Tracked through this thread about a week ago and hand sketched a rough schematic of the preamp to aid my understanding. Today I've tidied it up a bit and put some bogie values on various components. About half from various photos shared and the rest somewhat educated ballpark guesses. I believe it's close but a few things puzzle me. The cathode & plate resistor values for V1 triodes seem large with an effective 3K & 440K per triode. Borrowed assumptions posted by others that the power stage is a rocket, albeit with negative feedback and perhaps different component values.
Posting to share my sketch since nothing similar showed up when I searched through the forum. And also hoping for some feedback on my assumptions/errors/etc. And just to stir the pot a bit.
Out of curiosity, why do you guys think Ken used some negative feedback on this model.... just for a bit more headroom, or any other reasons you can think of?
Hope everyone's holiday season is going well!
Adam
First 25 years of playing, I never got along with EL84's...Now I think, WHAT THE HELL WAS WRONG WITH ME????
Single-ended amps can get a bit unwieldy without the use of negative feedback, especially if there is a lot of gain on tap (even from a 2-stage preamp). The power amp will have a response that is much higher up in the frequency range than is necessary which can be trouble (oscillations, weird responses and very large spikes in voltage!). The negative feedback not only reduces the bandwidth but it linearizes the response as well so things are much more predictable and controllable. I usually add a switch, the negative feedback is nice when things are gainy but when you're running a clean sound I quite like the high-end sparkle that is added from removing the negative feedback. You can even put it on a pot to fine tune things to your particular setup. FWIW this is all just from personal experience but I'm sure Ken ran into the same problems I have in building SE amps with a decent amount of gain on tap.
Feedback will scrub some grit particularly in higher frequencies, which increases "clean" headroom. Cliff's comments were absolutely spot on about the benefits --- except that songwriter and rocket are both push-pull class AB (quad of el84). So experience/observations with single-ended class A amps is great but slightly distorting to the discussion.
martin manning wrote:Cliff, NFB reduces gain but increases bandwidth, no?
Yes I was wrong there, NFB will increase bandwidth while linearizing the response. Good catch . The linearizing of the high end response is what helps tame things, especially when we usually don't know/consider how the leakage inductance and interwinding capacitances of the output transformer play into the high end response.
Also yes I thought the Songwriter was a SE amp and was wrong there. I definitely dig the lack of negative feedback in quad EL84/6V6 amps, it gives them a certain grit throughout the gain range that sounds very pleasing. Kinda neuters them when you add negative feedback. I'm not sure if the bandwidth is inherently lower in these push-pull quad output stages but they sure sound great (and the cut control is pretty darn effective with this setup).
Randal not yet I ordered a different 3ML pot with longer shaft but could only find a 2.5ML pot with 3/8" long shaft so I'll use that. No I didn't find any of the long shaft ones that I could cut down.
I the last time I fired the amp up it was running about half power for some reason I'll get back to it now that all the Christmas stuff is done.
565]http://i1372.photobucket.com/albums/ag327/didit_/amps/SWpreamp_zps12e6f310.jpg[/img]
I'll get back to it now that all the Christmas stuff is done. 