SRPP - determining load and balancing

[vitalstates]

I have been trying to be a bit more scientific than usual when nailing the load for an srpp driver circuit. In previous amps I have taken published circuits as gospel but having discovered a few descriptions of balancing methods I have found 2 equations which disagree. The balancing revolving around correct load(Rl).

1. Rk = 2Rl + Ra / (mu - 1)

or

Ra + 2Rl
2. Rk = -----------------
µ - 1

by the rules of bodmas one of these cannot be correct.

Can anybody shed some light on which is correct or whether both are wrong or another way of determining the output load value for srpp.

Ed

[vitalstates]

nobody? btw the graphic is equation 2, hand drawn as pure text entry screwed up.

Ed

[Svein_B]

If you want to be a bit scientific, and understand SRPP, I guess you may have already read what John Browskie has to say:
The Tube CAD Journal,SRPP Decoded


I had imagined that you would just select a nice operating point for the tube,
which would decide the current, Vgk and Rk the conventional way.
... but I understand there may be more to it than that.

Svein

[Koifarm]

I use this spreadsheet.

http://www.ict-net.net/tube.xls

Ronny

[Merlinb]

The hand-written formula you attached is correct for an SRPP with unbypassed cathode, and an additional resistor in the anode equal to 2Rk. However, it doesn't make a lot of difference if you leave out the extra resistor, except for really low-mu triodes.

Incidentally, both of the formulae you typed are correct! If you apply BODMAS to the first one you typed, you will end up with the second...

[vitalstates]

many thanks chaps

and many thanks merlinb for pointing out my bodmas error

the reason I ask is because I was looking at the old Angela 6SL7/2A3 circuit and that uses Rl of 470k......mmm?

using the equation above I get 6SL7 with 2k2 Rk to come out at Rl = 54k....well thats a long way from 470k so I was a bit intrigued....

any chance of verification of that result

Ed

[Merlinb]

Originally Posted by vitalstates

the reason I ask is because I was looking at the old Angela 6SL7/2A3 circuit and that uses Rl of 470k......mmm?

using the equation above I get 6SL7 with 2k2 Rk to come out at Rl = 54k....well thats a long way from 470k so I was a bit intrigued....

any chance of verification of that result

Ed

There was a fad for SRPPs for a while, which seems to have dies down a bit now. However, most designers didn't/don't realise that the SRPP is not a generic driver stage, or even that it has an optimum load. Fortunately, if you use a load resistance which is too large the distortion only increases a little bit. If you use a load which is too low then the distortion increases very rapidly. Obviously the simplicity of the SRPP appealed to many hobbyists, even if they didn't appreciate what it is for (a small power amp for fixed loads).

[regal]

I hate to see SRPP in line stages and DAC's, glad you guys posted this thread to shed a little light on why they never sound right in those applications.

[hesener]

Hey, I built a linestage with 6922s in SRPP, 4x parallel per channel and an output transformer (4:1), and I am listening to it right now. And it sounds pretty good !( but that might be because I built it myself ;-)))

[Brit01]

Have a doubt regarding the B+ and load line on a SRPP.

if I have 300V on the top tube and 150V on the plate of the bottom tube which B+ do I draw the load line off?
300v?

eg 12AU7
300V B+
6 volts on bottom cathode (150v on plate).
910R