sonolink

Advice for gain stage and Master output with tube preamp
« on: March 09, 2020, 10:14:14 AM »
Hello,

I built a clone of a Fender Showman preamp section (also known as Alembic Bass Preamp F2B) for a bassist friend of mine. He wants to use it as a distortion/overdrive/coloring pedal. The unit has a gain control and an TMB stack.
He is very happy with it except for the fact that it has a lot of gain compared to his other pedals/stompbox preamps and would like to be able to level the output.

This is the circuit: http://moosapotamus.net/images/AlembicF2B_SCH.gif
I'm using a switched power supply that provides a B+ of 190V, and 6V for the heaters.

My question is: what would be the right approach? A "Master Volume" control at the output? Would that be a 1M pot connected after R9 on pin1, the wiper connected to the output jack and pin3 to GND?

Thanks for your time and help
Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou


Rocinante

Re: Advice for gain stage and Master output with tube preamp
« Reply #1 on: March 09, 2020, 11:21:56 AM »
I built the Alembic but in a two channel rack unit. I believe I used a dual gang 500k or 50k for the master volume that went between the Alembic output and the solid state power amp circuit I used.  My bassist loves the amp and I too really like it.
If there's a harder way to do this, I haven't found it yet.

sonolink

Re: Advice for gain stage and Master output with tube preamp
« Reply #2 on: March 09, 2020, 11:38:00 AM »
Excellent! The connections are correct as I said, right?

Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

Fuzz Face

Re: Advice for gain stage and Master output with tube preamp
« Reply #3 on: March 09, 2020, 08:07:20 PM »
I would remove R9 and replace with 1M pot. Lug 3 goes to C4, lug 2 becomes output, lug 1 to ground.

The pot will work backwards the way you described, unless you name pot lugs opposite to what I’ve known as standard

sonolink

Re: Advice for gain stage and Master output with tube preamp
« Reply #4 on: March 09, 2020, 09:12:38 PM »
I would remove R9 and replace with 1M pot. Lug 3 goes to C4, lug 2 becomes output, lug 1 to ground.
The pot will work backwards the way you described, unless you name pot lugs opposite to what I’ve known as standard

I think you're right. Thanks Fuzz Face ;)

Out of curiosity, won't the Master volume pot affect frequency response, especially high end? If so, how about a split plate load resistor on any of the 2 stages to tame the gain?

Thanks for your input
Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

PRR

Re: Advice for gain stage and Master output with tube preamp
« Reply #5 on: March 10, 2020, 12:53:58 AM »
.

sonolink

Re: Advice for gain stage and Master output with tube preamp
« Reply #6 on: March 10, 2020, 06:56:58 AM »
Thanks a lot for chiming in PRR  ;)

Could please explain? I'm always eager to learn. Thanks again :)

Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

Re: Advice for gain stage and Master output with tube preamp
« Reply #7 on: March 10, 2020, 09:21:52 AM »
Wont the 220k and 10k pot in series form a potential divider ?
with full rotation of the volume control you will be getting a 22:1 reduction in the voltage at the output .

http://www.sengpielaudio.com/calculator-gainloss.htm

You could use a pot with a pull switch to cut in a resistor that changes the ratio ,
Lets say we use 250kohm as dropper resistor ,with a 50k pot thats a ratio of 5:1 or 14db loss
if you used the  pull switch contact to cut in a 12.5k resistor across the pot  we  get  a 25:1 ratio , or 28db loss ,now you have a pull switch volume boost for where you want to overdrive the following device in the chain a bit more.

https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/

EmRR

Re: Advice for gain stage and Master output with tube preamp
« Reply #8 on: March 10, 2020, 10:15:27 AM »
Wont the 220k and 10k pot in series form a potential divider ?
with full rotation of the volume control you will be getting a 22:1 reduction in the voltage at the output .

Yes, that's the entire point.  Tweak that value to deliver the range you want out of the volume control. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: Advice for gain stage and Master output with tube preamp
« Reply #9 on: March 10, 2020, 11:37:44 AM »
Arranging a cathode bypass capacitor to be switched in and out of circuit is another way of reducing stage gain found in some preamps.

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

Theres a neat calc I just found , it brings up a graph ,you can plumb in all the most vital circuit parameters , simply enter 0 in the capacitance field to calculate the gain unbypassed . Apparently its a formula the guy dug out of the great book itself ,
The 'no bullsh*t' 'does exactly what it says on the tin'  calc interface doesnt feel like I'm on the  internet at all , it just has its  own beautifully uncomplicated head space ,no distractions ,a rare thing online in this day and age . Looks like the author has whole site full of usefull stuff for amp tweekers.
« Last Edit: March 10, 2020, 11:41:11 AM by Tubetec »


sonolink

Re: Advice for gain stage and Master output with tube preamp
« Reply #10 on: March 10, 2020, 11:51:40 AM »
Thanks for the input Tubetec and EmRR. All this is very interesting, but raises one question in my head: would that serve as a Master Volume? I mean, my friend likes the gain saturation the preamp produces, what he asks for is to be able to handle the output signal to avoid bad clipping and untameable volume.

