High plate voltage- Princeton

[shoestring]

I'm restoring a 68 Princeton Amp, and I've encountered a problem. The plate voltage is at 477- too high for the Tung Sol 6V6s in it now. They're poorly matched- more than 6 mA difference in bias current, so they will be replaced, but isn't 477v too high for any 6V6? I tried replacing the GZ-34 rec tube with a 5Y3, but that only knocked the plate v down to 458. What can be done to bring it down? 

Thanks for your interest, everybody. I've done my best to answer all your questions, and will post more information as I can. One thing- this amp has been the victim of more than one tweaker. When my friend gave it to me, it didn't work at all and someone had tried to convert it to run 6L6s. Perhaps they did a circuit tweak somewhere to jack up the plate voltage? Anyway, it sounds fantastic and it wants to live on. Too bad I can't keep the high pV and just pop in a really stout pair of 6V6s. The sound of slowly-melting tubes is SOOOO sweet!

[Willabe]

Got a model #?

Should be on the tube chart inside the cab.


             Brad     

[EL34]

What is the bias current set at?


Plate voltage is related to how much current is running in the amp

[tubeswell]

One of the screens could be knackered if the tube current is way different. What is the tube current overall?

[Fresh_Start]

What is the bias current set at?


Plate voltage is related to how much current is running in the amp

Bias voltage at junction of two 220K grid leak (return) resistors might be helpful too.  I serviced a slightly older Princeton Reverb whose bias circuit filter cap was almost a dead short.

Dumb question:  is the 477 volts with the power tubes in the amp or without them?  Unloaded, the B+ voltage could be that high.  I ask because I'd expect more than 20 volt drop going from GZ34 to 5Y3 (unless it's a modern Sovtek 5Y3 which really is a different tube anyway).

Respectfully,

Chip

[shoestring]

Got a model #?

Should be on the tube chart inside the cab.


             Brad     

The tube chart is shredded, unfortunately. No model or date stamp survived. It's an AA964, though, based on a production date code stamped inside the chassis "T351968" and date codes on the original parts. The power transformer is a replacement.

[shoestring]

What is the bias current set at?
I set the bias so that the strong tube is at 18.9mA, weak tube at 12.6

Plate voltage is related to how much current is running in the amp

[shoestring]

One of the screens could be knackered if the tube current is way different. What is the tube current overall?

Screen grid resistors are well matched and okay as far as I can tell. These are my spare 6V6s- sound okay and run safely in any amp. In other words I know them well, and they always run more than 6mA apart no matter what amp I put them in.

[shoestring]

What is the bias current set at?

I set the bias so that the strong tube is at 18.9 mA, weak one at 12.6. I replaced the bias resistor and the range resistor when I put in the bias pot.

Plate voltage is related to how much current is running in the amp

Bias voltage at junction of two 220K grid leak (return) resistors might be helpful too.  I serviced a slightly older Princeton Reverb whose bias circuit filter cap was almost a dead short.

That's on my suspect list. The 4X20 uF can cap is a fairly new CE and it tests okay at no load, but its heat rating is only 55C, not 85C like the original caps were. I spoke to the good folks at CE (acquired by our friends at AES I believe) and they told me they are unable to use the toxic, carcinogenic materials that made the old caps work so well for so long, and they are in the process of working out substitutes

Dumb question:  is the 477 volts with the power tubes in the amp or without them?  Unloaded, the B+ voltage could be that high.  I ask because I'd expect more than 20 volt drop going from GZ34 to 5Y3 (unless it's a modern Sovtek 5Y3 which really is a different tube anyway).

Tubes in. The 5Y3 is an ancient Raytheon, from 1961 or so.

Respectfully,

Chip

[Willabe]

What's the AC voltage at pins 4 and 6 on the GZ34?

What's the number stamped on the PT?


             Brad   

[shoestring]

What's the AC voltage at pins 4 and 6 on the GZ34?

What's the number stamped on the PT?

M022772. Hmmm- wrong transformer, isn't it? I never even thought to check that.
The EIA number is 606-939. According to the Fender Forum, it's the correct PT for a Princeton.

             Brad   

[Willabe]

What's the AC voltage at pins 4 and 6 on the GZ34?



                 Brad   

[Willabe]

Did a search and found this;

"The 022772 is the correct FMI part number for a Princeton Reverb. I'm not sure of the "M" prefix. The EIA Schumacher Woodard 606117 could be '71 or '81."


            Brad   

[EL34]

You still have not told us what the bias current is set at?

