18W amp is super boomy Please help me mod it, schematics and sound clip included

[paintballnsk]

Hi,

I have an Ace Pepper Thundertweak Buzzbomb.

It's a fantastic sounding little amp. It's got a lot of clarity and I just love the thing. However, I play rock and metal. It's a moderate gain amp and with an overdrive it gets me there as far as gain goes. But the bottom end is super boomy. Like there's an unreasonable amount of <80hz and just feels really sloppy to play. Like, way worse than a Dual Rectifier with no overdrive kind of sloppy.

I've had it modded, I've tried a bounch of different preamp combinations including a 12AT7 in V3. The mod he did helped the bottom end a little, but not enough. He also modded something to help tame the high end, but I'm going to revert it. It sounds like the tone knob on my guitar is rolled way down and it's really lacking upper mids now.

I'm not familiar enough with components to understand how they affect the tone of amp. I'm pretty handy with a soldering iron and I'm not afraid to dig in and give it a try myself. I figure worse case scenario, I screw something up and I'll take it back to my amp tech to fix it. (don't worry, I know how to bleed the caps and all that stuff to keep myself safe). I'd even be willing to swap out the transformer and sockets for a pair of EL34's, but that's kind of my last resort.

I've attached a schematic of the amp and a .mp3 of something I recorded with it a couple weeks ago. Usually when I record guitar, I prefer to dial in a mic placement and blend of SM57 and Royer121, and then tiny EQ tweaks in post. I had to use a lot of post EQ to balance out the boom and the high mids. And standing in front of the amp, I've frustrated myself quite a bit trying to dial it in.

For a point of reference, I'd love to get it pluckier pick attack like a Splawn Quickrod and punch like a Peavey 6505. I know we won't get 100% there, but right now it just plays a little too sloppy.

https://soundcloud.com/crossfirerecording/thundertweak-buzzbomb-preview

Thanks,
Nick

UPDATE: I'm putting the mods I've done in the OP. See the photo attachment BuzzBomb_Mods_NSK

[tubenit]

Get rid of the .1 coupling caps after the LTPI and put in .02

Switch the first coupling cap after the first gain stage from .02 to .01.  Don't use anything higher for cathode caps on the 12AX7's then 5uf.

I think that will get you there.

I thought your guitar playing sounded good.

With respect, Tubenit

[DummyLoad]

try this:


--pete

[paintballnsk]

I found the schematic of the mods that were done.  I attached it to the original message. You can see the 2 mods that were done to the original.

Which one of them is killing my mid highs? Or should I just revert them both?

[paintballnsk]

Get rid of the .1 coupling caps after the LTPI and put in .02

Switch the first coupling cap after the first gain stage from .02 to .01.  Don't use anything higher for cathode caps on the 12AX7's then 5uf.

I think that will get you there.

I thought your guitar playing sounded good.

With respect, Tubenit

I put the modded schematic in the original post. Do you know why it wouldn't work? Is it because it's just a modification on the input EQ?

Forgive me, I know a lot about tone, and I've done my share of experimenting with amps and guitars. But I'm still a noob that had to go look up what an LTPI is (google is my friend).

I thought this was a class A amp. Does that mean it's actually A/B?

[Willabe]

I'm very sorry paintballnsk, I messed up 1 of your posts. 

You were asking if you could buy the coupling caps at Radio Shack.

My answer;

Maybe, they need to rated for at least 400volts.

Our forum host Doug has his on line store at the very top of the page and has excellent coupling cap choices and his delivery is very fast!

[paintballnsk]

I'm very sorry paintballnsk, I messed up 1 of your posts. 

You were asking if you could buy the coupling caps at Radio Shack.

My answer;

Maybe, they need to rated for at least 400volts.

Our forum host Doug has his on line store at the very top of the page and has excellent coupling cap choices and his delivery is very fast!

HAHA it's cool. I've moderated forums a few times myself. We've all done it! But thank you for answering the question. I'll check out his online store.

[Willabe]

Thanks. 

[Willabe]

I've used the Malory's on several builds and like them, so do a lot of other builders.

I talked with a member here a few weeks ago that has built dozens of amps and he loves the Mojo caps.

