Yes good advice indeed.
Ill have to look around and see what people have done in their builds.
I have not come across any such schematics, but its easy to implemt.
Would just like to see how people are doing it and any wiring considerations that are required.
It means I can make use of my OD3 VR tubes to regulate the grids to 300V, but I think I can also get away with the grid at 400V by the looks of it.
807 original screen grid rating was 300V (Beam Power mode, some call it Beam Tetrode, that is the same thing)
807 data sheets later added a rating for Triode Wired Mode: 400V screen.
I would estimate that for UL, a similar conservative rating for the screen would be about 350V.
Many operating amplifiers do exceed these ratings by a lot, and they work.
Not all 807 tubes are the same quality.
Your Mileage May Vary.
807 data sheets later added a rating for Triode Wired Mode: 400V screen.
I would estimate that for UL, a similar conservative rating for the screen would be about 350V.
Many operating amplifiers do exceed these ratings by a lot, and they work.
Not all 807 tubes are the same quality.
Your Mileage May Vary.
As far as UL mode goes, the voltage should not matter, the williamson amplifier in the schematic i posted earlier specified 440V on the anode.
My guitar amp has the screens running at 400V with no issues, but the anode voltage is 400V, im not sure how it would go with 400V on the screens and 440v on the anode, AFAIK, it still should be OK.
I dont know what the benefits are by runnning the grids at a higher voltage, I think it brings more linearity to the output with higher screen voltage, so dont know if its worth bothering at 300V in tetrode mode if its not going to run that optimal.
6A3summer is talking about screen voltage in UL, which is not any issue in UL.
See here for this williamson amp project that has a UL option.
http://www.tubebooks.org/file_downloads/stancor_ul_schematics.pdf
(see first paragraph on page 5)
I've now tested some 7581A's in my circuit which is pretty similar to the dynaco modified mullard. It works fine from what I can tell, with just shy of 430V on the plates and screens from 43% UL taps on a 6.6K output transformer. The sound doesn't seem to differ from the EL34/KT77/6CA&'s I've already been using. (sounds great)
The main tweaks that my amp has are in the feedback to suit my particular output transformer, a CCS in the tail of the LTP and altered bias arrangement to slightly stiffen it up whilst reducing the idle dissipation.
I think if you follow the voltage and dissipation limits of the 807, it should be fine.
The main tweaks that my amp has are in the feedback to suit my particular output transformer, a CCS in the tail of the LTP and altered bias arrangement to slightly stiffen it up whilst reducing the idle dissipation.
I think if you follow the voltage and dissipation limits of the 807, it should be fine.
Certainly, but I am refereeing to what you posted, which was about anode voltage.
I bought up anode voltage in response to him because I thought it was relavant on the limits of the screen voltage with the 807, at the time when I last was working with 807's, I was told the screens handle 400V fine if the anode voltage was close to 400V, so my concern was whether or not I could use 400V on the screen with a higher anode voltage say around 440V.
TBH, I dont even know if there is much benefit going with a higher screen voltage in this instance anyway.
Some say they like 300V quite fine, so not sure the exact reason that 807's were not popular with the 300V limit.
The higher you run the screen voltage the higher is the internal gain of the valve, for reasons I have already stated.
This is why TV tubes with low voltage but high gain screens can be screen driven.
It's totally pointless to run the 807 at 400V as a strapped triode because it defeats the whole object of the top cap, you do better to fit a 6L6GC immediately and it's a better valve!
It's a sad fact the 807 only really starts to sound any good when close to melting...meaning running the anode at 20-22W at idle with screen around 320 fully stabilised....
Which is of course what they were designed for - for CW transmitters.
Add to that the production spread of 807 was ridiculously bad. Ask me how I know!
Out of 25 brand new ones I bought (STC MIL version) 2 ARCED OVER 4 were low emission, 1 v low and I could just about make 2 matched quads.
If you can do better good luck!
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I think you still need to learn just the basics before conceiving a custom transformer shop. When paralleling two 8 Ω secondaries of the same transformer, the transformed output impedance still is 8 Ω, and it is 32 Ω if connected in series.
