Boxy / woofy sound from newly recapped amp

[JBeast]

Hi,

I recently completed recapping all electrolytics and I can't seem to exorcise the boxy sound coming out of it.

I have tried switching cabs to no avail. This amp never sounded boxy until I recapped it so I'm inclined to think it's the new Filter caps (Kemet, Nichicon and Panasonic). But I also blew a power tube prior to recapping and replaced the power tubes to KT77's, which I am unfamiliar with. Most of the other changes to the amp I made before the recap. I have not tried other power tubes. All I have currently are EL34 types. Are KT77's known for being boxy or woofy?

So, basically I changed too many variables at once and can't change it back to find out what might be causing this woof I thought I'd ask to see if anyone knows what kinds of things might contribute to an overly boxy sound.

Amplifier: Was a Crate Blue Voodoo (BV120H).

I have done so many mods to it that it's nothing like a Crate BV120H. It's now more like a Frankenstein child made up of Mesa Mk5, Engl Blackmore, Soldono SLO100 and Marshall JCM800 values. So, it's very marshal-like with a Fenderish tone stack. It sounds killer and I've managed to get 90% of the way toward the sound I wanted... accept for this woofy/boxy thing. The amp has plenty of dynamics, deep bass and brightness so it's not lacking in any of that. There's a very present and distinct midrange frequency that sounds, well, "boxy", like punching a cardboard box or like the speakers are mounted in cardboard or the speakers are made of cardboard.

Here is the whole story on this amp - sorry to make it more complicated.
I'll start by saying that I decided a few years ago to use this amp as a project for learning repairs, mods and tube amp theory. So, I basically sacrificed it's stock tone to squeeze it into something it was not. I've learned a lot, broken it a lot & fixed it after breaking it.

Anyway, before I recapped it the amp seemed to lack midrange complexity to me despite which tubes were in it, it was never really a great amp to begin with. I changed out the OT to a Magnetic Components JCM800, new coupling caps (Sozo), changed all of the carbon film resistors on the plates, cathodes and input to metal film to quiet the hiss. I also put snubber caps on the filters just before the preamp on the advice of an AX84 person who had experience with this amp. The amp was sounding pretty good and the midrange seemed more complex and richer as I had hoped. Fast forward a year and I start having signs that the filter caps are losing it (High pitch oscillations, microphonics, poor frequency response, etc. I changed out the tubes and the symptoms were all still there so I thought maybe the caps were going bad after 10+ years. I wanted to do a cap job just to learn how to do it anyway so I ordered the parts and did it. After it was all said and done I started it up, open and upside-down on the bench, I didn't really notice any boxiness because I was overjoyed that it even worked after my first recap. It worked fine for a few minutes until I dropped a screwdriver on the power tube pins and blew a tube along with a screen resistor and a fuse (facepalm). I had the KT77's so I thought I'd give them a try. Got everything working again and got a chance to play it for a few weeks. I've changed out tone stack values all over the place but this boxiness can't really be dialed out, not even with mid-scooped tone stack values. I haven't gone with smaller values than .022 for the mid cap because I'm thinking the problem is in the midrange somewhere, I don't want it to be worse by increasing the midrange content.

I have been using the Duncan Amps Tone Stack Calculator and by all rights I should have this figured out by now but something is telling me it's not in the tone stack. Now that the filter caps are all brand new and strong I'm thinking that it now has all of the frequency range spectrum available to it and my previous mods are standing out as being too midrangey or something but I'm not sure about that. Before recapping it I had snubber caps on the filter caps for a few months and it sounded pretty good, I've been thinking about soldering those back on the new caps but I didn't think I'd need them after the recap.

What I have already tried to eliminate boxiness:
- Tone-Stack Midrange caps .022 and .047
- Tone-Stack Treble Caps 220pf and 450pf both silver mica
- Tone-Stack Bass Caps .047 and .1
- Tone-Stack Slope resistors 100k, 47k, 33k
- Changed Power tube screen resistors from 480 ohm to 1k
- Preamp tube rolling: about 5 different varieties JJ, Tad, Mullard, Jan-Phillips, Tung-Sol
- Different speakers, Avatar OS 2x12 w/ 1x Jensen C12K and 1x Eminence c.1980 Jensen Clone
- Sozo coupling caps

Has anyone ever had to deal with this kind of thing before?

[EFK]

Check to make sure that in the midst of all your tone stack mods, you didn't accidentally lift the ground/leave the ground disconnected. That usually will equal all mids, all the time.

