Master volume in a Princeton aa964

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bluesbear
Hey get your own solder
Posts: 330
(2/16/04 6:59 pm)
 Master volume in a Princeton aa964
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 Hi. How would one go about adding a master volume to a Princeton aa964, preferably after the tone recovery stage and before the PI.
I'd like to know how to add one post PI with this kind of phase inverter, also, just to know how.
Thanks!
Dave
 
ganzonimx
Hey get your own solder
Posts: 387
(2/16/04 7:30 pm)
 Re: Master volume in a Princeton aa964
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 I don't know if a MV in this circuit makes any sense as it has not to much gain stages but I think you could

1. put a 1M pot in front of the first gain stage, that would be the volume pot. The actual volume pot after the TS would be a MV, ore

2. if you add a Paul C mod to the PI you could replace the 1M resistor from the grid to ground with a 1M pot, the wiper to the grid..


Just my mind juggle ... Cris
 
bluesbear
Hey get your own solder
Posts: 331
(2/17/04 12:08 am)
 Re: Master volume in a Princeton aa964
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 "2. if you add a Paul C mod to the PI you could replace the 1M resistor from the grid to ground with a 1M pot, the wiper to the grid.."

Already had the Paul C and John Stokes mods. This MV works perfectly! I don't want a lot of distortion, just an edge. This does it.
Thanks,
Dave
 
Jonas Gemini
Hey get your own solder
Posts: 302
(2/17/04 12:41 am)
 Re: Master volume in a Princeton aa964
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 whats the John Stokes mod?

Jonas
 
bluesbear
Hey get your own solder
Posts: 332
(2/17/04 8:34 am)
 Re: Master volume in a Princeton aa964
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 Here it is:

members.aol.com/blusbear2/stokes.jpg

Here's the text that goes with it

"A little more volume can be gotten out of Princetons and Princeton Reverbs by raising the plate voltage on the split load phase inverter. In Princeton Reverbs there is an untapped filter stage that is ideal for this purpose. By moving the plate supply for the phase inverter back to the unused filter stage, the plate voltage changed from 195v to 250v. The higher voltage gives the phase inverter more headroom.

This is not a tremendous difference, but it may allow the phase inverter to drive the power tubes closer to saturation. And, unlike converting to a long-tailed pair phase inverter, it doesn't change the variety of tones that you can get out of this amp.

Locate the wires marked in blue on the diagram (they won't necessarily be blue in the amp). Unsolder them from the end of the 56K resistor, lift them off the board, solder them together and heat-shrink them.

Now connect the violet wire to the end of the 1 watt 18K resistor at the top left of the fiber board to the 56K resistor where you just removed the 2 blue wires.

Note:
There was a post on alt.guitar.amps that warned about possible excessive voltage on the cathode when doing this mod. The poster said that a 12AX7 tube could take up to 100v on the cathode but not more. His schematic of a Princeton listed 65v being present on the phase inverter's cathode in the stock design, and thought there would be enough headroom so that the 100v max wouldn't be exceeded.

I didn't measure the PI's voltage before doing this mod, but my schematic listed its voltage at 50v. It's currently at 65v after doing this mod, so it doesn't appear that the tube is being stressed."

This mod is "safe as houses", as they used to say in England.
Dave
 
lowtech
Senior tube assistant
Posts: 177
(2/17/04 12:15 pm)
 Re: Master volume in a Princeton aa964
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 As far as pre-split load inverter master volume goes, get a 1 meg pot and use it as a voltage divider between the driver stage and the input of the phase inverter. This is how Fender did it on the Champ II and Super Champ amps. If you can find either schematic its real clear, although the Super Champ uses a 6C10 instead of a 12AX7. If you cant find either schematic and want, drop me a line and I'll mail you a copy. The Champ II is almost identical to a Princeton AB1164, with the exception of silicon for rectification.
 
