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Old 1st June 2011, 04:55 PM   #51
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Bingo! I would have preferred if he'd be willing to admit this himself.
Unless you declare that you have no intention of offering your heating boards for sale, that is a grossly hypocritical thing to say.
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Old 1st June 2011, 04:57 PM   #52
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Rod. At no point did I say my solution was optimum. If I did, please point to it.
You said:

Quote:
So same sound quality, lower cost, less heat to get rid of... The switcher sounds like a clear winner to me.
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Old 1st June 2011, 05:03 PM   #53
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You make the assumption that what he is measuring is due to this "current mixing effect" that you have thought up. Based on my understanding of how vacuum tubes work I just don't see that argument as being rooted in anything that goes on physically in the tube.
Yes, the cathode current provides electrons for the filament coating to release. The filament coating releases electrons because it's heated by the filament current (DC or AC). But the process by which these electrons are released to the space-charge cloud surrounding the cathode is completely random. It's a stochastic process. To argue that you can distinguish one electron from another and follow this one electron as it travels happily from the heater wire to the anode is absurd and not supported by science.

Why do you think the current mixing occurs in the space-charge? You are imagining it if you think anyone claims that.

Filament current and Anode-cathode current mixing occurs in the substrate wire of the filament, which conforms to Ohms law, just like any other piece of wire.

I have explained it numerous times already, so any interested readers can look further up the thread for the workings of it.
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Old 1st June 2011, 05:13 PM   #54
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Gentlemen, with all due respect...

This thread is about Tom's switcher and I'm very interested in its practical aspects. The theoretical arguments about its merits vs other schemes should be discussed separately in other threads.

Tom, thanks for sharing your work and please continue to share as you finalize your regulator.
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Old 1st June 2011, 05:20 PM   #55
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I'm measuring 0.05~0.06 % THD+N with DC heating at 1 kHz, 1 W into 8 ohm. This is measured on this 300B amplifier speaker output using an HP8903A audio distortion analyzer. Once I get further with this filament heater design, I will be posting THD+N vs frequency and output power just like I have for any of the other amplifier designs I've posted here. I will also be posting complete schematics of the amplifier and filament regulators as I have for my other designs.
Tom, this is the heart of the trouble.

If you want to design amplifiers based on a single parameter - THD, that's your choice, but to me that's no better than snake oil.

Unless you design for a listening experience, the outcome is not fit for purpose.

Single-parameter judgement is suspect at the best of times, but THD?

Most importantly, why bother with DHTs? Why not delete the 300B from the schematic and fit a nice LM3886, or even a class-D switcher. I mean everything sounds the same - but the measurement just gets better!

you could get a 35dB improvement in THD, lower cost, less heat to get rid of... The chip-amp sounds like a clear winner to me.
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Old 1st June 2011, 06:05 PM   #56
tomchr is offline tomchr  United States
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Originally Posted by dgta View Post
Gentlemen, with all due respect...

This thread is about Tom's switcher and I'm very interested in its practical aspects. The theoretical arguments about its merits vs other schemes should be discussed separately in other threads.

Tom, thanks for sharing your work and please continue to share as you finalize your regulator.
Thank you. I would like to see this thread return to a build thread as well. I will continue to post data, schematics, and such as it becomes available.

~Tom
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Old 1st June 2011, 07:52 PM   #57
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An interesting thread... I'm also in the process of designing a 300B amp using Western Electric (300B) tubes. So this (thread) has been a good read and some food for thought. What I've not seen is a discussion of the various DHT types, i.e., the filament structure, alignment within the tube itself and what coverage the filament has within the confines of the grid/plate structure. Not all (DHTs) share the same filament/cathode construction and as such some heater drive circuits may work better with one type vs another.

