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6GH8-6CZ5 Tube Amp Project
6GH8-6CZ5 Tube Amp Project
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Old 8th February 2018, 04:26 AM   #11
Eli Duttman is offline Eli Duttman  United States
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A Dyna ST-70 board configured for 6GH8s is reasonable. However, the 7199 parts values from the SCA-35's power section could be best. Like the ST-35, the SCA-35 uses EL84 "finals"

Would kindly you link to the driver board under consideration.

The Sovtek 5AR4 will serve you well, provided you install the series SS diode tweak. I suggest you use UF4007s, instead of the 1N4007s shown, given the concept that less SS switching noise, from the outset, is better, even though the vacuum rectifier blocks the noise.
Attached Images
File Type: jpg Dyna SCA35 Schematic (left).jpg (123.7 KB, 69 views)
File Type: jpg Dyna SCA35 Schematic (right).jpg (96.9 KB, 61 views)
File Type: jpg SS Series Diode Tweak.jpg (106.0 KB, 62 views)
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Old 8th February 2018, 01:42 PM   #12
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
Thanks Eli, attached is a link to the 6gh8 driver board I want to use, also attached is a schematic of the PC-3 board with changes.

DYNACO ST-70 6GH8 6GH8A ECF82 Driver board PCB PC-3 KIT Dyna ST70 | eBay
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File Type: jpg PC-3-6gh8.jpg (424.9 KB, 58 views)
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Old 8th February 2018, 01:56 PM   #13
Eli Duttman is offline Eli Duttman  United States
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Be prepared to "pad" the volume control with some additional resistance. The pentode voltage amplifier may do its job too well and yield more gain than is necessary. The symptom of this situation is barely moving the volume control from 0, before the sound is too loud. FWIW, this is called a "hair trigger" volume control.

BTW, the 470 Kohm resistor shown across the I/P is replaced by the volume control. The control's wiper connects to the voltage amplifier grid.
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Old 8th February 2018, 04:24 PM   #14
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
I have an unbuilt Glassware Volume Attenuator A3 that should work nicely in that spot.

How about one of these automatic bias boards?

Automatic bias board.

Couldn't this solve some problems?
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Old 8th February 2018, 09:47 PM   #15
Eli Duttman is offline Eli Duttman  United States
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I'm dubious about automatic bias boards. They may be OK, but I'm dubious. Setting things manually, on a periodic basis, is well known to be reliable.
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Old 13th February 2018, 11:18 PM   #16
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
Attached is the corrected schematic that I wish to use. I may have to change some values.
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File Type: jpg 6gh8-6CZ5-GZ34.jpg (155.1 KB, 22 views)
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Old 14th February 2018, 03:56 AM   #17
Tom Bavis is offline Tom Bavis
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Adjust the value of the 820K screen resistor if needed to get 75-100V on the 6GH8 pentode plate. The 1K bias trim pot won't do anything unless you add a resistor to ground at the junction of R9 and R10 to make it a voltage divider (I'd reverse the two, few K fixed resistor from bias supply, 10K pot to ground). Bias at about 30 mA - also I'd bypass the 6CZ5 cathodes (or use separate bias pots to each one, delete the 100 Ohm cathode trim pots - 2 pots instead of 3). That's assuming there's enough bias voltage - minus 23V should be enough.
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