[shoestring]

You still have not told us what the bias current is set at?
Okay, educate me. Where and how do I measure?

Did a search and found this;

"The 022772 is the correct FMI part number for a Princeton Reverb. I'm not sure of the "M" prefix. The EIA Schumacher Woodard 606117 could be '71 or '81."

Yes, I was just there- Blues-n-Rock I think, posted in 2009. I'll go check those voltages. Back in 20 min. Thanks.

            Brad   

[Willabe]

What is the bias current set at?

I set the bias so that the strong tube is at 18.9mA, weak tube at 12.6

He had the quotes tangled.


                    Brad   

[EL34]

You have to bias the power tubes to a certain current flow in order to set the bias

Plate voltage means nothing if the tubes are not biased correctly
Plate voltage goes up and down with how much current the power tubes are set at.

This will help
http://el34world.com/charts/fenderservice5.htm

[shoestring]

You still have not told us what the bias current is set at?

Thought I did. 12.6mA on the weak tube, 18.9mA on the other. If you're asking for something else, you'll have to educate me.

[EL34]

Sorry, I did not see that because you are wrapping your post in quotes as if someone else is saying that


You must be typing your post inside the quotes brackets

[EL34]

Sorry, I did not see that because you are wrapping your post in quotes as if someone else is saying that

You must be typing your post inside the quotes brackets

The text above is a quote from a post, now I am typing below that quote
You can raise that bias current up to 25ma easily and your plate voltage will be lower

[shoestring]

What is the bias current set at?


Plate voltage is related to how much current is running in the amp

Bias voltage at junction of two 220K grid leak (return) resistors might be helpful too.  I serviced a slightly older Princeton Reverb whose bias circuit filter cap was almost a dead short.

The bias voltage at that point measures -50.1

Dumb question:  is the 477 volts with the power tubes in the amp or without them?  Unloaded, the B+ voltage could be that high.  I ask because I'd expect more than 20 volt drop going from GZ34 to 5Y3 (unless it's a modern Sovtek 5Y3 which really is a different tube anyway).

Respectfully,

Chip

[shoestring]

What's the AC voltage at pins 4 and 6 on the GZ34?

                 Brad   

367v


Edit; untangled quote, Brad

[Willabe]

When you hit the 'quote' button you see a line of code at the top, then the quote text. At the very bottom you'll see [/quote]

Type your response AFTER that last bit of code.

Your adding your text into the original quote.



                     Brad   

[shoestring]

Sorry, I did not see that because you are wrapping your post in quotes as if someone else is saying that

You must be typing your post inside the quotes brackets

Okay, I don't know what that means. I'm just typing in the box that comes up when I click "quote". What's the right way to reply? There's  no "reply" tab or link.

Edit; See post above. Brad   

[shoestring]

When you hit the 'quote' button you see a line of code at the top. At the very bottom you'll see

[/quote]

Okay, is this better? Makes it hard to reply to questions individually.

[Willabe]

Yes but you have the [/quote] typed in twice, you only want it typed once.

Go back to that post, hit the 'modify' button and erase the 2nd [/quote], then hit the save button bottom right.


               Brad   

[EL34]

Yes that is correct
You only need to quote when you are answering a question inside the quote
Otherwise, just hit the reply button if there is no need to quote


And as Brad said, type your response below the slash quote


You can also edit the text inside the quote
There's no need for all of the text if you are just answering a small portion of the text inside the quote

The reply button is at the bottom of all the post
See my screen shot, it shows the reply button

[shoestring]

Okay, got it. This just puts a new reply at the bottom of the stack, though. How do I reply to a specific post?
Never mind. Our posts crossed. The answer was one post ago.
Thanks
Steve

[Willabe]

There's  no "reply" tab or link.

Yes there is, very bottom right.


                Brad     

[EL34]

Okay, got it. This just puts a new reply at the bottom of the stack, though. How do I reply to a specific post?

You use quote to reply to a post up the page somewhere so people know what you are replying to

[EL34]

I'm restoring a 68 Princeton Amp, and I've encountered a problem. The plate voltage is at 477- too high for the Tung Sol 6V6s in it now. They're poorly matched- more than 6 mA difference in bias current, so they will be replaced, but isn't 477v too high for any 6V6? I tried replacing the GZ-34 rec tube with a 5Y3, but that only knocked the plate v down to 458. What can be done to bring it down? 

