Hi
As mentioned here before I´m collecting parts for a 4x 807 PP bass guitar amp. I´ve been advised to try UL without NFB.
My question is: Can I run 807´s UL from a 470-500V PSU without melting the screen grids?
I´ve seen schematics of triode wired 6L6´s running from similar voltages, but I want to be sure.
Cheers
Daniel
As mentioned here before I´m collecting parts for a 4x 807 PP bass guitar amp. I´ve been advised to try UL without NFB.
My question is: Can I run 807´s UL from a 470-500V PSU without melting the screen grids?
I´ve seen schematics of triode wired 6L6´s running from similar voltages, but I want to be sure.
Cheers
Daniel
Check the 807 datasheet. The maximum voltage You can use, is the maximum voltage allowed for the screen.
sajti
sajti
According to the datasheets it´s 300V.
Way too low to get any power out of it.
Seems like I´m stuck to pentode connection then.
Thanks for the reply
Way too low to get any power out of it.
Seems like I´m stuck to pentode connection then.
Thanks for the reply
Oh, heck, 807s are cheap. Give it a try anyway. Vg2(max) is something commonly exceeded with UL and triode operation... to the extent that the 807 is really a rebased 6L6, it might even work.
They are cheap, but they are not that easy to find in quads in this corner of the world though.
I´m sure they would work fine at 470-500V, but the question is for how long?
Tnx
Daniel
I´m sure they would work fine at 470-500V, but the question is for how long?
Tnx
Daniel
807 are not 6L6 refitted, they have an improved g1 and g2 to handle the RF, and a better g3 with a different disposition.
Maybe you can put them in UL mode with a small resistor in series of the screens, just to limit max dissipation, because it's that the problem, I don't think that an 807 will arc internally with such a voltage. 470ohm or 1k will work.
Good Luck!
Maybe you can put them in UL mode with a small resistor in series of the screens, just to limit max dissipation, because it's that the problem, I don't think that an 807 will arc internally with such a voltage. 470ohm or 1k will work.
Good Luck!
I would limit that 3.5-watt screen to 400 volts as I have viewed triode connected at 400 volts in data sheets.
I'd keep it to the 400V triode spec too unless you want to (potentially) sacrifice some in testing.
There goes my idea of a nice zero NFB UL output stage
Well, at least I will get some serious power out of it if I runt them as true pentodes (or is it tetrodes?)
Here is a novel idea
Because you said it was for a guitar amp I'd not recommend using the screens through limiting resistors at this elevated voltage. Guitar amps can put output tubes outside of the SOA even with proper voltages applied in the hands of a talented musician. 😉
I have been meaning to try an idea I came up with of using a gas VR tube in between the screen tap on the UL output xfmer and G2 to allow the use of horizontal sweep TV tubes that usually have a low G2 limit of 250 volts or even less yet have big plate dissipation and high plate voltage ratings in ultralinear audio output applications. Zener diodes might also be tried. I had an opportunity to discuss this idea with Morgan Jones during a recent visit to my place and he thought it worth trying. In your case maybe try a pair of VR-150's (octal) or 0A2 (7 pin miniature). Anode to UL tap, cathode to G2. You might try a very small NP capacitor like a 0.1 uf across the VR tube to stiffen it against audio modulation. Too big a cap will turn the VR tube into a relaxation sawtooth oscillator. Hey, for a guitar amp, that might be a neat effect. 😉 You will also have to observe that dynamically the voltage swing with signal on the UL tap doesn't drop so low as to throw G2 below the level of valve cutoff or to the point where the VR tube extinguishes. This would make rather rude noises in the speaker.
Because you said it was for a guitar amp I'd not recommend using the screens through limiting resistors at this elevated voltage. Guitar amps can put output tubes outside of the SOA even with proper voltages applied in the hands of a talented musician. 😉
I have been meaning to try an idea I came up with of using a gas VR tube in between the screen tap on the UL output xfmer and G2 to allow the use of horizontal sweep TV tubes that usually have a low G2 limit of 250 volts or even less yet have big plate dissipation and high plate voltage ratings in ultralinear audio output applications. Zener diodes might also be tried. I had an opportunity to discuss this idea with Morgan Jones during a recent visit to my place and he thought it worth trying. In your case maybe try a pair of VR-150's (octal) or 0A2 (7 pin miniature). Anode to UL tap, cathode to G2. You might try a very small NP capacitor like a 0.1 uf across the VR tube to stiffen it against audio modulation. Too big a cap will turn the VR tube into a relaxation sawtooth oscillator. Hey, for a guitar amp, that might be a neat effect. 😉 You will also have to observe that dynamically the voltage swing with signal on the UL tap doesn't drop so low as to throw G2 below the level of valve cutoff or to the point where the VR tube extinguishes. This would make rather rude noises in the speaker.
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