1962 Brown Princeton 6G2

eolon

Gold Supporting Member
Messages
509
A customer brought me his Brown Princeton 6G2 to look at. There wasn't anything wrong with it, other than a blown indicator lamp, but he didn't remember what all the modifications were, and wanted me to determine what was modified, and how.

The chassis number dated it as 1962. The date stamp was December, 1961 - one of the first Princetons of 1962.

It hadn't been played in years, so I brought it up slowly on the Variac.

I plugged a Stratocaster into it. The most glorious sound I have heard in a long time. I would place this in the top three amp tones I have ever heard. I have heard quite a few.

The circuit was a real mess of modifications. The amp has added reverb and a master volume. All the "tone" caps had been changed. I assayed and blueprinted the amp. So, at least theoretically, now I know why this amp sounds so good.

I have written about the amp and its modifications on my website, here.
http://hardway.quantum-foam.com/1962-fender-brown-princeton.html

Down the page near the end, is the section titled "THE SECRET 6G2 CIRCUIT CHANGES FOR GOD-LIKE TONE". The layout drawing there documents the changes to this great amplifier. You might check it out if you get a chance.

Best Regards,

Don

IMG_1100.jpg

IMG_1112.jpg


.::.
 

Darkness

Member
Messages
2,429
Such an awesome circuit. I've had a Li'l Dawg Chocoprince 6G2 for a few years now. The tone is indeed glorious.
 

VJF

Member
Messages
1,620
Does the owner have any idea who did the mods? Sounds like they knew what they were doing.

Would love to hear clips.
 

drbob1

Gold Supporting Member
Messages
30,228
A couple of notes on your nice description of the amp:
1. The circuit is very similar to the tweed Vibrolux, especially in the PI/bias so they sound very similar
2. The 6G9 was actually built for 4 years, the earliest I've seen was 61, and the latest a "Tuxedo" dress one from 64 that I owned.

Another though, would you consider writing up the circuit with the reverb and the master volume not included, just the tone/power section mods? I don't necessarily see reverb as essential to the tone of this amp and it makes the mod much more difficult. And the master volume is also something I could live without...
 

eolon

Gold Supporting Member
Messages
509
A couple of notes on your nice description of the amp:
1. The circuit is very similar to the tweed Vibrolux, especially in the PI/bias so they sound very similar
2. The 6G9 was actually built for 4 years, the earliest I've seen was 61, and the latest a "Tuxedo" dress one from 64 that I owned.

Another though, would you consider writing up the circuit with the reverb and the master volume not included, just the tone/power section mods? I don't necessarily see reverb as essential to the tone of this amp and it makes the mod much more difficult. And the master volume is also something I could live without...

Yes, that's a good idea. It would be fairly easy to leave off the reverb and MV, they are just sort of "stuck on" there anyway. I strongly suspect that the real tone benefit is from:

1) The values of the preamp bypass caps and other "tone caps".
2) Biasing the power tubes at 100%.

Thanks, and

Best Regards,

Don
 

eolon

Gold Supporting Member
Messages
509
Does the owner have any idea who did the mods? Sounds like they knew what they were doing.

Would love to hear clips.

The modifications were done by a local shop, Toledo Music Service (Toledo, Ohio) that I'm pretty sure is now out of business. Yes, they certainly knew what they were doing. They could have done a better job with lead dress, though - it's a mess.

I may try to make some sound clips of the amp in the future.

Thanks, and

Best Regards,

Don
 

eolon

Gold Supporting Member
Messages
509
By the way, the power cord on this Princeton is the original two conductor cord. The ground switch is operational and the “Safety Capacitor” also known as the “Death Cap” is present and functioning properly. As you may know, many of these vintage amps are converted to a three conductor cord with ground wire for safety purposes, and the Death Cap is removed at the same time. I have done a lot of these modifications. The reason for this is that in certain conditions, as with a wall outlet that is wired incorrectly (reversed polarity) the potential for electrical shock exists. Musicians have died when holding their guitar, which is grounded to the amp chassis, and then touching a microphone which is connected to a PA system which is plugged into a different wall socket. In this scenario the guitar chassis ground is “hot” (at line voltage) and the guitarist completes the circuit when the microphone is touched. The current from the line voltage is more than enough to be fatal. People (myself included) have often been shocked when touching metal parts of equipment. The above situation is usually the cause; they are lucky they didn’t die.

