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Several Questions: heavily moddified Princeton

pfflam

Member
Messages
7,118
Hello folks,

I want to do some things with my heavily modified Princeton
- its now a 6L6 amp
- without a rectifier tube but a rectifier made up of some diodes on a tag board
- it has no bias control
- two pronger

So what I want to do:
- I want to add a bias control
- three pronger
- change caps
- possibly change the two parallel caps in the power section(?) that seem to be .47uf (Orange drops) to .01 (but I am not sure what the tonal differences are with different values in this place - I once knew but have forgotten . . . anyone want to remind me?)

So my questions:
- where do I put a bias control - would that go between the Orange drops and the bias section and how would I wire it up . . . any ideas?

- Three pronger: black goes to switch then fuse? or fuse then switch, and remove Death Cap? If I keep death cap in will it still be deadly? and wiill it serve some sort of line filtering? or is it best just to remove it?

- How hard is it to put in a 'Standby Switch" perhaps in the 'Death Cap'- ground switch hole?

Here are some pics of the inside:
I note that it looks like the power section (if that's the area with the Orange Drops) feed directly to the power tube . . . is that where I would intervene with a bias pot?
 

smolder

Platinum Supporting Member
Messages
14,399
Phsyco sent you in the right direction... but in short,

A standby switch offers you very little.

Put the bias pot in the polarity hole for external access (that way you need not pull the chassis each time you swap power tubes.

For the three prong, hot goes to the fuse, ground (typically green) to the chassis and the last wire goes to the OT.
 

pfflam

Member
Messages
7,118
OK, great, but the bias is situated between the where and the where?

BW: distaster?! perhaps, sounds great?! Yes!!
(which of course does not mean that fiddling with it won't make it even better)
 

smolder

Platinum Supporting Member
Messages
14,399
Hoffman has a page specifically detailing how to install a bias adjustment into a stock fender (princeton even I believe). It basically sets between the small tag board near the pilot light and the intensity control. You can also put a smaller trim pot directly on that tag board.
 

pfflam

Member
Messages
7,118
On an AB763 schematic (the one I am most used to working with) the Bias adjust goes between the 220k ressistors/.01caps and the bias board.

My amp has the Intensity knob between these points (since it is not using a Opto-Isolator type Vibe) Is this where I put the adjust?
Is the vibe circuit a problem with bias adjust?
 

pfflam

Member
Messages
7,118
I really appreciate the help folks . . . this site has helped me through all sorts of amp shenanigans over the years


hit my head in obviousness :bonk : any bias adjust along the 'Intensity' path would only act like the Intensity knob itself.

which then leads me to ask the question again: where would I put a bias adjust?

---

New developments -
I came across this:
Yes, the Princeton has a seperate bias board (you can see pics here: http://www.timeelect.com/vinfpa.htm). The bias resistor (27K) is mounted on that board. You remove it from the board and attach it to the bias pot as shown on Doug's page. You'll have to drill a hole through the chassis to mount the pot. I strongly suggest fitting the bias pot with a knob that has a clearly visible "pointer". Once you have the bias set, make a mark on the chassis and it will make it easy to see if the knob has been accidently moved. Also ggod for future reference. I found this fairly easy to do. If this still seems too complicated, you might want to get tech help.
and:
Pin 3 is the plate (high voltage), 8 is the cathode (mv).
So now I figure: find that resister (27k in regular Princetons), change it for a bias adjust with a resister attached, pop it in the Death Cap hole and ready to go . . . .
Does that sound good?
 

pfflam

Member
Messages
7,118
Wow . .. so I was right before my head hitting realization, and right also afterwards.
It seems strange that that would not result in mere Intensity changes.

Thanks - I kept looking at his page on biasing and missed that one, whereas the biasing page had too much language for me to understand . . :)
 

pfflam

Member
Messages
7,118
One more question: the way it is wired now there are two resisters instead of one
one is 22k the other is 20 k wired one after the other-

is this because the amp is 6L6 rather than 6V6?
Should I take that into account and work with a larger resistor and/or pot combination?
 

pfflam

Member
Messages
7,118
I'll do this:


Since it will let me make it adjustable from outside the chassis and I have assorted parts