Thanks for your interest, everybody. I've done my best to answer all your questions, and will post more information as I can. One thing- this amp has been the victim of more than one tweaker. When my friend gave it to me, it didn't work at all and someone had tried to convert it to run 6L6s. Perhaps they did a circuit tweak somewhere to jack up the plate voltage? Anyway, it sounds fantastic and it wants to live on. Too bad I can't keep the high pV and just pop in a really stout pair of 6V6s. The sound of slowly-melting tubes is SOOOO sweet!

This is a quoted reply to your post #1 at the top of the page

[Willabe]

Okay, got it. This just puts a new reply at the bottom of the stack, though. How do I reply to a specific post?

You can quote them and edit the quote down to just the question your answering.

Or you can;  @ Brad,  then type your answer.


                  Brad   

[shoestring]

Only problem, once I hit the reply button, the quote button is gone.

[Willabe]

Yes, so hit the quote button 1st then edit the quote text to what you want to answer and then type your answer AFTER the last end   [/quote]   code.

OR,  hit the reply button then scroll down to the post you want to quote, hit the insert quote button, on top right and it will appear in you reply box. Edit the quote as needed, then type your answer AFTER the last end   [/quote]   code.

                Brad   

[sluckey]

Here's another method that I like to use for quotes. Easy and quick even for multiple quotes. I usually prefer this method because I don't have to edit out unwanted quoted text.

[Willabe]

So that's how you get a quote without who it's from and the date/time.


             Brad   

[shoestring]

Okay, I think I've got a handle on replying. Would have helped to read the help posts first, but I was in a hurry to get to the bottom of this Princeton problem. Thanks for your patience.
I'll try adjusting the bias current up a bit. The last tube pair in that was EH's. I ran the amp on many gigs (hard use) for a year before I finally blew those tubes, and that was before I fixed the bias circuit. I never even checked the bias until after those tubes fried, so I don't know what they were running at, but the Tung Sols clocked in at (hot tube) >39mA- way hot. So I'm thinking a new pair of EHs. They sounded really good to me.

[shoestring]

Bingo! I adjusted the bias to about 25mA and put the 5Y3 back in. Everything is good. Plate voltage is down to about 425, and I'm back in business with this little gem. Thank you EL34, Willabe, and all! I'll be back next time I blow something up, probably soon.
Steve

[EL34]

Glad you got it sorted out

[shoestring]

Just a last bit of follow-up (for now) on the Princeton. I played the first full night (4 sets) with the new tubes (the EH 6V6s) last night. Re-set the bias and took a chance with this little amp in a fairly good-sized club. I brought my Deluxe as a back-up just in case the Princeton wasn't enough for this venue. No need. I worked it hard- on 8 all night, yet the little Prince filled that room (un-miked), and I got most effusive compliments from audience and bandmates on my tone. I like the EH tubes a lot- very warm, yet precise, with a boost at just the right midrange freqs. Short of a pair of NOS RCAs they're my faves. Thanks again to EL34 for steering me in the right direction.
Steve

[EL34]

Excellent

[Jim Coash]

Greetings:  I went through some of this with my Princeton Reverb project.  When we finished setting the bias it was clear that the 6V6 tubes were at their absolute max at over 400V.  Choices included getting new output tubes like the JJs that could tolerate more than the set I had or choose to remove the plug in solid state rectifier and re-bias. We were also a little worried about the output transformer.  With the 5AR4 back in, things were back to normal albeit with about 5 watts less output.  Rather than cook the tubes we left it that way and it sounds glorious.  When I do replace the outputs I can get better ones, pop in the solid state rectifier and re-set the bias again.  Maybe an upgrade output transformer too.  It is an amazing amp with truly wonderful qualities in a very compact package even with the Celestion AP-4 speaker in the box.  When we use either an E/V 12" or 15" (or both), it is truly astounding.  We stack it on top of the custom vented boxes I made either with one or both E/Vs, disconnect the built in speaker, run at four ohms and it not only fills a large room, it looks awesome too.  A lot easier to carry than our larger rigs and just a good in terms of sound.  It has impressed a lot of people!  Jim

[shoestring]

When I do replace the outputs I can get better ones, pop in the solid state rectifier and re-set the bias again.  Maybe an upgrade output transformer too.

