Need better overdrive channel Fender"pro tube series"

[fxloop]

I'm on a quest for advice on modifying the overdrive channel on my early 90's Fender "Super Amp" 4x10 from their Pro Tube Series. The amp is in great condition, works perfectly but i just hate the overdrive channel(very farty?). Any suggestions ? tried new tubes ect. I believe the circuit is the same or very similar to a Fender Concert II.

Thanks for your time guys!
Tim!

[Enzo]

Not very helpful I am afraid, but Fender is very well known for not having the slightest clue as to how to make a decent overdrive channel, but they make a good clean. This is of course my opinion.

A whole lot of guys use their Fender for clean and use a pedal out front for dirt.

There may be some great ideas coming, but I think you will be disappointed if you expect a couple of cap changes to fix things.

[tubeswell]

About the only way you can get it without a pedal or without biasing your output tubes really hot, is by adding more gain stages in one of the pre-amp channels. This involves another dual triode or small signal pentode, and making sure your PT can handle the extra current draw (esp in case of the latter). You might as well build a new amp JM2CW

[Robert M. Martinelli]

What Enzo and TW said.

I' ve been playing guitar for some 30+ years, and I field-tested many different amps along the way, and never found a decent overdrive-dirty channel on a Fender - looks like they designed their amps to be the ideal companions for a clean strat, and with this setup you can get very good cleans indeed, but that' s almost all ( IMHO ).

Your amp needs to be modified so deeply that I' m wondering if it's worth the effort; you' d probably better spend your money on an amp "targeted" at your real needs ( depending on the genre : Marshall, Mesa, Peavey, Rivera etc... ) and an amp-switcher to divert the signal to the two different amps.

Hope this helps

Best regards

Bob

[chipaudette]

OK, you're probably not going to find the overdrive sound that you really want...but it can be really fun to play around with your circuit to see how different things affect the sound. So, I'd dive in...

Caveat 1: I've only played around with making my overdrive sound better with humbuckers. If you play single coils, I don't have any great suggestions.

Caveat 2: Your circuit (http://www.schematicheaven.com/fende...er_concert.pdf) is a little unusual. OK, it's just plain weird. In both the clean and dirty channels, the volume controls and tone stacks are in unusual places. Usually, the volume controls follow the tone stacks. Also, in most Fenders, the tone stacks are earlier in the circuit. Or, if you're a Marshall, they happen later in the circuit. In this circuit, they're sorta in the middle...neither early nor late. Since the tone stack has a dramatic impact how how the signal sounds when ovedriven, it's location is very important. Being in the middle, I'm not sure how my suggestions will work out. But, here we go...

Suggestions for Playing Around:

I would do one mod at a time to get a sense for each effect. Sure, you'll end up with all the mods, just do them one at a time so that you can hear the difference. That way, you can decide if you need a little more or a little less of each spice.

Suggestion 1: No mod...just turn down the bass on the overdrive channel. I'm talkin, turn it down to zero. Turn the mids to 6. Turn the treble to 4-6. Still farty? Try the next...

Suggestion 2: Increase C10 (the treble cap for the tone stack for the overdrive channel) from 250pF to 750pF or 1000pF. You can simply add caps in parallel with the existing cap to add the necessary capacitance. Getting up to 750-1000 will make the sound much more nasally, which I think works really well for overdrive. In my amp, I have a switch that toggles between 250pF and 1000pF. Mesa Boogie also has this switch on many of their Mark series amps...it's called "Treble Shift". Like the boogie, I usually like to see the tone stack earlier in the circuit, but see how this tastes in your amp.

Suggestion 3: Add another voltage divider between V2A and V2B. Sure the tone stack really cuts back on the signal, but if you put in that 1000pF cap and turn up the treble knob, your signal might be too hot for the next stage. If the resulting overdrive sounds bad, try inserting a voltage divider between output of the treble pot and the input of V2B. Try a 470k/470K mix. Tweak the first 470K by factors of 2 to get more or less attenuation until the distortion smoothes out.

<interlude>

OK, I just spent more time thinking about your schematic. The above three suggestions are still fair suggestions but I got a new thought...new post below.

Chip

[chipaudette]

Alright here's my new line of thinking...[rhetorical question] where does the overdrive occur in your amp?

Looking at the schematic, It starts at V1A. That's clean. No distortion. Then, for the overdrive channel, it gets routed through the "gain" knob(s) and hits V2A. This'll get overdriven. After that, it goes through the tone stack (lots of signal loss!). At high enough settings on your tone knobs, though, you might get some overdrive in V2B. Next, you've hit the "volume" knob. Presumably, to keep you amp at a reasonable volume, you've got the "volume" knob dialed way back. When the volume knob is turned this far back, no overdrive happens in any of the circuits that follow after that.

