Thermioniclife
Member
As Artie Johnson from Laugh in would say, Very Interesting
Need Help with First Tube Build - WE91 300B Parafeed Derivative
- Thread starter EricS
- Start date
EricS
Member
My thought in using the thermistor is that it was "similar" to a resistor in that it would provide a path to ground while simultaneously providing a bit of ground isolation. But it is superior to a resistor because it can pass a TON of current without burning up. I've pulled more than 400w through a standard CL-60 in my big Class-A solid state amps without issue for more than a decade. A standard 3w or 5w power resistor might open when a fault occurs (potentially leaving the fuse intact), thus providing no protection for the circuit or the user.
I didn't try diodes with a resistor as Paul indicated, I was doing quick experiments and this is what worked most easily for me, so I'll leave it in for now.
Here are some images of the finished monoblocks.
I didn't try diodes with a resistor as Paul indicated, I was doing quick experiments and this is what worked most easily for me, so I'll leave it in for now.
Here are some images of the finished monoblocks.
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Thermioniclife
Member
Sweet, mirror image twins. Love the bases and the pinstripes. Beauty's to behold enjoy!
Yeah, big class A power amps have a pretty nasty turn on surge. Those monoblocks are looking lovely.
D
Deke609
Guest
Ditto. Those look amazing Eric. Nicely done!
cheers, Derek
cheers, Derek
EricS
Member
Thanks for sharing your kind thoughts, Karl and John.
Here are a few under the hood shots of the finished amp.
Paul, are the specs for the grounding resistor bundled with anti-parallel diodes something you can share, or is this information proprietary? I'm curious to explore other grounding configurations. Not sure why I'm seeing increased noise in the 20-40Hz range with the thermistor in place, though I am unable to hear it in my super-sensitive headphones. Maybe it's not worth playing around with...
Here are a few under the hood shots of the finished amp.
Paul, are the specs for the grounding resistor bundled with anti-parallel diodes something you can share, or is this information proprietary? I'm curious to explore other grounding configurations. Not sure why I'm seeing increased noise in the 20-40Hz range with the thermistor in place, though I am unable to hear it in my super-sensitive headphones. Maybe it's not worth playing around with...
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The antiphase diode/resistor/cap thing certainly isn't proprietary. Ideally you'd want a 10-20 ohm 5W resistor, a pair of diodes across the resistor anti-phase wired (non-schottky diodes that can do 20+A peak current), and a little 0.1uF ceramic cap to finish it off.
EricS
Member
Thanks, Doc and Jamie! It was a pretty solid 2-3 years to track down all of the necessary parts. I got lucky and scored a pair of TFA-2004 Mexico Jr. M4 pin stripes direct from Mike. The output transformers came from the great folks on the forum here.
I spent some time experimenting this morning with different grounding schemes (thermistors vs. diodes, resistors, caps) and none of the variations made much difference. I did note that when using diodes, the value of the resistor made a small difference of 2-3dB here or there, but we talking about something like -94dB vs -96dB. I could see it in the FFT, but certainly couldn't hear the difference with my headphones. For the record, I tried 4R, 10R and 20R. The 10R provided the lowest noise floor (at least for my environment), but only by a very small difference to the others. Overall, there is no appreciable difference between diodes+resistor and a thermistor to isolate the ground.
I'm not sure what to make of the FFT data in the 10-40Hz range, though - it just seems to flop around like the end of a whip. I don't know if my AC mains ground is just more "dirty" than others, or there is some house-borne contamination, or this is just part of what happens with this amp. I measured one of my solid state amps the same way and I'm seeing about -90dB in the 10-40Hz range as opposed to -55dB to -65dB with the tube amp.