Here's a possible problem I've come across before. If your filter caps get used to a tube rectifier, switching suddenly to a solid state rectifier can be a problem. The rec tube gives you a bit of a slow start every time you power up, which is a healthy thing for those caps. A SS rectifier slams full voltage onto those caps the instant you hit the switch. I took out the caps on my Super that way. If you really want the SS rec you might want to break out your variac and slow-start the amp once after you put in the SS rec; take your time, maybe 3 or 4 hours to bring it all the way up to 110V, to give the caps a chance to re-form. It wouldn't hurt.
Also, in the interest of my continuing education, are we talking about a vintage Prince? My understanding is that the old Princes were equipped with strictly 8-ohm OTs. If so, how do you manage a 4 ohm load?
Steve

[Jim Coash]

Good advice and some very important things to consider.  My amp is a mid-seventies, silverface, Princeton Reverb that has always been a favorite of mine and my son.  I had it fixed immediately when it began to exhibit noise problems a few years after I purchased it.  The tech in GR was a tube organ man and he had no problem locating the problem; the multi-cap can was going south.  Four new, larger value caps took that issue away.  The very next thing I did was abandon the original Jensen 10" for a Celestion AP-4, which is a 4 ohm speaker.  The instant improvement was breathtaking.  I began a total re-furb last year, following the same path I took with the Torres re-build of my Fender Twin Reverb, which was totally successful.  I did not buy a kit, I made my own, drawing heavily on various websites, including Doug's.  I have built a plug in substitute rectifier using two IN4007 diodes and my tech tried to set the amp up with that, but it was clearly too much for the 6V6 tubes in the amp.  There were no issues driving the 4 ohm replacement speaker but we went back to the tube rectifier so as not to overload the tubes or the output transformer.  My plan is to change out the 6V6 tubes with a pair of JJ units that will handle the higher voltages and I have built a 6 IN4007 rectifier with .01 caps and drain resistors that has worked so well in my other amps.  You can be sure I will heed your warnings.  Perhaps the output transformer from a larger amp would also be a good investment.  Jim

[Ed_Chambley]

Just a last bit of follow-up (for now) on the Princeton. I played the first full night (4 sets) with the new tubes (the EH 6V6s) last night. Re-set the bias and took a chance with this little amp in a fairly good-sized club. I brought my Deluxe as a back-up just in case the Princeton wasn't enough for this venue. No need. I worked it hard- on 8 all night, yet the little Prince filled that room (un-miked), and I got most effusive compliments from audience and bandmates on my tone. I like the EH tubes a lot- very warm, yet precise, with a boost at just the right midrange freqs. Short of a pair of NOS RCAs they're my faves. Thanks again to EL34 for steering me in the right direction.
Steve

For a lot of gigs today where we are playing a princeton is the perfect amp.  Lower stage volumes and people in the crowd simply prefer a more balanced sound.  Not like the days of cranking 100 watt Super leads.

I built one using a Deluxe OT with a toggle to lower the voltage.  off-on-on with the middle having a 320ohm 10 watt resistor landing my plate voltage at 373vDc for 6V6 with a 5Y3.  I can change the 5y3 to a 5u4GB and flip the toggle and change tubes to 6L6 and I get 474vDc, but you have to modify the tremolo to swing the 6L6.  Sounds a lot like a Pro.

This way I can be too loud in any condition.    Seriously, I use it far more than my Deluxe.  Princeton are great clubbing amps as they tend to have a more mellow high and doesn't seem to hurt the audience.

[Ed_Chambley]

Here's another method that I like to use for quotes. Easy and quick even for multiple quotes. I usually prefer this method because I don't have to edit out unwanted quoted text.

That is what I have been wanting to know.

[alerich]

Several years ago I went to GC to buy a Princeton Reverb and saw an early (SN #003) Goodsell Super 17 that I liked so I bought the S17. It was a nice amp but in the end I tired of the tone. I sold it for what I had in it and returned to GC and bought the Princeton Reverb I had originally sought out. It's what I should have bought in the first place. It is exactly the tone I had in my head when I originally went amp shopping.

There's a little bit left on the table inside the PR that can be released with a few small tweaks. I wanted tighter bass and I also wanted the amp to stay clean as far as possible up the volume control. I added an extra 22uf cap across the first filter cap to beef it up a bit. The GZ34 can easily handle this. I also moved the power connection for the PI and the stage preceding it to the unused power supply node. That gives those stages more voltage to work with. The amp has tighter bass and less breakup than it did before at the same point on the dial and may even be a touch louder. I tried the Allen/Bill M OT upgrade but I didn't like it and put the stock OT back in. I also prefer the stock Jensen 10" speaker. I mostly play the amp on the clean side and use a Jetter Red² pedal for overdrive.

I loved the tone of my old Super Reverb but I sure don't miss schlepping that anchor around.