Fender Bassman 135

[Rickenbacker 4003]

Anyone know if I can use this unit as a pre-amp, without using the speaker output , without blowing the output transformer(s)? My Ric just totally distorts when I get the volumes above 4.

[EWBrown]

I am not fully familiar with that particular amp model, I will presume that it is an all-tube amp, and not solid state. Also it would be helpful to determine whether the distortion is occuring in the presmp section, or in the power section of the amp. I'll go check "Fender Schematic Heaven" and see if they have this one covered...

There are two possible simple approaches here:

If you are just using the preamp section, and not the power section, you could disable the output section, by dsconnect the interstage coupling capacitor before the power amp; or use a hefty 4 or 8 power resistor to load the amp's output, in place of using the speaker. The second approach involves no "surgery" under the chassis. The power resistor(s) need to have a sufficient power handling rating.

Note: my next posting will eliminate the "disable the output stage" (green text) option, see below...
If you were to just remove the power tubes, the power supply B+ would run abnormally high; and the power amplifier should NEVER be run with no load, as that will cause excessive voltages to develop inside the output transformer windings, which will blow it out, and damage the output tubes, in the process.

HTH

/ed B

[Rickenbacker 4003]

My fault here: It is a Fender Bassman 135 and it distorts when I play my Ric 4003 through it with the volumes above 4. It has a line out in the back, which I wonder if I can use it as a preamp without using the power amp section. Basically, will the power section be okay, or does it need speakers plugged in to prevent the output transformers from being damaged? Wish I could word this better!

[EWBrown]

A quick look at the 135's schematic,
http://schematicheaven.com/fenderamps/b ... _schem.pdf

shows that the line/record output is taken from the speaker output, so that completely eliminates the "disable the power section" option that I mentioned earlier.


It would need to have the speakers, or an equivalent "dummy" load resistor plugged in, in order to prevent serious damage. The "dummy load" would allow the amp to run at the necessary power, but do so silently. However, the distortion may still be present, mostly depending on how hard the output stage is driven, perhaps with "line output" service only, you can run its overall gain at a lower level, and reduce the distortion.


Depending on your Ric 4003's output signal voltage form the pickups, the distortion could be occuring in the "preamp" section, and that can be determined by monitoring the line output through another power amp.

Or you can experiment with the correct combination of teh amp's individual channel and master volume control settings, and the Ric's volume / tone controls, in order to localize where the distortion is actually occuring.


/ed B

[Rickenbacker 4003]

Thank you so much for your input. I'm a bit challenged eletronically, but let me ask this: Can I make a "dummy load" out of a speaker cable with a 4 ohm resistor in the line? I really want to use a solid state power amp and use the tube preamp from the Fender. Here's yet another question: Is it better to run the master volume high and the channel volumes lower to prevent distortion? I've tried it before, but never seem to get the tone I'm after, it sounds "limited", no punch.

[EWBrown]

With four 6L6GCs in parallel push pull, I'd SWAG that your amp can peak out around 100 watts. So the "dummy" resistor will have to be able to handle that amount. at a 4 ohm outout, this translates to 20V at 5 amps.

You would need a cluster of wirewound resistors to handle this, and the total power handling capability should be at least two or three times that amount

I'd use something like twenty-five 100 ohm, 10 watt wirewound resistors in parallel, that will give you a pretty much bullet proof 4 ohm dummy load. THese ceramic boxy WW resistors are fairly inexpensive, 5% or 10% tolerance is perfectly good, it doesn't have to be exactly 4.00000 ohms... With that quantity of resistors, any tolerance errors will effectively cance themselves l out, so the end result whould be very close to 4.0 ohms.

Don't stack them close together like bricks, leave at least 1/4 to 1/2 inch airspace between them, they can be arranged in a 5X5 array, or whatever format you prefer. Without any convective airflow, the "inside" resistors will get very hot.

Connect it to a length of zip cord, and terminate the end with the appropriate plug (usually a 1/4 inch standard mono plug) to match ouur amp's output. And, unplug the speaker(s).

The resistive dummy load will get fairly hot, so don't let it contact anything flammable or meltable, and protected from any accidental skin contact..

As far as fine-tuning for the "tone", all I can say is to experiment with the volume (and tone) controls on the guitar, individual channels and master volume last. You should be able to create a nearly unlimited variety of tones, funk, grundge, etc by trying various combinations of setings.

Distortion generated in the (12AX7 or 7025) preamp and driver stages will have a diferent tonal quality, than that which is mostly created in the output (12AT7 and 6L6GC) stages. The master volume control will allow you to vary the latter effects.

FWIW, 7025 is a lower noise version of a 12AX7, and are otherwise completely interchangeable. and the "European" ECC83s can also be used.

I never got good enough to be able to fully experiment with trying to achieve that "special" tone, I was too busy just trying to get the correct notes, and keep in correct tempo with the rhythm 8^0

Another "trick" to slightly lower the overall 7025 driver stage gain is to replace any 12AX7s or 7025s with 5751s, they have a slightly lower overall maximum gain, of 70, rather than 100, and they are pin compatible . That equates to taking about 3 dB of voltage gain off the driver stages.

Don't change the phase inverter tube (12AT7) , that stage is more critical to the tube operating parameters.

If you plan to use a separate solid state power amp, just be careful not to exceed its safe input signal voltage, or the SS amp will suddenly become an expensive smoke generator. Tubes are far more forgiving of "abuse" than are transistors. Tubes just moan and complain a lot, while transistors will die sudden, horrible, deaths when pushed too far.

/ed B