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How high does everyone else run the screens on a 6L6G/A/B (not 6L6GC) in pentode mode? Do you limit it to +270 like the data sheets show? Also, do you regulate it, or just use a series dropping resistor from B+? What about the 6P3S-E? Is it reasonable to operate it at +400 to to +450 on both the screen and anode, or should the screen be dropped down as in the 6L6? I'm mostly thinking about longevity here, but if anyone has made distortion measurements at various screen potentials, that would also be helpful.
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Buy Chinese. Bury freedom.
Follow Ups:
Guess I'll regulate the 6L6 screens to their recommended voltage like Eli suggested. The amp previously used 7355s, so the power supply wasn't designed for significantly reduced screen voltage. Wish I knew more about the 6P3S-E. It seems to be more robust, but I don't have any definitive information regarding the real screen voltage limit.
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Buy Chinese. Bury freedom.
"The amp previously used 7355s, so the power supply wasn't designed for significantly reduced screen voltage. Wish I knew more about the 6P3S-E."Given the sort of O/P "iron" used with the 7355, adding a filament trafo and switching to EL34 category "finals" might be the way to go. Hey New Sensor, the 7355 is yet another type to bring back from the dead.
I believe the 6П3C-E (6p3s-e) is a solid 5881 equivalent, which is not quite up to 6L6GC capability.
Eli D.
Edits: 05/02/17
Eli, I agonized over that decision a long time. The EL34 is technically a better match for the OPT Z. However, I think the "benefit" is only more output power than the OPTs can handle (original rating 20W/ch continuous RMS).
I did build up a channel using 6L6GBs, and the result was very good. Output power is a tad less than with 7355s, but distortion is better. THD at ~4W is 0.05%, IMD is 0.18%. The 7355s measured 0.12% and 0.59% for THD and IMD respectively. At 20W, the manufacturer's THD spec is <1%. I measured 0.54% with 7355s, and 0.4% with the 6L6GBs. The latter is a minor difference, but the improvement at lower power levels is meaningful. So, all things considered, I think 6L6s will be a good choice for this amplifier. I just want to be sure my NOS tubes last a good, long time. :)
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Buy Chinese. Bury freedom.
Years ago, ran the 6s3p-e ultralinear and in triode mode in modified ST-70s, best tube we could find for these amps bar none. By ear, we liked them at 21 watts dissipation, and they have held up well over the years (more than 10), and they are still happy.
The Russians underrated their tubes considerably. Have seen posts where they had ~ 600V on the plates and were still not red plating (no info on the screen voltage). As far as the screen voltage limits, I have run these puppies in triode mode at 440V, and have held up very well over time. Similar voltage in ultralinear, so the screens were pretty close to that voltage. I don't judge things as much as you do by the numbers, but I can say these are really tough tubes, and sound great. Worth a try.
twystd
Yes, I really like the 6P3S-E, and I have a large stash. However, in initial tests some time back, my prototype amplifier with ~400V on the anodes performed better with a 6L6GB. Output power was about 20% greater, and distortion was much lower, about 1/3 as much at output levels below 5W. This might be due to the anode voltage of the amplifier (most users operate the 6P3S-E well above 400V), or maybe the low-Z output transformers (~3.8K P-P). Whatever the reason, the 6L6 types were clearly superior in this application. This is what led to my question regarding screen bias. At the time of the tests, the screens were operating through a dropping resistor at roughly 360V. There wasn't any damage to the tubes that I could detect, but I don't think it's a good idea to operate this far above the rating. I have uses for the 6P3S-Es later, but for now, I just wanted some idea of how others are using the 6L6.
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Buy Chinese. Bury freedom.
Edits: 05/03/17
Sounded the best biased to 5881 parameters. I ran 460-470VDC on plates and changed the screen grid resistors from 470ohm to 1k-ohm, 5-watt. Set idle bias dissipation to 15 watts (66% of 23 watt max), in a silverface Fender Bandmaster amp.
If I ran at 66% of 30 watts, 6L6GC standard or 20 watts, the amp overdrove too early in volume range. And had a much more compressed tone.
These are good tubes, if set to 23 watt max limit.