So, basically you've got one tube stage (two stages, max) getting overdriven. That is NOT a good recipe for a decent overdrive sound. Many people believe that you need a little distortion added in each of multiple stages...not tons of distortion added in one stage.

So, the best would be to add another gain stage (with appropriate voltage dividers to keep knocking the overall signal level down). In your amp, you don't have a lot of options. The only thing I see is that V1b is currently not used by the overdrive channel. It is dedicated to the clean channel. One could imagine, however, re-wiring the signal path used by the overdrive channel so that V1b is still used. That would allow you to get some distortion in V1b and in V2a (and some in V2b). That kind of signal re-routing is a pretty serious re-work of your amp. And, you could end up negatively affecting the tone of the clean channel in the process. That would be bad.

Another option would be to put a transistor or op-amp at the very start of the amp to act as a clean boost. That would allow you to get some overdrive in V1A in addition to the V2A (and sometimes V2B). That might work, but you'd want to add another relay to bring the transistor/op-amp out of the circuit when using the clean channel. While adding a relay is a pain, it's probably easier to do that than to re-route V1b as discussed above.

The biggest problem with the transistor/op-amp is...who wants to put solid-state in the a tube signal path? Some say that's heresy. I say, if it sounds good to you, go with it. You'd want the transistor/op-amp plus a fairly small cap afterwards to cut the bass before the following tube stage. Less bass early in the path is usually the key to acceptable overdrive. To get a taste of this solution, you can simulate the sound of this kind of mod with an clean-boost footpedal or with an EQ pedal set with all the sliders maxed out (roll off some bass on the EQ to do an even better simulation). If you like the sound, you can either keep the pedal and use it forever, or you can build the boost circuit into the amp.

So, either way (re-route V1B or add clean boost), we're talking about some serious modifications. If you're still interested, let us know. We can talk about butchering your amp. I've butchered my amp and I don't recommend it for the faint of heart. It's a lot of work for a 50/50 possibility of getting an improved result and a 100% probability of lowering the amp's reliability.

Chip

[tubeswell]

What Chip said about butchering amps not being for the faint of heart, if you want to do it, pick a cheapie. I've butchered and re-butchered and re-re-butchered a PV C30 ad-nauseum - it is fun (still might do it again) - of course its probably unsellable, but I enjoy it, so that's where it begins and ends. I don't know how I'd feel about butchering a real Tweed, Brown or BF Fender. Silver Faces ain't worth quite as much. Re-issues are good candidates for butchering.

In your case, looking at that scheme you might be able to do something if you re-route the o/d channel return by inserting it before the 2 stage in the clean channel, so that you get 4 stages before the clean ch tone stack. This will give you the groundwork for better OD and sustain, but will most likely require playing around with the padding between the stages to ensure you get a decent mid level OD sound (Too much bass in OD is farty/muddy and dull). This would be alot of trial and error because once you change the layout of the amp, you risk getting all sorts of unanticipated problems like unwanted signal coupling and hum and other stuff that you get from altering layouts. Also you have to be careful of a pc board mod, because it is finickity - easy to write-off those traces, or conversely to cause unwanted shorts

On the other hand I recently did a mod to my c30 where I used SLO100 values in the OD channel (with all the padding that goes with it). While this required changing the voicing of the clean channel and dumping alot of stuff from the stock C30 circuit, the OD has imroved outta sight.

[chipaudette]

Turned your C30 into an SLO100, huh? That's pretty cool.

My amp is a Fender 65 Deluxe Reverb Reissue that I jiggered into a Mesa Mark IIc / Mark IV. It's been a lot of fun.

Did you mod the power supply, too? I left my power supply stock, so the plate supply to each pre-amp tube is only ~266VDC instead of the ~350-400V shown for the Mesa. If you DID mod your power supply, did the change in voltages have a strong or weak effect on the tone of the overdrive?

