Critique this schematic...please.

[bigsbybender]

Hi everyone,

I'm trying to devise a unique bass guitar rig for myself.  I have decided that I want some vintage vibe with some of the ease and power of modern things.  So I want to make a tube preamp to output to a solid state PA-style power amp.  I want to start with the Ampeg B15 preamp section.  I stole from Joe Piazza's AMPEG schematic, removed the tube poweramp section, rectifier, and did some low-rent photoshop action to add a new power supply.

1st....I have a 120:120v Transformer setup that I'd like to use....for budget consciousness. Will the 150-170v B+ here be enough?  I couldn't find what the original B15 models ran on the plates.

2nd.... Is the line-out in this fashion going to sufficiently drive a common QSC or Crown power amp?

3rd....Errors on my rugged schematic that I may need to tend to?

4th.... Any other constructive comments and criticisms are welcome.

Schematic posted below

Thanks,

J.

[FYL]

B+ is way too low for the plate and cathode R's used, you'll have gain but no real voltage swing - app. 10 V ptp or so. Consider using a doubler if your PT can put out enough current. The B15 had a 450V B+, with 1K between plates and screens and 22K for the preamp, 23K drops 230V for 10 mA current.

Outputs are high Z : plate R (44K Ra typ.) + volume pot (1M) + 270K, at best you'll have 44K + (1M // 270K) = app. 256 K, at low levels more than 1M. You won't be able to drive anything w/o a buffer - a 6J5 cathode follower for instance, keeping the preamp all octal.

Filtering is way too low for a silicon recto, consider using at least 100 µF per main or decoupling node.

[PRR]

The two channels are identical.... why do you need both? Does someone else play though your amp at the same time?

The B+ probably should be 300V-400V, not 150V. It may "work" as shown, but I am sure the original ran higher.

The output impedance is high for a yard of cable, much less a squalid-state amp input. No simple change will fix this. Add a beefy cathode follower.

Filtering: as shown, all stages use 470K plate resistors, so the very lowest effective load on the power supply is 470K/4= 117K, and probably at least twice that, 200K. Take your dropping resistor as 5%-30% of the effective load, say 20K. Assume the ripple at first cap is much bigger than signal voltages (around 1V), that each stage has small ripple rejection, and you want signal ripple to be much-much-much-less than signal. You almost never want less than C-R-C-R-C filtering to a low-level preamp plate. 3-stage filter, C-10K-C-10K-C. "C" in uFd should be at least equal to load in mA, or 300V/200K or 1.5mA or 1.5uFd; use at least 20uFd. Do math, or breadboard and listen.

100uFd is good, I like my 470uFds, but at some point you get more filtering from more R-C stages than from giant caps.

> I have a 120:120v Transformer

Probably want some 6.3V too.

> at low levels more than 1M.

Zero on both pots is 135K output impedance. Still far too high for cable or QSC/Crown.

[bigsbybender]

Thanks for the replies!

Filtering is way too low for a silicon recto, consider using at least 100 µF per main or decoupling node.

   I should have known that, another example of the dangers of copy/paste.

Outputs are high Z : plate R (44K Ra typ.) + volume pot (1M) + 270K, at best you'll have 44K + (1M // 270K) = app. 256 K, at low levels more than 1M. You won't be able to drive anything w/o a buffer - a 6J5 cathode follower for instance, keeping the preamp all octal.

Thanks, that makes sense, but I'll need help designing it for sure, as this goes beyond copy/paste. 
I do have a couple 6J5 metal tubes in my pile that I could try.

The two channels are identical.... why do you need both? Does someone else play though your amp at the same time?

My original thought was to use one channel, but I then changed my mind. My thought here is that I have several basses. One active, one passive and one active piezo-equipped upright bass. With two channels I can leave one channel stock for vintage p-bass and be able tweak the other for use with the basses equipped with active electronics.

Probably want some 6.3V too.

Got that, I just didn't put it on the schematic..


Alright, back to the drawing (copy/paste) board...

j.

[jjasilli]

3-stage filter, C-10K-C-10K-C. "C" in uFd should . . . Yes: integrated amps (preamp + power amp in one box) may be deceiving.  There's usually 4 levels of filtering.  The first 2 filter nodes actually supply the power amp.  But the preamp stages benefit from those first 2 nodes.  That's why the first preamp stage is supplied by the last filter stage.  The last filter stage has the least B+ noise (ripple).  Any noise in the 1st preamp stage will keep getting re-amplified. 

My suggestion is to try to find a schematic for for a standalone tube bass preamp for some more ideas.

[PRR]

> find a schematic for for a standalone tube bass preamp

http://www.el34world.com/Forum/index.php?topic=7966.msg70990#msg70990

[bigsbybender]

Thanks for the help! I really do appreciate it!
I was also thinking that a transformer might be an option instead of the cathode follower. It may increase cost but it would lower overall parts count. I'm just kicking this idea around to see what you gentlemen think....

Thanks again,

j.

[FYL]

You may also consider a Mosfet follower.

[terminalgs]

suggestions/observations:

look at the Triode Transformers PA522 transformer.  $40 or so,  330-0-330 with a 12.6v/800ma filament winding.     if you want 165-0-165 & 6.3v,  just wire the two primary in series (as if to plug it into a 240v supply).      If you want to use 6v filament tubes,  put two in series on the 12.6v supply.   I've put 3 12v tubes and a 6v tube behind one this way.

also Edcor  makes a 180V@200mA & 12.6V(6.3-0-6.3)@4A tranny  ($45 or so ).

as far as using a transformer to turn your high Z to low Z,   run a cathode follower and forget about it,,,  otherwise, with a transformer, you need to figure out which ratio you want  spec one out with the correct frequency range, and pull the trigger on a purchase, which may/may not be the right transformer for you after your testing is complete.  a cathode follower won't disappoint, and unless you need balanced output,  a transformer isn't worth it.


I'm with PRR on the two channels.   sort of pointless.  even if you want different tonestacks for ea.  what about a rotary switch with 2 poles,  and 3-4 throws/positions ?  and switch between 2 or 3 tone circuits.    then you could use those other two tube stages for something else,,.  possibly two CF stages out to drive two different amps, or an amp and a direct out (might need an extra stage with a level adjust pot)?