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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.
Follow Ups:
I have been given one of these boards. Do you it will do okay driving a 6b4g in stock form? I have 6922 and 6cg7 to try as tubes. I know the 6922 is made for cascode but I generally like 6cg7 better.Yes elevating the heaters on the top tubes is better. Ditto on a negative rail and a better CCS.
I plan to try using no feedback but it would be nice to have enough gain to use some. I will be converting a regular dynaco 70 and the 6b4g's will be cathode biased. I'll probably use a 5u4 rectifier to drop B+ some.
Russ,
I think the PCB1A will drive 6B4G without any hastles. One thing to check is the max Rg1 value for the 6B4 in cathode bias but I don't believe that it will be a problem. Sonic Frontiers used exactly the same circuit to drive triode mode EL34s.
Cheers,
Ian
Some more thoughts.
You will notice that the cascodes are "Self Biased" relying on the 100 Ohm resistors plus internal impedance of the upper tubes to set the voltage across them. The more usual arrangement for a cascode is for the grids of the upper triodes to be wired to a voltage divider with the bottom arm bypassed to 0V. This arrangement has an advatage that you can also then add a resistor between the high voltage rail and the anode of the bottom triode to get more current through it and hence maximise gm and therefore the gain.With the existing self biased arrangement, if you increase those 100R upper triode cathode resistors to get the Vak voltages better then you may find that you need to bypass those resistors with large value electrolytics to keep the gain up.
Thanks for answers ! I will go on and experiment. I have a parallellconnected ECC 40 followed by a Baxandall tonestack and a 1 meg. pot. in front of my DUD 120 . Are using the amp as a guitaramp. It has a very clean, warm jazztone inspite of wrong balance in the cascodesplitter . The gain is a bit low so I´m thinking of splitting up that ECC 40 into two separate triodes with the Bax. tonestack between the triodes . Until now I have no feedback at all but guess I can have some after the change into two triodes in front of the DUD 120 . Stewen
Hi again! Now I´ve changed those 100 ohm resistors into two trimpots 1K. With about 450 ohms on the pots the cascodestages balance with 88 V over each ECC 88 . The amp sounds great with 40 mA through each of the quad 807 powertubes. Anodevoltage is 450 V. Screenresistors 120 ohm/ 2W . Triodemode. Funny that tubes of the same age like myself , 62, can sound better than many modern tubeamps in my lokal musicstore !
Stewen
you said:
Funny that tubes of the same age like myself , 62, can sound better than many modern tubeamps in my lokal musicstore !Not really surprising -
While many of the modern tube makers use resurrected equipment from yesteryear what has been lost is the experience of the people who used that equipment to turn out superior tubes.As for the tube amps in your local music store - a lot of the newer tube amps are comming out of China and they seem to be just building stuff to designs from the 1960s - we are not seeing any really new ideas which is a pity. There is a lot of scope for using modern circuit methods and ideas learned from solid state design in tube amps..
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