Fender Bassman 135
1) Not much on the restoration of this one, but some handy details.
So, is it really a 135watt amp? Actually no, unless you're Jack Bruce. It does make an unusable 135watts at just over 10% distortion but, at clipping, it's a genuine 100watts rms with a sine wave signal. That's still pretty impressive for a company that uprated the Twin Reverb from 70 to 100watts with a typewriter. Before you buy one though, just speak to someone who's had to pick one up.
2) Fender Bassman 135 Schematic
3) Fender Bassman 135 Layout
4) Fender Bassman 135 Speaker Options
The 135watt Fenders are unusual in that they have tapped output transformers. The speaker output jack sockets have automatic switching contacts which select either 4ohm or 8ohms windings.
It's not obvious from the labels but, if you plug anything into the EXT SPKR socket, it will automatically put both outputs in series and connect them to the 8ohm tap. The SPEAKER socket will just set itself to 'straight through' if not used.
5) A few common problems
For a lot of reasons, the bigger Fender valve amps went out of favour a long time ago. That lead to a lot of them being stashed away in dark, damp places and forgotten about until they rolled around into fashion again. Unfortunately, in many cases, the damage has been done and you're left with some odd damp/corrosion/age related problems to sort out.
Fender/CBS octal valve bases from this period tend to suffer from leakage between the pins caused by water and cleaning fluid absorption. They're an odd size and replacing them with something that didn't leave China two weeks ago can involve opening up the chassis holes. Not difficult when you've done a few but a bit of an adventure the first time. My favourite replacements that still look the part are new-old-stock McMurdo.
The other common part to give trouble is the mains voltage selector switch. The originals had a bit of a design problem with the lugs that retain the paxolin back. They tend to bulge out after a few decades and cause the internals to lose contact. Newer replacements have four lugs instead of three.
Quite often the whole lot will be chopped off and hardwired to one particular voltage. Perhaps the old 'health and safety' excuse was a handy way to save a few pounds.
Non-linear pots use a 'J' taper, making the amp sound louder at lower settings. All very well unless you're trying to get some in a hurry in Europe. Try not to fit ordinary log (audio) pots. They change the character of the controls and modern CTS ones don't actually do anything until you get towards number two on the dial.
There isn't a bias current adjustment, as such, on this amp. The 'O/P tubes matching' pot just balances the currents flowing through the two sides of the output transformer. You need to physically change R28 to alter the total current.
That's an easy one. Just adjust the pot for minimum hum through the speakers. Getting it wrong won't do any harm.