Fender Princeton Reverb Silverface: Need Help with tube selection and date of manufacture

[lefty bluesman]

(sorry long post)

hello,
In 1975, I purchased a Princeton Reverb Reverb (with a drip edge) not knowing the date of manufacture. I regrettably had someone modify the amp in the late 70's so that it had three vol controls, presence control, etc (it was intended to sound like a Mesa Boogey amp which was the rage at that time. A few years ago, I took the amp to tech in LA and asked him to restore the amp to its original condition, which he did. I received a huge baggie with all the parts he removed/replaced. It wasn't until more recently, when Fender amp prices skyrocketed that I was told it was a 1968, due to the drip edge. But after watching some of the Uncle Doug amp videos on YouTube, and I got curious about my amp. The tube chart on the amp, states the circuit is an AA764 with a GZ34 rectifier tube. There is currently an 5U4GB rectifier tube in the amp (but it's not the one that came in the amp). The serial number on the chassis is A23661, which according to the resource I looked at makes it 1969, and also the transformers have codes of 606-9-06, 606-8-51, 606-9-02. Is it safe to assume that the amp is a 1969, based on the dates on the chassis and the components? The thing that threw me was the presence of the drip edge, but I think I read that Fender continued using the drip edge into early 1969.

Also, in an Uncle Doug video, he was saying that if the amp is supposed to use a GZ34, you should put that in there (I know there is a lot of debate about that). Since the tube chart in my amp says to use a GZ34, should I do that? I also read that the tube chart on many of these amps were not accurate. Any suggestions on how to identify what the correct rectifier tube is for this amp?

 

[moosie]

Yes, I have an early '69 Drip Twin, so they continued using drip edge until it was gone (of course).

Those paper tube charts are notoriously inaccurate. They had a pile saying AA764, and kept using those, too, even after the circuit had changed. The only useful thing on the chart, IMO, is the two-letter stamp, which denotes year and month.

 

[lefty bluesman]

Yes, I have an early '69 Drip Twin, so they continued using drip edge until it was gone (of course).

Those paper tube charts are notoriously inaccurate. They had a pile saying AA764, and kept using those, too, even after the circuit had changed. The only useful thing on the chart, IMO, is the two-letter stamp, which denotes year and month.

hey, my paper tube chart doesn't have a date.So, if the tube charts are inaccurate, how do I know what schematic to use - or which rectifier tube should I be using? did yours change from 68 to 69? thanks!

 

[moosie]

Can you attach a photo of your tube chart please. It's not a date, it's a two-letter date code stamped onto the paper after it was printed.

Superior Music - Fender Amp, Amplifier Serial Numbers

 

[moosie]

@Wally might be able to help.

As I recall, the circuits changed a few times in this period, and not on any schedule. I think I concluded my Twin circuit was neither the blackface AB763, nor exactly the AC568. And I think it was too early for the AA769 and AA270.

Something of a mix. Of course there may have been a lot of hands in that amp, and little mods done over the years.

 

[Wally]

@bluesman13, the dates on your transformers indicate you have a 1969 amp. The drip edge cosmetics were used until the drip edge materials were expended….there is no set date when those cosmetic appointments ceased. It is not uncommon to see a 1969 amp with drip edge…especially an early ’79 as your amp is.
The date stamps on the tube charts were ceased with the introduction of SF cosmetics in April, 1967. Also, tube charts are not definitive information when dating amps. It is not uncommon to find tube charts that refer to earlier circuits used in amps with circuits that are newer. Your amp should have a 5U4 rectifier. There are two schematics for the AA1164 Prin Rev. The first calls for a GZ34 while the second calls for a 5U4. This second version was in use until 1970 When the B1270…December, 1970….schematic was drawn.
If you are curious, go here…

GGJaguar's Guitarium and Ampeteria Bibliography

and find the six articles entitled “Dating Fender Amps”. These articles provide detailed information concerning vintage Fenders. The serial number charts that are used today to date Fenders are the result of the work that brought those articles into being.

 

[Michael Smith]

There "may" be an ink stamp inside of your chassis, possibly near the power transformer indicating when the chassis was wired up. It should read something like T201369, which would indicate technician #20, 13th week of 1969. It's also possible the year is just a single digit. Assuming all of the transformers are original, then as others have commented, the earliest the amp could have been assembled was 1969.

