A lucky Bassman Ten owner!

It's been a long road, but I think I've just about finished working on my Bassman Ten. It's been a real learning experience!

I noticed, over the last couple months, it just wasn't as loud as it should be, or as I remembered it when I first got it. So last weekend I decided to try to figure out the overall health and finish up any maintenance it needed.

I started by pulling the chassis and taking voltage readings everywhere. I had located a recently redrawn schematic where you could clearly read all component values and voltage levels. Everything was within the Fender specified +/- 20%.

So I hooked up my Weber Biasrite to check the bias voltage and current and was very surprised to find .6mA on one powertube and 12mA on the other. It should be closer to 30 to 40 mA! It didn't seem loud because it was only making around 7 watts from 2 6L6 tubes!

Turns out the screen grid resistors had gone open. Some idiot had used 1/4 watt resistors in place of 1 watt resistors and the heat had caused them to go open! That idiot was me, apparently, but I honestly don't remember doing it and don't know why I did! I was very lucky they didn't short. It might have fried my brand new Weber output transformer I think, or at least the new JJ tubes in there.

So I replaced those (with 2 watt metal films), a few other resistors and finished swapping out all the electrolytics. I kept stock values on everything except the cathode bias bypass capacitor which sets the bias level of the second half of both preamp tubes. It was 5µF stock and I upped it to 22µF (62 volts).

I finally removed the death cap and put in a new power cord too. Someone had installed a grounded cord a long time ago, but never disabled the ground switch and removed the death cap. That's all done now. If there's anything left to do, it's swap out all the old diodes. I'm gonna ask around and find out opinions on that. They're around 30 years old now and I wonder about them holding up.

I should also check every carbon comp resistor I've got left to see if they've drifted too far, but I'm in no huge hurry for this because it sounds great, the voltages look good and I have no excess hum or noise. All the critical ones in the power section are new metal films, as well as a few others here and there.

This is really a very cool amp. It's pretty unique among Fenders. Channel two is voiced very close to a AB165 Bassman, Channel one seems to be pretty unique voicing (much darker than two) and the overall topology is close to a Bandmaster without Vibrato. Basically, it lacks the third 12AX7 gain recovery stage of a Bassman (and many others) before going into the 12AT7 phase inverter. Take the vibrato circuit (and tube) out of a Bandmaster and you have the same overall thing.

It's not a loud amp, especially given the two 6L6 tubes, and with the bias being fixed kind of cold, it's only pushing about 32 watts right now. The tubes should last a good long time. I may, at some point, convert it to adjustable bias and work those tubes a little harder, but I'm in no hurry. I think it makes a great rock amp as is...jumper the channels and you get rich distortion at reasonable volume levels. It's kind of difficult to describe the sound, but if you read up on the Bassman Ten around the net, you'll see most agree it sounds great and absolutely unique among Fender amps.

I'll eventually flesh out my blogger photo folder for it, but for now here's one overall shot (notice it's a head now, not the typical 4X10 combo), and a link to my photobucket account for it...

From Bassman Ten

I'm going to try to recount a list of everything I've done to this pawnshop prize here...

All electrolytics replaced
power cord replaced/ground switch dissabled
new tubes and bias balance set
all new screen grid resistors and power rail dropping resistors
new speaker output jack
new output transformer with only 8 ohm tap used
new head cab
negative feedback resistor upped to 1.2k (I think, it was upped anyway)
all mods from previous owners reversed or eliminated

There are a few I may have missed and a few more (like diodes and maybe adjustable negative feedback) that I may yet do.

All in all...very cool project!