Fender Amp Help

Jon Light · From: Saugerties, NY

OK. 72 Dual Showman Reverb. Turns out my wonderful rehab job has some issues. The reverb channel (which I have bridged so that both channels now have reverb) bass control is basically non-functioning. I changed the Mid pot from 10K to 25K. It works but the overall sound needs a bunch more bass and it ain't there.
One thing I'm looking at that is really confusing is the wiring of the bass pot. Lug #1 goes to the center lug of the mid pot. Lug 2 & 3 are jumpered together, then go to the treble pot and to the .1uF tone cap. Layouts I'm looking at for aa270 & aa769 look nothing like this! My Ch 1 doesn't jump the two bass pot lugs. Unfortunately I don't have a layout for this model (100W master volume) and I'm not real good at reading the schematic. BTW--I did not do any of this wiring, other than the mid pot installation. Is this a hack or mod of some sort? I can't honestly say that I'm positive about how the pot behaved before although I thought it worked properly. But this needed a cap job pretty bad and everything was too bassy and muddy so I'm not real sure. Ch 1 sounds pretty great. I'm in a bit over my head, I fear.
Any observations?
BTW--the only thing I did to this amp that was outside of routine is some reverb modifications that I was none too sure of myself with--operating on some fairly sketchy info and trying to read between the lines. I think I improved it some but I could have done something weird there---I clipped the 560pF cap, changed the 680ohm resistor to 1K, bypassed with a 25uF cap.
Anyway, if anyone has any observations, I'm all ears.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Well, the lugs of the bass pot being tied together (or not) is of no consequence. It works the same either way. Your big mistake was raising the value of the mid pot to 25K. That took away a lot of your "scoop", and most of the classic Fender tone. The other changes aren't that far out of the normal range, but it would help to know how (where) you tied the two channels together to have verb in both.

Quite often, the Fender published chassis layouts and the schematics don't agree, but the differences in performance are moot.

Jon Light · From: Saugerties, NY

Well, it seemed to me, conceptually and from a bunch of stuff that I've read that at lower settings the 25K pot would perform normally enough, if maybe a bit more sensitive, and would give more mids at higher settings. BTW--the original pot shaft broke which is why I was messing around with the values in the first place. With a spare reverb channel, there was no reason not to try a different flavor. I'm thinking of putting in a proper 10K just to revert to correct specs as I try to troubleshoot this problem.
The reverb mod was done per a standard drawing that I've seen online and have received from other techs. It worked like a charm. (I think--I'm sure of nothing as long as this problem continues.)

Michael Brebes · From: Northridge CA

I like having the 25K for Mids. Hopefully you made sure that it is a linear taper pot. With the 25K pot, "4" is the same as "10" with the stock pot. Late Tweeds used the 25K Mid. Higher positions on the Mids should also give you more bass, that's the way the Fender tone stack circuit works. I have found that it increases the available bass in the amp. Fender Blues Deluxe/DeVille and HotRod Deluxe/Deville use the 25K pot.

From what I have read in your post, it sounds like the Bass pot is wired correctly. It is possible that the pot is bad.

One thing I recommend is cutting the 120pF cap off the Master Volume pot. It acts as a Bright switch and isn't out unless you are at "10". You should be able to find your needed schematic at www.schematicheaven.com which is a great resource for amp schematics.

[This message was edited by Michael Brebes on 19 May 2004 at 08:37 AM.]

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Well, here's the caveat when dealing with Fender amps. Yeah, the net is replete with discussion boards about how to "tweak" them for the best sound, but you have to remember that most, (probably 99%) of the mods were developed by straight guitar players, and not pedal steelers. What constitutes a good straight guitar sound today is miles away from a good pedal steel sound.

Now, as to the mid control. While it's true that the total amount of mid reduction available is roughly the same for both pot values (10k and 25k), the significant difference is where the mids break. With a 25k pot, you'd have to run it at "2" or "3" to get the same amount of scoop you get with a 10k pot at "5". Maybe you're not concerned with the "scooped sound" at all, and if that's the case, you can stop reading now.

