Fender Princeton Amp 6G2 Bias and other ?s

[waldner]

I'm working on my 6G2 Fender Princeton and it appears to be all original but the power transformer is an M-022772 / EIA 606-632.  Wondering if the PT was replaced at some point or if this was a Fender substitute for the 125P1A, which the schematic lists as original?


I measured the plate voltage with a new matched pair of Sovtek 6V6GT tubes and am getting:  Vp=389/388, Ik=28/27, Output= 10.5/10.4 watts.  The output wattage seems pretty good (maybe a tad high) but my understanding is that the Plate voltage should be down at about 315.  Also when I ran some NOS tubes that were not well matched I was getting a Vp of closer to 418/420 and only about 4/6 watts of output.  Any recommendations?  Maybe just use the good tubes and leave it alone?


I haven't changed out any of the original caps yet as everything seems to be in the realm of working ok.  I installed a grounded 3-prong power cord and removed the death cap and that's about it so far.  It's tempting to just keep running all the old caps on the board until one presents a problem, but I think for safety I will replace the multi-cap can with a new 20-20-20uf.  The original schematic says 30-30-30 but that's not what is in there now, which seems to be original from Fender. 


I'd appreciate any insight or recommendations into any of this.  Thanks!

[sluckey]

The PT has been changed, wire nut and heat shrink splices are a dead give-away. Voltages and bias look fine to me. Since you don't have the original PT I would not get hung up on the 315V number seen on the Fender schematic. Do you have a real NOS 5Y3 in the amp? If not, get one. It will lower the B+ numbers.

Nice looking amp. I would not replace anything as long as the amp performs satisfactorily. Nice looking bias meter too!

PS... I would swap positions with the two wires on the fuse holder.

[waldner]

Thanks Steve. Since I made that post, things went downhill fast!  I tried another couple of pairs of NOS tubes to see what numbers I would get for Vp, Ik and Wattage and when testing one of them I started getting crazy high readings on one tube for Ik and wattage.  I shut down the amp and changed tubes back to the matched new Sovtek pair and now that was getting really high readings.  It could also be that something fried in my Compu-Bias tester, or both?   

The one tube was showing just constantly rising current numbers and wattage until I shut the amp down.  That tube actually felt cold. The other tube was hot to the touch.  Maybe my testing and repeated on/off cycles stressed some of the old caps and something went?  I had a guitar plugged in and making reasonably normal sound, even when the readings went really high, right before shutting off. 

Not sure what to do now.  Help!

By the way... the wire nut was part of my changing to a grounded 3-prong cord.  I didn't have any heat shrink so I just used that instead.  I didn't realize the fuse connection order mattered?  Or does it just look weird and backwards?

[pdf64]

If checking the fuse and it’s good, it stays live until it’s pretty much completely removed from the holder.
Swap the wires over and it’s de-energised as soon as it’s pulled out just a couple of mm.


With the output valves removed, are the voltage readings at both sockets reasonable and matching?

It looks like the power cable is being retained by a cable tie? If so, what’s preventing the cable rotating?

There’s only one safety critical connection in an amp, and it’s been put at about the most inappropriate place, under a power transformer mounting nut.

[waldner]

Good to know on the fuse holder pdf64.  I’ll swap those wires.   I will also get a proper strain relief for the power cord. Regarding the power cord ground, I thought that under one of the PT mount screws was the norm, even preferred place to ground, short of soldering to the chassis, which my iron may not be able to do. 


I will measure the power tube locations with tubes removed and report back.


Thx!

[sluckey]

When checking the voltages on the output tube sockets, we're especially interested in the voltage on pin 5 of both sockets. Also check the voltage on each filter cap under the doghouse. Report.

Give us some more info on the amp... Things like have you been using this amp regularly? Or has it been sitting in a closet for years? Or did you just buy this amp and have no idea about it's condition or history? Things like that may be important to know when we're offering suggestions of things to check.

And get rid of that wire nut! It belongs in an electrical box in your house. Besides, it's killing the good looks of that amp. 

[sluckey]

Move the green wire. Should not require drilling any holes. See pic...

[waldner]

Ha, yes I will heat shrink that connection in place of the hideous wire nut tomorrow when I’m back at my shop.  I will also take those measurements and report back.


Interesting history: This amp belonged to Charlie Ryan who wrote the song “Hot Rod Lincoln.”  He died in 2008.  I bought it from a friend who bought out his guitar related estate.  I’m not sure how much play it’s gotten in recent years.  Probably not much.   I was going to build a 6G2 at some point but this was too cool to pass up.


.... and I will move the green wire!  Thx

[pdf64]

Good plan!
Regarding the power cable and the wire lengths from it, it’s best to make them such that if the cable did get yanked out, the ground is the last connection to break, live then neutral will be torn out first.

[waldner]

Ok, after taking measurements I think the news is good, but you guys tell me. 


with no power tubes installed - preamp and rectifier only
V3-6V6 pin 5= 38.4v
V4-6V6 pin 5= 38.5v


with power tubes installed


V3-6V6 pin 3=392
            pin 4= 386
            pin 5= 38.2


V4-6V6 pin 3=392
            pin 4=386
            pin 5=38.3


V1-12AX7 pin 3=1.64
                pin 8=1.707


V2-12AX7 pin 3=2.4
                pin 8=78.5


Main 20uf-20uf-20uf cap can- 350
                                            386
                                            395




The amp seems fine with no red plating or any observable weirdness, so my guess is that my Compu-Bias is what was fried.  Bummer, cause I really like that thing.  I'm going to have to learn to figure out how to measure and determine output dissipation (wattage) the hard way.  Here I come Uncle Doug    I played through the amp after taking these measurements and it sounded great.  Anything else I should measure or do to optimize things?

[jordan86]

There may be better links on bias but this one is helpful too. Uncle Doug’s video is also great.

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Also, does the Compubias require a battery? If so, it may just need a fresh one. I’ve had my fair share of panic moments and crazy voltages that seemed too high to believe. Turns out my multimeter battery was just dying and causing crazy readings. Drag battery, problem solved.

[waldner]

Good thinking Jordan.  I didn’t even think of the battery because the display was working.  It would sure be nice if a new battery made the Computer-Bias work properly.  I’ve also read of the octal sockets being fragile and maybe one of mine has a short or something.


I must be a serious geek because I really enjoy Uncle Doug videos and think he’s nearly a comedian.

[jordan86]

I must be a serious geek because I really enjoy Uncle Doug videos and think he’s nearly a comedian.

You’re in good company. I find Uncle Doug’s humor just as enjoyable as his amp work. Maybe more so. I always enjoy his videos.

[pdf64]

Uncle Doug seems a nice chap, great presenter and a competent enough tech to fix amps etc. But viewers really shouldn’t mistake him for a competent EE, a valid technical reference source.