Plate Choke SRPP…. What do think?

[sgerus]

A while back a bought a pair of plate chokes to use with a 5842 as the input stage of a 2A3 SE amp. I decided not to use the 5842’s so I’ve been experimenting with the plate choke and a 6SN7.

Using a plate choke between the upper cathode and the lower plate of the SRPP improves the input sensitivity by about 20%, improves the high end frequency by 4KHz compaired to the normal SRPP.

And, It sounds really good.

Here are the details for a 6SN7GT:
390V upper plate
159V upper cathode
151V lower plate
4V lower cathode
680R = 5.76mA

Some general test data with a 6SN7

Input for 1 watt, Freq@-2db
Single section with plate choke @9mA: .9V, 44KHz
Both sections in // with plate choke @8mA each: .9V, 57KHz
Normal SRPP, 360VB++, -5.5V, 5.5mA 1.16V, 61KHz
SRPP with plate choke as described above .9V, 65 KHz

So it looks like the SRPP with the plate choke is the way to go, But I have never seen any circuits that use this configuration. Any thought’s / comments?

[revintage]

This subject has been up several times here. And it seems to be a nice configuration.

But you should take a closer look on the lower cutoff frequency. With this tube and the low current, you should need a fairly high inductance.

Personally I would choose a tube with lower Ri.

[Eli Duttman]

Personally I would choose a tube with lower Ri.

Agreed. X_L of the loading choke at 20 Hz. should be at least 3X R_P. An ECC99 meets the low R_P need, at a similar mu. A beta follower, as described by Morgan Jones, might be better with the 'SN7,

[revintage]

This could be a serious version with Zout below 150ohm at a standing current of ca 12mA. 6N30P has a Ri of between 1k and 2k. So with an inductance of 100H we are on the safe side as this is around 13kohm at 20Hz.

[sgerus]

Thanks for the replies so far.

I should mention it’s got to be an octal based tube. I gave up on the 5842 just because I didn’t like the looks of the 5842 next to the 2A3-40….. and my top plate is done.

I’m checking into using a 6F8G in place of the 6SN7 just because I like the way it looks!
How’s that for a good technical justification.

[revintage]

The 6N30P is available as both octal and noval.

The 6F8G will be totally useless in the proposed configuration. It´s got even higher Ri than 6SN7.

If you want an old cheap tube, 6BL7 would maybe be worth checking out.

How much current can your choke take? If you have a LL1668/100H current will be no problem.

[sgerus]

The choke is good for 20mA.

I need a valve with a "u" of at least 20....

What is the tube number of an octal 6n30p?

Thanks

[revintage]

Why not google for: 6H30 and octal ?

[kevinkr]

I've used the MQ exo-001 100H/20mA choke with the 7N7 or 6SN7 for quite some while, there is however some roll-off in the lows, but otherwise it sounds quite good. Electrically this behaves a lot more like a mu-follower than an SRPP, and I really liked the overall performance. A larger choke would improve the LF performance significantly.. IIRC I think the response was down slightly more than 1dB at 20Hz due to the choke, gain was very close to mu as you would expect, and I was running them at 10mA..

[revintage]

It could also be worthwhile checking THD at 20Hz when using high Rp tubes. Did a sim on 6SN7, 10mA, B+450V, RDC 680ohm and got below 0,1% at 1k and below 0,5% at 20Hz at 4Vrms. It does not roll off though, with -3dB at 2Hz.

Did some sims with the LL1667 (with two separate windings) in mind.

This one outputs 50Vrms with a THD of in the ballpark of 0,01% at 4Vrms out and below 0,2% at 20Hz. Zout ca 600ohm.

Goes down low with -3dB below at 0,8Hz.

Gain just above 15dB.

Wonder if it will work this well IRL ?