OD 715's seem a little cleaner to my ears.

I bought a few Jupiter caps to test against other brands and I like them very much.

I have not tried the Xicon caps yet but others here have and like them.

Since you only need like 3 or 4 caps maybe buy a few of each brand and see what your ears tell you? Just leave the cap leads uncut and tack solder them in to hear how they sound to you. When you decide on which cap brand you like trim them to size and install them. Then you can reuse the caps you don't use.

It's always good to have a few extra parts on hand just in case.

[paintballnsk]

I've used the Malory's on several builds and like them, so do a lot of other builders.

I talked with a member here a few weeks ago that has built dozens of amps and he loves the Mojo caps.

OD 715's seem a little cleaner to my ears.

I bought a few Jupiter caps to test against other brands and I like them very much.

I have not tried the Xicon caps yet but others here have and like them.

Since you only need like 3 or 4 caps maybe buy a few of each brand and see what your ears tell you? Just leave the cap leads uncut and tack solder them in to hear how they sound to you. When you decide on which cap brand you like trim them to size and install them. Then you can reuse the caps you don't use.

It's always good to have a few extra parts on hand just in case.

Great idea. I accidentally killed my question...

I'll grab a few of each.

Do you have any other suggestions for mods before I place the order. $7 shipping is best spent in one shot haha. I'm going to grab a pair of new tubes while I'm at it.

[Willabe]

Maybe get a few small/low value cathode (K) electrolytic bypass caps?

I think Doug has 1uF, 2.2uF, 4.7uF and 10uF? They would go on pin 3 and pin 8 of V1a and V1b. 

[Willabe]

Get rid of the .1 coupling caps after the LTPI and put in .02

Switch the first coupling cap after the first gain stage from .02 to .01.  Don't use anything higher for cathode caps on the 12AX7's then 5uf.

I think that will get you there.

I thought your guitar playing sounded good.

With respect, Tubenit

[paintballnsk]

Maybe get a few small/low value cathode (K) electrolytic bypass caps?

I think Doug has 1uF, 2.2uF, 4.7uF and 10uF? They would go on pin 3 and pin 8 of V1a and V1b.

What does that do? Is that similar to introducing a brightness cap?

And can you be more specific with where they go? Do I have to put the same caps on both pin 3 and 8, one or the other, or both?

[Willabe]

A lot guitar amps use a 25uF K bypass cap on the preamp tubes K. 25uF is large enough to let all frequencies from the guitars open E string and up be fully amplified. As you use a smaller value less low frequencies are fully amplified.

So we can tune/strip out the amps low end response/mud by using different values for the bypass cap.

Your amp has separate K bypass caps. On a 12AX7 tube pin 3 and pin 8 are the K's. You solder the positive end of the bypass cap to the tube socket and the other end to ground. They go across the K resistor in parallel.

Do a search for cathode bypass caps.   

[paintballnsk]

A lot guitar amps use a 25uF K bypass cap on the preamp tubes K. 25uF is large enough to let all frequencies from the guitars open E string and up be fully amplified. As you use a smaller value less low frequencies are fully amplified.

So we can tune/strip out the amps low end response/mud by using different values for the bypass cap.

Your amp has separate K bypass caps. On a 12AX7 tube pin 3 and pin 8 are the K's. You solder the positive end of the bypass cap to the tube socket and the other end to ground. They go across the K resistor in parallel.

Do a search for cathode bypass caps.

I see. Those are the caps and resistors on the left of the schematic right? Or are you saying this should be in addition to these? When I had the amp modded, he swapped out the 100mf for a 22mf@25V

[paintballnsk]

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

[sluckey]

Here's the schematic...

[Willabe]

Those are the caps and resistors on the left of the schematic right?

Yes.

22uF is still way too big IF you want to cut your bass end.

Look at what Tubenit said.

[Willabe]

Thank you Sluckey. 

[paintballnsk]

Thank you Slucky, that helped a lot.

I've attached what my amp currently has. Basically no bottom end filtering on the input.
And I've attached the same with a 4.7. It looks much more appealing. I'll buy a few other ones as you suggested so I can play with that.

I've tried using a 9 band EQ on the guitar and scooping out similar frequencies. Is that similar to what modding the V1A mod will do? Unfortunately it only gets me half way there.