[Krwkka]

You might try running the preamp out to another amp to define where the boxiness is located. I just put a shorted plug in the return so that the amp I'm working on is quiet and run a cable to a little Peavey Decade I have.

If the boxy sound comes from the other amp, you know it's originating in the preamp. If not, try running a preamp from another amp into the effects return on the Voodoo.

This will help your focus as you troubleshoot.

[Enzo]

EFK makes a good suggestion. The bass and mid controls each have their own path to ground. Check if there is 50k more or less from the junction of C4,C5 to ground with the mids maxed, and more or less zero with the mids at zero. And don't just measure across the control, measure from the farthest away point still connected. That will test not only the control but all the connections to it.

I have to wonder though, you replace all the filter caps, but change to a different type of tube, and you want to think the change in tone is because of the caps?

Some thoughts. You destroyed one power tube? The other three still would work, wouldn;t they? Stick two of them in the amp and see if it still sounds funny.

You changed tube types. Did you also change the bias setting to one appropriate for the new tubes?


I worry about altering the tone stack as an approach. If the boxiness was not there before, but is now, if the change did not follow a mod to the tone stack, then changing it to solve the problem is to not address the problem itself. If I put on a shirt in the morning, and notice the buttons seems to be missing, I could sew on more new buttons, but that ignores that the old buttons are still there, I just put the shirt on inside out.

A rule in the shop is that while you work on something if it develops a new symptom that was not there before, it will be because of the work you did. Sometimes it might be a missed solder connection, or a part installed backwards, or a wrong part installed. How many thousands of times has someone installed a "470k" resistor, then pulled their hair out until noticing that it was really a 470 OHM resistor.

A fellow once sent me a Fenderish amp kit he had built, and it just wouldn;t work right, no matter what he did. He had done a very nice construction job - parts neatly placed, smooth even solder work, wires neatly dressed. Turns out on the eyelet board, he had a place where a resistor and cap sat next to each other, one end of each even sharing a common eyelet, and he had swapped their places. Cap where resistor ought to be and resistor in place of cap. To the eye, nothing looked weird. And once he had looked over the board and nothing looked out of place, his eyes would not see anything as wrong each time thereafter.

[JBeast]

Cool, thanks for all of the great suggestions.

Ground connections are all good.

I just checked the resistance from "the junction of C4,C5 to ground" on the clean channel. I got 49k dimed and .01k at zero. I also checked the #2 distortion channel tone-stack at junction C16,C17 to ground and got around 23.8k at max which seemed weird until I saw the other two resistors in the midrange circuit (R26, R101). R101 is 47k in parallel with the pot so it's about right that the pot value ends up being 23 or so at max. I've been trying to figure out how this tone stack works because it seems a bit unconventional albeit somewhat Fenderish but I'm still trying to understand why they didn't stick with a traditional Fender circuit. What does their version do to change the sound? As far as I can tell there is a treble bleed cap in the mid circuit that's meant to pass high freq to ground if it makes it that far. The bass control also has it's own separate ground in series with a resistor but I'm not sure what that would do to the tone compared to a fender stack. I'm assuming the resistor to ground on the bass pot keeps the bass at a minimum value instead of sending it all away to ground. Once again, I'm not sure why they did this instead of the Fender thing unless they wanted "truer" three band separation with less interaction between controls...

I did re-bias the amp when I put the KT77's in it (@ around38mA). I did ask if KT77's are known for sounding somewhat boxy, no? I am not familiar with them so I'm not sure how they sound to other people. Yes, one of my tubes fried. Unfortunately it was a single 6L6 that was mixed with one of those Eurotubes "Integrated Quads". I have more EL34 tube types around (Tung-Sol & JJE34L) but not any 6L6's. I'll try a fresh set of E34L today and report back. I guess it's a bad ASSumption to think that all EL34 types will have a similar quality, hahaha.

[JBeast]

Ok,
I put EL34's in it and it didn't help.
So I tried .066uF cap in the mids and it definitely took the woof and box out, weird. I normally hate scooped tone stacks but this one in particular seems to do well with .047-.066 while still having plenty of mids for cut.

The on-board value is .022 so I put in an on/off/on DPDT with .022 on one side and doubled up 2x .022 for .044 on the other. When the switch is off the value is .022 (Very boxy!). I don't totally hate the boxy sound, I know that some people strive for it, it's just that I couldn't dial it out when I wanted to. Now I have a scoop switch on my amp ;P Not really the effect I was going for but there is so much mid content between cap types, tubes, speakers and my guitar (alnico pickups in Black Limba w/Maple cap) that the box effect was more than I could handle.

