ralph power pp amp with 807 ?

[Jaap]

http://www.meta-gizmo.com/Tri/interstage.html

What do I have to change in this design to be able to use my 807 tubes who are patiently waiting ?

thanx in advance

jh

 

[Miles Prower]

Depends on how you want to run the 807s: pentode, ultra-linear, or triode. You will almost certainly need to change the output xfmr, and possibly that interstage xfmr.

The PS will likely need to be modified for different DC out, plus V(sgsg) if you use pentode mode.

 

[Jaap]

I am thinking triode

 

[Miles Prower]

Here ya go: 807 App Notes. This is quite the comprehensive information on that. Includes both RF and AF operating modes, and plenty of data for pentode/triode operation at various DC voltages, Q-Point bias, and data on estimated THD for each.

 

[amperex]

Page wont open to review schematic.................

 

[Brett]

Page wont open to review schematic.................

No trouble here, but here's the drawing anyway.

 

[Brett]

And PSU......

Built this and similar. Good little circuit.

 

[PRR]

> www.meta-gizmo.com/Tri/interstage.html

How old-fashioned.

> What do I have to change in this design to be able to use my 807 tubes

807 is nothing but an ugly 6L6. You will have to separate the heater from cathode function: study any normal indirect-heat output stage. Triode-strap it with a 100 ohm resistor G2 to Plate. Adjust the cathode resistor (shown in the power supply section) to an appropriate value for 807/6L6 and the supply voltage you like. 300 ohms may be a good trial value for 275V supply.

> how you want to run the 807s: pentode, ultra-linear, or triode.

The intent seems to be to run Triode. Or at least I would try that first.

> You will almost certainly need to change the output xfmr, and possibly that interstage xfmr.

There are not a lot of interstage transformers on today's market, the Lundahl will be fine. It may be "too good" in that the 807/6L6 can be driven with less voltage swing than 2A3/6B4, but there isn't a smaller model. The load is the same: two naked grids. Overall gain will be a couple dB higher: no big deal. Input overload voltage will be a little less, but I assume there is a Volume control somewhere up-river.

The output transformer is only specified as "Sansui 1000A". I'm guessing that it is the usual 5K or 8K winding, which is appropriate for self-biased 807/6L6 also. Triodes are not that fussy about load.

2A3/6B4 will make a little less power than 807/6L6 working with the same supply and load impedance, due to higher plate resistance. This may be several dB different, not a lot.

 

[Jaap]

807

PRR, thanks for your input.

I am thinking about the 807 because i have some lying in the cellar. I have the impression that this tube has a good reputation.

I suspect that you have some ideas of bettering the circuit in a more modern fashion or that you have suggestions for a better output tube. I have the Lundahl LL1660 in mind as interstage transformer.

Feel free to share your ideas with a not so experienced tubenut.

regards,

JH

 

[PRR]

> I suspect that you have some ideas of bettering the circuit in a more modern fashion

Oh, No!

"Old fashioned" is not bad. Many of the Old Ways are still the best. I think it is a fine amp.

My only "change" would be to suggest using the 2A3/6B4 instead of the 807/6L6. But if you have a cellar full of 807, they will certainly work well.

If you are power-hungry, you can raise B+ on 807 toward 400V. Use the suggested conditions in the 807 app-note that Miles showed us.

 

[Tubes4e4]

Hi PRR,

807 is nothing but an ugly 6L6.

Ugly?

Look here

Tom

 

[Miles Prower]

807 is nothing but an ugly 6L6.

No it isn't. The 6L6 is a whimpier version, the 807 being an RF type, having greater P(D) and P(SG) ratings than the 6L6 (25W/19W; 3.5W/2.5W) and a higher voltage rating (750/360). Even though it's an RF type, the 807 has better THD specs than the 6L6. If you don't push them real hard, 807's can give THD figures below 1.0%, whereas the 6L6 doesn't even come close, never going below 2.0%.

This is probably due to the 6L6's being more of a "consumer electronics" device operated at lower voltages, higher currents, and lower impedances (cheaper iron all the way around) than is the case where 807's are used. Even though both types can do 26.5W in class AB(1), I'll take the extra 6.0W margin and lower THD.