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In Reply to: RE: 211 Build posted by deafbykhorns on November 07, 2014 at 16:42:05
Ok, after running a 211 thru SE CAD, it seems the most I can get out of that tube without too much distortion is 18watts.
Would everyone agree? Seems the grid starts going pos at just over 50 volts.
After about 75 volts, the distortion is probably unacceptable which is around 25 watts.
A 10K OPT seems to have much lower distortion but I was thinking about a compromise and try 7K.
Follow Ups:
To improve the efficiency, you must us a higher voltage and lower current, along with the appropriate higher impedance output transformer. This also improves the damping factor. That's probably why the Ongaku used a 16K transformer.
If you are constrained by available plate voltage, then the optimum load impedance is twice the plate resistance, hence around 7.5K ohms.
A 12AX7 is really not the choice I'd suggest to drive a 211.Have you read the B-52 amp by Grego Sanguinetti in Sound Practices, using a similar toplogy as you post above with a single 7788 and an interstage XFR ? He lays out the measured peak to peak driving voltage, with UBER low distortion, in the actual Sound Practices article. Read it !!
Jeff
Edits: 11/08/14
I haven't studied the 7788 data so the question is, can it drive a 211 into A2
I don't have any Sound Practices material,
Let me suggest looking at the 7788 data. It is a wee little tube, suitable for A1 operation perhaps. The transition into grid current takes a few volts, and sees the input impedance drop drastically. That sort of discontinuity in the load requires a stage capable of stand-alone amplifier service IMO.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
When my computer gets back from repairs, I'll try to send you the article or a copy thereof. Within a week.
Jeff
With a 10K Magnequest OPT, my 211s clip at approximately 30W output.
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Buy Chinese. Bury freedom.
Have you tried a 3c24 to drive a 211?
I forgot I had a couple of these laying around
It might be difficult to find a spice file for this tube
How's your hum AC heating the 211?
I've had some good luck with Rod Coleman's filament heaters.
Edits: 11/09/14
I played around with Loftin White driver using 12ax7
Here's another with IT
The green trace is at the grid of the 211
How do you model the fixed bias in ltspice when the models have only one pin on the filaments? Or is that wire trace virtually connect?
What is your max voltage at the grid of the 211 to get 30watts?
What were the benefits of grid basing both those tubes? To lower distortion in A2?
Edits: 11/08/14 11/09/14 11/09/14
"How do you model the fixed bias in ltspice when the models have only one pin on the filaments?"
The schematic I posted is only that - a schematic. It is not a SPICE file per se. I have a model of the 211 named "GL211.inc" that I've used for simulations. It's only good for approximate, small-signal interpretations of the real thing, but it does model the tube as an IDHT, which makes things easier. FWIW, I designed and built these amplifiers the old fashioned way, with manual measurements and a calculator. That was well before I was using SPICE for VT circuitry.
"What is your max voltage at the grid of the 211 to get 30watts?"
I didn't record that specific number. However, my notes show that at 25W, grid swing is roughly 180V p-p.
"What were the benefits of grid basing both those tubes? To lower distortion in A2?"
Grid bias at the 211 maximizes the available B+ and eliminates the need for a cathode bypass cap. The driver is direct coupled for A2 operation. An IT could be used instead, but at somewhat more expense and IMO no benefit.
"How's your hum AC heating the 211?"
The early designs with 60Hz filaments created about 80-100mV hum. There is no audible hum with the 65kHz ultrasonic supplies I use now.
"I've had some good luck with Rod Coleman's filament heaters."
I tried DC on these tubes, but didn't like the sound. I have no first-hand experience with Rod Coleman's product.
Regarding the title, bandwidth of this design for audio purposes is essentially determined by the pre-driver stages and OPT. I performed manual, point-by-point analysis during design. I don't have those numbers, but I was satisfied with the results. The amplifiers are currently apart for certain revisions. I now have the ability to do swept FR/THD/IM graphing, so a more comprehensive analysis will eventually be possible.
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Buy Chinese. Bury freedom.
This is a simplified schematic showing the pre-driver circuit. It's slightly different than my previous version. The input stage has been modified to include a tube-based CCS, and the totem pole is now a 6SN7. I previously used a 6BL7 in this position for the sake of expedience (it avoided the need to raise the heaters). Previous tests with the 6SN7 in this arrangement indicate less than 1% THD @ 200V p-p output, and I suspect that can be improved with a little work.
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Buy Chinese. Bury freedom.
Very creative using one of those Halogen lamp supplies!
Reminds of a Infinity Servo Static I once repaired using a bug zapper from Walmart.
Thank you for the ideas, I'm going to try the grid bias since I like the idea eliminating cathode caps.
TK is only showing you the last two stages. If I recall, its complex, a four stage amp with two cathode followers and two, maybe three cap couples. Oh well, to each his own.
Jeff Medwin
My amplifiers now have 23 coupling caps and 12 cathode followers. The entire ex-staff of Stereophile and Hi-Fi magazines and the ghost of Norman Crowhurst have all been to my house and proclaimed the sound to be superb.
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Buy Chinese. Bury freedom.
Did A D Blumlien's ghost decline the invite, or did you neglect to send?
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I'm saving him for the revisions. :/
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Buy Chinese. Bury freedom.
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