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?
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?
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.
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.
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.
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.
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..
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
?
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
?Attachments
As a Certified Tube Head, I've never understood that myself. Interstage is perhaps the second-worst place to put a transformer, and unlike The Worst (output), there's rarely a good reason to do so.
A plate choke can get you more swing, but 99% of the time (like here), it's used where you don't actually need more swing.
Frankly, I'm amused that the same guy who goes into conniptions because of some steel in a resistor endcap will then put a kilogram chunk of iron right in the signal path.
A plate choke can get you more swing, but 99% of the time (like here), it's used where you don't actually need more swing.
Frankly, I'm amused that the same guy who goes into conniptions because of some steel in a resistor endcap will then put a kilogram chunk of iron right in the signal path.
Lars, if you have time, please do a sim of a 6sn7 sprr with this data:
390V B++
70H Choke, 1365 DCR @ 1Khz
680R, 220uF on bottom cathode = 4V @ 5.76mA
I've done some more testing.... in general, the SRPP w Choke
(between the top cathode and bottom anode) = 4Khz better high end freq resp than a choke loaded 6sn7 in parallel
Im taking the output from the upper cathode... if I take the output from the lower plate, the freq resp is the same as the 6sn7 in parallel.
Kevin.... is there a name for this SRPP / Choke configuration?
390V B++
70H Choke, 1365 DCR @ 1Khz
680R, 220uF on bottom cathode = 4V @ 5.76mA
I've done some more testing.... in general, the SRPP w Choke
(between the top cathode and bottom anode) = 4Khz better high end freq resp than a choke loaded 6sn7 in parallel
Im taking the output from the upper cathode... if I take the output from the lower plate, the freq resp is the same as the 6sn7 in parallel.
Kevin.... is there a name for this SRPP / Choke configuration?
I played around with interstage transformers and chokes but eventually returned to RC coupling, you just can't beat a flat frequency response with no ringing for sound quality. Consider that using an inductance as a triode load will give you low distortion at high freqs and high distortion at low freqs apart from other problems.
sgerus,
This is no SRPP, it is to be looked upon more like a mu-follower.
Did a quickcheck at breakfast. It all points towards using a 330ohm cathode resistor. Given the fact your choke has a DCR on the high side we will have to accept fixing it up with a little asymmetric clipping. Change the cathode resistor and cap for a string of a 1N4148 and a red ca 2.1V LED. This will be the same biasing as using 330ohm. Far better than your proposed 680ohm//cap with to low current. This one places the 6SN7 at a more linear working point.
You will get 55Vrms out before clipping at 1% THD. With 2V out you will get below 0,05%THD. This with 50k load through a 3,3uF cap. At 20Hz 2V out I get an impressivly low value of THD below 0,1%. Loading with 10k is still well below ,1% at 2V out. Clipping occurs at ca 50Vrms both at 20Hz and 1k.
Can not say anything about the highfrequency-response as this depends on the capacitance of your choke. Your measurements are good enough. Low end with 0,1dB drop at 20Hz.
Zout in the ballpark of 400ohms.
So go for it, good luck! Iam sure this one will sound great!
And don´t mind the grumpy old men that didn´t succeed with iron !
This is no SRPP, it is to be looked upon more like a mu-follower.
Did a quickcheck at breakfast. It all points towards using a 330ohm cathode resistor. Given the fact your choke has a DCR on the high side we will have to accept fixing it up with a little asymmetric clipping. Change the cathode resistor and cap for a string of a 1N4148 and a red ca 2.1V LED. This will be the same biasing as using 330ohm. Far better than your proposed 680ohm//cap with to low current. This one places the 6SN7 at a more linear working point.
You will get 55Vrms out before clipping at 1% THD. With 2V out you will get below 0,05%THD. This with 50k load through a 3,3uF cap. At 20Hz 2V out I get an impressivly low value of THD below 0,1%. Loading with 10k is still well below ,1% at 2V out. Clipping occurs at ca 50Vrms both at 20Hz and 1k.
Can not say anything about the highfrequency-response as this depends on the capacitance of your choke. Your measurements are good enough. Low end with 0,1dB drop at 20Hz.
Zout in the ballpark of 400ohms.
So go for it, good luck! Iam sure this one will sound great!
And don´t mind the grumpy old men that didn´t succeed with iron
!Sorry, forgot to tell you, at +55 I also belong to the group!
About this specific circuit I was at first surprised myself that the lowend had so good indications. Then you must remember it is after all working in pushpull. Its a completely different issue when feeding via a platechoke against B+.
!About this specific circuit I was at first surprised myself that the lowend had so good indications. Then you must remember it is after all working in pushpull. Its a completely different issue when feeding via a platechoke against B+.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.