I mean, I know that a cathode follower for instance would also reduce gain, but then again he would loose the distortion wouldn't he?

What I'm trying to say is, shouldn't he be able to turn the volume down instead of the gain to keep the saturation color in the signal and have a manageable one for recording or feeding a different impedance input or whatever? (I hope that makes some sense)

Thanks a lot for all the input :)
Cheers
Sono

Why sleep when you can mix?
M.P.Stavrou

fazer

Re: Advice for gain stage and Master output with tube preamp
« Reply #11 on: March 10, 2020, 12:11:07 PM »
PRR,s suggestion doesn’t affect the gain of the tube pre,  it affects the output level which should be reduced  to drive the Ss amp input.  It also loads the output of the tube to 230k so as not to load the preamp at 10k input impedance of the SS power amp.   It’s a very good voltage divider to go between the the plate drivin high impedance of the 12ax7 and the 10-20k input impedance of the power amp. 

sonolink

Re: Advice for gain stage and Master output with tube preamp
« Reply #12 on: March 10, 2020, 12:35:08 PM »
Thanks for your reply Fazer but I was talking about the cathode bypass capacitor suggested by Tubetec really. I was wondering if that would suit the purpose as well

I'll definitely try PRR's suggestion as soon as I get hold of a 10k pot 😉

Thanks
Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: Advice for gain stage and Master output with tube preamp
« Reply #13 on: March 10, 2020, 04:01:56 PM »
Hi again,

In my build for this friend I didn't implement the switches (SW2 "Brite" and SW1 "Hi Cutoff" on the schem). I was thinking about implementing them with a single On/Off/On DPDT switch (got a couple laying around).

My question is: In what position happens the cutoff in SW1? When the switch is open or closed? Same for SW2? In what position does it add Brightness? When it's open or closed? IIRC you have to add capacitance to add brightness and shimmer, but TBH I'm not sure about it.

I would have to wire the switch so that in one position SW2 is open while SW1 is closed, in the OFF position they're both out of the circuit and in the remaining position SW2 is closed while SW1 is open. Is that correct?

Thanks a lot for your time and help
Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

fazer

Re: Advice for gain stage and Master output with tube preamp
« Reply #14 on: March 10, 2020, 04:06:34 PM »
Quote
but I was talking about the cathode bypass capacitor suggested by Tubetec really

Sorry didn't understand.   Yes it would bring down the gain of the tube pre and therefore the overdrive sound of the preamp as well as the output level. 

Regardless, the impedance matching with the pad/volume 220k/10K divider would still  be needed for the interface.  You have a 38k output impedance of the 12AX7.  230k load is better for the tube  to see than a 10k input impedance of the SS Amp input regardless of gain.  You usually want a 10 times the output impedance.  230k is close enough.   

Sounds like you get this already from your response. 

fazer

Re: Advice for gain stage and Master output with tube preamp
« Reply #15 on: March 10, 2020, 04:09:47 PM »
Sw 2 boost when closed causing the Cap to bypass highs to the output.
SW1 cuts when the switch is open.

sonolink

Re: Advice for gain stage and Master output with tube preamp
« Reply #16 on: March 11, 2020, 07:13:34 AM »
Sw 2 boost when closed causing the Cap to bypass highs to the output. 

You're saying that when SW2 is closed Bright is ON, correct?
It's just that I'm not sure I understand clearly "causing the Cap to BYPASS highs".
Bright ON is SW2 closed right?

Sorry for my language barrier  ;)

Thanks for your help
Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

sonolink

Why sleep when you can mix?
M.P.Stavrou

Re: Advice for gain stage and Master output with tube preamp
« Reply #18 on: March 11, 2020, 08:27:00 AM »
The bright cap allows the highs to bypass the pot at lower settings of the dial , the leakage effect of the cap is reduced as you increase towards maximum on the dial .

The same treble bleed cap is often found on guitar volume pots .

sonolink

Re: Advice for gain stage and Master output with tube preamp
« Reply #19 on: March 11, 2020, 03:43:14 PM »
Oh I see......so this switch is really useful at low gain settings isn't it?


Would moving the 1M resistor to ground (R1) before the 68k change anything? If it's ok I could mount it on the input jack t save space...
Why sleep when you can mix?
M.P.Stavrou


 

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