I spent some time experimenting this morning with different grounding schemes (thermistors vs. diodes, resistors, caps) and none of the variations made much difference. I did note that when using diodes, the value of the resistor made a small difference of 2-3dB here or there, but we talking about something like -94dB vs -96dB. I could see it in the FFT, but certainly couldn't hear the difference with my headphones. For the record, I tried 4R, 10R and 20R. The 10R provided the lowest noise floor (at least for my environment), but only by a very small difference to the others. Overall, there is no appreciable difference between diodes+resistor and a thermistor to isolate the ground.
I'm not sure what to make of the FFT data in the 10-40Hz range, though - it just seems to flop around like the end of a whip. I don't know if my AC mains ground is just more "dirty" than others, or there is some house-borne contamination, or this is just part of what happens with this amp. I measured one of my solid state amps the same way and I'm seeing about -90dB in the 10-40Hz range as opposed to -55dB to -65dB with the tube amp.
I haven't seen that kind of response myself, what happens if you run a frequency response sweep instead of the FFT?
EricS
Member
I wonder if this is an artifact of new computer hardware I'm using. I have an old computer with FFT software and a sound card that I'll try for a comparison. I should also be able to do a frequency sweep with REW.
This 10Hz-40Hz behavior is similar on both amps. For each of these measurements, the input RCA is shorted and the output is connected to an 8R resistor, then connected to my FocusRite analog to digital box. All metal parts in the amp are grounded to the chassis except for the caps.
This 10Hz-40Hz behavior is similar on both amps. For each of these measurements, the input RCA is shorted and the output is connected to an 8R resistor, then connected to my FocusRite analog to digital box. All metal parts in the amp are grounded to the chassis except for the caps.
EricS
Member
Here is a 5Hz to 20kHz frequency sweep of the 300B amp that I made with REW. Using the 1kHz dB level as a reference (about 61dB), it looks like the -3dB points (58dB) are about 21Hz and about 16.5 kHz. This is rather what I expected it would look like...
An FFT made with my old computer set up looked pretty much like the ones I've already posted: high noise floor in the 10Hz-40Hz region.
An FFT made with my old computer set up looked pretty much like the ones I've already posted: high noise floor in the 10Hz-40Hz region.
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EricS
Member
Now I'm thoroughly confused ???
I made an FFT with my old computer that looked similar to my previous graphs - output peaks in the 10-40Hz region.
So, using the integrated sound on my new computer (just built it last week), I'm getting what looks like much better FFT performance. Maybe the FocusRite (ebay purchase) that I was using previously was abused by it's first owner... Or the built in sound in my new computer is just not as sensitive... I'm really not sure what is going on here. I think I need to make a few more comparisons.
Anyhow, here are three new FFT plots using the built in audio of my new computer:
1) Noise floor of internal sound card, no audio connections
2) Noise floor of 300B amp, power on, input shorted
3) Noise profile of 300B amp, 1kHz test tone
At first I thought the odd harmonics in the third FFT graph are characteristic of the pentode driver tube, but after exploring other amps, it looks like this is an artifact of the built-in sound on my computer.
Do these graphs look better?
I made an FFT with my old computer that looked similar to my previous graphs - output peaks in the 10-40Hz region.
So, using the integrated sound on my new computer (just built it last week), I'm getting what looks like much better FFT performance. Maybe the FocusRite (ebay purchase) that I was using previously was abused by it's first owner... Or the built in sound in my new computer is just not as sensitive... I'm really not sure what is going on here. I think I need to make a few more comparisons.
Anyhow, here are three new FFT plots using the built in audio of my new computer:
1) Noise floor of internal sound card, no audio connections
2) Noise floor of 300B amp, power on, input shorted
3) Noise profile of 300B amp, 1kHz test tone
At first I thought the odd harmonics in the third FFT graph are characteristic of the pentode driver tube, but after exploring other amps, it looks like this is an artifact of the built-in sound on my computer.
Do these graphs look better?
Attachments
Paul Joppa
Moderator
The low frequency behavior looks very believable - the gentle rise (about 10dB/decade below ~400Hz) indicates i/f noise, which most electronic devices have.