I ran 1952 JAN Sylvania 6L6GA in a 1966 blackface Fender Twin Reverb. I dropped the voltage to the screen, from B+, using 1500-ohm wire-wound, 5-watt resistors. OEM resistors are 470-ohm.Plate voltage was OEM. 450-460VDC. I adjusted the idle bias dissipation to around 12-13 watts per tube (about 65% max dissipation).
I did mod around 2000. AFAIK, that amp is still running strong. Owner has two matched quads of the Sylvania JAN-CHS GA's. I'll email him, to see which set his is on, now.
He plays Jazz gigs, at local bar. He really likes the tone of the GA over the GC. Better midrange. Lost of output is not an issue in smallish gigs. He is running 1965 vintage, reconed alnico Jensen P12N speakers.
One of the best Twins. I ever heard.
8^)
Edits: 05/01/17
Distortion measurements are going to be the result of the g2 voltage and delivered load. Drive the load out under the knee and odd HD is going to take off.
On pentode signal amplifiers, I once built a linestage around a 6AV5GA. Idling at 50 mA, with a g2 voltage of 75V set by a 0A3. Load was 470R...:) It could drive *ANYTHING*. With a near vertical load line, its distortion spectrum was dominated by 2nd...though it was quite low.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Get the best possible open loop linearity out of full pentode mode, by employing regulated g2 B+. Once g2 B+ is regulated within published limits, you can take some liberties with the anode voltage limit, provided you scrupulously honor the plate dissipation limit.
Eli D.
Hi TK, i hope you don't mind if i jump in here as long as it jives with your query, if it doesn't, i will start a new post. I have been looking to try one for awhile but not able to get something that would be suitable for my needs. I am looking for a circuit that can do between 250 and 450v. If anyone cares to share that would be awesome. regards, Dak
Hang a properly rated MOSFET, with its gate adequately clamped from a voltage divider between B+ and ground. Use it as a source follower. Get a 12-position switch from Radio Shack so the resistance of the lower leg can be adjusted( while the amp is OFF ), so g2 voltage can be adjusted quickly.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
DAK, are you referring to a circuit that can be built into an amplifier, or do you mean a variable bench supply (250-450V)? There are lots of circuits around for the latter. Building a supply like that into an amplifier is awkward due to size constraints. Semiconductors aren't generally very reliable at those voltages, although I suppose it can be done. I've never tried, because I usually flesh out new designs on the bench, then use a fixed voltage in the gear.
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Buy Chinese. Bury freedom.
I'm going to disagree. I used MOSFET voltage regulators for B+ as I find it makes for better sound. I basically modded a Nelson Pass design (Nelson only does SS). it worked fine, I retrofitted the amps I had and put it in all subsequent builds. I go overkill when I engineer, so I use MOSFETS rated for more voltage than I think the device will see, and reliability has not been an issue.
Hi TK, i am considering an on amp design like the "maida regulator" or Kevin at K&K, sells one too. I am thinking about giving the K&K one a go on my amp to see if it is worthwhile to do it.
Are you interested in the solid state regulator because it's variable? If I remember correctly, the Maida site states 10%. That's a much smaller window than the 250V - 450V numbers you mentioned earlier. Actually, I'm not sure what the SS version buys you that a little experimentation with gas regulators wouldn't accomplish. Besides, they glow! :)
0D3 and 0A2 (150V): pink/lilac
0C3 and 0B2 (105V): pink
0B3 (90V): purple/indigo
0A3 and 0C2 (75V): orange
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Buy Chinese. Bury freedom.
Initially i was going to go with them and i bought various ones like you outlined, but besides the additional space required i read that tvr produce a fair amount of "noise" or distortion. I am not sure if this is true or maybe circuit dependant, but adding up the pros and cons i now feel that it maybe easier to go with the SS voltage reg. Just a few wires going in and out.
Mike Maida's seminal article is well worth reviewing.
http://www.ti.com/lit/an/snoa648/snoa648.pdf
If you want something with all the kinks worked out, follow the link and pay the man his money.
https://www.neurochrome.com/21st-century-maida-regulator-rev-2-0/
Eli D.
This is a nice regulator! I have used these in the past, well worth the cost!
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