Wednesday, February 3, 2016

1967 Fender Bassman amp

  Here's an amp I used to see all the time and haven't seen much of in a while. A good old '67 Bassman head built by Fender in Santa Ana California. These are to me some of the most consistently good sounding amps built by Fender, and are still relatively a bargain when compared to the earlier black panel amps. These are commonly known as the 'drip edge' series due to the aluminum edge around the grill cloth.

  This one is an AB-165 circuit. They started building this right around 1965. Not as desirable as the earlier AA-864 which, I ain't gonna lie here, is a better sounding amp. But these aren't bad by any stretch of the imagination. The AA-864 is relatively hard to find. I would describe it as more open sounding, the classic 'black panel' sound. These do have a bit more of a stuffy sound to them. You can convert one of these, it's a pretty big task so I wouldn't attempt it unless you have some serious skills. I've done several over the years with satisfying results. There is one thing I don't see talked about in literature and that is the output transformer may be wired out of phase on the AA-165. Once you do a conversion when you fire it up if it sounds like Armageddon (bombs falling, explosions, people screaming!!!) simply switch the blue and brown anode wires on pin 3 of your 6L6 tubes. Seriously, it's a scary sound!

  This amp just needed service. It had been off for about 25 years in storage. 5 out of the 6 tubes were original to the amp and unfortunately the 6L6 RCA tubes were shot. Always sad to see those go. I replaced them with Sovtek 5881 tubes, the thin base ones. They are rugged and sound decent. I like them the best cause I never get returns. All other glass is risky. The 12AT7 phase inverter tube I replaced with a NOS Sylvania. There are plenty of NOS 12AT7 tubes around, and in a Fender amp these, especially in the reverb driver circuit, take a lot of abuse. I get annoyed when I see new glass in these positions. The JJs and Sovteks fail. Don't be cheap and don't be lazy and besides, these often COST LESS than new glass. If you don't need premium Telefunken or Mullard, any garden variety RCA, Sylvania or G.E. will do.

  This amp received a full re-cap. Get those old, crusty electrolytics out of there! Yeah, they look cool and have those date stamps but, they need to go.

  Also check the power supply resistors. These were okay so I left them. Always replace the screen grid resistors though.

  I didn't convert this amp but did convert the bias supply. This is the era where Fender started to be more concerned with balancing the output tubes than actually biasing the amp properly. One tube stays constant, the other is adjustable. Not a terrible idea, Marantz has an individual bias pot for each tube, I like that. To do this really right you would need to add another pot to adjust overall bias, but we're just going to make it like the older amps.

  Now, on a late 70's model you need to leave that balance pot alone and add a bias trim pot. Here's why. Those amps are Ultra Linear. When one side drifts out the amp hums. I've seen people pay for a re-cap on one of those when all it was is the poor tubes aren't matched up right. I'm not a fan of ultra linear output transformers in guitar amps. The idea is low triode distortion at high pentode power. The result is an amp that is good for pedal steel or jazz but not as likable as the previous models.

 So Jef, what is Ultra Linear???

  Read this:

The Marshall Major, Park 150, Sunn amps, these are all Ultra Linear. I like these for bass. For guitar I like my distortion thank you!

  Anyway back to our Bassman. Once I got the amp re-capped and biased the amp ceased to hum and sounded great but still more needed to be done. Snap, crackle pop, hiss....... Here's an example of a big corporation trying to squeeze out every profit they can. Leo Fender was a perfectionist. He chose the best components available to him be it pickup wire or resistors, you know, the little things. CBS decided to buy resistors from Joe's Discount Resistor Shack, they probably just got a better deal on them. It's not unusual to have to replace EVERY load resistor in one of these along with the 220k and 470K resistors in that additional gain stage. Check them. In this amp all of them had drifted way out of tolerance. I replace them with new carbon composition resistors to retain that old, warm sound. There are plenty of choices. You can go with new Allen Bradley or Nichicon, Kemet, they are all good and I'm grateful I can buy these at Mouser, Antique Electronic or Mojo Musical. When I first got in the game I had to scour dirty, disgusting and often depressing HamFest or some poorly lit electronic supply houses. Ok, I did enjoy the HamFest and the surplus places, many of which are now gone were pretty special places. You met some really eccentric folks there. But that's ironic progress for you. A demand for a resistor that is by all means antique has created a new supply for us with picky ears! And that's exciting.

  It's the little things....


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