I do not think that a vintage amp such as this Princeton necessarily needs to have a 3-conductor cord installed. If the user just plays the amp in a basement or living room occasionally, and without other equipment like a PA system or another amp in use at the same time, then there is little danger from a reversed polarity shock. On the other hand, if the amp is used routinely with a working band, and plays venues with questionable power outlets (this is common) then the amp should definitely be converted to 3-conductor and the capacitor on the ground switch removed, and the ground switch disconnected (the ground switch is not needed when using a 3-conductor cord, in fact it is dangerous to leave it connected).
 

drbob1

Gold Supporting Member
Messages
30,228
In my basement, in stocking feet, I can just feel a little buzz off of a few vintage fenders that haven't been converted yet... Worries me a little.
 

zenas

Member
Messages
8,871
In a basement I like the grounded cord. Not many guitar players over 50 play barefoot on concrete but children often are and touch things. Even a switched off two prong amp can shock because the switch only works on one of the two wires. Depending on how it's plugged in or the ground switch is flipped.
On an amp with a power transformer (like a Fender) you'll only see full voltage and current if the "death cap" is shorted. You will get wall voltage though a "good death cap" just not full current. It still shocks though!
I'd NEVER recommend not putting a grounded cord on a vintage amp! It can be used safely by someone who knows how. But I'm not going to explain it to some young guy. Anymore than I'm going to explain how to start an old car without electronic fuel injection.

The "death cap" can be left in place with a proper ground. If it shorts then it blows the breaker in the fuse box.
 

NewLeaf09

Silver Supporting Member
Messages
2,609
In my basement, in stocking feet, I can just feel a little buzz off of a few vintage fenders that haven't been converted yet... Worries me a little.

Avid amateur here - I got a nice little lip shock from an older amp with a two prong plug when I leaned into a mic running through another amp in my carpeted music room. Not worth fooling with the risk when it's so cheap and easy to prevent and there's nothing to be gained by not fixing it. I enjoyed the occasional unroped solo but there was a return to that risk.
 

treeofpain

Gold Supporting Member
Messages
7,649
Very cool Princeton. And I understand your point about the 2 prong cable. I have left them in place for super clean vintage amps that were not going to be used for gigging, but let the owners know they represent a shock hazard. Most amps I have worked on are NOT mint vintage amps, and they get a 3 prong grounded cord as standard procedure before they are sold. I include the old cable in a ziplok bag if it is the original cable. A future owner can reinstall it if they like.
 

eolon

Gold Supporting Member
Messages
509
Don't get me wrong - I think this amp should be converted to a three-conductor power cable, as I think all vintage amps should be. I just think that in some circumstances, as I mentioned, or if the customer insists, that is not necessarily bad to use the original two-conductor cable, if the safety cap and switch are performing correctly.

I'm sure I'm not alone when I say that as an old guy who used to play when all amps had two conductor power cords, you just flipped the plug over in the socket if the hum was too loud or if you felt a tingle from the amp.

We also used to plug another amp into the "courtesy" outlet on the chassis of an amp. Unfortunately, the "courtesy" outlet, at least in Fender amps, was wired before the ground switch, which gave you the potential nasty surprise of having the two amps running reverse polarity from each other, even they were plugged into the same outlet. I'll never understand why Fender stayed with what I consider to be a hazardous design flaw.

Best Regards.

Don
 

eolon

Gold Supporting Member
Messages
509
Yes, that's a good idea. It would be fairly easy to leave off the reverb and MV, they are just sort of "stuck on" there anyway. I strongly suspect that the real tone benefit is from:

1) The values of the preamp bypass caps and other "tone caps".
2) Biasing the power tubes at 100%.