I think this will work well enough for me. Next time I'll have to try Sozo's blue Fender style coupling caps instead of the Marshall yellow, I wonder if they are more natural in the mids and not so supernaturally middy.

[Krwkka]

I still think you should try the effects loop stuff I mentioned above. It doesn't make sense that it sounds boxy now but didn't before, especially since you've gone back to EL34s. Something is wrong, but where?

I have heard that boxy sound from bad tubes - both power tubes and preamp tubes - from a bad cable, from a speaker with a poor solder connection (although that probably isn't the case here since you tried other cabs), and from bad solder joints on the circuit board. I also once had a coupling cap that sounded weird like that.

Usually, for me, it's been tubes or cable. Could it be the guitar?

I've found the yellow caps to be a bit less bright, but I wouldn't say they were super-middy sounding, but from what you've written that isn't a factor here - unless you also just replaced the coupling caps. If not, and the amp didn't sound middy before but does now then it's something else.

And like the big E says, if you haven't yet, you need to check the things you recently changed.

... A rule in the shop is that while you work on something if it develops a new symptom that was not there before, it will be because of the work you did. Sometimes it might be a missed solder connection, or a part installed backwards, or a wrong part installed. How many thousands of times has someone installed a "470k" resistor, then pulled their hair out until noticing that it was really a 470 OHM resistor.
_Enzo

[Enzo]

I see one advantage to that tone stack. If you turn all the EQ knobs to zero in many Fenders, you get no sound through the amp. This curcuit won't do that. Maybe.

[JBeast]

Man, I am obsessive about any project I commit to; when I say obsessive I mean it in the truest sense of the word, no joke.
I've been pouring over every connection on the board for a couple of weeks now for hours every day. If anyone really knew how much time I have put into this amp... whew! I even flowed new solder over all connections - new and old - and cleaned the tube sockets with DeoxIT. I've tried preamp tubes out the wazoo and made sure the outer foil of all coupling caps are oriented correctly. I've gone over every connection in the preamp with a continuity tester looking for breaks or accidental connections. All grounds are connected, polarity is correct on all of the new filter caps, filter cap values and voltages are correct replacements, etc, ad nauseum.

I replaced all electrolytics and put in new tubes, it doesn't seem to me that this should be the cause of "boxy" either but I'm seriously at a loss for where to look next. I'm sure that it's probably something simple, like putting my shirt on inside out but I've run out of places to look; who needs buttons anyway? All of the mods I did on this thing were related to improving reliability, dynamics and increasing midrange and bass. With the old worn out filter caps in place my mods were perfect. With the new filter caps and new tubes there is way too much midrange, so much so that I have to scoop the mids to get any relief. The only thing I can think of is that the JCM800 OT I put in there was too much to begin with. Unfortunately I threw the old one out, it always got so hot you could cook an egg on it and everything sounded buzzy through it even after eliminating the two cold biased gain stages. Like I said previously, I now love most everything about this amp (too bad I built it from Crate parts) and it sounds a bit more crisp and slightly dynamic than my Brunetti 059, accept for the boxy thing... Hey, if I can take a break from this thing then I will be ecstatic as it's a MUCH needed break, hehehe.

As for my guitar, it is a bit heavy in the mids but no more so than any Les Paul might be.
I built the guitar myself, it's a PRS scale, Black Limba body, flame maple top, Black Limba set neck, Macasar Ebony fingerboard, Stetsbar trem and bridge with 1x BL500XL, 1x Qtuner and 1xSD PRails with a Bill Lawrence Q-Filter, finished in nitrocellulose; There's that obsessive thing again, it took me five years to make the thing due to hit and miss shop access and saving up to buy parts and planning and such. I used to be a cabinetmaker by trade so the learning curve was a bit smaller for me. I tried my PRS CU 24 on the amp too and it seemed to be a bit more harsh (ceramic pups) and slightly less middy than my handmade but still boxy. If I use my Jensen loaded cab, scoop the mids and load the preamp with Chinese tubes then it sounds MUCH better but those TAD 12AX7A-C's are a bit harsh in the high end and a bit boomy in the low end.

