Please help me bias my Princeton Reverb...Please

[Josey]

Would someone be kind enough to help me walk through the steps of biasing my '66 Princeton Reverb? I know how to use a voltmeter and how to take measurements safely but for the life of me I feel like I'm either skipping a step or not doing the calculations correctly because my results just don't add up.

I've printed out the schematic and layout (AA764) and understand all the components but I'm confused by some of the readings I'm getting. Would someone please be kind enough to walk a noob through the process and explain the steps? I've tried watching YouTube clips and reading other posts but i can't grasp it.

Here's what I'm working with:
NOS Mullard GZ34 rectifier
(2) JJ 6V6 S power tubes

I'll gladly take measurements and post the results if some of you with years of experience and knowledge would be kind enough to walk me through it.

Please no negative karma...I'm already pulling my hair out trying to figure this out.

 

[clintj]

Let's get a baseline here. First, do you have a bias probe? Second, are you fully comfortable with pulling the chassis out enough to get a plate voltage reading on the power tubes, and working safely in a live amp?

 

[BobbyZ]

I bet a 66 isn't an AA764 it's an AA1164 with a left over tube chart most likely.
One way to tell is the AA764 has a center tapped filiment and the AA1164 doesn't. So have a look at the pilot light if there's an artificial center tap there it's an AA1164.
That's just a fancy way of saying two 100 ohm resisters going from the pilot light to ground.

I know this because I have a real 64 AA764 sitting on my bench with a burnt up PT. That's what happens when you combine poor eye sight, a weak little PT, a bad tube and a 3 amp fuse sure looks likes a one amp fuse.

Muchxs will hopefully chime in here to tell you for sure but my guess is the GZ34 isn't the rectifier for this amp. But don't stick a 5u4 in it untill he says your PT is ok with it !

Last thing you want to do is replace a PT in an amp worth what a clean BFPR is. Expecially a no logo early one !
Yeah I'm getting reading glasses.

Ok bias. Have you put a bias pot in yet ? Or are you going to sub resisters?

To measure bias with just a multi meter you can do a transformer shunt. But it's not the safest method in the world. You have to rember when one probe is hooked to 450 volts the other probe is hot when set to miliamps.
Say for instance you put the live probe in your left hand so you can write down the miliamps you're getting.
Also if you touch anything wrong you'll fry the little fuse inside your meter. So buy a bunch of THE RIGHT ONES before you start.

Gerald Weber actually has a video on YouTube of this method on a Deluxe Reverb. Of couse that has a bias pot but otherwise it's the same process.
Weber Speakers has an online bias calculator or they did? A Google search will find that.

Hoffman amps has the info for the bias pots somewhere on his site.

Again hopefully Muchxs will chime in about the PT and shead some light on the AA764 vs AA1164 rectifier situation. He had it nicely laid out in a thread but finding it would take some time.

One last thing. Are all the electrolytics fresh? Expecially the cap can and bias cap. If not stop now and do that first.
Rember that PT is from a Champ don't push it.

 

[jhundt]

and does your voltmeter work at 600VDC? I have noticed that many voltmeters today are limited to 250V. That won't be enough to measure your plate voltage.

 

[bobscribbly]

Simple way to tell if your Princeton is biased correctly is if tremolo actually works as the trem on these is directly affected by amps power tube biasing [2 hot no trem - 2 cold no trem] pretty easy to tell really - if only all amps were this simple to bias right.

 

[Josey]

Thanks guys.

(clintj) Yes I have a BiasPro tube bias probe that a just purchased from A-Sharp Fretworks. It's the MQ13 version with two probes and one probe has the switch so I can flip between plate voltage and cathode current readings. I purchased it because I figured it was "fool" proof so I couldn't screw up. I have a couple of tube amps so I thought it would be a good investment.

I've also pulled the chassis and understand how to drain the caps for safety and how to safely take measurements from a live amp. My voltmeter also works at 600VDC so that's no issue. I tried biasing just using my voltmeter and some calculations but thought maybe I was doing it wrong so that's why I forked out the cash for the Bias Pro.

The problem is that the results from my BiasPro readings and my voltmeter measurements aren't corresponding. So either my tools aren't functioning properly or my method is wrong. I'm trying to figure out which is which.

(BobbyZ) Yes it has two 100 ohm resistors going from the pilot light to ground. But these were obviously added later or replaced at some point because they are metal oxide resistors and everything else is carbon. (see photo) I've also ran a 10k trimmer pot in series with the 22k resistor on the bias board to give me some adjustment. I have another 27K/10k trimmer combo that I can drop in there if I need more resistance but if i need more that 37k I'll have to order something larger.

