Bassman 135 Deep Switch - What were they thinking? (Nerd content...)

[KenB]

I had some time on my hands today, so I thought I would post a question that I've been wondering about for years, relating to the operation of the DEEP switch on my Fender Bassman 135 amplifier.
I have researched this question several times over the past few years, with no good answer, so today I'm giving it another shot...

So:
The Bassman 135 has two channels, BASS INSTRUMENT and NORMAL.
The BASS channel has a DEEP switch, and the NORMAL channel has a BRIGHT switch.

The BRIGHT switch works as I would expect, obviously increasing the treble when in the "up" position, which is what I would think should be "on" (towards the word BRIGHT above the switch).

The DEEP switch, however DECREASES the low-end fullness quite noticeably when in the "up" position, no matter what settings of the tone controls I choose. I have always preferred the "down" (off?) position, because the amp sounds really wimpy with the switch on/up.

It is as if the DEEP switch is wired into the circuit backwards!

So today I've indulged my inner nerd/geek and played around with the schematic, online resources, and the Seymour Duncan Tone Stack Calculator program, to see if I can figure out what's up (or down ).

So, DOWN the rabbit hole we go!

Here is the relevant part of the Bassman 135 schematic, showing the DEEP and BRIGHT switches:

I am assuming that the Fender schematic follows the usual convention that the switches are shown in the normal (off) position ("Open" on the schematic = "off").

Here is a photo of the switch in the amp chassis:

It APPEARS that, when compared to the schematic, the wiring & resistor on the switch is NOT as I would expect for "down" is "off".
According to the schematic, closing the switch (DEEP on?) should short the 1M resistor to ground.
The switch in the picture shorts the resistor in the DOWN position.
Hmmmmmmm...

So I decided to fool around with some online schematics and the Seymour Duncan Tone Stack Calculator program to verify my suspicions.

The DEEP switch interacts with the MIDDLE control in the Fender tone stack circuit.

Here is an analysis of what happens to the circuit components with the switch on and off, and the MIDDLE control at 0 and 10:

Basically, with the DEEP switch off the lower 47nf cap is out of the circuit, like the DEEP switch doesn't exist. And with the switch on (shorting the 1M resistor) and the MID control at 0, C3 is increased to 100nf.

This Seymour Duncan Tone Stack Calculator circuit can approximate what happens with the DEEP switch on and off, and the MIDDLE control at 0 and 10 (it's not a perfect simulation, but it's close enough to give an idea of what's going on):

The Tone Stack Calculator shows that when the DEEP switch is in the ON position (according to the schematic), there is a 5dB CUT to all frequencies below 500Hz, regardless of the position of the MIDDLE control.

This is EXACTLY what I've been hearing from my amp, and what I would expect when examining how the actual switch is wired into the chassis.

The darn thing is wired BACKWARDS!
Up is off (low cut), and down is on (DEEP), which explains everything.

Why the heck would the engineers do such a thing?

And why couldn't I find any info about it in my searches? SOMEBODY must have been wondering about this!

And, do the DEEP switches in other Bassman models work opposite as well?

Thanks for listening...

 

[pfschim]

I'm not an expert by any means and I'm guessing that a real expert will be along to discuss this with you in depth.

I do know that the Fender tone stack is kind of an odd duck in reality with Bass, Mid and Treble controls seriously interacting as knob positions change. Its almost as if the Fender EQ controls act as attenuators on the other EQ areas rather than simply boost or cut what we think they would do from the labels (Bass, Mid, Treble). I am also aware that, counterintuitively, many deep controls actually do not add low eq content but cut certain bass EQ frequencies. I don't recall the engineering rationale for it, but I do recall that its true.

anyway, the experts will mosey into your thread soon and offer all kinds of great detail.

 

[agedhorse]

Open on the schematic would be deep on.

It's a simple low frequency voltage divider shunt filter.

 

[Wasnex]

The switch appears to be connected properly.

Here's a layout I found.

This seems consistent with your drawings.

Since there is confusion on whether switch closed = on or off = up or down...it would be better to talk about the condition of the switch (open/close). My guess is switch down = switch closed. A quick check with a multi-meter should resolve the confusion.

This page talks about how the TMB functions, but does not cover how the Deep Switch works. Maybe it will help you understand what is going on. TMB Tone Stack

If the circuit is wired properly and works backwards from what you want, it's an easy enough mod.

One possible explanation. The Deep switch is actually a mid scoop of sorts. The Ampeg SVT "Ultra Low" circuit, is actually a huge mid scoop so there is precedence for this. Or perhaps you could think of it as changing the upper knee of the bass control. That is what I am seeing in the graphs more so than a cut across the range of the bass control.

Trace Elliot and Ashdown use a circuit that's a bit different to change the upper knee of the bass control. I think it works at a bit higher frequency. They call it a mid shift. But if you have the Bass control all the way off, it has no effect on the mid range. As you turn up the Bass, the switch affects how far the Bass control reaches into the low mids.

From the Series I Trace Elliot V-Type schematic.

The Deep switch sucks out mids and AFAIK is pretty much equivalent to the Deep switch on the Bassman, although different components are selected so the sound and behavior of the circuit is a bit different.

 

[JTE]

The manual said that switch was for "bass heavy" instruments. My conjecture is they put it in to try to make a Gibson EB0 sound good

 

[KenB]

The manual said that switch was for "bass heavy" instruments. My conjecture is they put it in to try to make a Gibson EB0 sound good

The manual for the Bassman 135 just says for the DEEP switch:
"Provides deeper tone (more fundamental, less harmonics)."
It doesn't say whether up or down is "on".

It definitely SOUNDS like "more fundamental, less harmonics" with the switch in the down position.

