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Old 23rd August 2011, 07:09 PM   #81
Krughka is offline Krughka  Latvia
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I experimented with SRPP stage with GU48 & 250TH in lower part and GMI11 & GU81on the top. Cascade supply 4200V-50mA. SRPP with two GU48 didnít give me enough gain. Best that Iíve got was with 250TH & GMI11. GMI11 isnít the best choice of course also, Idea required some indirect heated pentode, but I just had these. There are oscillograms with 180 & 1680pF of load on 20kHz. Not too bad for first experiment.
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File Type: jpg 1680???.jpg (101.2 KB, 303 views)
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Old 24th August 2011, 05:21 PM   #82
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Very Cool !!!!

jer
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Old 31st March 2013, 02:54 PM   #83
bzal is offline bzal  United States
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Default ESL Direct drive

I have owned a pair of Acoustat speakers for over 7 years . I've tried many different amplifiers throughout the years to drive them to bring out the best sound. Tube amplifiers deliver the best sound so far. I built a pp amp using 6550 output tubes and 6sn7 as the drivers. Using a long tail pair phase splitter with a 6SL7 triode. B+ is at 500 volts in UL mode. Using Hammond 1650R transformers. I have eliminated the step up transformers in the MK121 module and connected the stators directly to the plates of the 6550's. I reversed the diodes in the bias power supply so that the B+ from the amplifier adds to the bias voltage. The sound level is more than adequate for above normal listening levels. The measured bias voltage is at 5700 volts with the amplifier voltage added. West Marine has high voltage wire available rated at 15000 volts. This was the best modification I have done for these speakers.
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Old 31st March 2013, 03:08 PM   #84
tyu is offline tyu  United States
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Way Cool... thanks for the info....No roll off in the topend?
Hows the Base?
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Old 31st March 2013, 03:41 PM   #85
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Some people have has success using a tube amplifier with 32 ohm or 70 volt taps.
Starting at 70 volts RMS is better than 30.

I read an article long a go where they used a 7000 ohm CT tube amplifier transformer for the step up.

Another option is to find a an output transformer with untralinear taps and drive the 50 or 40% taps. Use push pull 572Bs at 1500volts will get you into the +- 2600, 5400 peak to peak range.
You probably want to load the secondary a little to avoid arcing.
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Old 2nd April 2013, 12:51 AM   #86
bzal is offline bzal  United States
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Default Frequency response ESL amp

Here is the frequency response from 10hz to 20khz
I used a lower level for testing the output cause it gets too loud. This response is with the speaker connected.


Freq Output Volts Plate to plate
10 107.5
20 107.5
30 101.0
40 100.0
50 100.0
60 100.0
70 100.0
80 99.5
90 100.0
100 100.0
200 99.5
300 99.5
400 100.0
500 99.5
600 100.0
700 99.5
800 99.5
900 99.5
1000 100.0
2000 100.0
3000 100.5
4000 100.0
5000 100.5
6000 101.5
7000 102.0
8000 103.0
9000 104.5
10000 105.5
11000 107.0
12000 109.0
13000 111.0
14000 113.0
15000 116.0
16000 118.0
17000 121.5
18000 124.5
19000 127.5
20000 131.0
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Old 19th April 2013, 07:25 AM   #87
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bzal could you share more info about ?
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Old 19th April 2013, 07:27 AM   #88
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Krughka could you test 211 or 813 ?
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Old 19th April 2013, 08:18 AM   #89
Krughka is offline Krughka  Latvia
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I haven't these tubes at the pocket and don't want buy specially. 211 can't give me enough output signal. Later on I performed few experiments with SRS326 and RS391 on the top with better results and less power consumption, but still didn't go far ahead due lack of time.
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Old 30th May 2013, 08:03 PM   #90
bzal is offline bzal  United States
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Default ESL Amp Schematic

Here is the schematic of my current amp using 6SN7 and 6550 tubes
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