Low Voltage Plate Follower 6DJ8 Buffer Power Supply - diyAudio
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Old 24th July 2007, 03:05 PM   #1
mwl6m is offline mwl6m  United States
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Default Low Voltage Plate Follower 6DJ8 Buffer Power Supply

I am designing a tube buffer for a Buffered Non-Inverted Gainclone monoblock.

My design is a plate follower circuit with a gain of just over 1. I have chosen to use a B+ voltage of 70 volts.

This is my first amplifier project, and I am unfamiliar with what type of power supply would be necessary for the buffer. I have been having a hard time finding power supply schematics for such a circuit. Any help would be greatly appreciated.

My initial circuit is attached. Any comments on the circuit design would be great, as this must be completed for a college project. Thanks again for any help.
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Old 24th July 2007, 04:18 PM   #2
SY is offline SY  United States
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Do you really want to run the tube with that low of a plate load? Yes, the feedback will help linearize things, but you're starting with something pretty nonlinear. What's wrong with a cathode follower?

In any case, a simple supply is all you need. I generally use something like a Maida regulator as outlined in the infamous National Semiconductor LB47. You probably don't need a Darlington.
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Old 24th July 2007, 04:24 PM   #3
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It looks like you are using the 1K5 resistor to introduce lots of feedback.
May I suggest that a better way to do it would be something like the JLTi gainclone, though there are better buffers than the simple CF which Joe Rasmussen suggests, unfortunately the full JTLi is a copyrighted design and its up to yourself to work out the component values.

http://www.customanalogue.com/diytub...lone/index.htm

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Old 24th July 2007, 04:43 PM   #4
mwl6m is offline mwl6m  United States
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The idea of my project was to use low voltages for the 6DJ8 tubes. It seems as if I will have to use a much higher B+ voltage for a cathode follower, and I wanted to try and have a gain just above unity. From other forums I have heard that the 6DJ8 runs well from lower supply voltages. Any suggestions for a higher B+ voltage, or even a better circuit topology for a slightly above unity gain buffer?

Also, I had a question about the wiring for the heater. I am only using one triode in the 6DJ8, and I wanted to know how to wire the heating up. The datasheets say that a parallel supply of 6.3V @ 350 mA is needed. But where do I set the reference for this voltage? to ground or the cathode? Does this mean I need 700 mA for the heaters?

Thanks again for all the help.
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Old 24th July 2007, 05:04 PM   #5
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You don't need a higher voltage for a CF. The same 70V rail will work fine. Use a constant current sink in the cathode to run the tube at 6-8mA, and pull the CCS to the -35V rail you have for the 1875.

You'll need to power the entire heater of the tube, 6.3V terminal to terminal at 300-365mA (depending on the flavor of ECC88/6DJ8/whatever) that you use. The heater supply can be referenced to ground, but for lowest noise, it's better to reference it to 20-40V above ground.
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Old 24th July 2007, 05:31 PM   #6
mwl6m is offline mwl6m  United States
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So you are suggesting to use a CCS that feeds into the cathode from the -35V rail of the LM3875. I am not too familiar with constant current source circuits, but this does seem like a good idea. Do you know of any helpful links for CCSs or any basic schematics that I could learn from.

Also, how would you suggest to reference the heaters at 20-40V. I am somewhat confused by this, but I have seen this suggestion around the internet. Any more insight would be great.

Thanks.
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Old 24th July 2007, 05:31 PM   #7
mwl6m is offline mwl6m  United States
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Attached are the results for the 70V B+ cathode follower from John Broskie's TubeCad. So would I basically replace the cathode resistor with the CCS that was suggested? I am going to do a few Spice simulations to obtain some performance characteristics of the circuit. Any suggestions about the circuit or how to implement a CCS to feed the cathode would be greatly appreciated?

Thanks again for all the help from such a great group of people.
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Old 24th July 2007, 05:40 PM   #8
mwl6m is offline mwl6m  United States
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Here is the attachment. Sorry about that.
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Old 24th July 2007, 06:20 PM   #9
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In order to create your +40V reference you need to just create a resistor voltage divider between +B and ground. With a 70V +B you could use 300K 400K. with the three hundred tied to +B and the 400K tied to earth. The reference is taken from the junction and tied to one side of your heater winding. The ECC88 is insensitive to heater induced hum so no other special precautions will be needed. Infact you can probably get away with just referencing to earth.

As noted in another thread the ECC88 has extremely wide bandwidth and so will tend to oscillate if not wired up correctly. Grid stoppers are essential and should be wired straight onto the valve base. If you decide on a CF and there is no load resistor, I would advise a small resistor be wired onto the anode pin and between the anode and +B, something in the order of 10 - 100 ohms.

Its easy to make the ECC88 behave well if these precautions are observed.

Shoog
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Old 24th July 2007, 06:32 PM   #10
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Quote:
Originally posted by mwl6m
So you are suggesting to use a CCS that feeds into the cathode from the -35V rail of the LM3875. I am not too familiar with constant current source circuits, but this does seem like a good idea. Do you know of any helpful links for CCSs or any basic schematics that I could learn from.

Also, how would you suggest to reference the heaters at 20-40V. I am somewhat confused by this, but I have seen this suggestion around the internet. Any more insight would be great.
For free, and worth every penny, do a search on this site for my "Heretical" line amp. For money, and worth much more, grab a copy of "Valve Amplifiers" 3rd edition (Morgan Jones).
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