Princeton Reverb bias pot... as per Doug Hoffman

[King Fan]

Just cuz I like drawing in DIYLC, and really like Doug's external-adjust bias pot in my Princeton Reverb and similar amps, I offer this. Any errors are mine, not his.

Doug's classic diagram (his link here has helpful notes):

My DIYLC:

Details: The pot can go in a hole drilled near the bias board, or in an empty hole (I used the ground hole in the back panel), or mounted bushing-up (see wiring note) for internal adjustment. Since the pot requires a skinny screwdriver to adjust (and the slot is set well back in the bushing) there's no risk someone will give it a curious twist. OK, I have seen someone try to plug in a cable, but, um, nothing happened.

Doug's 27K tail resistor often seems to work nicely in a PR; in amps like the 6G2 and 5F11, a smaller resistor (15-22K?) is sometimes needed to center the bias range. And note the 100K resistor here is a bigger, 'knockdown' resistor for the high voltage from the rectifier -- if you have a ~50V bias tap, you need to downsize.

 

[Lowerleftcoast]

high voltage from the rectifier

This phrasing and the "From rectifier HT" on the layout, may give the wrong impression to the uninitiated. (Remember back to when we didn't *get* the rectifier somehow makes DC?)

"high voltage from the PT" would be less confusing.

Doug's layout reads, "Pin4 - Rectifier tube socket". Perhaps your layout could suggest a pin on the rectifier socket as a place to acquire the HT from the PT.

HT = high tension = high voltage. HT can mean either AC or DC.

For those of us that are *ground* challenged. (You know where I'm going KF. hah.) Doug's layout shows a *schematic style* ground triangle. Your layout suggests a certain type of ground scheme. Maybe yours can be more generic? Again, I am thinking a different ground scheme may be in front of a newbies eyes, making this layout confusing.

 

[Ten Over]

You might consider a 1M resistor between the wiper and the lug going to the diode just in case the wiper should happen to fail. Or you could design it such that the bias defaults to coldest upon wiper failure like this:




The 10k screwdriver actuated pots that I've used just about forever are accessible from either side so that they are always adjustable externally or internally.

 

[Ten Over]

A couple more:

 

[King Fan]

This phrasing and the "From rectifier HT" on the layout, may give the wrong impression to the uninitiated. (Remember back to when we didn't *get* the rectifier somehow makes DC?)

"high voltage from the PT" would be less confusing.

Doug's layout reads, "Pin4 - Rectifier tube socket". Perhaps your layout could suggest a pin on the rectifier socket as a place to acquire the HT from the PT.

HT = high tension = high voltage. HT can mean either AC or DC.

For those of us that are *ground* challenged. (You know where I'm going KF. hah.) Doug's layout shows a *schematic style* ground triangle. Your layout suggests a certain type of ground scheme. Maybe yours can be more generic? Again, I am thinking a different ground scheme may be in front of a newbies eyes, making this layout confusing.

You might consider a 1M resistor between the wiper and the lug going to the diode just in case the wiper should happen to fail. Or you could design it such that the bias defaults to coldest upon wiper failure like this:

View attachment 1039333
View attachment 1039334

The 10k screwdriver actuated pots that I've used just about forever are accessible from either side so that they are always adjustable externally or internally.

A couple more:

View attachment 1039335
View attachment 1039336

Thank you, gentlemen! Those are 100% excellent ideas, and do three great things: a) better reflect Doug's original wisdom b) variously upgrade and improve and generalize the diagram c) give me an excuse to play some more in my DIYLC toy chest…

 

[King Fan]

Test revision:

@Lowerleftcoast , does this work for your good ideas about the rectifier voltage and the ground scheme issue?

@Ten Over , I chose the jumper approach, since it mirrors Rob, is super simple, and costs $0.00. Does this make sense? The wording of notes on diagrams is tricky -- more information? or more chances to get confused?

Also, you're absolutely right that slotted-actuator pots are adjustable from both sides -- if mounted in a through hole. We do see a fair number mounted bushing up for internal-only use. Trying to make the clockwise=hot thing work for both mountings introduces more potential confusion. I've reworded my former cumbersome note into a different cumbersome note.

Keep those cards and letters coming in -- IMHO this simple mod is one of the very most useful for this class of amp...

 

[Lowerleftcoast]

I hope that should do it.

Now you have to show the design when a lower voltage bias tap from the PT is utilized.

 

[King Fan]

I hope that should do it.

Now you have to show the design when a lower voltage bias tap from the PT is utilized.

Good idea (though the diagram needs another note like, um, you know). Let's see, am I right that the standard Fender bias board (with a ~50V bias tap) uses a 470Ω 'range' resistor? I have a pic in my notes -- somewhere.

 

[Lowerleftcoast]

I always forget that terminology... *range resistor*. It is not part of a proper voltage divider so it is not easy to explain how to calculate a reasonable value. Yes, 470R was a value Fender used but they also used 6.8k, 10k, etc. It depends on the voltage of the tap and the value of the tail resistance.

 

[Ten Over]

Test revision:

View attachment 1039613

@Lowerleftcoast , does this work for your good ideas about the rectifier voltage and the ground scheme issue?

