Hi Dave;
I just finished measuring everything after a lengthy temperature test - so far my new heat sink/bottom plate is passing the test. The valves used are a different pair of Ei's than the ones used initially. One of the first pair of Ei's developed a problem apparently related to shoddy construction and a loose internal element after the first performance stats were documented.
Okay, here goes:
Current
1. Total current drawn from PS (sum of all secondary currents) = 83.33mA
2. Ip for triode = .503mA per triode section
3. Ip for pentode = 34.01mA each
4. Ig2 = 6.79mA
Voltage
1. Raw B+ = 245.3VDC (measured after the pi filter); HT before the pi filter measured as 371VDC with a large AC ripple component present.
2. Vp for pentode (relative to cathode) = 210.7V
3. Vg2 = 197.5V (relative to cathode)
4. Vp for triode (relative to cathode) = 81.7V
5. pentode's cathode floated 15.9V off ground
6. triode's cathode floated 1.2V off ground
Dissipation
1. Plate dissipation is now 7.16watts at idle. This is a trifle hot, since RCA spec is 7 watts, as is the Svetlana spec. If it proves desirable to lower it, the easiest way IMHO would be to drop the B+ to the pentodes by about 5V-10V. Even at the highest power dissipation operating point experimented with (7.8 watts on the plates) no evidence of red glow or other misbehaviour was noted in the plates, nor was there an early onset of distortion.
All the best,
Morse
I just finished measuring everything after a lengthy temperature test - so far my new heat sink/bottom plate is passing the test. The valves used are a different pair of Ei's than the ones used initially. One of the first pair of Ei's developed a problem apparently related to shoddy construction and a loose internal element after the first performance stats were documented.
Okay, here goes:
Current
1. Total current drawn from PS (sum of all secondary currents) = 83.33mA
2. Ip for triode = .503mA per triode section
3. Ip for pentode = 34.01mA each
4. Ig2 = 6.79mA
Voltage
1. Raw B+ = 245.3VDC (measured after the pi filter); HT before the pi filter measured as 371VDC with a large AC ripple component present.
2. Vp for pentode (relative to cathode) = 210.7V
3. Vg2 = 197.5V (relative to cathode)
4. Vp for triode (relative to cathode) = 81.7V
5. pentode's cathode floated 15.9V off ground
6. triode's cathode floated 1.2V off ground
Dissipation
1. Plate dissipation is now 7.16watts at idle. This is a trifle hot, since RCA spec is 7 watts, as is the Svetlana spec. If it proves desirable to lower it, the easiest way IMHO would be to drop the B+ to the pentodes by about 5V-10V. Even at the highest power dissipation operating point experimented with (7.8 watts on the plates) no evidence of red glow or other misbehaviour was noted in the plates, nor was there an early onset of distortion.
All the best,
Morse
Hi Rick;
>>>...a 4k7 R resistor on the pentode section of the tube. If that was not there, would you have the full B+ on the screen? Would it draw about 6.8 ma without the resistor due to internal resistance?...<<<
You're right - the 4k7R resistor is there to set the voltage on the screen grid. If it weren't there, the full B+ would be applied to the screen, which would increase power at the expense of increasing the screen dissipation. I'm not sure what the draw across the screen would be without it, but I think it would exceed the ratings for the valve.
As it is, the screen grid dissipation is within the manufacturer's ratings, and the overall plate dissipation is very slightly over (about 7.1 watts total plate dissipation and a manufacturer's spec of 7 watts IIRC). If that bothers you, you could lower the Vp slightly.
After a lot more listening, I'm impressed with how musical the circuit is at moderate listening levels - the biggest problems are the lack of really deep bass response due to the OPT's (not an issue given the original intent of the Gnat) and the amount of heat present both in the 1k5R PS dropper and the 270DX. I'm still considering choke loading the PS on mine and lowering the value of the dropper (can't recall off the top of my head what I worked out to be the right value). Biggest reason I've not done so is $$, which is ueber tight for me right now.
