I need a double check on something here.....

[G._Hoffman]

I'm working on a new amp design, and I'm at least mostly done with it, but I need a check on one thing.  What I'm concerned about is in the cathode follower (V3B).  More specifically, I've got a couple of mix resistors going into it (R40 and R41), but I'm concerned that the grid leak resistor (R43) is going to end up putting DC on the volume pots (P4 & P5).  So, how do I get around that?  Or, is it even a problem?  If so, can I fix it by putting a cap between the pots and the mix resistors?


I think that's the only question I have, at the moment.

Thanks in advance,


Gabriel

[tubeswell]

I'm concerned that the grid leak resistor (R43) is going to end up putting DC on the volume pots (P4 & P5).

The 470k grid leak is bootstrapped to the cathode resistor so it picks up the wobbles and feeds them back to the CF grid to improve the CF input impedance. You should put this 470k to the CF grid and have a coupling cap 'in front' of' the grid leak resistor. (And this would make sense because the bootstrapped CF stage is cathode-biased, so you want the grid voltage to be set w.r.t. the cathode).  I wouldn't bother with the 68k - you already have two mixing resistors (which will be in front of the new coupling cap, so blocking distortion shouldn't be a problem). You could even make the mixing resistors 270k or maybe 220k for less hiss/noise. JM2CW

On 2nd thoughts, since you have a high load on the pentode channel (what with the pentode's high output impedance'n'all), which will be effectively in parallel with the load on the triode channel, making something of a mega-load all together, then why bother with a CF stage to mix the two. Why not just have a normal inverting stage instead to pick up the signal from the tone stacks and kick it in the guts?

[G._Hoffman]

Thanks, I'll give that a try.


Gabriel

[DummyLoad]

...and if SW4 is closed, you'll have a trickle of DC at g1 of the pentode. you'll need cap coupling between wiper of P6 and R40 and cap coupling between wiper of P5 and R41. basically, isolate your summing network. since Zin of bootstrap is very high, CC can be very smallish - 1-2nF.

--DL

[G._Hoffman]

...and if SW4 is closed, you'll have a trickle of DC at g1 of the pentode. you'll need cap coupling between wiper of P6 and R40 and cap coupling between wiper of P5 and R41. basically, isolate your summing network. since Zin of bootstrap is very high, CC can be very smallish - 1-2nF.

--DL

Any reason to use two before the mix resistors instead of one after?  I can't think of one, and space is always at a premium.  I've got plenty of places in here to muck about with the tone, coupling cap wise.

It is nice to know that my intuition (not to mention my understanding of what I'm reading) is on track here, though!  It almost feels as though I'm learning something, as unbelievable as that may sound!!!


Gabriel

[sluckey]

I'd just use one cap after the mix resistors. I'd do it like this...

Replace R42 with the cap. Then disconnect the left end of R43 and reconnect it directly to V3-7. You don't need a dedicated grid stopper on this circuit.

PS... Sunn did a circuit similar to your SW4 NFB switch as part of their Mid Boost switch. Take a look. It might give you some more ideas...    (if you want 'em)
http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

[G._Hoffman]

PS... Sunn did a circuit similar to your SW4 NFB switch as part of their Mid Boost switch. Take a look. It might give you some more ideas...    (if you want 'em)
http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

Always.  I have a theory about DIY stuff:  if you do it to save money, you're an idiot;  If you do it to learn and have fun, you are doing it right.  So, the more education, the better.

It seems like that switch would provide an awful lot of negative feedback.  Aren't you going to loose an awful lot of gain that way?  Although, I suppose with a cap that small it would be pretty frequency specific.


Gabriel

[sluckey]

The 12pf cap will take a little bite off the high end. Losing a little high end can be perceived as boosting the mids. But, at the same time, the other half of the switch puts a 1.5µF cap on the cathode and that will definitely boost the gain. Net results is a mid boost.

[DummyLoad]

LTPI will be caddy-wompus unless C35 is grounded.

[G._Hoffman]

LTPI will be caddy-wompus unless C35 is grounded.

I haven't actually done anything with the PI yet, other than to copy paste it from another amp to make the rest of it a bit easier to draw, and I may have cleaned it up a bit.  I have plenty of work to do on this amp - I haven't even started looking at the PS.


Gabriel

[G._Hoffman]

The 12pf cap will take a little bite off the high end. Losing a little high end can be perceived as boosting the mids. But, at the same time, the other half of the switch puts a 1.5µF cap on the cathode and that will definitely boost the gain. Net results is a mid boost.

Gotcha.  Makes sense.  Cool.


Gabriel

[sluckey]

LTPI will be caddy-wompus unless C35 is grounded.

That's the famous Hoffman 18W Stout circuit. Sure wish Doug would speak up and say if that was intenional.   

[DummyLoad]

LTPI will be caddy-wompus unless C35 is grounded.

That's the famous Hoffman 18W Stout circuit. Sure wish Doug would speak up and say if that was intenional.   

i think he has confessed - i remember in a thread a long while ago, he mentioned it was initially a wring error and i'm quoting him from memory; "...it sounded so good i just left it". hopefully he'll chime in and set us straight.

 

--DL

[G._Hoffman]

LTPI will be caddy-wompus unless C35 is grounded.

That's the famous Hoffman 18W Stout circuit. Sure wish Doug would speak up and say if that was intenional.   

i think he has confessed - i remember in a thread a long while ago, he mentioned it was initially a wring error and i'm quoting him from memory; "...it sounded so good i just left it". hopefully he'll chime in and set us straight.

 

--DL

Well, then, maybe I should put a switch on it!   


Gabriel

[DummyLoad]

LTPI will be caddy-wompus unless C35 is grounded.

That's the famous Hoffman 18W Stout circuit. Sure wish Doug would speak up and say if that was intenional.   

i think he has confessed - i remember in a thread a long while ago, he mentioned it was initially a wring error and i'm quoting him from memory; "...it sounded so good i just left it". hopefully he'll chime in and set us straight.

 

--DL

Well, then, maybe I should put a switch on it!   


Gabriel

actually, buttery built a stereo stout that had a switch select for that cap. he grounded the cap proper for clean playing.

yes, another switch, you should...   

--DL

[G._Hoffman]

Well, then, maybe I should put a switch on it!   


Gabriel

actually, buttery built a stereo stout that had a switch select for that cap. he grounded the cap proper for clean playing.

yes, another switch, you should...   

--DL

Eh, I've got too many already, and since I kinda want it to be a cleaner amp, I'll probably just ground it.  Only kinda, mind you, since largely what I'm REALLY doing is horsing around...err, experimenting.


Gabriel