Fender Blues Deluxe/amps with their EQ before the preamp

[oilpit]

So for those that don't already know, many Mesa/Boogie amps, most notably their flagship 'Mark Series' have their EQ controls before the preamp in the circuit, as opposed to the vast majority of amps that have them placed after. The result being that they have ridiculously sensitive tone controls, that sometimes require settings that look really weird, and would probably sound just as weird on amps with a more traditional circuit.

Personally I am a huge fan of Boogies, and of course their sound comes from a whole lotta factors besides where they put their EQ, but the concept in general really intrigues me and I've been wondering if there are any other amps that employ a similar design.

From the mostly unsuccessful research I have done, I think that the Fender Blues Deluxe may be wired in a similar fashion. If you look at the controls on the top panel, the actual gain knob is after the EQ, which obviously doesn't really tell us anything, but this amp is the only Fender that is laid out that way, literally every other amp they currently produce has the gain before the tone controls. After noticing that I downloaded a PDF of the user manual and they specifically state that the EQ is more sensitive than most other Fender amps, and as the gain is increased, the bass should be lowered dramatically, which is pretty much exactly what the common wisdom is for EQing Boogies.

This would also make sense considering the fact that the original Boogies were just modified Fenders, but I think they used Princetons, which just have a singe tone knob, it's possible pretty much everything was added during the modification process.


So basically I'm wondering if you know of any amps that are wired this way or have a super sensitive EQ for some other reason.

Bonus points if you know if I'm right about the Blues Deluxe

Thanks!

 

[eclecticsynergy]

Not an expert, but my understanding was that prior to about 1978 all guitar amps had the tone stack before the gain stages. This is why earlier designs tend to feel looser when pushed, and can get tubby in the lows at higher drive settings. The original Boogie design was that way too. AFAIK beginning with the Mark II and the JCM800, most designs of the day switched to having the tone controls post-gain to allow the bass to be opened up more without flab, and for a tighter response overall.

Fairly sure all the later Mark series also use that type of architecture.
Not 100% positive - as I say I'm no expert, just a longtime Boogie man.

I do agree the Mark series tone controls are not just very sensitive but also highly interactive, with each other and with the Gain knob.
Boogies are notorious for their steep learning curve; I think it's primarily the Marks that are responsible for that.

 

[FuseG4]

From the schematic on blues deluxe I think there are two gain stages v1a and v1b for the clean channel, with the volume between them, and then the eq. For clean then it goes to phase inverter.
When on drive then it goes as above through the first 2 gain stages (with the clean volume grounded) then the eq, then to the drive pot which controls how much goes to v2a. So eq is after two preamp sections but before the third one that adds the gain channel drive. Then phase inverter
no cathode followers. V2b is not used.

 

[Ewizard]

He is correct in that YES, the tone stack ( in the Mesa Mark series ) is placed technically before any real gain stages, but the tone stack in all cases is placed AFTER the inputs gain stage usually noted as V1A. In days of yore when designers didn't utilize 6+ tubes in the preamp, it was simply to have an input gain stage ( V1 ) and then place a tone control ( usually a Hi-Cut filter ) and then have a makeup gain stage to make up for insertion loss of the tone control/s and send it to the power section to finish the job. As more stages of gain were added in subsequent designs, the tone stack moved around depending upon a few factors. In earlier designs, as noted, it was simply left right after the V1 position, and then stages were cascaded to acquire distortion Ala Dumble style and Mesa style preamps. Marshall with their JCM and other similar designs favored the late tone stack driven by the cathode follower. The idea in these designs wasn't so much tighter bass, or better control as much as acquiring LOTS of distortion with little or no tone stack insertion loss. As this topology developed, most of the preamp sound was created by sizing the coupling caps ( usually smaller ) and using snubbing caps or other small, low-loss tone tweaks.

The Mesa Mark series preamp circuit is bassy in general utilizing .047 and .022 coupling caps throughout most of it. Because the tone stack is early in the preamp though, the tone is shaped prior to the creation of distortion. Each subsequent stage simply adds its own harmonics on top of that, which ultimately you will try and EQ to taste. With a late tone stack, it is not as effective mainly because there is a metric crap-ton of harmonics and gain that it must cut in order to provide effective results. Or you could think of it as it is so late in the circuit that it has to work that much harder to shape the tone as you would like it.

Where you place the tone stack is rather important to how the amp will distort and ultimately sound. The insertion loss of a full tone stack is HUGE and you can think of any subsequent stage after it as being nothing more than a makeup gain stage that simply recovers the voltage loss from the tone stack. So in the Mesa, any real amplification isn't occurring until the third triode, which means any real distortion isn't occurring until the 4th triode. This is why there are 5 triodes to create the distortion that it has. The last two triodes are where the majority of all the distortion is occurring. The fully bypassed cathodes on most all of the gain stages help a LOT in getting a massive amount of gain/distortion.

The Blues Deluxe is a little different from all of those where the tone stack is placed mid-circuit. In this case, V1A drives a subsequent triode ( V1B ) into distortion ( if desired ) and then the tone stack is placed. The next triode ( V2A ) is used as a recovery stage to make up gain for the tone stacks insertion loss. It is not really providing much if any appreciable gain. Now if you were to turn the volume all the way up and turn all the tone stack controls to 11, you may at that point actually incur distortion from a multitude of places including the makeup gain stage V2A and the PI, as well as potentially the power tubes. The tone stack in this case can be utilized as a volume control of sorts if you can tolerate the sound of it. If you wind down all the controls you are cutting LOTS of volume and even with the volume control turned up, you can reduce or eliminate any downstream distortion from occurring; it would all be coming from the V1B triode. Conversely, diming the tone control pots will allow the volume from the first two triodes to fly through nearly unimpeded, potentially slamming the recovery stage with enough gain to cause it to distort as well, which then just slams the PI and so on.

You can get a surprising amount of distortion out of one triode and the sound of the PI distorting and the amount of distortion it provides and how complex it can be is also surprising. I designed an amp that has a single input triode gain stage that goes straight into the PI ( via a volume control ) and the amount of distortion from the PI is much more than you'd think it would be. When pushed with a boost pedal it can easily get you the Plexi type sounds many love. In my design, I went for a lossless tone control topology. I have a Lo-Cut and a Hi-Cut tone control. The Lo-Cut is a cut-only control and it pulls only the bass out of the signal from V1A. The Hi-cut is your typical Vox Top Cut tone control. So not totally lossless, but is because it technically doesn't shunt any signal to ground and the only voltage loss that occurs is because there is less total bandwidth being amplified. The tone controls do not drag down the signal, it simply reduces the passing of a particular frequency range onto the next stage. Amazing pedal platform amp! When I finish making the headshell, I will post more about it.

 

[oilpit]

Holy crap Ewizard, what an amazing answer! Thanks so much!

 

[Erlend_G]

Yeah! Thanks alot Ewizard! Greatly appreciated! ♪

-Erlend

 

[Erlend_G]

(Personally I play a Blackstar HT-20 amp, that I find lacking alot in the midrange register. I use an EHX "allied OD" pedal with 3-band EQ, and dime the mids on the pedal, in front of the amp. That gives me the "classic" 80's "marshall" screaming mids sound!)

-E

EDIT: Until I crank the amp. Then it takes on a... Dr Jekyll & Mr. hyde character- and gives the jolliest, low-end square-note fuzzed out notes, and a bright, loud and rude overdrive! . Hehe. But I can't crank it here in my apartment.