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

[808State]

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...

The Deep switch is not designed to be a Bass Boost.

It changes the Mid Cut Frequency to do as described.
Remove more upper harmonics more Fundamental.

I have older sims of B15 tonecontrols and Hiwatt DR506
Usually they set the mid dip Around G4 391 Hz

Of course you only get so close with available standard value capacitors
and the mid point will slightly shift with the other tone control positions.

Fender Sets the Mid Dip Up higher Closer to A4 440hz
with Deep switch Engaged it changes down to G4 391hz

The Reason why there is more of a bass cut with Deep engaged
is because of the way the passive tone stack interacts.
Also the first gain stage on the 135 has a Cathode Bypass Capacitor for more Gain
with a 5u capacitor
If you wanted the Bass cut to be less obvious with deep engaged
use a larger bypass capacitor like 20 to 25u like Earlier Fenders
instead of 5u. This Will also boost More Sub Sonic Content
Could be to Much Bass. 5u was a good factory Value.

Below Graph shows Deep switch Engaged
on Both Curves
Green line is Factory 5u bypass cap
Maroon Line is 25u bypass cap
Full Spice Emulation Using Tube Gain Stages and Factory Voltages
Duncan Tone Stack cannot Emulate Pre/ Post Tube Stages
I believe your sims show 250p Treble Capacitor 500p
is used on the 135 Bassman

Main Issue is This Tonestack has a Wider Q for the Mid Cut.
So lowering the Mid Point to G4 will always have a Affect on Bass.
A inductor Based circuit or Different Tonestack Could
make it a more narrow notch. More similar to a wide boost,
narrow cut Active EQ. And would affect the Bass Less

 

[Wasnex]

I just did some more "research"... I looked through a bunch of schematics for various Bassman models, both online and in my Aspen Pittman Tube Amp Book.

It looks like Fender has used the same basic arrangement for the DEEP switch for some time, starting, I think, with the Bassman 50.

I just looked at a bunch of online amp chassis photos, and, in every case, the DEEP switch is wired so that the switch is CLOSED in the down position. Meaning that the 47nf "DEEP capacitor" is connected directly to ground with the switch down.

With the switch UP, the capacitor is either disconnected, or connected through a large resistor to ground (essentially disconnected).

So, in a nutshell, this is what happens:

View attachment 4133122

It appears to me, that when the 47nf cap is connected to ground (switch down/closed), it provides a path for both mids AND treble frequencies to be shunted to ground (bypassing the tone stack), and thus subjectively boosting the lows.

With the cap disconnected, those mids and highs are no longer shunted to ground, and the tone is both louder and brighter.

That is EXACTLY what I hear when I toggle the DEEP switch on my 135.

If I run pink noise into the amp input, with the switch down the tone is full sounding, with plenty of lows, but balanced. When I toggle the switch up there is an obvious increase in upper mids and highs, and a definite increase in overall volume.

The effect does NOT sound to me like a "mid scoop". My Rumble 200 has a "Contour" switch, which IS a mid scoop, and the effect is very different.

So, anyway, I guess I've got my answer...

The Bassman DEEP switch INCREASES mids & treble when in the up position, and CUTS mids & treble when down. So the effect is to make the tone "DEEPer" when DOWN. Almost like a BRIGHT switch when up.

So backwards from what I would expect!

And all Bassmans apparently do it the same way.

SO WHY HAVEN'T I HEARD ANY DISCUSSION OF THIS BEFORE?

Great post. It's very clear what position the switch is in and what you are hearing.

Here is part of a schematic from one of my quirky amps.

One of the really quirky aspects of this amp is the Volume control works like an HPF.

Here is a circuit for an HPF

You will note that if you change R to a variable resistor it's the same circuit as my amps Volume control.

Here is the formula for the cutoff frequency of the HPF.

Therefore, as you open up the volume control, there is less attenuation, and the knee of the HPF moves to a lower frequency, so the lows fill in.

