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Old 14th November 2009, 03:04 PM   #171
martyh is offline martyh  United States
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I am looking for a simple filament supply for the 26 that I can build with the parts I have on hand. I would appreciate any comments on this one. The diodes are high speed soft recovery types and the inductor has a ferrite rod core. I’m not really looking for ultimate performance just something quiet enough to get me started.

Thanks,
Marty
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Old 14th November 2009, 03:12 PM   #172
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Originally Posted by martyh View Post
I am looking for a simple filament supply for the 26 that I can build with the parts I have on hand. I would appreciate any comments on this one. The diodes are high speed soft recovery types and the inductor has a ferrite rod core. I’m not really looking for ultimate performance just something quiet enough to get me started.

Thanks,
Marty
There'll be a little noise, I would expect, but there's no reason to hold you back on building. More L would help if the gyrator is out of reach. Increasing the C to at least 10000uF may also prove worthwhile.

Maybe look out for a logic level FET when you're next shopping, and work up a gyrator, if you want to go further.
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Old 14th November 2009, 04:37 PM   #173
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by martyh View Post
I am looking for a simple filament supply for the 26 that I can build with the parts I have on hand. I would appreciate any comments on this one. The diodes are high speed soft recovery types and the inductor has a ferrite rod core. I’m not really looking for ultimate performance just something quiet enough to get me started.

Thanks,
Marty
I would up that inductor to a couple of mH as a minimum and increase the capacitance to 10,000uF or so, otherwise I think it would work ok. In this sort of application I usually use a voltage regulator ahead of the ccs to provide a little additional noise reduction.. Generally very quiet.

Small chokes (a couple of mH) between this supply and the filament will provide some isolation from high frequency line disturbances. (You can also use common mode choke and a single choke for both diff mode and common rejection improvements.)

I also like to use a "pseudo" center tap across the filament using a pair of 22 ohm resistors connected to a 10 ohm resistor to gnd for measuring the cathode current. (Thevenin equivalent resistance seen by the cathode is about 21 ohms) Note that you do need to account for the filament current that flows through these resistors.
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Old 14th November 2009, 08:05 PM   #174
martyh is offline martyh  United States
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Thanks for the help guys. I went back to the junk drawers and I could do this with the parts I have. Unfortunately though it brings up a few more questions. Is there a problem with using the 100 Ohm 2W pot rather than a pair of 22 Ohm resistors?

Kevin, when you say that I need to “account for the filament current that flows through these resistors” Do mean that I need to adjust R set or that I need to account for the plate current flowing through the resistors that make the ct when I measure bias? Or….both? I have to admit that when there are multiple sources of current in a network I am totally lost.

Thanks Again,
Marty
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Old 14th November 2009, 09:33 PM   #175
Sheldon is offline Sheldon  United States
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Is there a problem with using the 100 Ohm 2W pot rather than a pair of 22 Ohm resistors?

Kevin, when you say that I need to “account for the filament current that flows through these resistors” Do mean that I need to adjust R set or that I need to account for the plate current flowing through the resistors that make the ct when I measure bias? Or….both? I have to admit that when there are multiple sources of current in a network I am totally lost.
You have two current paths: Current that flows from the B+ supply, through the plate, through the filament and cathode resistor, then to the B+ return. In the filament supply path, current flows through the filament from the filament supply, and is returned to the supply.

I think that the 100R pot should be OK. As it concerns the plate current loop, the 100R pot is essentially two parallel 50R resistors, that are then in series with the 10R resistor (this assumes that the pot is near center). It will move your bias point a bit, as your cathode resistor will be in effect 25R+10R.

The 100R pot (doesn't matter where it's set) will be in parallel with the filament, in the heater supply current loop. Since the filament has a resistance of about 1.5R under operating conditions (1.5V at about 1A), you can probably assume that the parallel 100R will not be significant.

Sheldon
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Old 15th November 2009, 07:41 AM   #176
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Going back to the question of using common mode chokes, when I was trying out different kinds of filament supply with an EE friend (from this site) we found that putting a common mode choke AFTER the LM1085 ccs degraded the sound, so we left it out - seemed that the ccs needs to be the last link in the chain. We didn't try out various points of putting the CMC before the ccs. On the other hand the CMC improved the sound when used as the last element after a voltage reg used in the usual way.

andy

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Old 15th November 2009, 09:50 PM   #177
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Apropos Pots and resistors across the filament. Heating with CCS can make DHTs sound much better than ac-heat - but (on the DHTs I've worked with) connecting a resistor across the filament can undo some of the advantage.

With 300Bs and CV1166s ( = Tungsram LP2) the resistor should be more than about 500 Ohms. I haven't experimented with the 26 (would like to now!) but would suggest using a higher value than 100Ohm, at least as an experiment. also, try just connecting the cathode resistor/cap to the negative side only, and compare the sound.

The macro difference between current source heating against dc (voltage) or ac is that the CCS presents a high dynamic impedance to the filament, over all audio frequencies. Perhaps there is a natural distribution of the cathode current across the cathode coating that is disrupted by low dynamic impedance schemes?

One thing I do know: if you convert a 300B amp to CCS heat, you can sit back and enjoy better sound. But if you then connect 1uF across the filament, the sound degrades all the way down to ac-heat level, and maybe even lower.
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Old 15th November 2009, 10:02 PM   #178
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One thing I do know: if you convert a 300B amp to CCS heat, you can sit back and enjoy better sound. But if you then connect 1uF across the filament, the sound degrades all the way down to ac-heat level, and maybe even lower.
Very interesting. Could you provide a schematic of your circuit please? Do you mean 1uf to ground or to pins 1,4? With our without any resistors attached to the cathode? (fixed bias?)

Thank you, as I like to directly ground one pin of the filament while using CCS supply, and am curious of your findings.

Last edited by greenvalve; 15th November 2009 at 10:02 PM. Reason: spelling
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Old 15th November 2009, 10:42 PM   #179
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Very interesting. Could you provide a schematic of your circuit please? Do you mean 1uf to ground or to pins 1,4? With our without any resistors attached to the cathode? (fixed bias?)

Thank you, as I like to directly ground one pin of the filament while using CCS supply, and am curious of your findings.
I use cathode bias, with resistors on both sides. They are different values, to let the cathode current make two voltage drops, 5V apart. The values are on the schematic.

Connecting the 1uF from F- to F+ wrecks the sound! I do have a cathode bias cap though - this old schematic shows it on the +ve side, but I think I found the -ve side sounds better.
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Old 16th November 2009, 12:25 AM   #180
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This is killing me. I have an AC heated 46-PP driving a 300B-PP with about 500uV of 120Hz hum, and I'm ready to rebuild the whole works with these current sources.

Enough already !
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