A Bassman 100 on the bench.
Missing PT (burnt, I suppose …)
Thanks to www.wallybill.com for the layout, thanks to Chris A. for pointing out an error
This model of Bassman was made from 1972 to 1976. This one seems to be from 1975 (Schumacher date code 606-5 … on the OT) :
OK, once the new PT is in place (a Hammond 290FEX) and the four 6L6GC changed, time to rebias this thing. On this model, there is a balance bias pot and not a “real” pot to set the bias. Balance bias is easy : the minimum hum, the better !
Unfortunately, this does not allow to set the current bias, and on this Bassman, the current was as low as 18mA (for 440V to the plate). So I replace the 15k resistor by a 1k “security” resistor and a variable resistor (a 50k I had, but a 20k should be even better) :
Before :
After :
Hence, I can set balance AND bias !
I replaced the 40 years old Mallory caps (I keep them, maybe someday I will try to reform them ?)
I noticed a wire going … nowhere ! Soldered at one end, cut at the other end ! A previous mod ? A “factory” mistake ? Anyway, I removed it (and the amp works well after having serviced it)
New caps …
Another small issue : a very nice blue light in a 7025 tube :
Air inside (and the getter is half gone, btw) : little break in the envelope … and one of the triode does not work (bass channel does not make any sound) :
Didn’t have any 7025 left, so I used a JJ 12AX7 “V1″ (low noise)
Anyway, the baby can sing again !
IT11 Audio Web Site
“I noticed a wire going … nowhere ! Soldered at one end, cut at the other end ! A previous mod ? A “factory” mistake ?”
Noticed the same wire going nowhere in my ’77 fender PA100, so factory mistake
Yes, that’s weird !
and on my bassman 100 73 also has this wire to nowhere…
Thanks for the information !
That little wire going nowhere is a “TPI” connection. Its where you would connect a scope
OK, thanks for the information !
What is « TPI » ?
Thanks for the pictures! My 1974 Bassman 100 has no “wire to nowhere.” Just replaced all 6L6GCs with a matched JJ quad and smoked one of the caps you replaced. I’ll be replacing all five.
Oops … for the cap !
Let us know if everything is right after having new caps replaced !
Thanks for the information about the weird wire …
In the schematic, which is very nice btw, a wire on valve 8 goes to pin 3 instead of pin 1. I know it’s from http://wallybill.com/Bassman100.html.
Err I mean layout, not schematic.
Hi Chris,
I’m afraid you’re right ! I uploaded corrected layout (I hope !)
Also on channel 1 bass channel the bass and treble pots are vise-versa…..also note non original choke , which might be an incorrect transformer.
Hi, noticed on the above layout – the 6L6 screen grid connections on the two outermost sockets aren’t connected to the supply, just to each other (ie pin 6 to pin 6 to nowhere). Otherwise this has been a lifesaver! Thanks
Thanks Matt. I already corrected the layout (orange “error”), but you are right, his layout is very useful (thank to its author) but not perfect
I’m looking at building a Fender Bassman 100 and have been studying pictures, the layout and schematic, and while I’m no expert I think I have a theory as to the mystery white wire. On domestic (US) versions of the amp, there is a ground switch. The layout notes that from this switch comes a white wire, which enters the filter capacitor section. However, on export versions of the Bassman 100 (with switchable voltages), as the schematic notes, there is no ground switch. My guess is that the deletion of the ground switch on your amp, which appears to be an export version, renders the white wire redundant – even though it’s still been installed. Further, Wally Bill’s layout must have been drawn from a US domestic Bassman 100, hence it’s inclusion on the layout.
Thanks Tony, I’ll have a look !
I’ve also noticed that Wally Bill’s layout has the bass and treble pots on the bass input channel located in the wrong place!!
Since my last post I’ve actually had a Fender Bassman 100 on loan, and when comparing what was inside that amp to Wally Bill’s layout noticed another discrepancy – the Bass Channel’s Deep Switch on his layout has the 1M resistor going to the middle pin of the volume pot, contrary to the amp I had open in front of me. After checking with others in a Facebook amp builders group, I’ve been advised that Wally Bill’s layout in this regard doesn’t match the schematic and won’t function correctly.
The white wire going nowhere would be going to the tremolo circuit on amps that use the same chassis as the Bassman 100 such as the Dual Showman etc.
Oh ? thanks !
Hi, thanks for your usefull informations, I actually repair this amp for a friend, I changed all the Polarized caps and now I did the mod for the bias pot.
Normally I use http://www.tedweber.com/webervst/tubes1/calcbias.htm for bias amp and from what I read of you’re measurement, 440V on plate have to be set approximatively to 47,7mA, is it what you’ve set? It seems so far from the 18mA at the begining, a 20K trim is enough?
Thanks a lot, sorry for my bad english I’m French
Yes, I think 20k is OK !
I’m just wondering why you didn’t beef up the filter cap values around the output stage?
Typically, the main filter cap is replaced with two 220uF caps.
The capacitors were 100uF (schematic as in the amp), I replaced by the same values …
Here is a link to my efforts to rebuild a Fender Bassman 100. I would be very pleased if it looks as clean as your amp.
https://ampgarage.com/forum/viewtopic.php?f=8&t=32901