What speakers are you going to be driving, anyway? I've used EPI 100's, RS 40-1354's (w/ a 40-1270D helper tweet in a 12 litre cab), cheapie 5-1/4" 2 ways by JVC, and Infinity IL10's. The Infinity's (as expected) were the pits due to the overly complex crossover (my 2A3 sounds as lifeless as one of the zombies out of a George Romero flick with them), but the others were all fine (though not acceptable for high SPL's - it takes some real efficiency for that with a 3 watt SET....).
Good luck and all the best,
Morse
>>>...a 4k7 R resistor on the pentode section of the tube. If that was not there, would you have the full B+ on the screen? Would it draw about 6.8 ma without the resistor due to internal resistance?...<<<
You're right - the 4k7R resistor is there to set the voltage on the screen grid. If it weren't there, the full B+ would be applied to the screen, which would increase power at the expense of increasing the screen dissipation. I'm not sure what the draw across the screen would be without it, but I think it would exceed the ratings for the valve.
As it is, the screen grid dissipation is within the manufacturer's ratings, and the overall plate dissipation is very slightly over (about 7.1 watts total plate dissipation and a manufacturer's spec of 7 watts IIRC). If that bothers you, you could lower the Vp slightly.
After a lot more listening, I'm impressed with how musical the circuit is at moderate listening levels - the biggest problems are the lack of really deep bass response due to the OPT's (not an issue given the original intent of the Gnat) and the amount of heat present both in the 1k5R PS dropper and the 270DX. I'm still considering choke loading the PS on mine and lowering the value of the dropper (can't recall off the top of my head what I worked out to be the right value). Biggest reason I've not done so is $$, which is ueber tight for me right now.
What speakers are you going to be driving, anyway? I've used EPI 100's, RS 40-1354's (w/ a 40-1270D helper tweet in a 12 litre cab), cheapie 5-1/4" 2 ways by JVC, and Infinity IL10's. The Infinity's (as expected) were the pits due to the overly complex crossover (my 2A3 sounds as lifeless as one of the zombies out of a George Romero flick with them), but the others were all fine (though not acceptable for high SPL's - it takes some real efficiency for that with a 3 watt SET....).
Good luck and all the best,
Morse
thanks Morse,
I currently have Altec Lansing 605 a, duplex, concentric 15" woofer with horn mid/tweeter in a 9 cuft box sealed. They are efficient enough (103 db at 4 feet). Also have some JBL pro two way at 92 db that I like. Looking to get some old Klipsch, though.
I will probably go with a choke since you have had some heat dissipation challenges.
Thank you for your comments,
Rick
I currently have Altec Lansing 605 a, duplex, concentric 15" woofer with horn mid/tweeter in a 9 cuft box sealed. They are efficient enough (103 db at 4 feet). Also have some JBL pro two way at 92 db that I like. Looking to get some old Klipsch, though.
I will probably go with a choke since you have had some heat dissipation challenges.
Thank you for your comments,
Rick
Banned
Joined 2002
Banned
Joined 2002
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Joined 2002
Dave's house is (was, appearently) like one of those antique/curio shops...wherever you look there is something of interest, especially downstairs
. ..and the location is spectacular. I'd give up a lot just to be able to drive that road whenever I felt like.
(I'm coming out that way this spring...I'll bring some horns with me)
. ..and the location is spectacular. I'd give up a lot just to be able to drive that road whenever I felt like. (I'm coming out that way this spring...I'll bring some horns with me)
Illusus said:Dave's house is (was, appearently) like one of those antique/curio shops...wherever you look there is something of interest, especially downstairs
(I'm coming out that way this spring...I'll bring some horns with me)
Interesting setting. Hasn't been a place like that in London for a few years now.
Banned
Joined 2002
605 A Duplex Altec
See this http://voiceofthetheatre.com/images/601C.602C.605A.2.jpg for the specs.
50 watts power handling on peaks and 35 watts continuous. They are very efficient -- 103 db at 4 feet. With just a small amount of power they can really rock! I would put them in some big enclosures (ported) and they will go down to high 30's hertz or use a sub.
Good luck!
Rick
See this http://voiceofthetheatre.com/images/601C.602C.605A.2.jpg for the specs.
50 watts power handling on peaks and 35 watts continuous. They are very efficient -- 103 db at 4 feet. With just a small amount of power they can really rock! I would put them in some big enclosures (ported) and they will go down to high 30's hertz or use a sub.
Good luck!
Rick
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