Bass guitar tube pedal. Baxandall NFB tonestack.

[nandrewjackson]

Starting a project for my Bass player.  I got this Baxandall NFB circuit from JohnLinsleyHood's "Valve and Transistor Audio Amplifiers" book.  I can't scan the page with my scanner, but the circuit in the book does not specify input Z or V range.  It starts the circuit with the 0.47uF cap. 

I have added the first tube stage, and the 200k?500K? volume pot. 

The values from Hood for the circuit are as shown.  I am guessing values such as 2K2 and 4K7 represent 2.2K and 4.7K, respectively.

Also, in Hood's book, the tube following the TS is EF86, I want to use 12AX7, or possibly 5751.

Does my placement of volume control make sense, or would it be better suited in parallel with the 1M grid to ground after the TS?

Any and all questions, comments, concerns are welcomed,
Noah

[LooseChange]

Copy the preamp circuit from a Ampeg B-15n... You will love the tone.

[PRR]

200K pot.

Bypass the second cathode, 10uFd. (Not for overall gain, for maximum boost when knobs are cranked.) Or use pentode....

I suspect the 4K7 should be 47K; look at other plans and observe resistor ratios. It isn't _real_ fussy, but 5K smells low to me.

> tube following the TS is EF86

Not "following", it is *embedded*. The tube does what the tone network tells it to do. As best it can. The tone network asks for maximum boost of 13. 12AX7 only has gain of 40-50. It will work; Fisher did a million like this. It won't give full boost. A pentode will give inside gain of 70-100, so will comply very well for boost of 13. Yes, yes, as a practical matter one 12AX7 makes more sense than two bottles.

Do NOT put the volume control "inside" the tone network loop, it foils the tone control.

Agree that Ampeg is a good target for plagiarism.

[nandrewjackson]

LooseChange; I'm sure the Ampeg circuit is awesome, and it uses a similar TS, but I really wanted to experiment with the TS in a NFB situation, not just passive.

PRR;  Thanks for the tips on Volume Pot and 2nd cathode.

There are quite a bit of typo's in that particular book.  I'm not necessarily speaking about the circuits, I can't decipher even 10% of the book's technical knowledge, but in the text there are obvious English typos, so I would gather that the drawings have the same amount of problems.  It's probably difficult for an Editor to proofread a work like that before it goes to print!

 
Duncan's TS calculator has a similar TS, and gives input Z as 38K, but again, the circuit isn't as symmetrical as the one used in NFB, and is for passive use.

My bass player, Mike, really likes pedals, mass produced as well as boutique, so I thought this would be a good addition to his arsenal.  I'm not really looking for boost, just a cool tube "preamp" pedal, I may add a transformer isolated XLR out as well as 1/4", just for kicks.  As long as the finished product gives a similar output level as other stompboxes, it should be fine.
*embedded*, I'll put that in the vault. 

Side note:
I really enjoy this forum for so many reasons, but one that stands above the rest is the Constructive Criticism, and lack of TROLLS.  So many forums people get "flamed" for misuse of terms, as I have done here.  It's such a relief to have a forum without the flaming trolls around every corner.  It really speaks to the character of the Admins and the members, themselves.
Noah

[nandrewjackson]

I'm thinking a good division between R1 and R2 will allow for simultaneous use of both 1/4" and XLR ouputs without the need for switching between the two, and without too much change in output if not using one or the other.

The transformer has DC resistance of 150 ohms, and is recommended to use a 1K Audio pot.
The transformer is being sold as an add-on for speaker cabinets, to take XLR output from a poweramp signal.

Am I headed in the right direction here?

I'm also thinking 3PDT footswitch and an on/off status LED powered from the heater circuit. (not on drawing).

[jazbo8]

I'm thinking a good division between R1 and R2 will allow for simultaneous use of both 1/4" and XLR ouputs without the need for switching between the two, and without too much change in output if not using one or the other.

A few suggestions:
1) the 1M input resistor should be tied to the pole of the bypass switch, so the input is always loaded;
2) take the volume pot out of the feedback loop as PRR already mentioned;
3) get rid of the 475k and R1, they are not really needed;
4) R2 can be the volume pot, 500kA should work fine;

The last item is trickier: change the 1kA pot to 1MA, perhaps a trim pot is better here, also make sure that you do not turn it down all the way, or the output will be shorted - just set it to the appropriate level and leave it alone.

As long as the 1/4 and the XLR outputs are loaded with reasonable loads, you can use both outputs at the same time. Since the Weber transformer is simply a 1:1 isolation transformer, it is perhaps not the best choice for DI output, but if you are careful with the load, it may do ok...

[nandrewjackson]

jazbo8;

yeah, I got the 1M on the bypass in mind, just haven't put it in the drawing yet.  Should it also have 1M to ground on the bypass signal?

I though PRR just meant not to put the volume on the 2nd tube grid to ground, and that it was fine where it was???

If R1 isn't there, then the 1/4" output will be parallel with, at most, 1K Ohm, and the 1/4" output would be really low, right?  I thought the xlr and 1/4" outputs should be at least in totally equal parallel circuits, with their own volume controls within.  1K for the XFMR and let's say 500K for the 1/4".

This is all theory as of right now, I've yet started to gather parts or start the build.

[nandrewjackson]

Like this,

R1 and R2 divide the signal to help equalize each output.
I'd like to use the 1K pot for the xlr, that's what's recommended for it, I'm a bit OCD that way. 

[DummyLoad]

Like this,

R1 and R2 divide the signal to help equalize each output.
I'd like to use the 1K pot for the xlr, that's what's recommended for it, I'm a bit OCD that way. 

another suggestion; make R1 (300K) a pot - like you have for HI-Z out. if you're playing in some venues you may want to send the LO-Z out direct to the house mixer and if you do that  you'll like having an output level control on the LO-Z side as well; also, suggest a ground lift switch. the altec 15095 is much better sounding x-former than that weber iron and you won't need the balance pot, another option is the cinemag CM-2810 also won't need a balance pot.

http://cinemag.biz/output/output.php

--pete

[jazbo8]

another suggestion; make R1 (300K) a pot - like you have for HI-Z out. if you're playing in some venues you may want to send the LO-Z out direct to the house mixer and if you do that  you'll like having an output level control on the LO-Z side as well; also, suggest a ground lift switch. the altec 15095 is much better sounding x-former than that weber iron and you won't need the balance pot, another option is the cinemag CM-2810 also won't need a balance pot.

http://cinemag.biz/output/output.php

--pete

I swear I saw an option for a larger value pot for the Weber transformer, anyway found another picture where a 5-100k pot/resistor is placed in series with the 1K pot (really? I have never seen one that low in value, but anyhow that's what it says). For the 1M input load, if  it's connected to the pole of the switch, i.e., the tip of the 1/4 jack, then the input will always be loaded whether the EQ is bypassed or not.

[nandrewjackson]

Jazbo and Pete,

Thanks for the input again,

I'll check out some of those other transformers.  I'm just hashing this out on paper now to see if i'm even in the ballpark.

I was thinking ground lift switch along side xlr level control.

Whichever way it gets built, I'm sure there will be more than one component swap before it's right. LOL

[PRR]

> Duncan's TS calculator has a similar TS, and gives input Z as 38K

No, that is the OUTput resistance of the 12AX7 stage often used to drive this tone control.

The input impedance of the tone control is high at low frequencies and low at high frequencies. (What you expect with a lot of significant capacitors.)

The image seems a good plan until you get to the 475K resistor. This gives (against a 1K pot) an overall gain much less than unity. If you reduce the 475K a lot, Q2 will struggle. I think you need more stuff to drive a transformer.