Thanks, and

Best Regards,

Don

Here is the layout of the 6G2 circuit modifications without the reverb or master volume - I hope you can read it.

princeton_6g2_layout_MODS_WO_RvbMV.jpg
 

calfzilla

Cynical Hack
Messages
3,870
A couple of notes on your nice description of the amp:
1. The circuit is very similar to the tweed Vibrolux, especially in the PI/bias so they sound very similar
2. The 6G9 was actually built for 4 years, the earliest I've seen was 61, and the latest a "Tuxedo" dress one from 64 that I owned.

Another though, would you consider writing up the circuit with the reverb and the master volume not included, just the tone/power section mods? I don't necessarily see reverb as essential to the tone of this amp and it makes the mod much more difficult. And the master volume is also something I could live without...

Yes, that's a good idea. It would be fairly easy to leave off the reverb and MV, they are just sort of "stuck on" there anyway. I strongly suspect that the real tone benefit is from:

1) The values of the preamp bypass caps and other "tone caps".
2) Biasing the power tubes at 100%.

Thanks, and

Best Regards,

Don

Ok, correct me if I'm wrong (just be gentle :D), but wouldn't the reverb section be integral to the special tone of this amp? Not due to the reverb itself, but because of the extra gain? Or is the 763 reverb different than the reverb that went into the Princetons? Honestly just curious and wondering! I have no idea if that's a correct thought or not.
 

eolon

Gold Supporting Member
Messages
509
Ok, correct me if I'm wrong (just be gentle :D), but wouldn't the reverb section be integral to the special tone of this amp? Not due to the reverb itself, but because of the extra gain? Or is the 763 reverb different than the reverb that went into the Princetons? Honestly just curious and wondering! I have no idea if that's a correct thought or not.

Yep, that's exactly right. The reverb section is another gain stage. Gain can be either positive or negative, though. The typical AB 763 reverb circuit actually loses a bit of gain; that's why there is another 1/2 12ax7 gain stage in the AB 763 to make up for reverb losses. The reverb circuit itself is really a treble booster, because only the high frequencies are sent through the tank - bass sounds muddy with reverb. This 6G2 amp uses a 12AX7 as the reverb driver instead of the standard 12AT7. The 12AX7 has an amplification factor of 100, compared to 60 for the 12AT7. So it's possible that this amp is seeing some +gain from the reverb. (people report that a 12AX7 as the reverb driver makes the reverb too "harsh", but it sounds good to me in this amp.)

Is this reverb modification integral to the tone of this amp? Yes, I would say so. If not due to any gain from the reverb, then because the reverb makes the amp sound better.

Best Regards,

Don
 

calfzilla

Cynical Hack
Messages
3,870
Yep, that's exactly right. The reverb section is another gain stage. Gain can be either positive or negative, though. The typical AB 763 reverb circuit actually loses a bit of gain; that's why there is another 1/2 12ax7 gain stage in the AB 763 to make up for reverb losses. The reverb circuit itself is really a treble booster, because only the high frequencies are sent through the tank - bass sounds muddy with reverb. This 6G2 amp uses a 12AX7 as the reverb driver instead of the standard 12AT7. The 12AX7 has an amplification factor of 100, compared to 60 for the 12AT7. So it's possible that this amp is seeing some +gain from the reverb. (people report that a 12AX7 as the reverb driver makes the reverb too "harsh", but it sounds good to me in this amp.)

Is this reverb modification integral to the tone of this amp? Yes, I would say so. If not due to any gain from the reverb, then because the reverb makes the amp sound better.

Best Regards,

Don
Ok, I just wanted some clarification since I knew that the old NR Princetons were "little twins" because they were missing that extra gain stage and never broke up. I just didn't want @drbob1 to be disappointed if he did the mod sans reverb and didn't hear quite the improvement he was hoping for. I know the 6Gx circuits were already gain-y to begin with, but didn't know if (in your opinion) this one had extra gain or "better" gain or more crunch or anything due to the reverb being in the circuit.

Good stuff!
 




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