Now take a look at the Treble control and note how similar it is to the volume control. One big difference is the output of the Treble control is taken before the capacitor, while the Volume control the signal is taken after the capacitor. I don't sense any significant interaction between these two circuits, although I would not be surprised if there is some that could be seen with test equipment.

IMHO the Deep switch on the Bassman is arranged similar to the Treble control on this amp, so I agree that it's bleeding highs to ground, but again you need to consider where the signal is taken off the tone control circuit.

The signal enter and exits from near the top of the circuit. The Deep switch may have some impact on the signal at the bottom of the Treble pot, but I don't think it's going to have any significant impact on the signal at the top of the Treble pot, which is where the highs pass.

So if you were to run traces with the tone controls in various positions, I still think the effect of the Deep circuit would best be explained as a mid cut. It may extend so high in the frequency response that it sounds like a high cut with bass guitar, but I don't really think it's functioning as an LPF overall. Rather it functions primarily as part of the Mid circuits band pass function. But of course, results depend somewhat on the relative position of the Bass, Mid, and Treble control since they all interact to some degree.

But I could be entirely wrong .

@Passinwind any chances you have done modeling for the Bassman (bass instrument channel) tone network, see redrawn circuit above? The silverface Bassman 135, 100, and Super Bassman all have the same circuit.

 

[Passinwind]

@Passinwind any chances you have done modeling for the Bassman (bass instrument channel) tone network, see redrawn circuit above? The silverface Bassman 135, 100, and Super Bassman all have the same circuit.

I have the Twin Reverb AB763 and a few F2B variants in my stash, so I'd be happy to model a thing or three for you. Might take a day or so though.

 

[Wasnex]

I have the Twin Reverb AB763 and a few F2B variants in my stash, so I'd be happy to model a thing or three for you. Might take a day or so though.

Awesome! It would also be really great if you could do a third model that removes the 1 meg resistor.

 

[808State]

Awesome! It would also be really great if you could do a third model that removes the 1 meg resistor.

Circuit Functions the same with or without the 1meg Resistor. It reduces popping noise when you toggle the switch.

Also Fender describes the Circuit Exactly how it behaves. It Removes upper harmonics and allows fundamental.
Fundamental is around 40 to 300 hz

It really makes more sense if you set the Tonestack flat. This case its around 1 , 10 , 1
then if you hit the Deep switch. It removes all the mids and treble. And only allows Fundamental.
Its passive, obviously there is a volume drop.
Hence Fenders Description= Removes Harmonics , More Fundamental


Graph below tonestack flat 1, 10, 1 = Maroon
tonestack flat 1, 10, 1 with Deep Switch = Green

 

[808State]

Maybe it makes even more sense if I compensate the gain.

Maroon = Flat 1, 10, 1
Green = Flat 1, 10, 1 with deep

If you use the deep switch....turn up your volume, its passive tone control

 

[Wasnex]

Circuit Functions the same with or without the 1meg Resistor. It reduces popping noise when you toggle the switch.

Also Fender describes the Circuit Exactly how it behaves. It Removes upper harmonics and allows fundamental.
Fundamental is around 40 to 300 hz

It really makes more sense if you set the Tonestack flat. This case its around 1 , 10 , 1
then if you hit the Deep switch. It removes all the mids and treble. And only allows Fundamental.
Its passive, obviously there is a volume drop.
Hence Fenders Description= Removes Harmonics , More Fundamental


Graph below tonestack flat 1, 10, 1 = Maroon
tonestack flat 1, 10, 1 with Deep Switch = Green

View attachment 4133618

Nice! This is not inconsistent with what I wrote in post #42. With the Treble control at 1 (off), the deep switch is sucking out mids and highs.

Switch closed, Bass 1(off), Mid 10 (max), Treble 1(off)

Green is the primary signal path and orange is tone sucking action of the Deep switch. Since the signal path is through the bottom of the Treble pot the Deep switch will have an impact on the highs.

