srpp input stage

[nhuwar]

Hi I'm building an srpp input stage with 6sn7gt's.

I am using 1k cathode resistors but I'm not sure on what the wattage of these resistors should could anyone give me a little guidance as to what the wattage should be.


The plate voltage is 400vdc.


Thanks

Nick

 

[Poindexter]

Half watt is plenty. Use more if you like, it won't hurt anything. Take the signal off above the upper cathode resistor, there's less hum and Zout that way.

Aloha,

Poinz

 

[nhuwar]

Thanks a million

Nick

 

[SY]

I'm building an srpp input stage with 6sn7gt's.

Why? SRPP is the topology of choice for very large swings, but small signal linearity is not its strong point.

 

[nhuwar]

Hi SY it's a starting point given by advice from someone who knows much more then me about audio circuits.

Nick

 

[SY]

It certainly is a fashionable circuit, but for performance in this position, it's questionable. YMMV.

 

[Miles Prower]

Hi I'm building an srpp input stage with 6sn7gt's.

As Sy said, the SRPP is a trendy circuit. However, there are an awful lot of folks out there who don't really understand it. The SRPP is really an active pull up/pull down circuit that works as a PP power amp. That's what it was originally designed to do: drive the high capacitances of T-lines.

Here is an alternative that gives both high gain, and has a Lo-Z output. Borrowed from SS, this is a bootstrapped amp that uses positive feedback to increase the effective size of its plate resistor many times over its DC resistance. The linearity is better, and the output impedance lower than what you'd get with an SRPP.

 

[arnoldc]

Hi Miles!

Care if I "steal" the circuit and graft into my 300B project which I've been thinking since last night what driver topology I would use next.

Can it drive a 300B for some 8 watts?

ps.

I will use 6SL7 because that's the only octal dual-triode I have left...

 

[Miles Prower]

Hi Miles!

Care if I "steal" the circuit and graft into my 300B project which I've been thinking since last night what driver topology I would use next.

Not at all.

Can it drive a 300B for some 8 watts?

ps.

I will use 6SL7 because that's the only octal dual-triode I have left...

I highly doubt that a 6SL7 would make a good 300B driver, and one of the causes of the often dissappointing results from 300Bs is inadequate drive. I would use a MOSFET source follower to drive the 300B grids. The 6SL7s might be able to drive the MOSFET if you choose one with a low reverse transfer capacitance, and run it with enough voltage to get those capacitances down.

 

[matejS]

Re: Re: srpp input stage

Here is an alternative that gives both high gain, and has a Lo-Z output. Borrowed from SS, this is a bootstrapped amp that uses positive feedback to increase the effective size of its plate resistor many times over its DC resistance. The linearity is better, and the output impedance lower than what you'd get with an SRPP.

Miles, can you provide more info on this circuit; I mean, some link explaining it, implementations and some sonic results - at the end this is what really matterrs (not the theory).

Thanks,
Matej

 

[nhuwar]

I am using the srpp as starting point I might change it I might not.

Right now Kevinkr is helping me design this amp sence I definantley need the help and he has been gracious enough to help me.


The tube it is driving is a 813 so there is a bit of capacitance there.



Nick

 

[Miles Prower]

Re: Re: Re: srpp input stage

Miles, can you provide more info on this circuit; I mean, some link explaining it, implementations and some sonic results - at the end this is what really matterrs (not the theory).

You can read more about it Here

Here's the conclusion:

The bootstrapped-compound circuit offered the highest gain and a fairly low output impedance and a much improved PSRR figure. (Definitely a sleeper circuit.)

As for sonics, I'd expect that to be quite good, since the whole purpose behind this is to increase the effective plate load. The closer you can come to a CCS situation, the better the linearity. Improved linearity will tend to make the amp more "transparant" by reducing the extra harmonics.

As for myself, I would prefer to use an actual SRPP in cases where I needed the full push/pull operation. Otherwise, something like what I described, or a Mu Stage or Kimmel might make more sense.