Help! With a Fender Bassmann 135 Clone

I feel that if you have blown 8 output tubes then the 807 was not a good choice. The RCA data sheet specifies 300V as the maximum for the screen grids.

 

Hi Francesco.

What Paul said.

Kudos for deciding to build that particular amp, most of us wouldn't have .
Nice craftmanship in any case.

That said, first of all, the tube choice is a bit odd unless You got a box-full of them stashed away.
Thinking outside of the box is always a nice thing to do, but practicality is IMO a better guideline.

Secondly, UL?
Why?
Sort of great for Hi-Fi, if normal class A designs aren't powerful enough, but there has been only a very few succesful MI UL applications. For a reason .
UL tends to suffer from the same shortcomings as the various OTL designs in MI use do, they don't sound the way we are accustomed to hear tube amps to sound. IMO anyway.


As for the hum, a few observations of points that I'd check first.

The individual heater wires to each tube are possibly one source, daisy-chaining them IMLE works better.
The ground "bar" also runs across the entire lenght of the chassis, a great antenna if there's any stray fields inside the chassis.

The choke is usually at an angle to the PT and OT, straight angle orientation may cause some interference.

Good luck, I'm sure it'll sound great when the bugs have been chased out of the system .

Regards
Sam

 

Wow. Hard to tell where to start looking for the problem. Here's some pointers/questions:

Make sure the B+ HV winding center tap (or bridge rectifier "-" terminal) goes straight to the "-" terminal of the first filter capacitor (bottom 350V cap). If you tie it to another ground point it can create hum problems.

Do your 6.3V heater windings have center-taps to ground, or did you reference them to ground through a pair of 100-ohm resistors? If the 6.3V windings are not somehow given a leakage path to ground, it can create noise problems.

Good luck.

Chris

 

Hi Francesco.

Sounds like a good enough reason to me .

UL is just a choice I probably wouldn't have made, that's all.

Way back when I had mine I used whatever 6l6's I had in hand. JJ wasn't around then yet.
Unlike Marshall Major for example, Fender UL is IMLE somewhat forgiving on the tube choices, even the unmatched CCCP tubes we could buy pretty cheap sounded quite good. That was about 20 years ago though, so my mind may be playing tricks on me .

Regards
Sam

 

I have a 135 bassman, and I loved its sound. I havent heard it for a while, though. Its been disassembled to re work the Power supply, and hasn't been reassembled since. I think I liked the Bass input channel with the mid knob... I remember reading something about radio frequency oscillation being a potential problem with UL design (could be bunk). I wonder if your tubes choice contributes to that? Do you have a scope that goes into MHZ?
My 135 doesn't have a choke, I was reading about UL taps helping the power supply out a bit. Should I add one? Its inline for some work soon.

 

From a technical perspective,Fender didn't actually run their "ultralinear" amps with output transformers that were actually implementing what we conventionally think of as "ultralinear" operation. People do like to call Fender amps "ultralinear" though, and I think that this is primarily because people see the extra screen taps and equate the presence of the familiar-looking topology with ultralinear operation.

Looking at the tube spec sheets, there isn't any way that the Fenders amps should be able to make their rated power based upon UL operation. UL operation produces less power, and the Fender amps are producing power output that's nearly as high as if they were running in pentode mode.

There's a reason for this -- When I took measurements of an actual Fender OT, I found out that it wasn't designed to operate anywhere near the 40% tap that we commonly think of as "UL" mode. The Fender iron is actually designed to operate very differently from a 40% UL OT. Fender actually designed the amps to operate in what tube engineers call "distributed load" mode.

In the big scheme of thigns, I think it's important to understand the difference bewteen distributed load mode and ultralinear mode. It explains why replacing a broken Fender OT with a generic UL OT will result in the amp not being able to make it's rated power.

Not that this solves your problem, I'm just pointing out that the common parlance of calling Fenders "ultralinear" is actually a misapplication of the term.

 

Actually addressing your problem, you need to calculate the internal power dissipation that's occurreing in each component of the tube to see which one is failing. Exceeding voltage and power dissipation ratings for plates or screens is a fast route to failure.

With that said, I am able to run 6L6GC with 530 VDC B+ in distributed load mode by keeping a handle on screen dissipation.

 

It looks like you have put a lot of work into the amp. These sort of builds are a lot of fun, more so when you get it working.

Is the ground bus grounded at only one point or are there multiple ground connections to it? On a bus, I like to order the connections from high current (power supply first node at one end) to low current at the other end (the first preamp tube) with the ground bus grounded at one end or the other. This helps the amp to be quieter and avoids ground loops.

Are the shielded cables grounded at one end only? If not you can get a ground loop.

As was mentioned with the heater circuit, either ground the center tap OR use an artificial canter tap or hum pot. Don't do both or you will have a ground loop.

Where is the speaker return grounded? I like to ground it close to where the phase inverter is grounded on the bus.

When you set the bias, are you measuring the cathode current or are you simply setting the bias voltage to some number?

You have to break the amp down and look at one section at a time. Try disconnecting the pre amp from the power amp section. Pull the power tubes. Then check just the preamp. See if it is working properly. Connect it to another amp and see if there is noise in the pre-amp.