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6GH8-6CZ5 Tube Amp Project
6GH8-6CZ5 Tube Amp Project
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Old 6th February 2018, 09:49 PM   #1
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
Default 6GH8-6CZ5 Tube Amp Project

Hello Everyone, I need some advice please. I have a Knight Kit Stereo 40, KA-55 Amplifier that is trashed. I want to use the Power and Output transformers for this project if feasible. The first RCA schematic comes up in a few places on line. The second schematic also shows up on the net and has a few voltages attached that some seem incorrect in the power supply. Also attached is an ultra linear set up from a Seeburg jukebox amp that uses the same value resistors for the 7199 tube as the rca schematic shows. What I want to do is use 6GH8 and 6CZ5 tubes in this amp and use it as a power amp. I started working on a schematic according to the Sylvania 6CZ5 tube data sheets and what I have on hand around here. Please be kind, this is my first project and I am stumped as to if this will even work. I have rebuilt several amps in the past. Thanks, Tom
Attached Images
File Type: jpg 7199-6973.jpg (122.8 KB, 315 views)
File Type: jpg AMPLI-RCA-7199-6973-5BC3.jpg (135.3 KB, 310 views)
File Type: jpg AMPLI-RCA-7199-6973-5BC3-2.jpg (489.7 KB, 300 views)
File Type: jpg seeburgshfa4 schematic.jpg (215.9 KB, 287 views)
File Type: jpg 6GH8-6cz5 amp.jpg (143.4 KB, 288 views)

Last edited by listener14; 6th February 2018 at 10:40 PM.
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Old 6th February 2018, 10:50 PM   #2
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
Default 6GH8-6CZ5 Tube Amp Project

Attached are the Sylvania 6CZ5 tube data sheets.
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File Type: pdf 6CZ5.pdf (163.6 KB, 35 views)
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Old 7th February 2018, 01:37 AM   #3
Eli Duttman is offline Eli Duttman  United States
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If possible, please provide the original Knight schematic.

A 100 Kohm log. taper part will be fine as R1. Perhaps the uploaded schematic fragment will help in sizing R8. Assume parasitic oscillation will be present and suppress that nasty with carbon composition stopper resistors on all control grids.

As it is unlikely your 6ZC5s are well matched, "stand" each O/P tube's cathode on a 1% tolerance 10 Ω resistor and employ individual bias trim pots., to match "idle" currents. Measure the voltage drop across the 10 Ω part and apply Ohm's Law to calculate the "idle" current.

Do you have any gas discharge regulator tubes on hand? Full pentode mode O/P tubes yield max. open loop linearity, when g2 B+ is regulated at a fraction of anode B+.
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File Type: jpg Fisher X100 part 3.jpg (401.9 KB, 125 views)
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Old 7th February 2018, 12:39 PM   #4
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
Default 6GH8-6CZ5 Tube Amp Project

Thanks Ed, Attached is part of the KA-55 schematic and the power supply schematic. I do not have the instructions for the kit. I do have around 20-30 tall bottle RCA nos 6CZ5 tubes and some Sylvania's also. I should be able to find a matched quad. I do not have any gas discharge regulator tubes.
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File Type: jpg Stereo40.jpg (451.4 KB, 98 views)
File Type: jpg Stereo40PS.jpg (526.8 KB, 70 views)
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Old 7th February 2018, 02:36 PM   #5
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
I can do the carbon composition stopper resistors on all control grids and put the output tube cathodes on a 1% tolerance 10Ω resistor and employ individual bias trim pots to match the idle currents.
Attached are the 6973 and 6CZ5 tube specs for type AB amps.
Attached Images
File Type: jpg 6973 AB.jpg (403.2 KB, 59 views)
File Type: jpg 6CZ5 AB.jpg (296.7 KB, 31 views)

Last edited by listener14; 7th February 2018 at 02:42 PM. Reason: add to
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Old 7th February 2018, 02:38 PM   #6
Eli Duttman is offline Eli Duttman  United States
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Even with well matched tubes, separate bias trim pots. are a good idea.

You may be in trouble using a 5U4 rectifier, which draws 3 A. of filament current. The OEM 5AR4/GZ34 draws 2 A. Also, the forward drop in a 5U4 is greater than that in a 5AR4. You can completely dodge the vacuum rectifier issue by using 2X 1200 PIV Schottky diodes.

You are fortunate in being able to FWCT rectify the bias supply. A couple of these Schottky diodes will do nicely.

If I read the 6ZC5 datasheet correctly, stacking a 0A2 & a 0B2 will take care of regulating g2 B+. A stack in each channel will surely work.
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Old 7th February 2018, 03:06 PM   #7
GoatGuy is online now GoatGuy  United States
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Quote:
If​ I​ read​ the​ 6ZC5​ datasheet​ correctly,​ stacking​ a​ 0A2​ &​ a​ 0B2​ will​ take​ care​ of​ regulating​ g₂​ B+.​ A​ stack​ in​ each​ channel​ will​ surely​ work.
BZX85B100-TAP Vishay Semiconductors | Mouser is I prefer, myself. But I freely admit not having affinities for the beautiful light show gas discharge tubes offer. I kind of like the 45¢ unit cost, and the fact that Zeners last essentially forever in service.

The only reason I didn't do a Mouser search for 220 or some other higher voltage is that I also like the added dissipation that stacked 45¢ Zeners offers.
_______

Since I completely respect your opinion(s) Eli, might it not also be reasonably good advice to float using series-with-cathode voltage regulator chips as "set-and-forget" constant-current references? Instead of 10 Ω current-sense resistors and potentiometers to adjust things?

We'd bypass them of course with both a nice mid-value electrolytic (220 μF at 33 VDC) and a polypropylene HF bypass (4.7 nF, 100 V). So the semiconductor auto-biasing scheme becomes acoustically “invisiblized”. Sorry about the made up word.

I agree with your assessment of vacuum rectification, and the utility of using hi-PIV Shockley diodes as alternate (and cheap, and good) rectification. It isn't clear to me why one wouldn't use the 45¢/ea parts for a full-wave bridge (4 diode) configuration, but hey… I trust your intuition.

Anyway, my 2¢
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Old 7th February 2018, 03:27 PM   #8
Eli Duttman is offline Eli Duttman  United States
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The Knight power trafo the OP has is configured for a FWCT bias supply. A bias tap on each leg of the rectifier winding is a nice touch.
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Old 7th February 2018, 08:57 PM   #9
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
Yes, I did not take into consideration a 5U4 draws 3 amps. I was looking at voltage drop. I can get a 5ar4, no problem
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Old 7th February 2018, 09:37 PM   #10
listener14 is offline listener14  United States
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6GH8-6CZ5 Tube Amp Project
Thanks for the ideas. Yes, the 20v taps are nice and convenient to have. If the 6GH8 configuration will work, I was thinking about building a Dynaco ST-70 driver board made for 6GH8 tubes for this amp.
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