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Old 11th November 2011, 04:46 AM   #111
tomchr is offline tomchr  United States
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If the regulator was indeed an ideal battery, SpreadSpectrum would be right. If one end of both cathodes is at ideal GND, then the other ends of both cathodes/filaments (assuming DHT here) could be tied to the ideal battery. The regulator is very close to an ideal battery -- at least at frequencies below the resonance frequency of the LC output filter (10-ish kHz). However, its output impedance is finite and non-zero. Hence the cathode currents from the two tubes could potentially interfere with each other. It really depends on the ground impedance as well as the output impedance of the regulator.

My advice would be that you need a separate regulator per directly heated tube. Or, say, you convert a design from AC heating to DC heating, you'll need one regulator per transformer filament winding.
For indirectly heated tubes, you can use one regulator to power several tube heaters as long as the heaters are to float at the same voltage.

Latest board incarnation attached. The multiple output caps are options. You'll see that only one is populated.

~Tom
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Last edited by tomchr; 11th November 2011 at 04:48 AM.
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Old 11th November 2011, 05:13 AM   #112
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Thanks for the response, Tom. It sounds like you are worried about signal(or some undesirable noise/distortion) coupling between the filaments in the shared supply. Is that what you mean by the cathode currents interfering with each other? Please explain if I have this wrong.

I am planning on using two regulators for four 841s. Each regulator would see a total load of 7.5V @ 2.5A. These are fixed bias, sharing a negative filament supply reference. They are also CCS-loaded so there would be very little plate current swing, just plate voltage swing.

I guess I could do four regulators but I only want to do so if there would be a real benefit. What do you think?
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Old 11th November 2011, 05:59 AM   #113
tomchr is offline tomchr  United States
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Quote:
Originally Posted by SpreadSpectrum View Post
It sounds like you are worried about signal(or some undesirable noise/distortion) coupling between the filaments in the shared supply.
More a coupling between the cathode currents. Let's say tube A runs at Ia AC anode = cathode current. This creates an AC voltage on the filament regulator output that's Ia * Zout, where, Zout is the output impedance of the regulator. If tube B's cathode is connected to the same regulator, this means tube B's grid-to-cathode voltage will have an AC component on it that's Ia * Zout. In addition to whatever AC component is in tube B's anode = cathode current.

I think the effect will be minuscule -- at least near DC, but I can't deny it's there. I would expect a significant effect at frequencies near the LC resonance.

Quote:
Originally Posted by SpreadSpectrum View Post
I am planning on using two regulators for four 841s. Each regulator would see a total load of 7.5V @ 2.5A.
First off, I suggest using WebBench (National Semiconductor | High-performance Analog) to work out the inductance and Cout. Note that the DC resistance of the inductor as well as the ESR of the output cap play a role in the overall stability of the regulator. So you wan't to buy the right parts.

Do the tubes operate in parallel? If so, you could tie the two tubes that are in parallel to one regulator. I don't see a big issue with that as the cathode currents of the two tubes in parallel should be nearly identical.

Quote:
Originally Posted by SpreadSpectrum View Post
I guess I could do four regulators but I only want to do so if there would be a real benefit. What do you think?
I think there would be a benefit, but I can't quantify how much. You could start with two boards and get another two if you don't like the results.

~Tom
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Old 11th November 2011, 10:37 AM   #114
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Hope you plan on getting quite a few of these made. I want to put them into my OPUS and My Preamp.
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Old 11th November 2011, 08:58 PM   #115
tomchr is offline tomchr  United States
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Quote:
Originally Posted by SGregory View Post
Hope you plan on getting quite a few of these made. I want to put them into my OPUS and My Preamp.
I'm expecting to get 30~40 boards for the first run. I can't have too much money tied up in inventory... Sadly, due to the economics of scale, this also means that the boards will be a touch pricey. I'm expecting the cost to come in around $7.50/each.

