Fender Amp schematic/layout problems

[Underwood]

Hello,
I have a Fender Concert amp 6G12 version. The treble and bass pots have some strange resistor/cap combo that are not on the schematic/layout. They are definitely stock from the factory, and was wondering if anyone knew what they were for, or what would happen if I took them out and went with what was on the layout. The treble and bass pot are connected by a 220K resistor and a disc cap that says 210m. Then they go to a .05 cap on the board. The treble cap has no 100K resistor or 250 cap as it says on the layout. The bass pot is consistent with the layout, but has another .001 cap  on it. Any help would be appreciated. Thanks

[kagliostro]

Can you post a pair of good quality photo ?


K

[Underwood]

I tried and it says the file is too big. Any help with that too? 

[AZJimC]

I tried and it says the file is too big. Any help with that too? 

Read the very top post on this board. The one about image sizes too big.

Jim

[Underwood]

Hers a picture

[eleventeen]

I'm not believing that stuff is stock. I'm not saying it can't or won't work, I have no way of knowing that. It doesn't look like the construction within any Fender amp I have seen. There were some rather unusual things in the brown-era. But realize that these amps were not especially inviolate, holy grail things in the early to mid 70's.


Why do you think it is stock...because the solder joints are old, with a grey patina on them? Those mods (as I am calling them) could have been done quite a while ago. Myself, I find the brightness of the coloration on those ceramic caps a little too bright to be stock...but that impression could be the result of lighting, or flash. Anyway, if those mods were done when the amp was 8 years old, the solder joints would look pretty old. When that amp was 8-10 years old it was just a "used amp".

While the amp is open, I would check the date codes on the pots just for grins. Though they look stock.

Why are you motivated to change it, assuming it works?

If you touch it, IMO, you probably embark upon a journey to return it to stock.

[Underwood]

This amp as been in storage since 1961. It was probably made in mid 1960 and everything has been untouched. Well, until now because I have changed all the the electrolytics in it and a few way off resistors. I have been studying the layout and schematic trying to keep everything perfect. I know this was a very transitional time for Fender and they were basically phasing out Bassmans into Concerts it seems. The transformers are all Bassman trannies and it has P10Q speakers dated the 17th week of '60. The was another weird 100K resistor coming from V3 to the board that on the schematic and other Concert chassic pics I have seen is just a plain wire. It was missing a .001 disc cap that went right next to it. I guarantee whoever solder the parts in at the factory read the .001 backwards or upside down and put a 100K.

[Underwood]

Here's a few more pics/

[Underwood]

The two new pics are from the normal channel and the vibrato channel. They both have these same caps and resistors.

[Underwood]

The caps on the pots that say 250 mm on the schematic, what would they be? On the board there is a 250 mm and it is an old domino silver mica. Do I use a 250pf silver mica or a 250pf ceramic? What is mm?

[sluckey]

250mm is an old school way to say 250pF. mm is micromicro, µµ, or 10^-12. pico is also 10^-12. In the shop we would call mm micky-mike.

I think Fender was experimenting with the tone stack. They made a big change between 6G12 and 6G12-A. If the tone controls work OK I'd just leave them as they are.
 

[Underwood]

What do you think the sound difference would be if I went with what is on the schematic and took the extra capacitors and 220k resistors off?

[meycom]

250mm is an old school way to say 250pF. mm is micromicro, µµ, or 10^-12. pico is also 10^-12. In the shop we would call mm micky-mike.

I think Fender was experimenting with the tone stack. They made a big change between 6G12 and 6G12-A. If the tone controls work OK I'd just leave them as they are.

Cool .. That's good to know!  mm-->pf ..  Dunca