I'm sure you meant for me to read this:
The 470 ohm bias range resistor value determines the sweep of the bias pot. If you want to change the voltage range that the bias pot sweeps across, change this resistor up or down in value to move the sweep up or down. A smaller value will give you more negative voltage in the bias circuit. More negative voltage means less current flowing in the power tube.
A larger value bias range resistor will drop the overall voltage of the bias circuit. Less negative voltage means more current flowing in the power tube.
Example: You can't get the current any higher than 20MA for each power tube and the bias pot is turned all the way up. You have too much negative voltage in the bias circuit. Make the bias range resistor larger in value and try again. Keep adjusting the bias range resistor until you can get the bias pot to sweep higher and lower than the desired voltage. If you want to be able to bias each power tube at 35ma of current, adjust the bias range resistor until you can get more than 35ma when the bias pot is turned all the way one way and less than 35ma when the bias pot is turned all the way the other way. Don't ask me what value bias range resistor you should use, you need to figure it out by following the instructions I just gave you. :)
 
Last edited:

pfflam

Member
Messages
7,118
So when I measure for sweep at what point on the amp do I measure voltage?
at pin #8 of the power tubes (without tubes)?
or pin #3

Should #8 read 35ma
(I am assuming that it is ok to measure without the tubes in, this is how I measured my Marshall build)

OR
should I merely be measuring post bias pot for a sweep that encompasses 34ish Ma and measure that at the tubes with my bias probe?
.
 

TweeDLX

Member
Messages
3,756
So when I measure for sweep at what point on the amp do I measure voltage?
at pin #8 of the power tubes (without tubes)?
or pin #3
I'm assuming by "measure for sweep" you mean the range or "sweep" of the bias pot. You need to mount a one ohm/1 watt resistor (Doug Hoffman has these and the test jacks) on pin 8 of each power tube. This requires removing the existing wire that goes to ground, and replacing it with the 1 ohm resistor. I run a wire from the tube pin to a test point, and run the 1 ohm resistor to ground from the test jack. Put one probe from your DMM in the jack (or between the tube pin and resistor if you don't use a jack), and the other probe on the chassis (ground). You should get a reading in mV. This translates directly into mA (your bias current). Leave the probe in the jack and turn your bias pot from all the way off, to all the way up. That's your range/sweep.
 

pfflam

Member
Messages
7,118
Thanks,

I was thinking of measuring the voltage before it hits the tubes -but then again I'm not sure this isn't what I mean . . . .
 

pfflam

Member
Messages
7,118
Well, I 've done the mods:
new electrolytics
three prong plug
a bias adjust

and it works fine

One thing I may want to do is change the capacitors that I am not sure what they are called, but they are in the picture and are Orange Drops, I want to tighten the bass up a bit - the schematic has these at .01
I have some Blue .047ufs and I believe I have some .022 . . . would either of these tighten the bass?



These Orange Drops are called?
 

TweeDLX

Member
Messages
3,756
These Orange Drops are called?
Billy and Tex.
I think you mean what is their function? They're the output coupling caps. The schematic should say they are .1uf, NOT .01uf. Using a .022 or .047 will REDUCE the AMOUNT of bass frequencies allowed to pass. You can also use a lower cathode bypass cap (4.7uf @ 25V or down to .68uf is good). To TIGHTEN bass, use a Cap can with 40uf/20uf/20uf/20uf instead of the 4 x 29uf you have.
 

pfflam

Member
Messages
7,118
Thanks,

which is the cathode bypass cap?

Right now the can is filled with 2x (22uf 500v) and 4x (100uf 350v)
if that is unusual I think it may be due to the 6L6 circuit
 

LarryN

Member
Messages
955
Thanks,

which is the cathode bypass cap?

Right now the can is filled with 2x (22uf 500v) and 4x (100uf 350v)
if that is unusual I think it may be due to the 6L6 circuit
I'd like to see the cap arrangement to figure out what you have going. Right now, "2x (22uf 500v) and 4x (100uf 350v)" doesn't make much sense to me. Usually you'd have 2x 220uf 350v in series plus 3x 20uf 500v in a 2X6L6 Fender.

Stock Fender cathode bypass caps are usually 25uf/25v on pins 3 & 8 of some of the preamp tubes. Since it's heavily modified, I don't know how your amp is configured.

Do this: The current is measured in mV across the 1 ohm resistor at pin 8 to ground. You can do this at the tube if you don't have test points installed. If you can adjust it to 32-35 mV with adjustability above and below that point, you're good to go. If not, you'll need to raise or lower the resistor value on the casing of the bias pot, until you can get a good mV range. You can't get a mV reading without the tubes in.

I would put a fresh bias supply cap in there, too. It looks like you have an old one in there. Don't forget positive to ground.
 




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