 

[King Fan]

@bluesman13 , you're getting great info here. First, congratulations. While exact dates are fun, the key thing is you have a drip-edge Princeton Reverb, probably the most desirable PR of the SF era. The PR didn't change much across the CBS event horizon, either, so a drip-edge can be considered pretty close to BF electronically.

Second, yes, Fender paperwork on these is horribly confusing. As noted, there never was an AA764 Princeton Reverb... or schematic.

But... danger. At one time Uncle Doug, along with much of the amp-web, fell for a photoshopped "AA764" schematic. And Fender schematics can be worse; some early AA1164 schematics show a 5U4. Why is that bad? Cuz as @Wally says, until the B1270 revision in ~1971, the original PT could not supply 5U4 filament voltages. A 5U4 in a PR before 1971 could be a PT-killer. So, photoshop aside, Uncle Doug is right. I would suggest you run a GZ34 or a (budget and voltage friendly) 5V4.

Finally, don't forget our motto here. Pics or it didn't happen...

 

[Wally]

until the B1270 revision in ~1971, the original PT could not supply 5U4 filament voltages.

I need to keep this in my memory. I do not recall working on anything but BF Princeton/Princeton Reverbs that had GZ34s or 1970s SF examples….never anything in between that was carrying a 5U4.

 

[King Fan]

I need to keep this in my memory. I do not recall working on anything but BF Princeton/Princeton Reverbs that had GZ34s or 1970s SF examples….never anything in between that was carrying a 5U4.

Yeah, the tube charts and schematics are super confusing, and so are most threads on the topic across all amp forums. The best discussion I know was by our friend muchxs (who also gave me the 5V4 idea). I'm lazy; I'll just paste it here, but leave out his notes about BetterCoil PTs that also sneaked in for a while.

TLDR? Unless you have a 1971+ (010020 PT), don't run a 5U4...

"There are three basic eras of Princeton Reverb power transformers.

BF era mostly used the 125P1B. That's '65 to '67. Amps with the 125P1B use a 5AR4 rectifier. The 5AR4 has a 2 amp filament. If you cheap out and use a 5U4G because there is a Princeton Reverb schematic that shows a 5U4G... your transformer will run hot. It tends to run hot, anyway. It's really a "Champ" transformer. It's asking a lot to run twice as many tubes in a Princeton Reverb.

"Intermediate" amps '68 through '70 generally use the 022772 transformer. They all came with "Champ" transformers. This one also has a 2 amp rectifier filament. That means you must use a 5AR4 rectifier with this one as per the tube chart as well. It's gonna run smokin' melt your transformer hot with a 5U4G rectifier."

....{discussion of BetterCoil PTs used in some 1968s}....

"1971 and up Fender got smart and used the 010020 "Export" transformer. It's kinda handy... move two wires, you're ready for Europe. The 10020 has a 3 amp rectifier filament winding so it will support a low buck 5U4G. There's a caveat... if you think you're gonna be cool and make it act like a '65 using a 5AR4 tube... don't. It sends the supply voltage way high."

 

[schmee]

Yeah, the tube charts and schematics are super confusing, and so are most threads on the topic across all amp forums. The best discussion I know was by our friend muchxs (who also gave me the 5V4 idea). I'm lazy; I'll just paste it here, but leave out his notes about BetterCoil PTs that also sneaked in for a while.

TLDR? Unless you have a 1971+ (010020 PT), don't run a 5U4...

"There are three basic eras of Princeton Reverb power transformers.

BF era mostly used the 125P1B. That's '65 to '67. Amps with the 125P1B use a 5AR4 rectifier. The 5AR4 has a 2 amp filament. If you cheap out and use a 5U4G because there is a Princeton Reverb schematic that shows a 5U4G... your transformer will run hot. It tends to run hot, anyway. It's really a "Champ" transformer. It's asking a lot to run twice as many tubes in a Princeton Reverb.

"Intermediate" amps '68 through '70 generally use the 022772 transformer. They all came with "Champ" transformers. This one also has a 2 amp rectifier filament. That means you must use a 5AR4 rectifier with this one as per the tube chart as well. It's gonna run smokin' melt your transformer hot with a 5U4G rectifier."

....{discussion of BetterCoil PTs used in some 1968s}....

"1971 and up Fender got smart and used the 010020 "Export" transformer. It's kinda handy... move two wires, you're ready for Europe. The 10020 has a 3 amp rectifier filament winding so it will support a low buck 5U4G. There's a caveat... if you think you're gonna be cool and make it act like a '65 using a 5AR4 tube... don't. It sends the supply voltage way high."

