and the other going down, then it must be that the plate currents also change exactly the same except for the direction (phase). With equal plate current change and equal plate resistances, the voltage change has to be the same.

The use of a metering resistance, or balance pot between the cathodes is what can disturb the balance of this circuit, because no CCS is perfect. In that case, you would have to use un-equal plate loads to restore the balance.

Of course to make up for an unbalanced output stage, you may want to un-balance the phase invertor in the opposite direction, in order to cancel the output stage's less than perfect balance.
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Old 6th March 2019, 05:13 PM   #23
AnatolyAA is offline AnatolyAA  Russian Federation
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kward, well, too far from Europe to buy

Can anybody advice on the choice of the KT88's? I prefer mostly rock, pop and classic (I presume that matters). Surprisingly I have found many good reviews on the Shuguang KT88Z and Psvane KT88-TII quads, but they seem to be most expensive. Alternatively I have at hand the Genalex, EH, Svetlana choice. Can you say some words on the most and less preferred for this type of amp?

Or may it be an option to build it and test it with something simple like Svetlana | EH and afterwards invest more expensive set of tubes?

Last edited by AnatolyAA; 6th March 2019 at 05:36 PM.
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Old 6th March 2019, 09:01 PM   #24
peteki is offline peteki  Australia
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Some KT88's had not been de-gassed properly - Russian Sovtek, whilst most modern ones do not use cathode material that lasts if run near maximum recommended dissipation. My experience with current Genelex has been reasonable (they seem to have a good vacuum).
However none of the modern ones last.
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Old 7th March 2019, 07:03 AM   #25
multi is offline multi  Australia
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A friend bought 40 Psvane KT88 : i spent weeks fixing his amps as the shorted out; would not touch with barge POLE. Lucky he bought them from a firm in The USA that replaced the lot. I would stick to Svetlana or EH. The friend gets about 4 years out of them and he listens a lot
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Old 7th March 2019, 05:44 PM   #26
Troy Madden is offline Troy Madden  United States
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I built mono blocks based on the VTA ST-120, using their octal driver board upgrade for Mk3's. Very happy with them, wouldn't change a thing.
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Old 7th March 2019, 07:55 PM   #27
AnatolyAA is offline AnatolyAA  Russian Federation
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do you happen to have the schematics?
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Old 7th March 2019, 09:34 PM   #28
Troy Madden is offline Troy Madden  United States
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I do have the schemas, but I don't have permission to share them. However, your initial schema is correct. I have built the ST-70, 120, and M-125. Also the Curcio M-100. To my ears, the ST-120 sounds the best. The 120 also reproduces 1K and 10K square waves better than the others too...virtually perfect at 1K and very good at 10K. I built mine as mono-blocks because the 120 destroys rectifier tubes almost as fast as you can install them, LOL. Now my rectifiers are hardly stressed. I use 5U4BG's, I can afford the voltage drop. Sylvania chrome dome 6SN7's and KT-120 outputs. I have tried the Lundahl 1679 PP transformer, its a good transformer but the ST-120 trans is just as good as far as I can tell.
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Old 8th March 2019, 07:04 AM   #29
AnatolyAA is offline AnatolyAA  Russian Federation
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Wow, that’s an experience!) and really good to know that the schematics is ok.

What tools did you use to tune your amp? The octal driver is unbalanced . Did that cause any issues?

Last edited by AnatolyAA; 8th March 2019 at 07:07 AM.
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Old 8th March 2019, 01:22 PM   #30
Troy Madden is offline Troy Madden  United States
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Theres not much to tune with the new CCS phase splitter driver board, except the bias. The older VTA driver boards had a balancing pot, but not these. It's my understanding that with the CCS, the splitter is inherently self balancing. Now that you ask, I may scope the output of the splitter though, if I invert one output and add the two I should be able to see any discrepancy. BTW, I ended up converting my M-100s to VTA's octal driver circuit, the regulated screen supplies in the M-100's went up in smoke. Pretty much all I kept was the iron.
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