@Ten Over , I chose the jumper approach, since it mirrors Rob, is super simple, and costs $0.00. Does this make sense? The wording of notes on diagrams is tricky -- more information? or more chances to get confused?

Also, you're absolutely right that slotted-actuator pots are adjustable from both sides -- if mounted in a through hole. We do see a fair number mounted bushing up for internal-only use. Trying to make the clockwise=hot thing work for both mountings introduces more potential confusion. I've reworded my former cumbersome note into a different cumbersome note.

Keep those cards and letters coming in -- IMHO this simple mod is one of the very most useful for this class of amp...

The stock load resistor is 27k. You wouldn't want to replace that with a 27k in series with a 10k variable resistor because that would only allow a colder range from 27k to 37k. I used a 20k resistor so that the load range is from 20k to 30k. With a 340Vrms HT secondary winding, I got a bias voltage range of -29Vdc to -42Vdc.

 

[Ten Over]

I always forget that terminology... *range resistor*. It is not part of a proper voltage divider so it is not easy to explain how to calculate a reasonable value. Yes, 470R was a value Fender used but they also used 6.8k, 10k, etc. It depends on the voltage of the tap and the value of the tail resistance.

Seems like everybody has their own terminology which is fine as long as they give some kind of clue as to what they mean by a particular term. I have called the "range resistor" the "current limiting resistor" for most of my life, but not very many other people do this. Since I am in a minority camp with this, I am way more prone to call this the "range resistor" these days.

Fender used 470R a lot, but they also used 1k and 1.5k in different individual amps of the same model. I would say that they didn't know what voltage was going to come out of the bias tap until they fired it up.

 

[Lowerleftcoast]

I have called the "range resistor" the "current limiting resistor" for most of my life, but not very many other people do this.

In keeping with the *real* function of the bias circuit, it is just DC rectified from AC, "current limiting resistor" is a more accurate descriptor. Continuing on, the *bias cap* is a reservoir cap. All of this reminds me of the "Brown Super Bias issue" thread discussion, where @SoK66 wrote of the 15k resistor. What descriptor would be proper terminology for the 15k resistor in a rectified (smoothing) circuit? We could call it a dropping resistor, R of the RC filter it creates, or R1 of the voltage divider with (R2) the 56k resistor.

Layout of Brown Super bias board:


If King Fan added a variable resistor to the Brown Super, the 56k resistance would be replaced with a pot and resistor like in his examples above.

I just noticed, the Brown Super doesn't have a current limiting resistor.

 

[King Fan]

The stock load resistor is 27k. You wouldn't want to replace that with a 27k in series with a 10k variable resistor because that would only allow a colder range from 27k to 37k. I used a 20k resistor so that the load range is from 20k to 30k. With a 340Vrms HT secondary winding, I got a bias voltage range of -29Vdc to -42Vdc.

Good point; the theoretic split-the-difference value with a 10KL pot would be a 22K resistor. I used 27K in mine cuz Doug drew it like that, way back in the DotMatricene era, and it apparently works — he never revised it, for one thing. Of course, he was also suggesting you could just re-use the existing resistor in a stock PR. But more than his old suggestion, and the re-use thing, 27K works nicely in my PR, with it's 330V HT. And I've seen quite a few other PR builds where it works. Admittedly I'll have to get in there and measure the bias voltages sometime.

Any other PR builders with experience using a 10KL bias pot? What tail resistor did you use? Maybe 22K would be more centered more often, but 27K is 'good enough'?

Doug mentions you may have to adjust one or both resistor values, and tells you how, but I'm not going to repeat that long discussion with my diagram. I should more clearly refer readers to his page for insights and adjustments...

 

[King Fan]

If King Fan added a variable resistor to the Brown Super, the 56k resistance would be replaced with a pot and resistor like in his examples above.

I just noticed, the Brown Super doesn't have a current limiting resistor.

Heh, this is definitely *not* about a generalizable bias circuit. Merlin notes there are almost as many bias-adjust circuits out there as there are amp models. BTW, for folks who want a simple, fairly universal circuit his elegant version (using a 10KL pot!!!) is both powerful and flexible.

 

[Ten Over]

...the theoretic split-the-difference value with a 10KL pot would be a 22K resistor. I used 27K in mine cuz Doug drew it like that, way back in the DotMatricene era, and it apparently works — he never revised it, for one thing.

The center of the resistance range is not where the center of the voltage range occurs. It's another one of those non-linear things.

Doug's strategy and my strategy are two different things. Doug's version doesn't change the load in the bias circuit and the variable bias voltage is achieved by moving the wiper. My version does change the load and the variable bias voltage is achieved by altering the bias circuit.

 

[Ten Over]

What descriptor would be proper terminology for the 15k resistor in a rectified (smoothing) circuit? We could call it a dropping resistor, R of the RC filter it creates, or R1 of the voltage divider with (R2) the 56k resistor.

I'm not the best person to ask about proper terminology. I would be tempted to call it a dropping resistor, but I would probably also call out the nominal value.

 

[Ten Over]

Here is a diagram showing the two different styles of variable bias discussed here.

 

[King Fan]

Thanks. As you can tell, I didn’t quite “get it” looking at the layouts.