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Old 1st July 2008, 03:56 AM   #1
kmaier is offline kmaier  United States
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Default More triodes

Well, I finally bit the wallet and purchased a new matched pair of Western Electric 300B triodes. These are nicely made, very nice indeed. They also adhere to specs well and are a very close match (direct from the factory). Here's a pic showing a nice trio of triodes.... NOS 45 (Sylvania made), NOS RCA/Cunningham single-plate 2A3 and new manufacture WE300B.

Click the image to open in full size.

I installed into a prototype design on breadboard and have found a few things... first, no way to get hum-free operation on AC filaments. Center-tapped filament makes it near impossible. This pair gets down to around 14mv into a 16-ohm load, which is pretty loud. Specs are fair for a first draft, within 2db from 20Hz - 50KHz at 4-watts output, max rated output 6 watts. Using Hashimoto iron, PT-160F, C16-150W and H-20-3.5U. Pic attached... yes, it plays.... but hums a lot.

Click the image to open in full size.

So, DC filaments are in the future as is some re-tweaking the input/driver circuit. Will be a while yet in the final making but results so far look promising.

Regards, KM
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Old 1st July 2008, 05:11 AM   #2
kevinkr is offline kevinkr  United States
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Default Re: More triodes

Quote:
Originally posted by kmaier
<snip>
I installed into a prototype design on breadboard and have found a few things... first, no way to get hum-free operation on AC filaments. Center-tapped filament makes it near impossible. This pair gets down to around 14mv into a 16-ohm load, which is pretty loud. Specs are fair for a first draft, within 2db from 20Hz - 50KHz at 4-watts output, max rated output 6 watts. Using Hashimoto iron, PT-160F, C16-150W and H-20-3.5U. Pic attached... yes, it plays.... but hums a lot.

<snip>

So, DC filaments are in the future as is some re-tweaking the input/driver circuit. Will be a while yet in the final making but results so far look promising.

Regards, KM
Cool, I have been designing and building 300B SE amplifiers for about 10 yrs and can echo your comments on current WE 300B. I used them in all of the commercial amps I sold. (Not many at those prices.)

I used 5V heating in my early designs with dual .47mH chokes to provide isolation between the filaments and the supply. My personal amplifier uses CCS heating for the 300B. (The only issue with this is that not all 300B have 1.2A filaments - JJ which I use in this amp comes to mind at 1.5A.) I also recommend trying fixed bias if you have not already, avoids the need for an expensive high quality cathode bypass cap and allows considerable flexibility in determining the best operating point for your load conditions.

My stereo amplifier are fed from a dedicated quasi dual mono outboard tube regulated power supply. (Shared power transformer HV secondary, after rectifier everything is completely independent.)

Hum balance pot on the filaments might help you to knock the hum down to about 5mV or so, but I would be surprised if you do better than this without feedback. (No don't go off and use feedback.. ) DC filaments are recommended - must be ripple free for decent performance.

I have one 2A3 single plate RCA, well worn, hopefully someday I will find its mate...
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Old 3rd July 2008, 03:09 AM   #3
kmaier is offline kmaier  United States
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Thanks Kevin,

Actually, there is a hum balance pot! Now that they have some hours on them, I'm able to balance down to 10mv into 16 ohms... still loud. I've used fixed bias before but the cost of the bias circuit would probably be the same cost using equivalent components. One goal is not to have any electrolytics in the finished unit. Requiring a DC filament supply makes that impractical.

I've fine-tuned the input/driver circuit and measured performance is respectable, sans the filament hum. Design point was 6-watts, running the 300B conservatively for long life. Output is 10-volts RMS into 16 ohms (or 7-volts RMS into 8 ohms and yes, you can have 5-volts RMS on 4 ohms). THD measures at less than 1% at 1-watt, less than 2% at 4-watts and less than 2.5% at 6-watts. Flat within 1dB from 20Hz-40KHz at 1-watt, down 2db @ 50KHz. Usable power bandwidth from 40Hz-40KHz. 3-stage design, no feedback. DC Filament supply is next.

Regards, KM
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Old 3rd July 2008, 02:48 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by kmaier
Thanks Kevin,

Actually, there is a hum balance pot! Now that they have some hours on them, I'm able to balance down to 10mv into 16 ohms... still loud. I've used fixed bias before but the cost of the bias circuit would probably be the same cost using equivalent components. One goal is not to have any electrolytics in the finished unit. Requiring a DC filament supply makes that impractical.

I've fine-tuned the input/driver circuit and measured performance is respectable, sans the filament hum. Design point was 6-watts, running the 300B conservatively for long life. Output is 10-volts RMS into 16 ohms (or 7-volts RMS into 8 ohms and yes, you can have 5-volts RMS on 4 ohms). THD measures at less than 1% at 1-watt, less than 2% at 4-watts and less than 2.5% at 6-watts. Flat within 1dB from 20Hz-40KHz at 1-watt, down 2db @ 50KHz. Usable power bandwidth from 40Hz-40KHz. 3-stage design, no feedback. DC Filament supply is next.

Regards, KM
Looks like very good performance to me, baring the hum issue. CCS heating is what I use these days and it requires relatively few parts, and seems not to injure the sound as has been alleged for constant voltage heating. (I used that in early amplifiers with chokes and it did not appear to be an issue there either.)

Measured performance of mine (also 3 stage) is relatively similar, but is tube dependent. I think my thd is a tad lower at 1W and a tad higher at 6W, but I have not measured this amplifier since it was built so I no longer remember exactly.

I just keep thinking that about 30W of 300B goodness is what I need with my current homebrew speaker system, but my 300B PP amps (designed for a long ago VTV article) are not quite as good overall as my SE. I think I will be trying 845 or 211 with IT at some point in a quest for more power.
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Old 4th July 2008, 10:22 PM   #5
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just keep thinking that about 30W of 300B goodness is what I need with my current homebrew speaker system, but my 300B PP amps (designed for a long ago VTV article) are not quite as good overall as my SE. I think I will be trying 845 or 211 with IT at some point in a quest for more power.>>

Getting a bit off topic, I'm in the same situation of needing more power for my Apogee Caliper Sigs. I'm also hoping PP 300b will get me there and am currently building one. If not, I've bought some 813. Have you heard one? Have you considered that option?

Andy Evans
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