Now what does changing the caps in the LTPI do? Will that give it a snappier feel?

[HotBluePlates]

... the bottom end is super boomy. Like there's an unreasonable amount of <80hz and just feels really sloppy to play. ...

Silly question: Have you fiddled the Earth control to reduce bass-boom? It's job is to introduce low-end resonance by decoupling the feedback at low frequencies...

Now what does changing the caps in the LTPI do? Will that give it a snappier feel?

Making them smaller cuts bass.

[2deaf]

The cathode capacitors say "mf", but that doesn't make much sense, so it must be microfarads, right?  The bypass capacitor across the 1M resistor between V1-B and V2 (missing on schematic) looks like it says 470nf.  That doesn't make any sense either, so it probably is 470pf, right?

Changing the plate resistor on V!-A from 220K to 100K will decrease the gain a little.  If the cathode resistor is 1uf, changing the plate resistor to 100K will create a larger relative cut to the frequencies that are attenuated.  It is a fairly dramatic difference.  Changing the cathode capacitor to 22uf is moving in the wrong direction if you want to remove bass.

The 1M resistor after the coupling cap. on V1-A is a problem.  Changing the coupling cap. from .02uf to .01uf changes the cutoff frequency from 4Hz to 8Hz with 2M in series like that.  A 0.001uf cap. with 2M would give a cutoff of 80Hz.  It would take a 470pf with a cutoff of 169Hz to really cut the bass down to where you would notice.  The 1M resistor is also a Miller Effect problem and it will shear a bunch of the highs off. 

[2deaf]

I've attached what my amp currently has. Basically no bottom end filtering on the input.
And I've attached the same with a 4.7.

R_G is the load presented by the input network for V1-B, in this case 2000K.  When you put 58K in there, it's the same as reducing the plate resistor on V1-A and it has the same effect on the frequency response.

[paintballnsk]

The cathode capacitors say "mf", but that doesn't make much sense, so it must be microfarads, right? ...

Here's some photos of the actual amp.

[Willabe]

... the bottom end is super boomy. Like there's an unreasonable amount of <80hz and just feels really sloppy to play. ...

Silly question: Have you fiddled the Earth control to reduce bass-boom? It's job is to introduce low-end resonance by decoupling the feedback at low frequencies...

[paintballnsk]

... the bottom end is super boomy. Like there's an unreasonable amount of <80hz and just feels really sloppy to play. ...

Silly question: Have you fiddled the Earth control to reduce bass-boom? It's job is to introduce low-end resonance by decoupling the feedback at low frequencies...

Yes. I've had the knobs everywhere. I've tried it on 4 different cabs (splawn, orange, genz benz, and mesa). It's even got a TS808 in front of it which usually really cuts a lot of the low boomy frequencies.

[paintballnsk]

I plan on changing the LPTI caps to .02.

But now I'm sufficiently confused about what to change on the first gain stage. What should I swap out?

As per the OP, the highs are definitely getting cut off, like my tone knob is being rolled way off. I don't know if this was before or after his mods he did.

[paintballnsk]

Looking at the amp more carfully, it doesn't look like the resistor he swapped is documented correctly. He swapped the 220 for the 100k in a different spot than highlighted :/

[sluckey]

Looking at the amp more carfully, it doesn't look like the resistor he swapped is documented correctly. He swapped the 220 for the 100k in a different spot than highlighted :/

You're right.

See that 22µF cap he put in? It's the black one on the left end of the board. Unsolder one end of that cap (doesn't matter which end) and leave that end just dangling in the air without touching anything. Does that help with the booms?

[tubenit]

Replace the .1 coupling caps with .02 first and follow Sluckey's advice:

See that 22µF cap he put in? It's the black one on the left end of the board. Unsolder one end of that cap (doesn't matter which end) and leave that end just dangling in the air without touching anything. Does that help with the booms?

If that lessens the boominess of the amp, then you can replace the 22uf with 5uf or 2.2uf.