Best regards!
nzoomed, for UL info on the KT-66, which isn't irrelevant for the 807, check the articles from the 1955 issues of Radiographics. Here's the link for the first article of three:
https://frank.pocnet.net/other/AWV_Radiotronics/Radiotronics_1955/1955_05_AWV_Radiotronics_20_05.pdf
https://frank.pocnet.net/other/AWV_Radiotronics/Radiotronics_1955/1955_05_AWV_Radiotronics_20_05.pdf
Screen voltage sets the maximum cathode current.
If you do not need maximum current, low screen voltage gives maximum voltage gain.
Thanks for doing this, thats good to know.
now i have to toss up whether I go with the dynaco design or use this other one here that was designed for 6L6.
The EF86+6SN7+6L6 push-pull circuit - Amplifier_Circuit - Circuit Diagram - SeekIC.com
OK, so it gives more gain, im assuming that naturally equates to higher power output?
I really was only considering adding the option of tetrode mode to experiment with and learn from, so Im not concerned if it doesnt work as intended. Kinda gives me 2 amps in one. Im not going to bother with triode mode.
Im going by what an old amp builder told me who used to build guitar amps for a living, his theory sounded correct to me, as i thought he was just treating it like you would with resistors in parallel or series.
Come to think of it, I think he actually talked about 2 ohm secondarys, that would make more sense then?
Thanks, will take a look.
If you want max (plate+screen) current, then you want high screen voltage? That would mean highest gm, so that makes sense.
For sweep tubes (in pentode), those are usually run with only 150V or 200V on the screens, but much higher voltage on the plates (450V, 600V, etc.). Does that mean they're set up for max voltage gain? But... Lowering the screen voltage reduces gm for both plate and screen, so how does that give you *more* voltage gain rather than *less*?
The plate load chosen (primary impedance of OPT) would also influence Ip and Ig2 as well as voltage gain.
Obviously, I'm confused.
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2+2= 4 doesnt it? How is that any different than the 4 ohm centre tap on an 8ohm secondary? How do you get 8 ohms from 2x2 ohm?
nzoomed,
The impedance of a transformer coil is proportionate according to: (the number of Turns) squared
A 25 turn 2 Ohm winding in series with a second 25 turn 2 Ohm winding is 50 turns.
25 squared = 625
50 squared = 2500
2500/625 = 4:1
2 Ohms x 4 = 8 Ohms
Of course, whether you series windings, or parallel windings, be sure to pay attention to phase.
2 identical windings in series that are connected out of phase, does not have any output.
2 identical windings in parallel is a Dead Short, if they are connected out of phase.
Example, there are some smoked 6.3V parallel windings that were connected out of phase.
The impedance of a transformer coil is proportionate according to: (the number of Turns) squared
A 25 turn 2 Ohm winding in series with a second 25 turn 2 Ohm winding is 50 turns.
25 squared = 625
50 squared = 2500
2500/625 = 4:1
2 Ohms x 4 = 8 Ohms
Of course, whether you series windings, or parallel windings, be sure to pay attention to phase.
2 identical windings in series that are connected out of phase, does not have any output.
2 identical windings in parallel is a Dead Short, if they are connected out of phase.
Example, there are some smoked 6.3V parallel windings that were connected out of phase.
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OK, I understand now.
That would make sense. I dont know what this guy was smoking, but I just trusted he was correct, since he was a manufacturer of guitar amps.
His theory was you needed to make full use of the secondary winding to make it work optimally. He said if you use the 4 ohm tap for instance, you are only using half the secondary, then rambled on about his method of wiring in series or parallel on the OT to match the speaker impedance.
Whenever possible, use all the secondary windings.
I have some output transformers that have 4 each 1 Ohm windings.
Using the proper connections, and all the secondaries, I can get the following output impedances:
1 Ohm
4 Ohm
9 Ohm
16 Ohm
Connecting it the right way is a little tricky, but all 4 windings are used for all of the above impedances.
I have some output transformers that have 4 each 1 Ohm windings.
Using the proper connections, and all the secondaries, I can get the following output impedances:
1 Ohm
4 Ohm
9 Ohm
16 Ohm
Connecting it the right way is a little tricky, but all 4 windings are used for all of the above impedances.
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