I agree that the switch in the down position corresponds to the "closed" position in the schematic.

It just seems counterintuitive that a switch with "DEEP" written over it would be "on" in the down position, with LESS "deeper tone" in that position.

If I were to have arranged it that way, I would have labeled the switch "LOW CUT"!

 

[PraiseBassing]

The manual for the Bassman 135 just says for the DEEP switch:
"Provides deeper tone (more fundamental, less harmonics)."
It doesn't say whether up or down is "on".

It definitely SOUNDS like "more fundamental, less harmonics" with the switch in the down position.

I agree that the switch in the down position corresponds to the "closed" position in the schematic.

It just seems counterintuitive that a switch with "DEEP" written over it would be "on" in the down position, with LESS "deeper tone" in that position.

If I were to have arranged it that way, I would have labeled the switch "LOW CUT"!

It'll be interesting to see if other amps operate the same or if the switch on yours was wired/installed incorrectly.

 

[KenB]

It'll be interesting to see if other amps operate the same or if the switch on yours was wired/installed incorrectly.

I am sure the switch is wired and installed in the chassis as intended.
I just wish I could find an "official" explanation of WHY it seems to work "backwards".

I am interested to find out how the DEEP switches work in Bassman amps other than the 135.

I've started a poll, but haven't had any responses yet...
Serious question about DEEP switch operation on Bassman amps...

 

[PraiseBassing]

I am sure the switch is wired and installed in the chassis as intended.
I just wish I could find an "official" explanation of WHY it seems to work "backwards".

I am interested to find out how the DEEP switches work in Bassman amps other than the 135.

I've started a poll, but haven't had any responses yet...
Serious question about DEEP switch operation on Bassman amps...

No, I responded. It just doesn't help you.

 

[Wasnex]

It appears to be wired correctly per the Layout I provided. My assumption is the switch is closed in the down position, which I agree seems counterintuitive. Can you confirm with a multi-meter.

My understanding is the Deep cap just bypasses some of the signal around the mid control. In other words, it's shunting some of the mids to ground. With the 1Meg Resistor in the circuit (switch open), the frequency of the mid cut is pushed lower, and the amount of attenuation is reduced. Let's keep in mind that 1Meg is quite a lot of resistance, so the amount of cut would probably be pretty minimal, but whatever cut does occur should be at a relatively low frequency.

When the switch is closed, the 1Meg resistor is bypassed, and the capacitor is shunted directly to ground. This raises the frequency of the mid cut, and increases the mount of attenuation. In other words, a more significant cut at a higher frequency.

The Bassman 100 and Superbassman have the same circuit, except the mid control is a fixed 8.2K resistor instead of a pot. The switch is shown in the open position on the Super Bassman schematic, so I don't think you can put too much into the meaning of the switch position--or maybe the Bassman 135 is showing the switch in the open position as well.

This layout shows a different switch connection than the previous layout I posted, but functionally I think it is identical.

FYI the Bassman 50 Deep circuit is similar to the Bassman 100 and Super Bassman. It has the fixed 8.2K mid resistor, but the 1Meg pull down resistor is missing. So the Deep cap has no affect unless the switch is closed.

 

[pfschim]

The manual said that switch was for "bass heavy" instruments. My conjecture is they put it in to try to make a Gibson EB0 sound good

tried and failed
nothing could make that instrument sound good.

 

[alaskaleftybass]

I know this much. In 1971 I bought my first Fender Bassman 50. Our guitarist would rehearse through the bright channel and I would play along in the deep channel. The deep switch seemed to push my low end down deeper and out of the way of the guitarist. With him using the bright switch we had two distinct sounds that worked well together. I don't know what the specifics of today's Fender deep and bright switches are, but I'm highly doubtful that they would design the deep switch to cripple the bottom end of a bass. Odd.......

 

[Matthias]

Well.. Not knowing anything about electronics (except for Ohms law..) I would approach it this way:
Bright switch in the up ("high") position => more highs
Deep switch in the down ("low") position => more lows
Makes sense to me

 

[Wasnex]

Well.. Not knowing anything about electronics (except for Ohms law..) I would approach it this way:
Bright switch in the up ("high") position => more highs
Deep switch in the down ("low") position => more lows
Makes sense to me

The problem is things don't always work the way we assume the work.

Here are various sweeps of the SVT tone controls.

The sweep with the big notch in the middle is with the Ultra Low Boost selected. The Service Manual describes it as -20dB at 600hz. It definitely makes the sound deep, but it's so overly mid scooped that I always considered it unusable.

My semi-educated guess is the Bassman's Deep circuit does something similar; essentially it scoops the mids, rather than giving you a huge low end boost.

 

[Slevage]

Looks like the switch cuts lows when closed.
If you want the instrument to sound bossier with the switch in the Ip position, see if it can be flipped over where it is mounted.

 

[Bushrod Johnson]

It's a simple low frequency voltage divider shunt filter.

I knew that!

 

[BassBrass]

If it is anything like the Ampeg deep switch, cutting mids to let in more fundamental, the perception will be a quieter sound plus the speakers have to be able to reproduce the fundamental, they can't...as we know, we're not hearing the fundamental on most bass guitar notes vs digital subwoofer stuff.

 

[Al Kraft]

This is why I really appreciate those manufacturers who put extra time into their manuals. Just a few sentences describing the design intent/functionality of the circuit and recommended use of the switch (in the up or down position) for various playing situations would be very helpful here.

 

[StereoPlayer]

The manual said that switch was for "bass heavy" instruments. My conjecture is they put it in to try to make a Gibson EB0 sound good

 

[Killing Floor]

Jaco didn't need a deep switch