But not everyone runs there Fender tone stack 1,10,1. As you turn up the Treble control, the signal path transitions progressively to the very top of the tone network.

The Deep switch does not suck out lows, so it won't have any effect on the lowest frequency range of the Bass control.

Also the Deep switch is not going to prevent the Treble control from boosting the highs.

 

[Wasnex]

Maybe it makes even more sense if I compensate the gain.

Maroon = Flat 1, 10, 1
Green = Flat 1, 10, 1 with deep

If you use the deep switch....turn up your volume, its passive tone control

View attachment 4133632

?? I assume the previous graphic shows the correct gain relationship. The Boost control cuts the mids and highs. This graphic suggest a Bass boost.

 

[808State]

[Invalid or Expired Link Removed]
Assuming a 20 fret Fender Bass, Note range goes from 1E to 4D♯
1E = 41.2 Hz
4D# = 311.1 Hz

Looking at the -3 dB response of the Deep switch in Flat 1, 10, 1 settings
52.3 Hz to 340.8 Hz
Pretty dang close to perfect fundamental response
Deep Switch = More Fundamental , Less Harmonics

 

[808State]

?? I assume the previous graphic shows the correct gain relationship. The Boost control cuts the mids and highs. This graphic suggest a Bass boost.

Yes Correct, First Graph shows correct Gain Relationship.
2nd graph shows gain compensation
Basically what would happen if you hit the Deep with Flat EQ and turned up your volume

 

[MIMike]

Thankfully I spent a couple of hours with my 72' Bassman 100 a few years ago before I started gigging with it. Had I seen this post before that, I wouldn't have known what to do!

I ended up letting my ears decide what channel to use (normal), and where to set the conterols (mostly flat).

 

[808State]

Thankfully I spent a couple of hours with my 72' Bassman a few years ago before I started gigging with it. Had I seen this post before that, I wouldn't have known what to do!

I ended up letting my ears decide what channel to use (normal), and where to set the conterols (mostly flat).

Sounds Good to Me. My favorite Fender Channels are the Bassman 50 Bass Channel , Deep Off
And Bassman 100 normal Channel , Bright Off

The Bassman 50 has a .01u or 10n across the 100k first gain stage plate resistor. I'll lower that to 1n or 500p
like a normal channel to let more highs in.
Also many Bassmans have a Highpass on the bass channel, they use 5u for Cathode bypass.
Ill increase that to 10u or 15u. Normal channel has 25u
Sometimes Ill lower the normal channel to 15u or 10u so it has little more highpass like the bass channel

 

[Wasnex]

Sounds Good to Me. My favorite Fender Channels are the Bassman 50 Bass Channel , Deep Off
And Bassman 100 normal Channel , Bright Off

The Bassman 50 has a .01u or 10n across the 100k first gain stage plate resistor. I'll lower that to 1n or 500p
like a normal channel to let more highs in.
Also many Bassmans have a Highpass on the bass channel, they use 5u for Cathode bypass.
Ill increase that to 10u or 15u. Normal channel has 25u
Sometimes Ill lower the normal channel to 15u or 10u so it has little more highpass like the bass channel

I learned to play on a blackface Fender Twin Reverb. My favorite setting was bright off, BMT at about 4,4,2. I currently own a TFL500D Showman Reverb which is essentially the same amp in head form. I also own a Orange AD200B. With the (master) Volume maxed out, I think it sounds and behaves a lot like the old Fender circuit, but with about double the power. I like BMT at 10:00, 3:00, 9:00 on the AD200B

 

[808State]

I learned to play on a blackface Fender Twin Reverb. My favorite setting was bright off, BMT at about 4,4,2. I currently own a TFL500D Showman Reverb which is essentially the same amp in head form. I also own a Orange AD200B. With the (master) Volume maxed out, I think it sounds and behaves a lot like the old Fender circuit, but with about double the power. I like BMT at 10:00, 3:00, 9:00 on the AD200B

Yeah Exactly 4, 4,2 sounds about right, and some cabs maybe 6, 6, 3 at the most.
I have never really understood players that are obsessed with some sort of tone magic with elaborate
amps with excessive amounts of tone control.
Not really necessary. Get more tone with the Bass and pickups.

