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An Excessively Complicated External Power Supply and Bias Circuit for 807 Amp
An Excessively Complicated External Power Supply and Bias Circuit for 807 Amp
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Old 16th July 2018, 10:39 PM   #1
H713 is offline H713  United States
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Default An Excessively Complicated External Power Supply and Bias Circuit for 807 Amp

What started off as a "simple" push-pull 807 build has exploded into a less-than-simple idea. First was the massive power transformer, and then a buddy of mine gave me some industrial wiring boxes that I will use for chassis (see attached photos). I'm going for a sort of industrial/transmitter look, so I think they'll work out okay. I'm not afraid of a little machining.

I won't be able to fit an entire amplifier in one chassis, and I only have one power transformer, so I'm thinking of using an external power supply. Attached are the schematics for both the amplifier and its over-complicated power supply.

These boxes came with some Harting connectors on them (see photos, not exactly sure what they are). I'm thinking of using them to connect the PSU and amplifier sections. Let me know if this seems unwise.

Also visible in my schematic is the bias circuit. It uses a voltage control on the power supply (using an LM337). Each channel has a balance control, and then there is a third control to balance the two channels. I thought it might be interesting to try, though it's seeming a bit complicated.

The screen grid and preamp/phase inverter stages all share a common 300V regulated power supply, using a 6N7 as a sort of PAS element. I'd like to find a tube with a top cap to take it's place (aesthetic reasons).

I calculated that I'd have a maximum of about 90 mA on that rail. The 807 datasheet specifies a maximum screen current of 16 mA under load. What I'm not sure about is how realistic it is to see this much current on the screen. At 300V, I think that would probably exceed the 3.5W screen dissipation rating of the 807.

I'd love to get some feedback on this slightly impractical design.
Attached Images
File Type: jpg 20180716_171420.jpg (473.7 KB, 253 views)
File Type: jpg 20180716_171408.jpg (404.2 KB, 240 views)
File Type: jpg 20180716_171400.jpg (375.7 KB, 229 views)
File Type: jpg 20180716_171313.jpg (466.9 KB, 225 views)
File Type: png 807 Rev2.PNG (34.2 KB, 250 views)
File Type: png 807 Power Supply Rev 1.PNG (36.4 KB, 191 views)

Last edited by H713; 16th July 2018 at 10:51 PM.
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Old 16th July 2018, 11:07 PM   #2
sony6060 is offline sony6060  United States
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Your design looks fine to me. Some tube regulator circuits can be noisy. I would be tempted to use 6x 0A2 instead (two in series each). At start up the low resistor value required may over current the 0A2 for around 8-10 seconds until the 807s draw screen current, but I have never damaged a regulator tube that way.


BTW- use low noise diodes on the 415 volt rail.

Last edited by sony6060; 16th July 2018 at 11:09 PM.
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Old 16th July 2018, 11:22 PM   #3
H713 is offline H713  United States
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What about the bias circuit? I've never seen it done this way, and I'm curious if there are any obvious issues.
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Old 16th July 2018, 11:46 PM   #4
sony6060 is offline sony6060  United States
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I have not seen a regulator used in bias either, but the regulator noise is about -90dB so it should work ok.
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Old 16th July 2018, 11:49 PM   #5
trobbins is offline trobbins  Australia
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Your schematics have some errors. No grid leak resistors back to bias points, and possibly too long an RC time constant for the bias voltages (possible hot start stress on 807's). Screen supply bridge rectifier wiring. 6N7 heater not connected to cathode. Main heater not connected to ground somehow. Is that 42VAC winding actually 21-0-21 ?.

OA2 can cope with a low level of bypass cap (eg. 0.047uF), and upper valve can have a resistor across it to help startup of the lower valve.
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Old 17th July 2018, 12:16 AM   #6
H713 is offline H713  United States
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Quote:
Originally Posted by trobbins View Post
Your schematics have some errors. Possibly too long an RC time constant for the bias voltages (possible hot start stress on 807's). Screen supply bridge rectifier wiring. 6N7 heater not connected to cathode. Main heater not connected to ground somehow. Is that 42VAC winding actually 21-0-21 ?.

OA2 can cope with a low level of bypass cap (eg. 0.047uF), and upper valve can have a resistor across it to help startup of the lower valve.
Didn't notice that bridge rectifier wiring error. That probably wouldn't work too well.

Why would the 6N7 heater be connected to the cathode? Other heaters are not grounded through a virtual center tap simply because I didn't want to draw it into the schematic. I do believe that the toroid is a pair of 20 or 21 volt windings. That centertap is not necessarily supposed to be grounded. Drawing error.

Didn't even think about the RC time constant for the bias supply, that's definitely worth looking at more carefully.

Also, think I'll need that choke on the 300V line with that regulator in place?

Quote:
Originally Posted by trobbins View Post
No grid leak resistors back to bias points,
Could you please clarify on exactly what you mean by that? I assume this is on the power tubes.

Thanks for the suggestions.
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Old 17th July 2018, 12:58 AM   #7
sony6060 is offline sony6060  United States
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The 807 grids needs a resistor to ground to prevent tube 'run away'. It is likely same as a 6L6GC. I think 100K to ground is low enough.
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Old 17th July 2018, 01:01 AM   #8
trobbins is offline trobbins  Australia
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Highly recommended not to float heaters. 6N7 datasheets don't typically show heater-cathode voltage limit, but a German datasheet shows a 45V limit.

42Vrms full-wave rectified could expose LM337 to too high a differential voltage.

Power tube grid leak is the resistance between the grid terminal and the grid bias voltage (the closest bypassed circuit node) - the schematic shows no such resistor.

Choke with regulator - depends on what level of mains frequency ripple you want to get down to in the output stage, and whether the regulator and NFB can do that sufficiently - I'd suggest you suck it and see what hum residual is in your output signal after you've tracked down all other hum contributors.

Last edited by trobbins; 17th July 2018 at 01:03 AM.
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Old 17th July 2018, 01:26 AM   #9
kward is offline kward  United States
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You have the right idea in your bias/balance circuit, but you can achieve the same thing I believe with a simpler circuit. (see attachment, but ignore the voltages and resistor/pot sizes and just look at the topology).

Also since this is a push pull amp, the bias supply doesn't need to be of the highest integrity. You can get rid of the three terminal regulator all together and instead add a small filtering/dropping resistor between the two 100 uF caps, say 1KΩ, or so. Then just feed that raw filtered bias voltage straight into the bias/balance circuit.

Also in your high voltage supply, the caps that are wired in series need voltage equalizing resistors across each cap. Exact value is not that critical as long as they are at least say 100KΩ so you don't draw too much unnecessary current through them.
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File Type: jpg Output stage bias-balance.jpg (48.6 KB, 165 views)
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Old 17th July 2018, 01:48 AM   #10
H713 is offline H713  United States
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I have no doubt I could do a simpler bias circuit, but I was interested in trying the topology I came up with.

I was under the impression that if a regulated screen grid supply is used, then it's best to have a regulated bias supply so it doesn't change relative to the screen grid when the line voltage changes.

BTW, is there a better tube for a PAS element than the 6N7? I think I have some stray 6L6GCs around, but it seems a bit overkill for this. It'd be cool to use something with a top cap just to keep with the aesthetic of this thing.

BTW, is there any reason I'd want to use an 0D3 instead of an 0A2? I'd use a bunch of zener diodes, but they don't glow purple.
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