How much power for a SE 807 amplifier?

[fdegrove]

Hi Joe,

Further to my e-mail to you:

It is recommended to use a 100R resistor as a plate stopper, this can be soldered in directly on the topcap.

A series R of 100R isn't necessary for pin #2, topcap (plate) and #2 can be strapped together.

Recommended plate voltage for the 807 SE Class A1 is 250V, Rg should be no higher than 500K (yours is 680K it seems) and when cathode bias is used a 500R resistor with appropriate capacitor bypass should be used.

Primary load impedance at 6K THD would than be 4% at max power of 1.1W.

If you can't download the sizeable file Brett posted, let me know and I'll e-mail you a much smaller one.

In fact all of the above can be found verbatim in the datasheets.

If this was connected differently to the B+ supply would this provide around 8watts?

Provided the cathode resistor of the 807 is adjusted so you have the tube idling at around 65mA plate current and screen current at around 3mA, yes that would yield about 8W with THD at around 12%.

Cheers,

[Positron]

I worked with 807s for several years, back in the 60s, and 304 volts on the plate/screen is no problem, as the design itself testifies.

However, with that said, the screen dissapation is the critical factor, so overdriving the tube could, it could put the tube in peril.

In general, triode mode is easier on the average screen dissapation than pentode mode as the screen voltage never goes higher than the plate, thus the peak screen current is lower.
In pentode mode, the screen voltage can be higher than the plate, depending on how much the plate swings (at low signal levels the plate voltage always remains higher than the screen).

[Original Burnedfingers]

So how do I connect it in Pentode mode?




J

[Sch3mat1c]

Run the screen to B+ instead of the plate. I don't know the pins, but someone else mentioned them already.
B+ will be found on the not-connected-to-plate side of the OPT primary (usually a red wire).
Since the voltages are the same, you won't have to rebias or anything.

Tim

[SHiFTY]

You need to connect the screen to its own stabilised supply. In the schematic below, the author uses Zener diodes to keep a constant voltage, however, you would need to calculate what voltage you need (your B+ is much lower.)

You could just try using a 5-10K or so resistor and a 10uF cap without the zeners, that would work fine...

Also your distortion would be higher, you might want to use some feedback.

All these things are very easy to do, you might as well experiment!

[fdegrove]

Hi,

(your B+ is much lower.)

Don't you mean much higher?

Also your distortion would be higher, you might want to use some feedback.

And triode mode is just that: local feedback.

Cheers,

[SHiFTY]

No, his B+ is only 315V?

Actually, come to think of it, your 807 is only dissipating 13.4 Watts (280V * 48mA)

You could decrease the value of the cathode resistor, and increase the current, giving more watts. 807s are rated at 25W maximum, so running it at 20-22 W dissipation would give you more power...

Just another idea

[fdegrove]

Hi,

No, his B+ is only 315V?

With respect to plate voltage I mean.

Cheers,

[Brett]

Originally posted by ShiFtY
No, his B+ is only 315V?

Actually, come to think of it, your 807 is only dissipating 13.4 Watts (280V * 48mA)

You could decrease the value of the cathode resistor, and increase the current, giving more watts. 807s are rated at 25W maximum, so running it at 20-22 W dissipation would give you more power...

Just another idea

Running it through Tubecad (trioded)

B+ = 315
Rl = 3k
Ia = 70mA
Pout = 1.87Wrms
Plate diss = 20.6W
Vg = -19.1V
2H = 5.2%
3H = 0.4%

807's sound really nice at these sort of op points, not like most 6L6's used at traditional 'max power output' op points. Not very efficient though.

[Sch3mat1c]

ShIfTy wrote:
You need to connect the screen to its own stabilised supply. In the schematic below, the author uses Zener diodes to keep a constant voltage, however, you would need to calculate what voltage you need (your B+ is much lower.)

You could just try using a 5-10K or so resistor and a 10uF cap without the zeners, that would work fine...

I don't recommend using resistors: you loose regulation, and this means the screen voltage will be pulled way south at maximum output. Besides that, any lower screen voltage than as before will require a rebias, and maybe even a different load impedance for maximum power output as well (not like that will make much difference anyway though).

Tim

P.S... Huh... didn't notice a new page of replies on this thread... workin too fast, boys