I also had a conversation with a tube guy about using the GZ34 rectifier instead of the 5U4GB. He was a tube guy not an amp guy, and I don't understand exactly how every component works, but what i took away from the conversation we had was that the GZ34 had some pros and cons when used. Con being that it would cause my power tubes to run hotter but because of the JJ 6v6s I was using, they could handle it. Pro being that it allowed the tubes to slowly warm up rather than being hit with current all at once as with the 5U4.

I may not be explaining that correctly so don't quote me, but that's how I understood it. I have an old General Electric 5U4GB if that's what I need to use instead. I can save the GZ34 for my Deluxe Reverb. I just want to use what's best for operation.

Yes all the filter caps and electrolytics have been replaced so I'm good there. Gerald Weber's video is actually the one I watched and took notes on his process, but it being a Deluxe with a bias pot he was working on and not a Princeton I thought maybe there is a different method to follow. I can type out the steps I took and give you my measurements if that would help decipher whether or not I am doing it correctly.

 

[Josey]

Simple way to tell if your Princeton is biased correctly is if tremolo actually works as the trem on these is directly affected by amps power tube biasing [2 hot no trem - 2 cold no trem] pretty easy to tell really - if only all amps were this simple to bias right.

Well I have noticed changes in the tremolo when trying to get the bias set correctly. It may become faint or sound very pronounced, but I haven't lost it entirely.

 

[BobbyZ]

Is that the original power transformer? The soldering looks a little funny on the chassis.
Rember all it takes to have the correct tranny codes is four screws and an end bell swap.

Anyway main reason I ask is because of the 5U4s higher current draw on the heaters vs a GZ34. At 3 amps vs 2 it's like running a GZ34 and a half. To me that's more of a concern that the warm up thing.

Really though Muchxs and Wally know a lot more about these things than I do.

 

[muchxs]

Please no negative karma...I'm already pulling my hair out trying to figure this out.

Negative Karma Poster Boy here.

I don't generally comment on "How do I bias my amp?" threads.

Sign on the wall sez,

"We fix it... $35 per hour.

You try to fix it first... $350 per hour."


If you don't have a bias probe you'll be poking around in high voltage with approximately 450v on the plates and close to 700v across the rectifier. I was pokin' around in a real '65 PR tryin' to dial in that last tenth of a milli... I didn't pay attention to where my fingers were. The high voltage actually arced from the first filter to my fingertip in a wake up call shock. It left a little white arc burn that became a blister. A week or so later it was a reminder of the incident whenever I fretted a note using that finger.

It's your best opportunity to damage yourself and / or a valuable vintage amp. With a bias probe it's still your best opportunity to damage a valuable amp. In particular there are too many things in a Princeton Reverb that can be exactly right or exactly wrong. There isn't a lot of wiggle room.

Again hopefully Muchxs will chime in about the PT and shead some light on the AA764 vs AA1164 rectifier situation. He had it nicely laid out in a thread but finding it would take some time.

PRs up to 1970 often have the 022772 PT. It has a 2 amp rectifier filament and will not support a 5U4G without getting hot. Expect around 440v on the plates, regardless.

It's Catch 22: It would be nice to be able to use a 5U4G to drop the supply voltage a little but you'll cook your transformer.

Today's Top Secret Tip: '71 and later Princeton Reverbs used the superior 010020 "export" transformer. While it's not a premium piece it's a cut above the earlier 022772 "Champ" transformer or the roughly interchangeable 125P1B.

Remember... it's a secret! Don't tell anyone.

I also had a conversation with a tube guy about using the GZ34 rectifier instead of the 5U4GB. He was a tube guy not an amp guy, and I don't understand exactly how every component works, but what i took away from the conversation we had was that the GZ34 had some pros and cons when used. Con being that it would cause my power tubes to run hotter but because of the JJ 6v6s I was using, they could handle it. Pro being that it allowed the tubes to slowly warm up rather than being hit with current all at once as with the 5U4.

I may not be explaining that correctly so don't quote me, but that's how I understood it. I have an old General Electric 5U4GB if that's what I need to use instead. I can save the GZ34 for my Deluxe Reverb. I just want to use what's best for operation.

I'll keep it simple 'cuz "The Easy Way Is Hard Enough!!"

DO NOT use a 5U4G in a pre- 1970 Princeton Reverb.