Passive tone controls all have this character. They don't "add" anything! They only allow you to cut out what you don't want. So, if you want more bass (and considering the bass control is already wide open), you lower the mids, (which are the most predominant with a pedal steel), lower the highs a tad, and then you jack up the volume. This is how you get more bass from most good amps.

My starting settings for Fender tube amps is ...

Treble...3-4
Mid......0
Bass.....6-10

This setting should give you plenty highs, and all the bass that's available from the amp. Now, adjust the mids up to where it starts to get "nasal" (usually "3" or "4"), and the fine-tune the tone with the treble control (the treble control also controls the mid-shift).

In closing, you said you wanted more mids. Can I assume you were already running the mid control at "10"?

I've seen many players say "This amp doesn't have much low end". Then, when I look at their settings, they have the bass control at "4", and the volume control at "3". Go figure.

Maybe Ken Fox will come in here soon, and give you some more ideas.

Jon Light · From: Saugerties, NY

Thanks Michael. The schematics I've got. I'm not real good at reading them yet so I'm trying to cheat with a layout but I still have yet to find one for this specific 100w MV model. I'll go ahead and clip that MV cap although I always run it at 10. And yeah, it's 25K linear.

Thanks for the lecture, Donny (absolutely no sarcasm intended at all). Actually, I've got an email thing going on with Ken on this too.
I'm not particularly searching for anything with the mid pot values. Just want to try a different flavor as long as I've already got one real good sounding channel and as long as I needed to replace the pot anyway. One thing I have discovered is that as much as I love the scoop, I think I exaggerate it at home. Then when I take it to the bandstand it just doesn't cut through the mix. So I'm still learning how to dial in a Fender on the stand. Not to say that it needs the 25K pot to get there. Like I said, just messing around.

When this stuff starts happening, you lose faith in your own sanity and powers of recollection. This amp was real bassy and muddy before the cap job. Now I can't swear that the bass pot was even working before. Maybe it was a bad mod? The way it is wired is definitely NOT as per any plans I've looked at so I'm going to study on this a bit more and then go ahead and try to wire it according to the layout I've got in hand. Then we shall see. If you hear a huge whoop coming from the direction of Brooklyn, you'll know what's up. Likewise if it's a "d'oh!!!"

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Sorry if my attempt to help sounded like a lecture, Jon. I sure didn't mean it that way.

Also, unlike Michael, I've never heard a Fender amp that gave more bass response with the mids turned 'way up. To me, when you crank up the mids, they just get nasal and "honky", string separation disappears, and the good solid bass goes out the window.

Maybe my old ears are working bass-ackwards! (LOL!)

Anyhow, I do agree that while the scooped mids sound great at home, they sometimes tend to get lost in live band situations. That is...unless you've got plenty of power and speakers!

I admit, I've shaken a few stages in my day. (I think Curls would have loved my old rig.)

Ken Fox · From: Nashville GA USA

Well, I msut admit I have tried the 25K scenario on the "Tone Stack Calculator" only. If it is correct you can boost the mids quite a bit more. Does not seem to affect bass or trouble response much. As Doony said, increasing mids is liek turing the bass and treble down. More mids might be nice to get the sound up in the mix a bit. A more common mod for steeler seems to be to move the mid notch from the standard 450hz range to around 750hz using a 56K slope resistor. I have done several amps with that mod. I like it even for guitar. If you want more bass out of a Fender amp, get a deep basket speaker, such as an EVM12L or EVM15L. I have heard these speakers dramatically improve the bass response. They are just plain more efficient in the bass region. The amp is putting out a lot of bass signal, it just needs a speaker that can reproduce it! The Eminence Delta Pro 12A and Delta Pro 15A seem to be a clone of the EV speakers. I just got two of the Delta Pro 12A speakers, they look and sound the part to me!
As Donny said, these are passive controls. They do not amplify the bass, treble and middle. They merely allow is to pass thru. Just turn all three controls to "0" and you will see what I mean, there will be no volume!

Ain't this fun! You guys are great. It is wonderful to have people with the experience Donny brings to our Forum. Like I said before, my experience level is far behind Donny and a few of the other guys on the Forum. So if I slip up, let me know!!!