eddiejg
Senior tube assistant
Posts: 227
(2/17/04 2:49 pm)
 Re: Master volume in a Princeton aa964
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 Guys-
I'm getting ready to build the AA1164 & I want to add a MV.
I won't have the extra stage already available as on the AA964, but can add it.
At the same time, I looked at the Super Champ schematic & it has a 100K pot for the MV.
So, can anybody give me the pros & cons of using:
1) 1 meg pot in place of the 1 meg resistor to ground
2) 1 meg pot in front of the 1 meg resistor
3) 100K pot in front of the 1 meg resistor
I'm figuring option 2 is going to increase gain to excessive levels.
But, options 1 & 3 both look promising.
I'm building it for the 1st time & will be wiring it stock 1st so I know what it's supposed to sound like.
Some feedback on these 2 methods will probably help me decide which way to go without trying both (although neither is a major change).
Thanks.
-Ed
 
bluesbear
Hey get your own solder
Posts: 333
(2/17/04 9:30 pm)
 Re: Master volume in a Princeton aa964
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 "So, can anybody give me the pros & cons of using:
1) 1 meg pot in place of the 1 meg resistor to ground
2) 1 meg pot in front of the 1 meg resistor
3) 100K pot in front of the 1 meg resistor"

I don't know about the others (2 and 3), but the 1 meg pot in place of the 1 meg resistor to ground sounds great...but only if you do the Paul C mod, of course, otherwise the 1 meg resistor doesn't go to ground. Also, I changed the cathode resistors for the both gain stages to 820 ohms for more gain. Between that, the Paul C mod, and the Stokes mod, you should get enough gain to make a master volume worth while. I see no reason why you couldn't put a dual gang, 1 meg pot after the PI. Looks simple enough on the schematic.
Dave
 
lowtech
Senior tube assistant
Posts: 178
(2/18/04 12:52 am)
 Re: Master volume in a Princeton aa964
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 Hey .. sorry but I mispoke earlier.. The Super Champ and Champ II do use a 100 k pot, not a 1 meg pot as I earlier mentioned ( I wasn't wearing my glasses - old age is Hell. ) There shouldn't be any problem using the fixed bias ( Paul C. thing) with the Potentiometer preceding if you isolate it DC- wise with a cap from the plate of the preceding stage and a cap between the output of the wiper on the pot, and the input of the phase splitter. That keeps the fluctuating resistance of the pot from messing with the dc-bias of the the driver and fixed bias on the grid of the phase splitter. Hows it sound? I dont really care for it- I usually set the MV wide open and go from there.
 
lowtech
Senior tube assistant
Posts: 179
(2/18/04 1:24 am)
 Re: Master volume in a Princeton aa964
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 Another thought as far as the " Paul C" fixed bias mod to the phase inverter goes. If you replace the 1 meg grid resistor with a pot applied as a variable resistor and use it as a "master volume" you are actually changing the dc bias on the grid . This changes the q-point of the phase inverter which will upset the balance of the circuit, which is sort of the point of doing the mod in the first place. If it sounds ok to you, I guess thats whats important, I just thought you might like to know whats going on here.
 
bluesbear
Hey get your own solder
Posts: 334
(2/18/04 7:46 am)
 Re: Master volume in a Princeton aa964
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 "you are actually changing the dc bias on the grid . This changes the q-point of the phase inverter which will upset the balance of the circuit, which is sort of the point of doing the mod in the first place"

Could you explain this to me, please? I'm scratching my head. I would have thought that every time you hit a note on the guitar, the signal that grid sees is changing and the circuit isn't being upset...as far as I know, anyway. Seriously, I don't understand what you're saying.

Thanks!
Dave
 
lowtech
Senior tube assistant
Posts: 180
(2/19/04 3:15 pm)
 Re: Master volume in a Princeton aa964
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 Yes you are right as far as the signal swinging both positively and negatively when signal is applied. What you are changing if you put a pot in and adjust it, thereby changing the resistance is the amount of dc bias to the grid of the phase inverter, which in turn changes the q-point of the phase inverter with no signal applied. All tubes must be biased in some way at some point in order to operate. The amount ( and sometimes polarity) of bias sets the quiescent operating point of the circuit. You do something similar when you set the bias on output tubes ( although in this case it is a set negative valued dc) As far as the "Paul C" mod is concerned, you are utilizing a fixed dc bias ( read voltage) obtained by a voltage divider from the plate supply, rather than the "self bias" configuration which is used in the stock Fender setup. This is nothing really new, people have been doing it for years . ( Ampeg di