Here's what I'm working with:
NOS Mullard GZ34 rectifier
(2) JJ 6V6 S power tubes

Something to be aware of with JJ 6V6s: They're a weirdo new 6V6 that everyone loves because they dissipate more than NOS 6V6s. They act more like 6L6s. A stock Princeton Reverb doesn't have enough current available to support 6L6s. There Ain't No Such Thing As A Free Lunch. JJ's data sheet says they pull 70ma on the plates in Class AB1 although you may exceed that before they redplate. If you have a 022772 PT... 70ma is all she's got.

 

[Josey]

I only assume that is the correct PT because everything else was period correct and as you said the codes matched up. How could I check for certain? If you're referring to the scratches and soldering on the chassis... that's where I cleaned up a spot and re-soldered the grounds when I was in there replacing the cap can.

I'm guessing the higher current draw is actually better for the life of the PT and output tubes? Like I said, I don't understand exactly how everything works but I am able to put things together and take them a part.

Kind of like a car I guess. I may not understand how the engine in a vehicle does what it does, but if I give my symptoms to a mechanic and he could diagnose the problem, I'm able to find the part and fix it. I guess I'm trying to save money on labor.

 

[Josey]

Well muchxs the problem for me is that I don't know of any techs in my area that I would trust working on my gear. The closest guy I know with a good rapport is an hour and a half away and lets just say the guys down at Guitar Center don't seem to be the most technically savy.

I have thick skin so don't worry with hurting my feelings. I can take it and dish it out. But I've read posts in the past where people ask for help and are met with comments like "if you don't understand what your doing you should take it to a tech". I'm not looking to build an amp from scratch but I would like to understand how to change the oil per se.

 

[jazzguitar]

I only assume that is the correct PT because everything else was period correct and as you said the codes matched up. How could I check for certain?

To my knowledge you can't. Hints are the plate voltage and the temperature in use. Using a 5U4GB rectifier and seeing whether the transformer gets really hot indicates it was not meant to use a 5U4GB. If your plate voltage is above 420 Volts when using a GZ34 that indicates it was intended for a 5U4. These are hints, not proof!

However, if your transformer is original in a 1966 PR, the correct rectifier is a GZ34.

I'm guessing the higher current draw is actually better for the life of the PT and output tubes?

On the contrary. Higher current capability is, but not draw. Use a GZ34.

As for biasing: if you use a probe (inserted between sockets and tubes) and the power tubes' idle currents are between 15mA and 24mA, and matched within 10%, your amp is OK.
Make sure your high voltage is in the 390V to 410V range.

I do not use a bias probe but read the voltage from the read output transformer lead to the blue, and from the red to the brown (blue and brown on the tube sockets pin 3) and make sure this is between 2.5V and 4V which corresponds to the currents listed above. You should have roughly -35 Volts on the grids (pins 5)


CAREFUL! Do not slip with multimeter probes, do not touch any metal!!! Best to wear thick rubber gloves.

 

[muchxs]

To my knowledge you can't. Hints are the plate voltage and the temperature in use. Using a 5U4GB rectifier and seeing whether the transformer gets really hot indicates it was not meant to use a 5U4GB. If your plate voltage is above 420 Volts when using a GZ34 that indicates it was intended for a 5U4. These are hints, not proof!

Consider that any Princeton Reverb in Germany has either been converted to 240v / 50hz or is running a convertor transformer. Actual measured plate voltage in vintage U.S. market Princeton Reverbs is usually an off the charts (the charts being the 6V6 data sheet) 440 plus volts. The "reissues" are right up there, too.

I have thick skin so don't worry with hurting my feelings. I can take it and dish it out. But I've read posts in the past where people ask for help and are met with comments like "if you don't understand what your doing you should take it to a tech". I'm not looking to build an amp from scratch but I would like to understand how to change the oil per se.

I work on vintage amps where the owners knew just enough to fearlessly dive in where they should have thought better of it. It's often a quick way to convert a $2500 amp into a $1000 project or a $1000 amp into a $300 project. Got one right here right now. It may have had one problem before the owner messed with it. It came in with half a dozen problems while it's unclear what the root cause of the original problem is or was.


"The Easy Way Is Hard Enough!!"

(link removed)

Considering 440v on the plates dial in 22.5 ma using your bias probe and you're there.

 

[telex76]

Is the tremolo on while you are trying to take readings?

 

[Josey]

Is the tremolo on while you are trying to take readings?

Not sure what you mean but if you're talking about the speed and intensity controls being on then no...both control knobs are dialed all the way down.

Which brings up a question I've always meant to ask... When taking readings for things such as biasing an amp, where should your controls be set? Should they all be dialed down or dialed up or half way or what? I've always dialed mine all the way down but not sure if that is proper procedure.

