Cascode input stage w/octal

[surfsup]

Was wondering if this is a waste of time or not...I am planning to use a 6SN7 and cascode both halves, into a standard CF, then tonestack then a gain stage. The circuit is RJ's Supre with an extra tube and putting the octal up front. I haven't put in the component values but I'm debating a cascode or just using the two triode halves as two gain stages before the CF. The appeal of the cascode is the "pentode" sound. What do you guys think, go for the cascode or just put in four normal stages?

Schematic edited and move to more recent posting.

[greekie]

I like the cascode circuits. A AT7 does the job well. A 6SL7 would probably work great. Haven't tried the 6SN7 yet. You'd better try both and see what you like.

Let me know what you find out.

[surfsup]

Well the heater to cathode rating is 200V so that's a pretty big swing so I think it will be fine but I'm going to run through the math this weekend. Could you elaborate a bit on what you like about cascodes?

[bluefireamps]

The cascode has great gain but doesn't really sound like a small pentode. If that's really the sound you're after, go with the pentode.
Dave

[surfsup]

Well I wasn't really going "for" the pentode sound but I hear the cascode adds a little to the tone. Here's the circuit after doing the math, would appreciate any comments from the gurus:

[img877]http://chicagocadcam.com/ChrisHahn/supr ... 20copy.jpg[/img]

[XgamerGt03]

It looks like you have your feedback connection drawn incorrectly.

Take a look at how they applied NFB in the Fender Champ
http://myfenderchamp.com/wp-content/upl ... ematic.png

Right now you have no ground reference for V2B.

[surfsup]

Whoops v2b has a ground there on the amp (alreadybbuilt and working without cascose input). The nfb might be going to the ground instead of cathode as you indicate i'll check that. Any suggestions on the cascoded input?

[tweedeluxe]

I've used 6SN7 cascode input in many in guitar amps. Go for it! They're very quiet and sound great but you can't get HUGE gain numbers like you can with a pentode, even when current-boosted. They make a good standard 12AX7 triode input sub though, almost no miller capacitance

They don't sound the same as pentodes, of course. 6SJ7 is a great-sounding (and cheap) octal pentode.

[surfsup]

Tweed, what do my voltages and component values look like to you? I am also debating a mosfet for the CF before the tone stack, then use the full second tube for a GS.CF in one tube after the TS to keep the lead dress cleaner. I'll prob go for that.

Its such a nice weekend in chicago though, been hard to stay inside. Maybe tomorrow i'll get it done.

[tweedeluxe]

looks like a good low-medium gain stage. I usually use Rp 47k and Rk 470Ω, grid-leak biased liked yours. I have tried current boosting the lower triode with resistors from its plate to the B+ node, but the tone changes in a negative way IMO. Interesting bootstrap of the CF there

[tubeswell]

It looks like you have your feedback connection drawn incorrectly.

Take a look at how they applied NFB in the Fender Champ
http://myfenderchamp.com/wp-content/upl ... ematic.png

Right now you have no ground reference for V2B.

Or look at the BF champ (AA764). All you'd need is 47R to ground from the junction of the Rk and feedback resistor.

[surfsup]

Tweed i biased both at approx center for more clean headroom...i just hope it sounds good.

tubeswell i checked it out, thx for pointing that out. This amp started as RJs Supre and has the 470 there.