Does anyone know where you find the actual specs of the 3 transformers for comparison? What are the voltages? Filament amps? etc?
Now days I think most companies just offer one replacement tranny for a "stock" replacement, regardless if it's a 65 or a 70's.. Although they often offer an "upgrade" for DR's and Princetons too, but I don't remember seeing any wording for "use this is you have a 5U4GB rectifier". Which voltage do they offer?

 

[King Fan]

Does anyone know where you find the actual specs of the 3 transformers for comparison? What are the voltages? Filament amps? etc?
Now days I think most companies just offer one replacement tranny for a "stock" replacement, regardless if it's a 65 or a 69.. Although they often offer an "upgrade" for DR's and Princetons too, but I don't remember seeing any wording for "use this is you have a 5U4GB rectifier". Which voltage do they offer?

I don't know the exact HV specs, but the 5V current specs were famously low (2A) until 1971. So tube choice is 'easy' once you ignore the ten tons of misleading info out there.

OTOH, for PT replacements, am I right to think modern PTs will spec their 5V current supply? If some 'pure clone' had <3A for the rectifier heaters, you'd want to go GZ34.

Of course, as muchxs said, getting the B+ / HV right is also complicated by the much smaller voltage drop of the GZ34.

 

[schmee]

I don't know the exact HV specs, but the 5V current specs were famously low (2A) until 1971. So tube choice is 'easy' once you ignore the ten tons of misleading info out there.

OTOH, for PT replacements, am I right to think modern PTs will spec their 5V current supply? If some 'pure clone' had <3A for the rectifier heaters, you'd want to go GZ34.

Of course, as muchxs said, getting the B+ / HV right is also complicated by the much smaller voltage drop of the GZ34.

"1971 and up Fender got smart and used the 010020 "Export" transformer. It's kinda handy... move two wires, you're ready for Europe. The 10020 has a 3 amp rectifier filament winding so it will support a low buck 5U4G. There's a caveat... if you think you're gonna be cool and make it act like a '65 using a 5AR4 tube... don't. It sends the supply voltage way high."

I was thinking about the statement of "too high voltage". If true, then wondering what do modern "one size fits all" replacements emulate? The later voltage or the BF era voltage?
In reality both layouts AA764 and B1270 show the same voltage... I haven't checked the schems though.

 

[King Fan]

"1971 and up Fender got smart and used the 010020 "Export" transformer. It's kinda handy... move two wires, you're ready for Europe. The 10020 has a 3 amp rectifier filament winding so it will support a low buck 5U4G. There's a caveat... if you think you're gonna be cool and make it act like a '65 using a 5AR4 tube... don't. It sends the supply voltage way high."

I was thinking about the statement of "too high voltage". If true, then wondering what do modern "one size fits all" replacements emulate? The later voltage or the BF era voltage?
In reality both layouts AA764 and B1270 show the same voltage... I haven't checked the schems though.

Oh gotcha. Yeah. I *think* muchxs was saying the higher *current supply* of the beefier 010020 would get pulled down less than the wimpy 75mA original, and then adding the (lower drop) higher output of a GZ34 would push already-high PR voltages 'way high.'

You're right, the schematics show both older and newer PTs with an HV of 340V (and that's on ~115 at the wall). Fender's schematic voltages, including HV, are not always realistic, and I tend to think of modern PR PTs as running like 330V. But even if it's only 330, going from 5U4 to GZ34 could add 30+V to your B+, like from 420V to 450 or more.

So maybe muchxs would use a 5U4 if he had a 010020 *or* a modern PT. For that matter, he often said his choice for a replacement when a vintage PR PT died was a beefier (100mA or more) PT with at least 3A for rectifier heaters but with somewhat *reduced* HV -- less abusive of both expensive NOS 6V6s and delicate modern ones.

 

[lefty bluesman]

hello,
thank you all for your comments/input. The numbers of the transformers 606-9-07 (7th day of 1969), 606-8-51 (51st day of 1968), and 606-9-06 (6th day of 1969), chassis serial number A23661 (1969), and "T30139" (13th day of 1969)written on the inside of the chassis. All this points to an early 1969 manufacture date.

In reading the above posts about the correct rectifier tube, I'm still confused - one post seems to indicate that the 5AU is the correct tube, and another the GZ45. Also, does my power transformer look damaged? One of the Uncle Doug videos I watched said that the wrong rectifier tube can make the PT melt. Is that what has happened here?