When I first played it with my band my drummer was the first to say, "Why does it sound so boxy". I couldn't give him an answer and I still can't but I know that every little thing I do to scoop the mids helps. My cable is even a Fender Pro Gold which is my most scooped cable compared to my Canare/Neutrik cables, hahaha. Oh well, I'm seriously at a loss. I am even considering that my obsession to "get the boxiness out of the amp" has overtaken reality. Since I've made all of these changes I have nobody to bounce the sound off of because my band dissolved last week. Maybe it's all become a subjective nightmare that I can't seem to escape... I don't know. I'm not very good at taking a step back and being objective until I think something is "Perfect" or until I give up.

[JBeast]

I still think you should try the effects loop stuff I mentioned above. It doesn't make sense that it sounds boxy now but didn't before, especially since you've gone back to EL34s. Something is wrong, but where?

I have heard that boxy sound from bad tubes - both power tubes and preamp tubes - from a bad cable, from a speaker with a poor solder connection (although that probably isn't the case here since you tried other cabs), and from bad solder joints on the circuit board. I also once had a coupling cap that sounded weird like that.

Usually, for me, it's been tubes or cable. Could it be the guitar?

I've found the yellow caps to be a bit less bright, but I wouldn't say they were super-middy sounding, but from what you've written that isn't a factor here - unless you also just replaced the coupling caps. If not, and the amp didn't sound middy before but does now then it's something else.

And like the big E says, if you haven't yet, you need to check the things you recently changed.

... A rule in the shop is that while you work on something if it develops a new symptom that was not there before, it will be because of the work you did. Sometimes it might be a missed solder connection, or a part installed backwards, or a wrong part installed. How many thousands of times has someone installed a "470k" resistor, then pulled their hair out until noticing that it was really a 470 OHM resistor.
_Enzo

Oh man, I forgot to do that effects loop thing and didn't address it, sorry. I got caught up in other things. You are totally right, I will try that later or tomorrow. Um one thing though, I have never done this before. Which ins and outs go where, is there a diagram online I can follow? Do I connect the output of the crate effects loop to the input of the effects loop of another amp and plug the power amp input with a dummy jack?

[Enzo]

You have to learn to put things aside when you are spinning your wheels. Spending hours and hours going over the same ground is just going to wear a rut in the path. Whatever you are overlooking is now out of your vision. You are looking at something which appears right, but probably is not, but it has become so familiar your mind can't see it as wrong.

In school if you took a test and couldn;t solve a problem, you - hopefully - didn;t remain on the one problem the rest of the hour. You moved on and came back to the problem later. A fresh perspective. That is standard procedure in the repair game.

[Krwkka]

I have never done this before. Which ins and outs go where, is there a diagram online I can follow? Do I connect the output of the crate effects loop to the input of the effects loop of another amp and plug the power amp input with a dummy jack?

You've got it, 'cept a shorted plug may be better. You just do one at a time - testing your preamp then reversing the wiring and testing the amp with the preamp out of the other amp.

[JBeast]

Enzo, you should write the tube amp book that never was "Zen and the art of tube amp repair". I hear what you are saying and that has been my life's lesson man. I've always been a really intense person and learning to step back has been a challenge for me as long as I can remember. Thank you for the reminder, it never hurts to hear from someone else when we have forgotten how to chill.

Krwkka, thanks for confirming that, I really appreciate it.

I got it figured out.
I had a wire short in my resonance control that had the control maxed out at all times (1 meg). So, basically I had too much speaker damping going on and the bass was far too resonant thus "Boxy" or "Woofy". I got it working properly now and I can go from boxy, closed & honky to thunderous, open and chirpy in 2.5 seconds - or however long it takes to turn the knob.

Thanks for all of the suggestions!

So, here is the Boxy/Woofy/Honky troubleshooting procedure:

1. Check tubes (preamp and power). Is there any red-plate or reason to think a tube might be bad. Try a few different tubes in V1 and the PI
2. Check cable, does it have a floating ground? If yes, is it plugged in with the grounded end at the amps input? Try turning it around. Try a different cable
3. Speakers: Are they wired in phase? Cabinet: Did you recently close the back, install new speakers or pad the inside? Try switching cabs
4. Connect the preamp to another power amp that you know isn't faulty. Still Boxy? If yes then the problem is in your preamp
5. Do you have a resonance control? If yes then make sure it's functioning properly. If not, did you recently replace the feedback resistor? Make sure it's value is correct and not too high
6. Did you recently change anything in the amp that can be easily reversed to rule it out as the cause?

Did I miss anything?