I'm toying with the idea of offering assembled boards at a few different voltages as well. I haven't even begun the math on the pricing for those. Just giving you a heads up of my plans.

The lead time on the boards is about a week or 10 days. So should I run out of boards, more can be ordered and be here quickly.

I expect to submit the board for fab this weekend. I'll keep you posted.

~Tom
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Old 12th November 2011, 02:51 PM   #116
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I thought I had this current coupling issue cleverly outmaneuvered since my plate load will be '10M90S cascode || 1.5M || 6pF.' Pretty high impedance, hence very little AC current swing to even worry about. However, this tube is going to require grid current to get the voltage swing I need so there will be cathode current bumps at low plate voltages due to grid current.

I'm using these because I need to develop 200Vrms+ at low distortion and low-gm DHTs tend to be linear across wide voltages (less curve bunching at low plate currents and high voltages). They develop the voltage swing to drive the 50% NFB output stage in a push-pull amp, so they are two push-pull pairs.

I think what I will do is try running a pair on one regulator. I will run a test applying a large signal to the first in the pair with no signal on the second. I will sweep through frequency-wise and watch for a peak in amplitude of whatever is there on the second tube's plate. Should be fun and interesting.
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Old 12th November 2011, 06:41 PM   #117
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Quote:
Originally Posted by SpreadSpectrum View Post
I thought I had this current coupling issue cleverly outmaneuvered since my plate load will be '10M90S cascode || 1.5M || 6pF.' Pretty high impedance, hence very little AC current swing to even worry about.
As long as the CCS has sufficient bandwidth. Yeah. That doesn't sound too far off. I'm still having some trouble accepting that you won't get any fluctuations in the cathode current, but I'm willing to be proven wrong on that.

If you're running a simulation of your amplifier, you could try including a circuit to model the output impedance of the voltage regulator in your sim. I would think that an LC parallel resonance with some series resistance would do the trick. You may have to have a large resistor in parallel with the L, C to tame the Q of the tank.

Quote:
Originally Posted by SpreadSpectrum View Post
I'm using these because I need to develop 200Vrms+ at low distortion and low-gm DHTs tend to be linear across wide voltages (less curve bunching at low plate currents and high voltages). They develop the voltage swing to drive the 50% NFB output stage in a push-pull amp, so they are two push-pull pairs.
If they are P-P pairs, I would think that you might be able to get away with one regulator for two tubes. If the tubes were perfectly matched, I would think that any change in cathode current of one tube would be outweighed by a similar in magnitude but opposite polarity current in the other tube. Except for the Region of Trouble when the tubes enter grid current.

Quote:
Originally Posted by SpreadSpectrum View Post
I think what I will do is try running a pair on one regulator.
Sounds like a plan.

~Tom
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Old 12th November 2011, 08:27 PM   #118
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Sure, I would get fluctuation is cathode current, but they would be as small as I could reasonably make them, certainly nothing like what you would see in a power amp stage.

The 841 is an interesting tube, very linear if the curves in the datasheets are to be believed, but pretty high impedance and you need to drive grid current to get any useful signal output, so unfortunately I will spend a lot of time in the region of trouble. There was probably a better way to do this, but I really want a white hot filament in my amp, but didn't want to pay for 801As, which were a more ideal choice.
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Old 13th November 2011, 12:01 AM   #119
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The deed is done. I have submitted the board for fabrication. With a little luck, I'll have it back just in time for Thanksgiving (Nov 24th).

I'm accepting pre-orders through my website (see my signature). I ship via USPS Priority Mail so if you order now, expect to have boards in hand by December 1st.

~Tom
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Old 20th November 2011, 02:03 AM   #120
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Tom,

I ordered a few of these. I am going to start putting together a parts list. I was wondering if you could post a list of board part sizes so I don't order the wrong size of something.

I have been playing in WeBench and have a list of parts generated so now I just have to make sure they fit the board and make sure I can buy them.
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