Fender Bassman 50 with Master Volume

[xtian]

Here is a schematic. I had to send this to Jeff Bober about 3 weeks ago he couldn't find it for his magazine article.

Yay, Mark!! This was not to be found via Google. Excellent.

Look at that "Ground switch", with its associated 0.047 cap, prior to the power tranny. Is that a valid ground switching scheme, or it that one of those "death caps"?

[rdjones]

I have never seen a Bassman 50 with Master Volume.

I wonder how many of those things were built?

In terms of a Bassman it was short-lived.
The Ultralinear 70 Watt was introduced in '77.
I think the Master was added to the 50 in '75.
So maybe 2 years.

rd

[rdjones]

Here is a schematic. I had to send this to Jeff Bober about 3 weeks ago he couldn't find it for his magazine article.

Looking at the schemo I see no master volume.
I'm curious if the 50 watt MV is more similar to the 70watt UL or the 50 watt non-MV.

Look at that "Ground switch", with its associated 0.047 cap, prior to the power tranny. Is that a valid ground switching scheme, or it that one of those "death caps"?

Properly grounded 3 prong AC cords appeared around '71 or '72, but the vestigial ground switch and so called "death cap" remained until the '80s.

On amps that already had a grounded cord I used to just disconnect the switch from the hot side, allowing off-off-cold connection.
This does nothing to protect from badly wired outlets, or the use of an AC ground lift.
Now I just disable the cap altogether and make sure the AC is wired correctly.

reddog

[Structo]

Yeah, I don't see a MV either.

That amp has the funky tapped pot on the bias as well.

[John_P_WI]

The Bassman Ten had a MV.

Check out:

http://www.turretboards.com/layouts_sch ... manten.gif

About 15 years ago I turned one of these (Bassman Ten) into a Vibro / Plexi with great results...

[xtian]

That amp has the funky tapped pot on the bias as well.

Yes, and I've seen suggestions to turn this into an adjustment for bias. Anyone have instructions for this?

[M Fowler]

http://www.enotes.com/topic/Fender_Bassman

Bassman 50 (1972-1977) - Silverface piggyback head - two 15" speakers, 50 Watts/RMS - Same specs as the original silverface Bassman heads produced between 1968 and 1972, except for the addition of a tailless amp decal and an AC568 circuit.

Bassman 70 (1977-1983) - Silverface piggyback head - Same as the Bassman 50, with 70 Watts/RMS and a master volume control.

Seems you just use the Bassman 70 MV schematic as reference.

[solderstain]

Here is a shot of my 1966 (AB165 circuit, with earlier version circuit label affixed to inside of head cab). I changed it to AA864 / AB165 hybrid.

I did exactly the same thing to my '66 Bassman a few years ago based on the info in that same link. KILLER! I also made both channels into 'Normal' channels and use an ABY switcher - set one channel for crunchy rhythm (Volume knob at about 4 1/2) and dime the other channel. Great fun!

BTW, I also tried the LarMar in that amp - mmm.... not so good. Took it back out again. I have YET to find any way to attenuate an old Fender that sounds good. The LarMar does work good in some applications, just not in old Fenders, IMHO.

[xtian]

Bassman has landed! Works as advertised, no obvious issues. First thing I noticed after playing for a few minutes, there was a lot of heat coming off the tubes. Power tubes (Sovtek 6L6WXT+) were glowing blue (not redplating).

1. So, on examining the interior, I see the 470R resistors on the power tube grids have been replaced with 1K (the blue ones at the top of the photo, outlined in yellow in the schematic). Why would someone do this? What's the net effect?

2. The 2000pF caps (circled in green) do not appear to be installed. One less thing to do.

3. The stuff circled in blue...is that the bias adjustment? I don't understand the function of the fourth lug on this potentiometer. What is this device called?

4. The value circled in red...could that be "-48V" ?

5. Keep or replace the white Mallory caps?

6. It has the original filter caps, too. Sound OK for now, but replace them anyway?

[xtian]

Still need help with my questions, above. Thanks!

[John_P_WI]

1) 1K screen resistors, good thing. Limits screen grid current, helps save the tubes. Better touch sensitivity too.

2) 2000 pF caps were to limit oscillation problems. If the amp is stable, leave them out.

3)Bias adjustment is actually a "balance" control. Fender did not use "matched tubes" nor expect matched tubes to be used. Anyone could "bias" the amp by adjusting this for minimum hum to balance currents / tubes.

4) Yes. -48v, raw bias voltage.

5) If the white Mallory caps sound good keep 'em. What are you looking for?

6) New caps are good, BUT the amp may sound brighter and have more punch. It will sound different...

[xtian]

Thanks, John!

If I change the hum balance circuit to the AA165/AA864 circuit, is that a true bias adjust, or is that just a variation on the hum balance?

How do I change this hum balance pot to a true bias adjustment?

[xtian]

I'm confused by statements like this: "165's used a bias balance circuit, not true bias. Best thing to do is convert the circuit to "true bias". Check the AA864 Bassman schematic for how this is wired."

The 165 and 864 schematics appear identical in this respect.

[xtian]

The 165 and 864 schematics appear identical in this respect.

I stand corrected. The AB165 appears to have the hum balance circuit, and the AA165 has the bias adjust.

[NickC]

The 165 and 864 schematics appear identical in this respect.

I stand corrected. The AB165 appears to have the hum balance circuit, and the AA165 has the bias adjust.

That's correct. Bassman AA864 schematic shows the bias circuit you want.