With respect, Tubenit

[paintballnsk]

The 1M resistor after the coupling cap. on V1-A is a problem.  Changing the coupling cap. from .02uf to .01uf changes the cutoff frequency from 4Hz to 8Hz with 2M in series like that.  A 0.001uf cap. with 2M would give a cutoff of 80Hz.  It would take a 470pf with a cutoff of 169Hz to really cut the bass down to where you would notice.  The 1M resistor is also a Miller Effect problem and it will shear a bunch of the highs off.

I'm ordering the parts I need today. So you're saying I may need to go as low as 470pf on that black cap on V1A?

Also, I do feel some high rolloff somewhere. It sounds like the tone knob is being stuck on 0 on my guitar. I wonder if it's that 1M resistor you're talking about. What should I change it to? Maybe that as a low pass filter is intentional and he just rolled it off too far?

If I used: f = 1/(2*pi*R*C), what am I plugging into that formula? 1M gives me an unreasonable number.

[2deaf]

I'm ordering the parts I need today. So you're saying I may need to go as low as 470pf on that black cap on V1A?

I wouldn't actually put a 470pf in for the coupling cap. on V1-A, I was just making a point about the 2M in series with it.  If you were to replace that cap., it would be the yellow .022 cap.

Also, I do feel some high rolloff somewhere.

You could try changing that 1M to 220K and see if that helps with the highs.

If I used: f = 1/(2*pi*R*C), what am I plugging into that formual? 1M gives me an unreasonable number.

R is in ohms and C is in farads.  For the .022uf/2M filter, R is 2000000 and C is .000000022 .  That will give you 3.6Hz.  If your calculator doesn't like that many zeroes, cancel a bunch of them out.  This frequency is where the attenuated frequencies are reduced by 3db with an RC high pass filter.

[2deaf]

You already have a 1uf on the cathode of V1-A, so you would want to try 0.68uf, 0.47uf, or 0.22uf to cut the bass more.  Doug sells these.  They have a higher voltage rating than you need, but that doesn't hurt anything (except the price).

[paintballnsk]

Are we talking about the same spot?

http://www.aikenamps.com/index.php/what-is-miller-capacitance

I thought you were saying the Miller effect was on the .02uf/1M going to the Gain pot from V1 pin 6? Also how did you get 2M again? Are you adding all of the resistors before and after the capacitor from input to ground?

If I wanted to lowpass a guitar signal, I'd probably want to aim between 7k and 15k. If I was smart enough, I'd experiment how to do that before or after the second gain stage, because that's where the bulk of the distortion is with this amp I think. It'd be interesting to know how it sounded to low pass before clipping and after.

[2deaf]

Let's give the components some names.

[2deaf]

C1 is 1uf right now and you would change it to a lower value (0.68, 0.47, 0.22uf) to cut more bass.  Halving this capacitor's value shifts the frequencies attenuated over one octave.

R4 has been changed from 220K to 100K and is a good change to cut more bass.

C2, R5, and R6 form a high pass filter that is .022uf/2M.  This is where it would take a 470pf cap. to have any significant bass cut.

R5 is the resistor that is a Miller Effect problem.  R6 will contribute to it when it is turned down.  R5 is the one you could try changing to 220K.

R9 is bypassed by a 470pf cap. that is shown on the layout, but not on the schematic.  It should compensate somewhat for the highs lost to the Miller Effect.

C3 is the 22uf cap. that has been changed.  This is the one that Sluckey recommends removing.  The increase in signal from changing R5 could be countered by removing C3.

[2deaf]

Silly question: Have you fiddled the Earth control to reduce bass-boom? It's job is to introduce low-end resonance by decoupling the feedback at low frequencies...

It sure sounds like that resonance control (earth) is causing the problem.  Maybe it is wired wrong or the pot. is messed up.  Does anything change when you rotate it to its extremes?  You could remove C11 from the sky pot. and hook the 47K feedback resistor (that I failed to name) directly to that point instead to see if that helps.

[Willabe]

With tweaking an amp you want to try 1 thing at a time not several at once.

Your getting yourself mixed up by looking at too many different places in the amps circuit.

Tubenit and DL told you what to try early on. Try what they said 1st. (I know you asked if their were any extra parts you might need before you ordered and I suggested a few caps but I was not trying to get to go off on chasing other things.)   

