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Old 22nd January 2011, 06:54 AM   #1
dfro is offline dfro  United States
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Default Basic tube power supply question

Hi all,

I am designing a tube amp for the first time and I have a basic question:

Does each tube's HT supply need to be isolated by a resistor and cap from every other tube's supply, or can several tubes be supplied from the same common HT supply?

I would like to supply 8 tube stages from the same 260V supply. Is this okay, or should I isolate each on from each other?

Thanks,
Dave
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Old 22nd January 2011, 07:13 AM   #2
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I would at least isolate left and right channels from each other with RC network. Also, providing separate supply to input stage and power output tubes is recommended. But there is no must
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Old 22nd January 2011, 07:16 AM   #3
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You may consider using PSUD 2 application. It helps much.
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Old 22nd January 2011, 07:21 AM   #4
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Quote:
Originally Posted by dfro View Post
Hi all,

I am designing a tube amp for the first time and I have a basic question:

Does each tube's HT supply need to be isolated by a resistor and cap from every other tube's supply, or can several tubes be supplied from the same common HT supply?

I would like to supply 8 tube stages from the same 260V supply. Is this okay, or should I isolate each on from each other?

Thanks,
Dave
Ideally you decouple (isolate) each section of an amp - for example the output section is decoupled from the driver which in turn is decoupled from the input.

For a good quick and easy to understand approach, read the power supply section of this site
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Old 22nd January 2011, 07:48 AM   #5
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Hi!

This depends on the actual amp and I would also balance it with the overall budget which is spent.

You mention 8 tube stages which sounds a lot, is this a preamplifier?
In a power amp it would be better to isolate the power stages with LC rather than RC. For drivers and preamps RC is ok.

It also depends on the power supply rejection ratio of each stage. In some cases it is quite ok to supply both channels from the same node. In preamps you can also supply two consecutive stages from the same node. Usually each stage inverst phase. So the interaction is less.

If three stages are supplied from the same node, you can experience motorboating. This is a kind of positive feedback through the common supply.

Best regards

Thomas
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Old 22nd January 2011, 07:48 AM   #6
dfro is offline dfro  United States
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Wow, those were some fast replies!

aardvarkash10, I have just purchased Merlin's books. I have been intently studying them for the last week. I am basing my PSU design on his examples. These books are awesome. He is a great teacher.

I hope he completes a trio of books by writing a book on the power amp output stage. I still have many basic questions about output transformers, power tubes, screens, etc.

I am going to go for a MOSFET regulated PSU, as Merlin describes in his preamp book. I want to have three Voltage levels regulated by three discrete MOSFET series regulators - 420V for the power tubes, 310V for the screen, and 260V for four 12AX7's.

In Merlin's preamp book, he shows several examples that have more than one HT supply for the preamp section. He mentions some limits on current for each HT section, but says nothing about how many tube stages can be supplied by each HT section.

I guess I could supply the first two 12AX7 input tubes with a separate HT section than the next two 12AX7's.

I don't know it this is a tradoff between a higher HT voltage versus some more noise filtering for the input stage. I have no "shop feel" for this part of the design.

yagoolar,
I will look into the PSUD 2 program. I hope it can run under Wine/Linux.

I have discovered LTSpice, which the author has made sure will run under Wine. In a couple of days I have learned enough to model my PSU section and all the tube stages. I am stuck on the output transformer and input transformer for the moment. I have been doing a lot of research on how to make a decent enough spice model.

I have learned more about tube amp behavior in the last few days using LTSpice than I have in a year. What a great program!

Thanks,
Dave

Last edited by dfro; 22nd January 2011 at 07:51 AM.
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Old 22nd January 2011, 08:16 AM   #7
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Quote:
Originally Posted by dfro View Post
Wow, those were some fast replies!

aardvarkash10, I have just purchased Merlin's books. I have been intently studying them for the last week. I am basing my PSU design on his examples. These books are awesome. He is a great teacher.

I hope he completes a trio of books by writing a book on the power amp output stage. I still have many basic questions about output transformers, power tubes, screens, etc.

I am going to go for a MOSFET regulated PSU, as Merlin describes in his preamp book. I want to have three Voltage levels regulated by three discrete MOSFET series regulators - 420V for the power tubes, 310V for the screen, and 260V for four 12AX7's.

In Merlin's preamp book, he shows several examples that have more than one HT supply for the preamp section. He mentions some limits on current for each HT section, but says nothing about how many tube stages can be supplied by each HT section.

I guess I could supply the first two 12AX7 input tubes with a separate HT section than the next two 12AX7's.

I don't know it this is a tradoff between a higher HT voltage versus some more noise filtering for the input stage. I have no "shop feel" for this part of the design.

yagoolar,
I will look into the PSUD 2 program. I hope it can run under Wine/Linux.

I have discovered LTSpice, which the author has made sure will run under Wine. In a couple of days I have learned enough to model my PSU section and all the tube stages. I am stuck on the output transformer and input transformer for the moment. I have been doing a lot of research on how to make a decent enough spice model.

I have learned more about tube amp behavior in the last few days using LTSpice than I have in a year. What a great program!

Thanks,
Dave
This has the foundations of an interest thread. Care to share what you have learnt or designed as it would improve my knowledge. The design books look good but expensive and I would have to wait for ship from US - you didn't spot an ebook version by any chance?
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Old 22nd January 2011, 08:39 AM   #8
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Quote:
Originally Posted by dfro View Post
Wow, those were some fast replies!

aardvarkash10, I have just purchased Merlin's books....

I am going to go for a MOSFET regulated PSU, as Merlin describes in his preamp book. I want to have three Voltage levels regulated by three discrete MOSFET series regulators - 420V for the power tubes, 310V for the screen, and 260V for four 12AX7's.
...
Thanks,
Dave
Hi Dave - the advantage of a world-wide membership is round the clock responses!

Three active regulators is probably overkill. The purpose of the regulator here is two-fold - to set the voltage (obviously) and secondly to remove power supply ripple.

imho, the highest level reg feeding your power tubes will do the majority of the work - everything after it can be very basic since they are invariably operating in class A and so are not cursed with variable current loads.

For simplicity, I would regulate the 310V to the screens - they respond well to tight regulation. Downstream from there at your voltage amps and phase splitter, simple dropping resistors and cap decoupling will do very well.

Of course, ymmv!
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Old 22nd January 2011, 10:58 PM   #9
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PSUD2 runs under Wine/Linux. Checked myself.

AFA simulation PSU is concerned - it is quicker and easier to start with PSUD2 than LTSpice. Yet you are limited to predefined topologies in PSUD2.

Last edited by yagoolar; 22nd January 2011 at 11:03 PM.
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Old 23rd January 2011, 01:26 AM   #10
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I don't know if it is the right approach but I have always thought it wise to decouple every inversion. So I would not have two CC stages fed from the same decoupling network but a CC and a follower I would power from the same node.
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