 

[lefty bluesman]

I was poking around this morning and found a thread that address my question about the correct rectifier tube: (some great info here)

Need a correct answer: what's the right rectifier for Princeton 69/70?

The tube chart in my Princeton (non reverb) 69/70 Silverface says the rectifier should be a GZ34, but it has a 5U4GB. There are lots of different opinions on this on the net, some say they used old tube charts in early SF's, some say they changed from GZ34 to 5U4GB on the SF's after a couple...

www.tdpri.com

I know that the existing 5U4 is not stock. I am wondering - can I replace the existing 5U4 with a GZ34,5AR4, 5V4, or 5Y3, or do some adjustments need to be made? thanks so much everyone for your input! Steve

 

[Michael Smith]

hello,
thank you all for your comments/input. The numbers of the transformers 606-9-07 (7th day of 1969), 606-8-51 (51st day of 1968), and 606-9-06 (6th day of 1969), chassis serial number A23661 (1969), and "T30139" (13th day of 1969)written on the inside of the chassis. All this points to an early 1969 manufacture date.

In reading the above posts about the correct rectifier tube, I'm still confused - one post seems to indicate that the 5AU is the correct tube, and another the GZ45. Also, does my power transformer look damaged? One of the Uncle Doug videos I watched said that the wrong rectifier tube can make the PT melt. Is that what has happened here?


View attachment 1175641

The last digits of the transformer date codes indicate the week. It does look your transformer has gotten hot at some point, but a lot of them do run hot. I guess you could measure the resistances to see how close to spec they still are.

 

[thekaziromel]

Yeah, the tube charts and schematics are super confusing, and so are most threads on the topic across all amp forums. The best discussion I know was by our friend muchxs (who also gave me the 5V4 idea). I'm lazy; I'll just paste it here, but leave out his notes about BetterCoil PTs that also sneaked in for a while.

TLDR? Unless you have a 1971+ (010020 PT), don't run a 5U4...

"There are three basic eras of Princeton Reverb power transformers.

BF era mostly used the 125P1B. That's '65 to '67. Amps with the 125P1B use a 5AR4 rectifier. The 5AR4 has a 2 amp filament. If you cheap out and use a 5U4G because there is a Princeton Reverb schematic that shows a 5U4G... your transformer will run hot. It tends to run hot, anyway. It's really a "Champ" transformer. It's asking a lot to run twice as many tubes in a Princeton Reverb.

"Intermediate" amps '68 through '70 generally use the 022772 transformer. They all came with "Champ" transformers. This one also has a 2 amp rectifier filament. That means you must use a 5AR4 rectifier with this one as per the tube chart as well. It's gonna run smokin' melt your transformer hot with a 5U4G rectifier."

....{discussion of BetterCoil PTs used in some 1968s}....

"1971 and up Fender got smart and used the 010020 "Export" transformer. It's kinda handy... move two wires, you're ready for Europe. The 10020 has a 3 amp rectifier filament winding so it will support a low buck 5U4G. There's a caveat... if you think you're gonna be cool and make it act like a '65 using a 5AR4 tube... don't. It sends the supply voltage way high."

Apologies for the bump, but I’m facing this exact problem with my recently acquired 1970 PR. The tube chart says that it’s a AA764 circuit and asks for a 5U4GB… but as you predicted, the original PT got fried at some point and it’s now running a modern, beefier PT. The problem is it’s also running a GZ34! Is that a ‘double kill’ that the previous owner’s tech has done? Am I eventually killing the filter caps (rated @450 on the Weber cap can) and the 6v6s? Attaching the relevant pictures.

 

[King Fan]

Apologies for the bump, but I’m facing this exact problem with my recently acquired 1970 PR. The tube chart says that it’s a AA764 circuit and asks for a 5U4GB… but as you predicted, the original PT got fried at some point and it’s now running a modern, beefier PT. The problem is it’s also running a GZ34! Is that a ‘double kill’ that the previous owner’s tech has done? Am I eventually killing the filter caps (rated @450 on the Weber cap can) and the 6v6s? Attaching the relevant pictures.View attachment 1232304View attachment 1232305View attachment 1232306

Good job doing all this homework.

Short long answer: "Are your voltages too high?" Long short answer: "If you have 3A on the 5V, run a 5U4."

 

[thekaziromel]

Good job doing all this homework.

Short long answer: "Are your voltages too high?" Long short answer: "If you have 3A on the 5V, run a 5U4."

@King Fan , thanks man! Much appreciated.