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Old 21st January 2015, 02:52 AM   #1
65 DegN is offline 65 DegN  United States
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Default 300B

I have a friend that has a pair of monoblock class A 300B amplifiers. I did a little work on the filament supply installing a 5V regulator with filtering. I noticed the filaments had a regular 20 watt wire wound resistor to ground, bypassed with an 80 mfd 450V German electrolytic.
I suggested non-inductive bias resistors and film caps. I am looking for unbiased 3rd party opinions on this to pass along to the fellow.
Thanks
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Old 21st January 2015, 09:54 AM   #2
DF96 is offline DF96  England
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Resistor inductance is rarely an issue at audio frequencies; about the only example I can think of where it might matter are the very low value resistors used as 'emitter' ballasts in a solid-state output stage.
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Old 21st January 2015, 04:45 PM   #3
Keit is offline Keit  Australia
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Not only is the inductance of resistors, wire wound or carbon film, negligible in tube circuits at audio frequencies, if it's bypassed with a cpacitor, the inductance can't do anything anyway.

However, the use of a 450 V electrolytic as a cathode bypass may be a concern. Electrolytics can de-form unless the operating voltage is a significant fraction of the rated voltage. You should choose an electrolytic that has a rated working voltage 1.2 to 2 times the voltage across it. Some brands are better than others in this respect.

There will be no audible difference between electrolytics and film caps in cathode bypass use. The equivalent series resistance in either case is negligible compared to the tube cathode inpedance, which is 1/gm. A film cap will be more reliable but this is pretty much a non-issue with modern parts.

Last edited by Keit; 21st January 2015 at 04:49 PM.
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Old 21st January 2015, 05:40 PM   #4
kevinkr is offline kevinkr  United States
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What Keit says about electrolytic capacitors is in general absolutely true, however I have found films perform measurably (in some cases) and audibly better (in most cases) than any electrolytics I have tried other than some now exotic unobtainium capactors from Elna and Blackgate. The problem appears to be varying ESR (ESL too) with frequency. I have gotten measurably flatter FR response with 300B and other dht avoiding electrolytics as cathode decouplers. Other things appear to be going on with DA related effects as well.

TBH I much prefer fixed bias operation as it removes a questionable capacitor from the design equation at the expense of just a little bit more complexity.
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Old 21st January 2015, 07:53 PM   #5
65 DegN is offline 65 DegN  United States
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kevinkr, I agree concerning filament (cathode) biasing. I much prefer fixed bias but I would not be allowed that much freedom to change the design of the amp.
I would think that simply the fact that electrolytics do not treat the positive and negative going parts of a waveform identically would be enough to warrant the use of films. Thanks guys for all your input. I appreciate all the different views on this.

Last edited by 65 DegN; 21st January 2015 at 08:01 PM.
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Old 21st January 2015, 08:11 PM   #6
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Quote:
Originally Posted by 65 DegN View Post
I would think that simply the fact that electrolytics do not treat the positive and negative going parts of a waveform identically.....
??? Don't get that one even in principle, you'll have to explain......and in this case there's always dc bias present and if all is well approx. zero ac potential. There is ac current, but why would that be asymmetric ?
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Old 21st January 2015, 08:49 PM   #7
Retsel is offline Retsel  United States
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Default Filament supply

I assume that this supply is DC for the 300B.

I think that the best DC filament supplies do not use a regulator. Instead, use a separate transformer and dial in the voltage you need by changing the secondary windings on filament transformer. After the transformer, use Schotky diodes, a choke (common mode), or two chokes, and good quality capacitors to reduce ripple.
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Old 22nd January 2015, 12:40 AM   #8
Keit is offline Keit  Australia
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There is NO need for DC on the filament of a power stage, PROVIDED that other aspects of the design are correctly done - the hum that is occurs with directly heated tubes can be adequately balanced out. The fundamental is balanced out by means of feeding the filament with a balanced supply. The 2nd harmonic can be balanced out in an SE design by feeding a controlled amount of 100 HZ ripple, filtered back to sine, to the driver anode. Usually what remains is inaudible.

However, once DC is employed, the voltage drop across the diodes, which is esentailly a constant amount, emphasises the variation in AC mains voltage. If you have used two schottky diodes with a centre-tapped transformer, you still have about 10% drop in the diodes. So +&- 5% variation in AC mains input becomes approx +&- 6% variation in DC filament voltage. That's ok.

But some folk make the mistake of using ordinary power diodes in a bridge rectifier. In this case, the diode drop is about 40% of the DC output. The +&- 5% AC main variation becomes about +&- 10% varition in DC filament voltage. That is not acceptable. It will shorten the life of the tube, and affect the linearity. So a regulator becomes necessary.

So, summing up: The best way is just use AC. It's simple, reliable, and does the job. If you can hear hum, don't abandon DC on the filament, figure out what you did wrong. If use must use DC, don't use a regulator, just use 2 schottly diodes in a push-pull rectifer fed by a centre tapped transformer (and teh filering of course). A choke input filter will assist in reducing the effect of the diodes in magnifying AC mains variation. This is not for beginners though - because it means a precision calculation of transformer secondary voltage and a non-standard transformer.

Note that my comments apply to power amplifiers. For those that use unusuall tubes, not originally intended for such use, in preamplifiers, the hum problem can be more difficult. However I still would not use DC heater supplies even then. There are other ways to eliminate hum.

Last edited by Keit; 22nd January 2015 at 12:47 AM.
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Old 22nd January 2015, 02:42 AM   #9
65 DegN is offline 65 DegN  United States
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Retsel, I agree with that. But I had to work within the constraints set by the owner. I presented him with that option and it seemed he really did not want to go to the expense of an additional xfmr. Also there wasn't much room on the chassis.
One thing I do like about the 3 pin reg. (with TO-220 shottky rectifiers) is that the voltage is rock stable. When I got the amp it was up around 6.5 volts with 6 VPP ripple. Now it is 5V with 0.1 VPP ripple. Much less hum.

Last edited by 65 DegN; 22nd January 2015 at 02:46 AM.
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Old 22nd January 2015, 02:56 AM   #10
65 DegN is offline 65 DegN  United States
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lucky, it is my understanding, although I have not investigated this for myself, that varying voltage will occur on the cathode in phase with the grid input signal if there is no bypass capacitor. If that is the case then there will be negative and positive fluctuations that the cap will need to deal with. Seems to me that a film cap would be better suited to that task than a polarized electrolytic.
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