Today we have a Musicmaster Bass amp from the 70's. The customer complaint was it smelled burnt and the volume was rather low. Also had a faulty input jack.
I used to live with a cat that played 2 of these. He was way too loud! These little cheap hand wired tube amps can pump it out even stock. With mods they can really do some damage.
So to start I changed all the electrolytic capacitors. Typical stuff. The burnt smell was most likely the 1k 1/2 watt resistor:
It doesn't appear burnt but it was reading 2.5k. It's the first power stage resistor between the output transformer and the screen grids. I upgraded to a 1k 5 watt:
Yes, I like overkill in my power supplies. This baby will NEVER burn out. I also changed the first filter cap to an 80@450V cap. Deep, rich tones with no hum. Oh, may lose a little sag compression but really, it's a silicon diode rectifier so why not. Besides, the owner needs this amp to play loud and kind of hold it together so it's a wise choice.
These amps stock are meant to be a cheap bass practice amp. I've used them before with good results in the studio for, yes, bass and guitar. They are only about 12 watts so most bass players won't touch them. Yes, they are considered 'vintage' (wank wank wank) but this is an amp where for guitar the tone is kinda limp. So I say: If your tone is a' starvin', git on in that amp and start carvin'!
First thing: The first gain stage cap is a .01 ceramic disc:
Change it to a .022 cap of your choice:
Ok, missed a good photo op, but it's the yellow cap on the right. I also changed the 2.2uf bypass cap to a 22uf.
Now looking at the above pic you see a 470k resistor connected to a .0047 cap. That's the input going to the first stage. Not acceptable! Get rid of that cap and that resistor.
Since we are changing the input jack I'll put the load resistor right on that. I prefer a 1 meg carbon composition:
Should look like this:
Gah! Again, sorry about the poor pic! But notice that the 470k resistor is gone and the wire going to your first gain stage 12ax7 is connected right to the 68k input resistors.
Next, the very tone robbing tone stack:
Get rid of that 100k resistor and .022 cap. I simply copied a tweed Deluxe:
I'm using a 250pf cap for the highs and if you notice, I'm re-using that .0047 input cap as the rolloff. I could sell the customer a new cap here but why? This one is perfectly good and I'm a reduce reuse recycle kind of guy. Seriously. These are made of plastic. If I drop a new one in there this either sits around my tiny space or becomes sea turtle food eventually.
So how does it sound now?
In a word: British. Jangly top end with that nice euphonic mid. Think 18 Watt Marshall with a $300 pricetag. The customer will be replacing the speaker, I would go with a Weber English series or a real Celestion. Great amp for a jazz player who doesn't need fusion volume. Rich, warm and solid. These amps can be a real treat for those on a budget or those who just like a wonderful tone!
Another nice thing about these is they use the rather easy to find 6AQ5 tube. Plenty of cheap NOS examples out there. The later ones used 6V6 tubes, I think these sound more English than a 6V6 model. I kinda prefer these. And, you can just buy a pair of tubes and drop them in. No biasing required.