Jon Light · From: Saugerties, NY

I've got plenty of bass in Ch 1. No issue there. There is simply something quite wrong in Ch 2. I'll need to spend time with it. I'm sure I'll solve this.
Hey Donny--this is the sort of thing that I want to try real hard to express well--if you don't dig the word 'lecture' I'll find another word. But I didn't mean it in any way other than an expression of appreciation--real and deep--of the knowledge and experience that you and others share here. I'm learning so much and I'm eating it all up. Thank you!

Brad Sarno · From: St. Louis, MO USA

Jon, make sure the big .1uF cap is good, as well as any connections to/from it. Did you put in a new Orange Drop?

Brad Sarno

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Y'all are too kind, thank you for the nice words. Sometimes, this old man just forgets that what he likes and does doesn't necessarily please everybody else. :embarrassed: Jon, you go right on "experimenting"!

Ken, I'm sure you have a lot more technical expertise than I do. Most of my electronic know-how came from doing just what Jon's doing, experimenting and changing this or that just to see what would happen. Back in the '60s, I remember modifying my Twin Reverb's negative feedback circuit. (I think I put in a small ceramic cap in place of the resistor.) Anyhow, I played steel on the next couple of gigs sounding like Roger Mcguinn's 12-string Rickenbacker. The treble would crack a glass at 40 feet!

Ken, I was just noodling with that "Tone Stack" whachamacallit (neat little program...wish I'd had that 40-odd years ago!), and here's something for you to try.

First, change the mid pot to 25k, and set the mids at 10, and leave the bass and treble at 5. Now, do the "snapshot" thingy. (That saves this curve.) Now, change the mid pot back to 10k, but set the bass at 3, mid at 10, and treble to 4 1/2. Notice anything about the curves? Yep! they're almost identical, only the volume is down by about 3db (just noticeable) with the 10k pot.

I realize that don't turn y'all on, but I think it's pretty nifty,

and it does show that setting the controls right on these amps can make all the difference where the tone is concerned, even with a significant change in the circuitry.

(I'm thinkin' a 1 meg bass pot might be the cat's pajamas, the bees knees, or whatever they say nowadays. I just might play with that little program all night!

)

Jon Light · From: Saugerties, NY

rad--yeah, it's an Orange drop. And it checks out ok. But I just discovered something that may be exciting---one leg of the .1 cap was in contact with a leg of the nearby cathode bypass cap. This may have happened after I went back in and probed around. But if it was like that as a result of my crappy work, I wonder if it might be the problem--or would the amp have blowed up instead? That would have been fun.

Brad Sarno · From: St. Louis, MO USA

Jon, the reason I suspect that cap is because it passes the bass part of the tone stack. If you had missing or lacking bass response, it seems that it may be connected to that cap somehow. Let us know if you track it down.

Brad

Jon Light · From: Saugerties, NY

Yeah Brad. I'll change it just on general principal. Some follow ups: That observation about leads touching came to nothing. I rewired the bass pot as per standard schemos. No change. Changed the .047 mid cap that wasn't measuring right. No change. An observation: the bass pot is a little more responsive when that 25K mid pot is at zero. Also, Ch 2 has less gain than Ch 1.
Well, back to the mines.

Ken Fox · From: Nashville GA USA

From what you just stated I would suspect the cathode bypass cap on channel two, V-2, first stage. An open cap would decrease the stages gain and change the frequency response (less bass).
The second stage shares a cathode bypass cap and resistor with the second stage of V-1 and is after the tone controls, so it is not likely suspect. You could temp a cap around the existing one with some jumpers to check it. Just a thought!

Jon Light · From: Saugerties, NY

Sounded like a strong idea, Ken. But everything onboard checks out--cap values, signal paths, it all seems to be as it should. BTW--my cathode electrolytics are 22uF rather than 25. I assumed this was not a problem?

Ken Fox · From: Nashville GA USA

Not a problem on the value. Still, I would bypass it, just to make sure it is OK. If you get a change in gain, then swap it out.
An interesting thing is that the 90's Vibrasonic had this cap was left out of the "Steel" channel. The idea was to compensate for the larger output signal of the steel pickup, I suppose.

[This message was edited by Ken Fox on 23 May 2004 at 01:54 PM.]

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