I have recorded a lot of amazing players, most just plug in and go.
Then there is the " talkers" fart around and chit chat, then spend 2 hours twiddling knobs.
To basically get a Fender Tonestack curve. Sometimes I get annoyed, most the times I tell myself
I just got paid 2 hours watching Mr Magic LOL. I even try to help and make life easier.
They usually dont listen. More advanced EQ only comes in with high gain or Distortion.
Im more picky pants when it comes to Speaker Drivers

 

[MIMike]

I edited for clarity; mine is a 100.
I know the EQ isn't flat with everything set mid, but I had the amp tech that went through it after I bought it tell me what the flat settings were. As it turned out, the best sound was not on the bass channel, deep switch in either position, but on the normal channel, bright switch up, master volume maxed...that's what I like.

 

[Bill Whitehurst]

Sometimes you have to ignore what it says it's doing and just listen to what you hear it doing.
Sure mid cut can make you think you have more bottom because it is more relative to the mids.
I think if you really want a low boost, the switch should kick in a second, parallel cathode bypass cap.
Then you have some boost, but maybe mud as well.

 

[Wasnex]

Just for fun, AB763 tone circuit for BF Fender Twin VS Orange AD200B tone circuit.

 

[808State]

Just for fun, AB763 tone circuit for BF Fender Twin VS Orange AD200B tone circuit.

View attachment 4133871

Yes !! At a quick glance those are very nice Values.

For players that like the possibility to have more Bass Boost, Changing from factory 250K to 500K
Will add slightly more boost.

Likewise players that Use higher bass settings, Or Treble settings.
The Mid Scoop is more noticeable.

The factory 10k Mid pot wont allow enough Mid Boost, I always used 20K double the value.
Look like Orange Really went a little Extra with 25K
Wonder if the Tiny Terror is the Same.
Recorded some kids lately, and he had a Tiny Terror. I enjoyed the Stack on that amp a lot.

 

[Wasnex]

Yes !! At a quick glance those are very nice Values.

For players that like the possibility to have more Bass Boost, Changing from factory 250K to 500K
Will add slightly more boost.

Likewise players that Use higher bass settings, Or Treble settings.
The Mid Scoop is more noticeable.

The factory 10k Mid pot wont allow enough Mid Boost, I always used 20K double the value.
Look like Orange Really went a little Extra with 25K
Wonder if the Tiny Terror is the Same.
Recorded some kids lately, and he had a Tiny Terror. I enjoyed the Stack on that amp a lot.

I don't have the Tiny Terror, but here is the Tone Stack from the series I Terror Bass.

AFAIK, M473 capacitor code = 47nF, 20% tolerance.

 

[KenB]

It really makes more sense if you set the Tonestack flat. This case its around 1 , 10 , 1
then if you hit the Deep switch. It removes all the mids and treble. And only allows Fundamental.
Its passive, obviously there is a volume drop.
Hence Fenders Description= Removes Harmonics , More Fundamental


Graph below tonestack flat 1, 10, 1 = Maroon
tonestack flat 1, 10, 1 with Deep Switch = Green

OK... You need to clarify one thing: You say "hit the deep switch", which "removes all the mids and treble".

What do you mean here, by "hit"? Does that mean push the switch up, towards the word "DEEP", or leave it down, in what LOOKS like it would be the "off" position?

I know this sounds stupid, but the distinction is the point of me starting this thread!

When the deep switch on my 135 is down, the tone is nice and full, well balanced, and sounds like there is plenty of fundamental. When I push the switch up, towards "DEEP" (what I would call "hitting the deep switch"), there is an OBVIOUS increase in volume and highs/high mids. NOT subtle, and definitely NOT "more fundamental, less highs"!