As an example, I'll use the 45 triode and then compare the original single plate 2A3. The filament design of the 45 is very simple. It uses a single filament wire which is arranged in a "M" configuration. The two top peaks of the "M" are tethered above the grid/plate structure and the two bottom ends of the "M" plus the center peak (or dip) are tied to wire supports on the bottom of the tube below the grid/plate structure. The filament voltage is applied to the two bottom ends of the "M". This arrangement gives the 45 a symmetrically aligned filament within the grid/plate structure. Also note that less than the entire filament wire is contained within the grid/plate structure, so any calculations and/or theories which consider the entire length of the filament wire and it's associated heating voltage as interacting with the grid/plate structure (current flow) would be inaccurate. With with the 45, you have 4 segments of the filament wire which can interact with the grid/plate structure. As the 45 has a symmetrical filament, using an AC heating voltage "should" be a preferred method, as the the AC signal for heating should cancel itself out as the ends are out of phase with each other. Using a fixed DC balance (as the filament wire should have an even cross-section and linear resistance) is logical and effectively balances the filament as a cathode. A balance pot to null any remaining AC component (which is caused by uneven coating of the filament wire, a slight physical alignment error, etc.) should result in quiet operation, assuming good quality tubes. By contrast, using a DC heating voltage skews the bias towards one end of the filament and any ripple component can not be reduced by the symmetrical filament. In the case of a 45, I would lean towards an AC heater supply as it looks to be a better fit and feasible to obtain quiet operation.

The single plate 2A3 has a different arrangement. The filament is a "series-parallel" arrangement per the RCA manual. It's filament has two "sets" of wires. Each set is arranged to make 4 vertical passes thru the grid/plate structure held by a tension bar across the top with a pair of pusher springs. Note it's not a "M" structure like the 45. The heater voltage is applied to the ends of the wire (like the 45). The two sets are sitting side by side within the grid/plate structure, i.e., one set covers the left side of the grid/plate structure and the other set covers the right side. Once again, the entire run of the filament wires are not contained within the grid/plate structure, so any calculations/theories are as above with the 45. The parallel reference is how the two sets are tied together. Where the two sets meet in the middle of the tube, the ends are tied together as one filament contact. The two outer ends of the filament sets are then tied together as the other filament contact. As a result, it's impossible to heat a SP 2A3 with AC and get anything close to an acceptable output noise level.

The Western Electric 300B has a similar filament arrangement and can not be heated with AC and achieve an acceptable output noise level. So, heating either tube will require a DC filament supply for quiet operation. As the purity of the DC can affect the performance of the tube, attempting to calculate it is difficult, as it's hard to measure what percentage of the filament is within the grid/plate structure. This makes it difficult to accurately calculate distortions, noise, etc. which are the result of filament supply noise. Whether constant current or constant voltage is used, you can probably make a good case for either but results may be more subjective than measurable.

Most designers use some common criteria for measurements and specifications which result in proper performance given a set of conditions. Among these measurements are: frequency response, output power, distortion, signal-to-noise, etc.. For my own amplifier designs, one goal is to achieve a minimum of 80dB signal-to-noise ratio referenced to 1-watt RMS output. Having very low THD, wide frequency response, flat power bandwidth, square wave response, etc. are important as well. Once meeting these specifications, I think it becomes a difficult discussion to quantify what audible distortions are attributable to the filament supply. Note that I'm not tossing stones at anyone's filament supply design, I need to have one at some point as well. The point being that one type of filament supply may not work best with all types and/or brands of DHTs.

Regards, KM
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Old 1st June 2011, 11:42 PM   #58
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Originally Posted by kmaier View Post
For my own amplifier designs, one goal is to achieve a minimum of 80dB signal-to-noise ratio referenced to 1-watt RMS output. Having very low THD, wide frequency response, flat power bandwidth, square wave response, etc. are important as well. Once meeting these specifications, I think it becomes a difficult discussion to quantify what audible distortions are attributable to the filament supply.
That lines up very well with my design goals actually. In addition to the measurements you mention, I usually measure the output noise spectrum of the amplifier with the input grounded. This allows me to detect, quantify, and hopefully minimize any residual hum/ripple/EMI resulting from layout, power supplies, etc.

When I design circuits, I tend to work block by block. I design, build, test, and characterize each block individually. But in the end, it's the system performance that matters. I.e. the performance of the completed amplifier is what matters.

~Tom
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Old 2nd June 2011, 02:02 AM   #59
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This little discussion gave me an idea about a different kind of filament supply. Instead of using straight DC or low frequency (50/60Hz) AC how about this.

A split supply of +/-5 volts and a basic mosfet bridge. Now this is just a concept so in reality you'd want a driver chip with some logic so both mosfets can't turn on at the same time. The 7404 is configured as an oscillator and then one gate is used to invert the signal.

I bet this hasn't been tried before
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Old 2nd June 2011, 05:33 AM   #60
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Yeah... You'll need non-overlapping clocks for that switch drive. That's a solvable problem, though. Then you add spread spectrum clocking to the deal.
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