The easiest thing to try and that you don't need a new part for is lifting 1 end of the 22uF K bypass cap like Sluckey said. Lift 1 end of that cap and see how it sounds. It's just a test to see if you hear any difference. If it's less boomy then you can change that cap for a smaller value to add some high end back in. Adding high end decreases the bass end you hear because it changes the high/low balance.   

If you still need more bass end reduction, then change the 2 PI caps from .1 to .02 then play through the amp again.

These tweaks won't harm anything in the amp as long as you can solder and not lay the iron on any wires.

[Willabe]

I think what 2deaf is saying is right but IMO I think you don't need to worry about those things yet.

Try the other 2 things 1st. Then if you still need to tweak the amp more you could try changing that 1M series R to 220K. Listen to the amp, then go from there.

You could add a bypass cap across that R just like the 470pF cap that's across the 2nd gain stages 1M series R.

You could also play with different values there, 470pF, 220pF, 100pF, .0001uF (= 1000pF).

There's several different places to tweak/tune the amps bass/treble response. BUT you try them 1 at a time and in order of least invasive and price and least complicated.

Silly question: Have you fiddled the Earth control to reduce bass-boom? It's job is to introduce low-end resonance by decoupling the feedback at low frequencies...

It sure sounds like that resonance control (earth) is causing the problem.  Maybe it is wired wrong or the pot. is messed up.  Does anything change when you rotate it to its extremes?

2deaf may also be right that the earth control is messed up. I re-posted what HBP asked about that control and your response was;  "Yes. I've had the knobs everywhere." That doesn't tell us anything about that control.

You could use your meter and check that all the parts in the earth and sky circuits are correct AND that their wired up correctly while you wait for your parts order.         

Can you post a few up close pics of the back of those 2 pots and of any parts on the turret board that are part of those circuits?

[paintballnsk]

I picked up some .022 and .01 Mallary caps and 220k and 100k resistors from my amp tech on my way home. That will get us started. He didn't have any .68, .47, or .22's unfortunately. So anything else I need, I'll need to order.

Sorry I thought you were literally asking if I had tried dialing in the tone with the earth control, not if I've modded it. I haven't even turned the soldering iron yet myself, so no mods have been made other than the 2 swaps shown in the photos.

The earth control adds some low end and some low mids. The amp overall lacks some low end growl and that helps a little bit keeping it at noon or higher. Turning it down to the extreme sounds weak, turning it up to like 9:00 has all the boom and none of the low mids, and turning it all the way up might sound good after we control the boom. I'm not sure. Maybe I'll shoot a video tonight so it's easier to show.

You guys are all amazing by the way. I can't thank you enough for sharing your knowledge. I'm a software engineer by trade, not a EE. I spend a lot of time in the lab and am familiar with scopes and some minor probing and stuff like that, and I'm pretty savvy at researching and learning quickly. I like to tinker. I also run a small recording studio, and do live sound on occasion and stuff. I think I've gotten my ears tuned enough where I can usually go "oh that needs less 60hz" or even to the point where I can say "the EQ is fine, but i want more dynamics at 80hz".But even when you say "... use your meter and check that all the parts in the earth and sky circuits ..."... i have to go google that because I wouldn't even know what to look for LOL. But dammit I'll learn!

V = IR FTW!!!

[Willabe]

Sorry I thought you were literally asking if I had tried dialing in the tone with the earth control,

Yes, HBP and 2deaf were asking what it sounds like playing around with the earth control.

Now you have explained it. 

A lot of guys here find that if they use a 25uF or 22uF (you'll never hear a difference) as a K bypass cap the amp will sound muddy on the bottom end, especially as they turn up the amp into distortion. (Guitar amp bass distortion is muddy.)

You have 3 K bypass caps in that amp, 2 are already 1uF. I wouldn't worry about getting a .68, .47 or a .22 caps yet. To me that's really fine tuning, that going from the 1uF to those values, you might or might not even here it.

The 22uF and the .1's are the main culprits to go after 1st.     

[Willabe]

But even when you say "... use your meter and check that all the parts in the earth and sky circuits ..."... i have to go google that because I wouldn't even know what to look for LOL. But dammit I'll learn!