 

[Josey]

I do not use a bias probe but read the voltage from the read output transformer lead to the blue, and from the red to the brown (blue and brown on the tube sockets pin 3) and make sure this is between 2.5V and 4V which corresponds to the currents listed above. You should have roughly -35 Volts on the grids (pins 5)

This sounds like the method I tried before purchasing the bias probe. Is this what's called the transformer shunt method? When I get home I'll carefully explain my steps and tell you my readings. My problem is that some of my readings never seem to stabilize which leads me to believe something isn't right. I'm not talking about fluctuating back and forth but rather a slow constant increase or decrease in the numbers.

 

[telemnemonics]

There Ain't No Such Thing As A Free Lunch.[/B] JJ's data sheet says they pull 70ma on the plates in Class AB1 although you may exceed that before they redplate. If you have a 022772 PT... 70ma is all she's got.

Does this mean that JJ 6v6 over tax an already overtaxed pre 1970 PR PT?

Would you put JJs in a '66 PR?

Edit: Sorry Muchxs for butchering up your post, I try not to butcher stuff up, but...

 

[muchxs]

Does this mean that JJ 6v6 over tax an already overtaxed pre 1970 PR PT?

Would you put JJs in a '66 PR?

Edit: Sorry Muchxs for butchering up your post, I try not to butcher stuff up, but...

HA! You're apologizing to the resident Post Butcher.

I could go off the reservation I mean way off topic and talk about cheap guitars or my latest epic dumpster dive.

I wouldn't put JJs in a Princeton Reverb because I have half a dozen matched sets of excellent NOS 6V6s all cued up and waiting for the next Princeton Reverb to show up.

We usually discontinue our excursions to Maine when the coast gets cold and inhospitable. If you need a stellar tube set let me know. Not a commercial post... I generally don't sell tubes. Any tube I sell inevitably costs me more to replace.


Further clarification on "I generally don't comment on 'how do I bias my amp?' threads:

Several years ago a few of us proposed an amp build- off like the TDPRI guitar build- offs. I think it was Printer2's idea but I 100% support a contest I'd surely win. Anyway, word came down from The Powers That Be and I paraphrase,

"The TDPRI does not sanction or endorse endeavors that promote experimenting with dangerous voltage and / or electrocution hazard."

 

[bparnell57]

Several years ago a few of us proposed an amp build- off like the TDPRI guitar build- offs. I think it was Printer2's idea but I 100% support a contest I'd surely win. Anyway, word came down from The Powers That Be and I paraphrase,



"The TDPRI does not sanction or endorse endeavors that promote experimenting with dangerous voltage and / or electrocution hazard."

Probably a few months ago, I got together with some members and asked if we could do an amp build challenge. We got the same response.

The way I see it, my machining practices are a lot scarier than even a few thousand volts. I'd rather be knocked out than lose an arm up to the elbow.

Routers, belt sanders, band saws, drill presses, and mills, are all very dangerous without proper precautions.

I remember I asked to open up an amp that had been serviced under warranty at Sam Ash, and they told me I couldn't even take the back panel off due to the danger of high voltage.

Next time, I'll keep my tools in my car. I love seeing people get so uneasy when I start talking about 2.5 kilovolt microwave power transformers, which can deliver that voltage at a half amp. Basically an electric chair...

Well it's not dangerous if you take every precaution. It's just easier to be stupid with something you can't see rather than a giant saw blade.

Worst I've ever been hit was by a powerful Van de graph generator out of one of those novelty arc lamps that make the Lightning bolts go towards your hand when you touch the glass. Well the glass came off and I picked up the still on lamp by the top. Yowza...

Then again I also witnessed someone drill into the back of their hand and pull the bit out with a foot long ligament wrapped around it that had been ripped out of their arm up to the elbow.

ITS ALL DANGEROUS

Message of the day: be safe when working on anything.

 

[Josey]

PRs up to 1970 often have the 022772 PT. It has a 2 amp rectifier filament and will not support a 5U4G without getting hot. Expect around 440v on the plates, regardless.

It's Catch 22: It would be nice to be able to use a 5U4G to drop the supply voltage a little but you'll cook your transformer.

Today's Top Secret Tip: '71 and later Princeton Reverbs used the superior 010020 "export" transformer. While it's not a premium piece it's a cut above the earlier 022772 "Champ" transformer or the roughly interchangeable 125P1B

My PT is the 125P1B which I'm guessing is the precursor and cross reference to the 022772 you mention.

Going back to BobbyZ's first post he talks about a centered tapped transformer vs an artificial center tap with two 100 ohm resistors leading to ground from the pilot light. The schematics I've seen look as if these were written in and not part of the original design. Can someone explain these to me? Should I leave them in place or remove them?