What Does This Resistor Do? + 62 Year Old Mistake

[rp]

Now I just have remember what it was...

Ginkgo Biloba

5C8 has NFB routed to one power tube grid directly, and in a roundabout way to the other.

Martin, I wanted to ask about the NFB, as you brought it up, how exactly does it get to the other tube? What’s with the large value 1 meg?

and it made the amp ungodly loud and harmonically rich.

This thing is really loud too for 380V cathode bias w/ 6L6G/ 5881. And yes harmonically rich but in an in your face way, I thought the latter was just a quality of the four octals but maybe it's the circuit after Firestorm’s report. I keep wanting to tame it. It's too touch sensitive for a hamfisted guitarist like me, and I don't play electric much anymore so I'm sloppy when plugged in. Has high string noise, you need even, controlled fingering otherwise it's too dynamic, hit one string a touch harder than the others and it'll explode out the amp. I've been running it through an Eminence Black Mt., a crisp sounding 101db alnico as I like the twangy bright but that makes my fake country runs even worse! I have to find a really suave jazzy country guy to give it good test drive before I can properly judge it.

I keep thinking it should be smoother and compressed, more Bassman/JTM but I chose to build an early ‘50s tweed and really it's just up to me change to it not visa-versa. IME all these old 40s early 50s styles were bright, clear, and fast and then they got brash when pushed, except maybe the 5E3 which is actually pretty smooth when goosed. Smooth overdrive seems to come at the end of the ‘50s. Guess the musicians were asking for it and Leo had the good sense to listen to them.

[martin manning]

NFB: The 1M from the speaker goes to the upper power tube grid, and then through a 250k and 6k8 to ground. The upper power tube grid therefore gets about 1/5 of the speaker voltage applied at its grid. The 250k and 6k8 divide the combined NFB and inverting-side PI output voltage down, and deliver it to the grid of the non-inverting side. The lower power tube then gets the combined signal and NFB from the output of the non-inverting side in the same proportion as the upper power tube.

[Firestorm]

'50s and '60s Fenders were volume driven: they wanted them as loud as possible - run the outputs over voltage, buy speakers that could take it. Sometimes they tweaked. Sometimes it's undocumented.

[rp]

But they did something out of the ordinary (to compensate for no presence, maybe) and it made the amp ungodly loud and harmonically rich. Now I just have remember what it was...

I'm not so sure it could be just a single resistor somewhere. I learned a lot with this thing, after mucking around here and there it always remains what it is, like you described. I think it's just that paralleled plate driven 2nd gain stage w/o a cathode follower to add smoothness and some compression, maybe it's the paraphase too, but it has unity gain if I got it right, so that might shape the tone but not make it so energetic. It certainly does have that early MI meets PA clean linear amp thing happening - I was playing my ipod through it and it wasn't half bad! Beats me, it's different. 6SC7s are sui generis too.

BTW I replaced the Stancor A3851 it had with an Edcor CXPP30-MS-6.6K which was supposed to be for this amp from the start. Then I chickened out using over-qualified, plastic bobbin, 20-20 hifi iron @6.6K and went with the smaller Stancor. I figured given the modest operating points of the ancient Twin it would be more than adequate and maybe better - more grind and bite. So much for the seasoned old iron must be better stuff. The Edcor is in every way better, took away the brash bark and balanced out the bass and treble, it now sounds how a 1950s premium amp should. And it didn't make the too clear and sensitive part all that worse, a tiny bit and now the pick noise is more pronounced along with string noise. The acid test for a "too good" OT will be flat-out overdrive, if it's buzzy and plasticy, can't do that at home, and flat out OD may never be this amp's forte, but for now the swap appears 110% worthwhile. If I hate the OD I can alway try a Heyboer Bassman OT to safe be or a Pacific Express OT at 6.6K and keep gambling.

Lesson of all this: If Leo went with an oddly little OT you should go with an oddly little OT, if Leo went with an oddly big OT you should go with an oddly big OT. Don't argue with Leo and his Bakersfield Tonemeister Test Drivers.