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Old 29th September 2009, 07:36 PM   #1
gja is offline gja  Spain
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Default 6j7g 832a amp

Hi everyone,
need some help with a project:
6j7g driving 832a.just canīt seem to get the correct circuit...
anyone con draw a basic schematics to help me out?
thanks
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Old 29th September 2009, 08:09 PM   #2
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SE? PP? A few more clues on your parameters would be good...
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Old 29th September 2009, 08:51 PM   #3
gja is offline gja  Spain
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Default 6j7g 832a

pp,forgot to tell...
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Old 29th September 2009, 10:13 PM   #4
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start here - My version of the Simple EL84 or rise of the anti-triode.
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Old 3rd October 2009, 06:25 AM   #5
gja is offline gja  Spain
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thanks for the link.interesting.
Not sure I should use 6j7g in triode or pentode mode...
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Old 3rd October 2009, 06:41 AM   #6
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in shoog's design, pentode. In fact, there is a loooooooong and interesting thread on the use of Schade-esque feedback and the collective agreement is that a pentode at the front is a close-to-absolute requirement for plate to grid feedback.
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Old 3rd October 2009, 06:20 PM   #7
gja is offline gja  Spain
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ok,let´s go for pentode,which is what I have done so far except my circuit must be completely wrong. anyone have a schematics lying around?will try and post mine but that might take a while...
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Old 12th October 2009, 06:29 AM   #8
gja is offline gja  Spain
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Default schematics

Ok, this the basic schematics...probably full of mistakes but then the point of the forum is to get some help,isnīt it?
I know some caps are missing,driver section seems quite ok.I īve included the voltage readings.output trannie comes from a magnavox 6eu7/6v6 pp amp.
psu is rated 260v,readings say 280v.
Anyway just though Iīd post something in order to get some help!
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Old 12th October 2009, 08:00 AM   #9
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ok a few design issues... terminal 6 of the 832 should go to ground, not to the driver anode.

I would lock down the G2 voltage a good deal more firmly than a resistor is capable of. A solidly regulated supply voltage is required here for best performance, whatever voltage you select.

Haven't done the numbers, but I suspect that the 6J7 will not have enough drive to punt the G1 capacitance across the required p-p range - you need a driver tube that is capable of a bit more current capacity. If you are working with available parts (like I mostly do) you may want to consider augmenting the 6J7's anemic drive capability with a MOSFET source follower. Others more capable than I should comment here.

Finally, you are discussing a full Class A system here - output is not going to shift planets, and every volt and milliamp you can push counts. Having said that, I think your present biasing of the output tube puts you into some seriously restricted territory - are you SURE you meant -3.88V bias? I hope you like the sound of clipping!

Heres a clue - work it backward. Start at the OPT, figure out its parameters. Then move back to your output tube. Work out what it needs to swing to maximise the performance of the OPT. Then move back to the driver and ensure that it can supply the needs of the G1 of the output tube. Once you have all that under control, do your power supply to give a rock solid voltage and current supply to your good work.

THere are a heap of good articles and resourses to assist you in understanding this - take a good look around! Start here with Crowhurst. They are classic
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Last edited by aardvarkash10; 12th October 2009 at 08:12 AM. Reason: added crowhurst link
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Old 12th October 2009, 10:41 PM   #10
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Some serious problems with this design not the least of which is the 6J7 needs to drive a phase splitter to drive an 832 in PP - as drawn I bet it makes almost no sound at all. Alternately you can make the 832 do the phase splitting as well as power amplification.

Connecting pin 6 to ground without removing the 832 cathode bypass capacitor means that only one half of the 832 is doing any useful work, and the other is just loading half of the primary and providing cancellation of the dc flux in the output transformer.

If you want to use the 832 for phase splitting as well as power amplification duties I would replace that cathode resistor with a CCS using at least an LM317 set to sink an appropriate amount of current for the tube based on supply voltage (dissipation limited) and the most appropriate load line for that OPT. (Using the 832 for phase splitting means you must run pure class A with a CCS to get decent performance.)

In this application particularly if you make the 832 do the phase splitting tightly regulated screen supply is a good idea. (At minimum a power zener or a couple of gas regulator tubes. Think 0A2 + 0B2 for example.)
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Last edited by kevinkr; 12th October 2009 at 11:05 PM.
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