Chip

[tubeswell]

I kept the C30 output stage the same - I just changed the pre-amp, which soughta lends itself to the SLO100 preamp topology (if you hold the schematic at arms length and kinda squint). I had to solder extra caps and resistors on the back of some of the boards and on one of the pots and swap bits around here and there and cut a couple of traces, but it works pretty smoothly (although it is understandably hissier due to the higher gain). I did try tweeking the B+ by raising the voltage, but it worked better with the stock voltages, so I put the supply resistors back to stock c30. - I left the schematic on another thread in this forum. It sounds pretty decent

http://music-electronics-forum.com/s...ead.php?t=9416

[fxloop]

WOW

Thank you guys soooo much! Been looking for a little advice long and hard and the pro's on here came to bat! Thanks for the schematic link!!! I'm going to look into the tone stack values and pre-amp resistors to see if i can do a little tweaking on the amp ( it is quite bassy ) i'm not really looking for the saturated type gain just maybe a little more sustain/pushed sound and more of a boost from the drive 2 stage. Every other aspect of the amp is great!, have two original alnico's and two Jensens in the 4x10, sounds awsome otherwise.
I guess i could try other tube types( 12ax7, 12at7 ) ect. You guys have any tried and true resistor/cap values in the pre-amp you'd suggest?

Thanks again!
Tim Grover

[Guitarist]

As per your handle, you could insert a stereo jack/loop prior to that dirty channel circuitry~ the grid, and plug in a quality EQ.
Grab an insert cable: 1 TRS to 2 mono.
I just read a few hours (again) on the VH tone and so thought of offering this for your case. It is cheap, easy and will work. Now you got a got a 3 channel amp...

[Gtr_tech]

Looking at some of those newer Fender schematics makes my head hurt....all those relays to trace in the signal path. I'd say you have enough "real estate" to work with. First thing I'd do is get the large value cathode bypass caps out of the OD ch stages. SOmething .68 to 1u will not muddy up the tone. Maybe put in 2.7k instead of the 1.5k K resistors. I'll have to look closer at that print and come up with other options.

[Steve A.]

Alright here's my new line of thinking...[rhetorical question] where does the overdrive occur in your amp?

Looking at the schematic, It starts at V1A. That's clean. No distortion. Then, for the overdrive channel, it gets routed through the "gain" knob(s) and hits V2A. This'll get overdriven. After that, it goes through the tone stack (lots of signal loss!). At high enough settings on your tone knobs, though, you might get some overdrive in V2B. Next, you've hit the "volume" knob. Presumably, to keep you amp at a reasonable volume, you've got the "volume" knob dialed way back. When the volume knob is turned this far back, no overdrive happens in any of the circuits that follow after that.

So, basically you've got one tube stage (two stages, max) getting overdriven. That is NOT a good recipe for a decent overdrive sound. Many people believe that you need a little distortion added in each of multiple stages...not tons of distortion added in one stage.

So, the best would be to add another gain stage (with appropriate voltage dividers to keep knocking the overall signal level down)...

Well, I would recommend trying a lower gain variety of 12A_7 tube or tubes in the preamp:

MU FOR 12A_7 PREAMP TUBES

12AX7 100
5751 70
12AT7 60
12AZ7 60
12AY7/6072 44
12AV7/5965 37
12AU7/6189 20

Those are just nominal values but they could be used as a good starting point. I'm not sure which Fender amp is the subject of this thread: the Super 60 from 1988 or Super/Concert from 1994. So I can't be more specific about which tubes to cut the gain on and by how much.

Preamp tube swaps are completely non-invasive- no need to get into the circuit board at all. (I do that all of the time with tube driven stomp boxes since I like a *lot* less gain and distortion than the designers do! And the modern ones are a total Sarah to work on! )

Steve Ahola

P.S. I agree with Enzo's evaluation of Fender overdrive channels- better to just use pedals with the clean channel. But I also extend that statement to most of the modern production amps, with a few exceptions like the Mesa Boogie Lone Star Special. But that is just my own tastes which are the old blues and early rock sounds (maybe up to Bad Company? You know, when hard rock went corporate... )

[boonestunes]

'94 Super 'gain channel' mods?

glad I found this thread....I have the same '93-94 Super 410...same probs with gain channels.

the last post about changing preamp tubes.....is swapping out the 12AT7's for 12AX7's, will that give more gain to the 'gain' channels? or will it similarly effect the clean channel as well?

I've been resigned to using pedals out in front of the amp, and i'm getting closer. all my other amps I use the amp's gain channels for my main sounds (marshall 2210 and 3203) so I'm kinda partial to using amp gain vs. pedals.

[pdf64]

I can't find a schematic for it, but I'd guess that one 12AT7 is the reverb driver, the other is the phase splitter (nearest the power tubes).
Neither will affect the pre amp gain / voicing, the reverb driver really does need to be a 12AT7.
You want more gain from the overdrive channel?