1961 6g13 - Resistor values off...

[bullkelp]

Im servicing my 6g13. Going well. Bias is spot on and I identified a microphonic/intermittently shorting 12ax7 in V3.

What im a bit stumped by is what seems like factory incorrect resistor values.

1. Instead of the schematic 2.2Meg following from the second triode plate of the first 12ax7 in the harmonic trem there seems to be a 4.7M

2. IN the vibrato channel 2nd triode 12ax7 there is a 22k instead of a 10k at the voltage divider where the B+ enters the plate.
( I think)

3. Third question relates to how much upward drift can one tolerate in these carbon comps in the non 5% tolerance resistors? I really dont want to change them out.

Link to the schematic
https://schematicheaven.net/fenderamps/vibrasonic_6g13a_schem.pdf

[bullkelp]

First the 2.2m for 4.7m replacement

[bullkelp]

and an image

[bullkelp]

And the 10k now 22k

[bullkelp]

and layout

[tubeswell]

Fender made a continuous series of little tweaks over time. Maybe these ones were factory tuning the amp to the original customer's preferences? Who knows? The secret is lost with time.  If the vibrato works fine, I'd leave it alone.

[bullkelp]

Thanks

The vibrato is sublime!

Im battling a little fizz tho when pushed. Also a loud thump when the trem intensity is dialed to zero. Thoughts ?

[tubeswell]

The thump could be EMF cross-talk from lead dress around the vibrato circuit interacting with the signal circuit, or it could just be a bad preamp tube (which could also account for the fizz, so trying rolling each of the preamp tubes before breaking out your soldering iron).

[HotBluePlates]

Im servicing my 6g13. ...

What im a bit stumped by is what seems like factory incorrect resistor values.

Fender documented their amps more/better than nearly any 1950s-1960s guitar amp brand (with the possible exception of Ampeg).

Even so, until the mid-60s or so, Fender seemed to have a lot of minor circuit-changes that didn't get documented on schematics.  The guys who heavily collect vintage amps & also pay attention to tech-details are the ones who seem to notice these incremental versions floating around.

The resistor style, solder joints, and position of other parts suggest that the resistors you noted are factory-original.  That makes them "Right" no matter what the schematic says.

... how much upward drift can one tolerate in these carbon comps in the non 5% tolerance resistors? I really dont want to change them out. ...

If the amp isn't malfunctioning, then there's no reason to change the resistors.

Take any amp, and you can likely halve/double any individual resistor and barely notice a change occurred.  If "double-resistance" is barely-audible, then there's no sonic goodness to be had in tight precision.  That's usually a fixation of Novice Techs, because they assume "something must be done" and they're not quite sure what to tinker.

I, too, assumed everything needed to be refreshed & back to factory-original when I first started out.  In hindsight, I wish I would have not-changed the cathode bypass caps in that 1967 Princeton Reverb, because the dried-up caps measuring 1µF sounded better (and less bloated in the low-end) than after the new 25µFs were installed.  But I didn't have experience against which I could evaluate that amp & see it needed no tinkering (I got it in the first place because it sounded perfect after playing the first chord).