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Re: DUD 120 ampmods

Posted by Gingertube (A) on March 6, 2007 at 19:17:27

In Reply to: DUD 120 ampmods posted by Stewen on March 6, 2007 at 14:03:30:

Stewen,
Firstly your English is very good. I had no trouble understanding.
The DUD120 you refer to, I assume is the DUD120 on The EVATCO Website Project Pages.

This is my design or at least my implementation of various historical designs. The front end is a "Hedge" circuit with a current source added and is very similar to some Sonic Frontiers designs. The 6SN7 cathode followers were added so that low value Rg1 resistors could be used on the output tubes (to keep their Dc operating points stable) and to drive the relatively large Miller capacitance of the output tube grids.

I did try the 6SN7 as a high current differential amplifier driver instead of cathode followers (using 22K anode load resistors) but found that the Miller Capcitance of that stage then limited overall high frequency response and so I reverted to cathode follower configuration for the driver.

There is one mistake on the posted schematic - ZD1 in the High Voltage Regulator for the cascode diff amp is drawn the wrong way around. Cathode should go to the base of Q1 and anode should go to the LM317T adjust pin.

The schematic is an earlier schematic and in fact the prototype has the upper tube of the cascodes heaters sitting at approximately +100V. This required cutting some tracks on the CAE PCB1A PCB.

It seems to me that 45V over the upper triode in the cascodes is indeed too low and that making those 100R resistors in the upper cascode triode cathodes a bit higher may help fix that. You may need to experiment BUT you really want about 80 or 90 volts across each ECC88.

I put the prototype of this amplifer back on the shelf a couple of years ago meaning to come back to it. It has a couple of potential problems.

1) It is marginal on gain, to apply more than about 6dB of global feedback. To get more gain you need to run the cascodes at higher current. (The gain of a cascode is largely determined by the gm of the lower triode and the load resistor value at the upper triode anode - more current means higher gm) To do that you need less voltage on the cathodes of the lower triodes. That would reduce the "compliance" voltage (the voltage the circuit has to work with) of the Current Source.
That brings us to the second problem.

2) The maximum signal input (the signal which can be applied to grid 1 of the cascode on the input side) is limited by the cathode voltage. This triode MUST NOT cut off. I always intended to experiment further with the input stage by returning the bottom of the cureent source (that is the 3K( base resistor and the 100R emitter resistor) to a small well regulated negative supply.

I do intend to take this amp off the shelf some time soon and to try some further mods and build the second monoblock.
I did try Plitron VDV2100 Toroidal Output Transformers in place of the Hammond 1650T vut found that the amps output impedance was higher (VDV2100 is 2000:5 ohms whereas the 1650T is 2200:4 ohms) and with the circuit as shown I did not have enough gain to add more feedback to the VDV2100 version to get Zout down to acceptable value and so went back to the Hammonds.

I hope this is of some help to you,
Cheers,
Ian

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