'75 Fender Bassman 100 question

[Randall]

I got a Bassman 100 in because a couple of the power tubes fall out. Turns out they are metal based 6550's which are too big for the bear claw tube retainers. I t also has a pink wire under the dog house that looks like it was never stripped or connected, which is odd because the amp is pretty quiet.

My question is, what is the consesus on leaving those tubes in there? The amp has the dreaded CBS bias balance circuit, as well as all original caps.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

The schematic calls for 6L6's. I'd be worried about the extra nearly 3A filament current. Coupled with the fact that the tubes don't fit with the tube retainers, I'd probably install a new set of 6L6's.

[vintagekiki]

It is unknown how long tubes 6550 worked at the previous owner, and how long they will work for the current owner.
The question is what next.
I think that for future operation and maintenance, the amplifier should be returned to the original schematics (6L6GC), despite the fact what now has a powerful, very clear sound (6550) like the Marshall amp.

[Leo_Gnardo]

No! If I'm not mistook, 6550 filament current is about double that of 6L6. An extra amp or two, Fender's transformers will put up with. 4 amps? You risk melting the filament winding. And of course there are those bear traps, pushing the 6550 out of the sockets, what kind of bad idea is that?

Pick your favorite 6L6GC, or if you want to try something a little different I can support JJ's 5881. I've used them quite a bit over the last 3+ years, so nearly 100. One was bad out of the box. And a pair of them noticeably worn in one customer's amp. He plays all day long, gives lessons, writes & records, and played in 6 (!) bands before covid hit the scene. So he legitimately wore them down some over the course of 18 months or so. Although the specs may suggest otherwise, I've found they handle the voltages in Fender amps of all sorts. Is this not enough? I've found they also deliver more power than expected, typically surpassing what we expect out of 6L6GC. In a Bassman 100, or Twin, or other similar amp I suggest bias to the cool side, say 25 to 30 mA, just a bit past the point where you see crossover distortion disappear on the scope. Yeah, the glass envelope is on the small side, looks like JJ's 6V6. Small and mighty, that's my conclusion. All I can hope is JJ doesn't drop the ball on quality control with these.

Bias balance control? We've just been through that, right? You know what to do. I''ll spare folks having to look it up, simply substitute the 33K resistor on the back of the balance pot with a combination trimpot and fixed resistor that can cover the bias range. Say, 22K with a 20K trimmer. I use the half watt Bourns "blue cube." Link below. Solder & smile!

652-3386W-1-203LF

[g1]

Where is the unconnected wire going at the other end?

[Randall]

We have decided to 6L6s back in it, do the cap job and tweak the bias circuit, so that's all good. The unconnected wire is soldered at the junction of the choke, screen resistors, 2.2K and 20uF cap. It snakes up between the second 100uF filter cap and the first 20uF filter cap and is pointed at the +/- junction of the two 100uF filter caps, but the end is cut clean and is not stripped or ever soldered. Weird.

[Axtman]

I have seen a few Fender amps with cut wires. I am not sure why. Probably a bunch of filter cap boards were wired up at the same time and cut if not needed for that particular amp.

[Leo_Gnardo]

Yes, snip that errant pink wire. It's a gift from the factory. Like a 2 inch piece of braid wire I found in a '66 Super Reverb last week. It was up near the hi voltage power supply, stuck to the chassis floor apparently with wax or whatever goop we find in the corner crevices. Lucky it didn't wander over to the B+ node and short things out. Murphy's law says it should have done that or other mayhem sometime in the last 56 years, but Murphy loses this time. No point in keeping your pink wire scrap. Disappear it before it leans on the chassis, sparks & pops the fuse. Worry no more, one quick chop with the dikes & you're home free.

[Randall]

So I'm thinking it was a quality control mishap?

[Enzo]

Probably an assembly oversight. QC generally checks amps for function, but likely won't pull the cover off the doghouse to look for extra wires.

[Randall]

Here is another question about this amp. It has two 5uF and one 25uF cathode caps. Why did CBS go with 5uF, when the blackfaces were mostly all 25uFs? I don't have any 5uFs on hand, why couldn't replace them with 25uFs?

[vintagekiki]

With 5uF in Bass channel and with 500pF in the treble control and with .0047 in volume control, the amp has a less deep (harder) tone, and it sounds similar to Marshall.

With 25uF in Normal channel and with 250pF in treble control, the amp get powerful deep tone, and it sounds like a good old Fender.

It's all a matter of taste. Feel free to replace 5uF with 25uF

[The Dude]

.......... or, instead of 5uF use the more common 4.7uF.

[Randall]

I have some 10uF on hand, I will go with that.

[Jazz P Bass]

Why not install the correct value (4.7uf)
The 10's are going to pass more bass.