The 'sky' is a common presence control and the 'earth' is a bass resonance control, as already stated by HBP and 2deaf. They are both injected at the same place, the PI's (V3a/b) tail ground connection.

Here's a hi-lighted copy of your amps schematic;

[paintballnsk]

So...

I swapped in the .022 for the PI caps and I decoupled the black 25uf.

Umm. If you could see my smile. It's one of those "OH MY F%^&!" moments. It sounds 10x better.

It's almost there. There's still a little flub in the low ends. and still a small lack of high end, but again.. 10x better... Kind of like a clean guitar is punching through the gain instead of clipping with the rest of it. It's hard to explain but adds a sludgy tone to it.

So first question, can I leave that 25uf cap off there, or will it attenuate the bass response farther by putting a smaller cap on there?

If I swap that 1M resistor going to the gain pot with a 220, will that kill my distortion? I feel like the amp needs just a hair more distortion as it is. I'm running it about 9/10. If I go to 10 it gets a little annoying. so maybe more gain isn't the right maybe what I need to to add a little gain but spread it out over the gain phases so it's not as brittle? It's not brittle now, just needs a little "more". I can add a little by turning the gain up on my TS808, but it make it too compressed.

I'm going to try swapping the 1M resistor now

[Willabe]

So...

I swapped in the .022 for the PI caps and I decoupled the black 25uf.

Umm. If you could see my smile. It's one of those "OH MY F%^&!" moments. It sounds 10x better.

Ok, good. The guys were steering you right.      

I wish you would have made 1 change at a time and THEN listened, that way you would know which change made the most difference and by how much. 

That way you can HEAR what your ears tell you what each change does, but, maybe next time. 

It's almost there. There's still a little flub in the low ends. and still a small lack of high end, but again.. 10x better... Kind of like a clean guitar is punching through the gain instead of clipping with the rest of it. It's hard to explain but adds a sludgy tone to it.

I think I hear you. The top end is NOW coming through because you stripped out the bottom end distortion which we hear as mud. It's about balance.   

[paintballnsk]

So...

I swapped in the .022 for the PI caps and I decoupled the black 25uf.

Umm. If you could see my smile. It's one of those "OH MY F%^&!" moments. It sounds 10x better.

Ok, good. The guys were steering you right.      

I wish you would have made 1 change at a time and THEN listened, that way you can HEAR AND LEARN by what your ears tell you what each change does, but, maybe next time. 

It's almost there. There's still a little flub in the low ends. and still a small lack of high end, but again.. 10x better... Kind of like a clean guitar is punching through the gain instead of clipping with the rest of it. It's hard to explain but adds a sludgy tone to it.

I think I hear you. The top end is NOW coming through because you stripped out the bottom end, distortion which we hear as mud. It's about balance.   

I can put the 25uf back on if you'd like to know. From what I'm understanding, it's not doing much at 25uf anyway.

Caps are still reading 400v, so I gotta wait a bit before I try the 220k swap.

[paintballnsk]

Is there a 3rd/4th gain stage? what is the 2nd 12AX7 doing exactly?

Is there anywhere I can put a 100k or a 220k to get more gain out of the 2nd or 3rd gain stage.

[Willabe]

I'm going to try swapping the 1M resistor now.

Ok, I'll say on line with you for another hour or so. 

[Willabe]

Is there a 3rd/4th gain stage? what is the 2nd 12AX7 doing exactly?

No. Slow down.

Is there anywhere I can put a 100k or a 220k to get more gain out of the 2nd or 3rd gain stage.

You will get more gain by changing out the 1M series R that's in between the 1st gain stage and the 2nd gain stage, try that 1st. 

Blue circle 1st, then if you need more gain red circle next.  You can go down to 100K or less if you need to, BUT try 220K for the blue 1st, then IF you want/need more, try 220K for the red and listen. 

[sluckey]

Totally remove that 22µF and put a 2µF in it's place. The gain will increase.

[paintballnsk]

And another stupid questions. the caps still read 400. on my meter they read 400 then like 85 then 0 and it just keep repeating. It's been 20 minutes. I don't have any alligator clips. Is there an easy way to discharge them so I can try this tonight? it's unplugged and I took the tubes out.