Making an amp brighter

[SoundmasterG]

Hi guys and gals,

I'm working on a build for a customer that is loosely blackface based, and I can't figure out why it is doing this. Everything is wired correctly and all the part values check out. It is using the 250pf, .1uf and .047uf caps in the tone stack along with a 100k slope resistor....I've played with raising voltages, adjusting cathode bypass caps and resistors, coupling caps are all same as a Blackface Vibrolux Reverb....but the darn amp sounds like the treble is full off and bass is full on all the time. You can move the treble and bass controls and they work like they should....but the amp always sounds dark....I haven't been able to figure it out yet..... Voltages in the preamp are currently around 200v on the first stage and up from there, but I've had it as high as 230v and still have the condition. Transformers are customer supplied Mercury Magnetics...amp is 30 watts RMS and is sort of intended as a blown up Deluxe Reverb with 6L6's. The customer likes clean with lots of headroom and lots of reverb....right now it isn't getting the treble response it needs though it sounds good and is stable otherwise. I thought about adding the 470k - 470pf combo like I a Marshall...but I shouldn't have to....since it is blackfacey mostly it should sound like those do....at least thats what logic says....anyone have any ideas?

Greg

P.S. I remember reading somewhere that you can parallel a cap with the 47 ohm or 100 ohm resistor in the feedback network to ground and control the treble response of the whole amp....I can't recall where I saw it however and don't know what cap values to start with. The amp currently has a 47 ohm resistor in that position along with an 820 ohm feedback resistor. The NFB can be disconnected and of course the amp gets more aggressive but it still has this treble mask over it.

[jjasilli]

1.  Are you sure the tonestack has good ground connection?

2.  The tonestack is an RC filter.  Besides the slope resistor, the pots are resistors.  Stating the the value of the caps but not the pots, and/or fixed mid-resistor, is insufficient info.  Use the Duncan Tonestack Calculator (Google for free download).  Consider [posting a ost a schematic.

3.  Otherwise treble is bleeding off elsewhere.  Things to try.  Temporarily disconnect "unnecessary" circuits such as reverb, tremolo, effects loops, etc., to make sure the main signal path is functioning. Use a listening amp (signal tracer), or an oscilloscope, one preamp stage at a time, including the input jack, to locate the point where treble vanishes.

[Ed_Chambley]

Do you have a schematic?   You can always reduce the cap/resistor values in the bypass of v1 and moving down the signal path doing the same.  Are you getting any speaker flap when the bass is set on the high side?  This is very common with the AA1164 circuit when using beefy iron.  What speaker(s) at what wattage?  Is the PI a LT pair?
Vibrolux values everywhere, but what circuit?  What is the MM OT specs?  What voltages do you have on the Power Tubes and PI?

[eleventeen]

I would be kind of in Ed's school with this....changing the cathode bypass caps in the first preamp stage or two could produce some positive results. It's also possible that some component somewhere is robbing all your treble response. Got scope and signal gen?

[12AX7]

Maybe a bad/mislabeled/misread coupling cap? That would do it in a fender. Unlike marshalls that use large couplers and tiny cathode bypass caps to remove flab, fenders always seem to do the opposite and use tiny couplers in the pf range and huge cathode bypass like 22uf. If a coupling cap was allowing a much fuller range thru, with those big bypass caps on the stages would make the amp sound like a freakin bass amp.

[SoundmasterG]

Thanks for the replies guys!

jjasilli:

1. Yes I am but I will check again. The tone stack works....the treble and bass pot function, but it sounds as if the treble is full off and bass is full on when the treble is full on and bass is full off...and it just gets worse from there when bass is turned up.

2. I realize that not stating the pot values makes it hard to figure the whole picture out...sorry about that. They pots are 250K-A just like in the Blackface Fenders. The mid resistor is a 6.8k, just like in a Deluxe Reverb. I would love to post a scehmatic but its all hand drawn and I don't have time to draw something up on the computer for 6 weeks due to school.

3. The amp does have reverb, and bias modulating trem on the output tubes. I figured the treble is bleeding away somehow but am not sure where. I do have a broken Heathkit signal tracer that needs a power transformer...it would be very useful at the moment...though I could probably cobble something together that would sort of work. I do have a scope and sig gen and have been using it. I can see the effect the tone controls have on the signal and see that they work (and hear it too) but the overall response sounds like there is a blanket on it.

Ed:

I do have a schematic but it is hand drawn. The first stage and tone stack are just like a Blackface DR AB763. The second stage has a 2.7k cathode resistor and a .47uF cathode cap at the moment in an attempt to add treble and clean up the amp output a bit. There is not overwhelming bass like most Fenders. Then it has the standard Fender Reverb circuit and values, and a gain stage after the reverb recovery with the same values as the Fender circuit except the cathode resistor is a shared 1.5k, again to reduce overdrive. The reverb recovery shares that resistor, and the higher value resistor is cutting down on a tendancy of the reverb to feedback at max levels. With the reduced cathode cap level on the one stage the speaker flab is absent. The customer gave me the speaker which is one of these: http://www.musiciansfriend.com/accessories/eminence-maverick-fdm-tone-adjustable-12-guitar-speaker--8-ohm though I've been doing testing with a closed back cabinet and a Celestion Sidewinder because it has a pretty neutral sound. I also tried the amp through a C15N that was in a 1960 Pro I had on my bench last week, and while the Jensen was brighter than the other two speakers, the 1960 Pro was FAR brighter than this amp through the same speaker. The PI is a Fender LTP direct out of the VR AB763, though I am thinking of going with a larger cathode resistor to get more clean headroom...maybe a 680 ohm...will have to experiment. The MM OT is the FBFDR-O and the amp is using a couple of 6L6GC's, which is what the customer requested. I have not tried it with 6V6's. The PT is a MM FBFDR-P-FS fat stack. The preamp plate voltages have varied from 160v to 230v at times, and the power tubes have about 430v on the plates, screens at about the same. The dropping resistor between the screen supply and the PI supply is a 1k, and the supply voltage for the PI is 412v. The PI is using 100k's for both plate resistors and the voltage on the plates is 206 and 201 with a 12AT7. NFB is on a switch and is an 820 ohm and a 47 ohm to ground when switched in circuit.

Greg

[SoundmasterG]

eleventeen:

I do have a scope and sig gen....how should I use it to find what is going on with the missing treble? Vary the freq response out of the sig gen and see what each stage is outputting?

12AX7:

I am wondering if having a mislabeled part might be the case. The coupling caps are Mallory 150's for the most part with a couple 6PS Orange Drops in there too. I had a ceramic .001uF going into the PI but change to an Orange Drop awhile back. The ceramic the customer gave me to use was WAY off value.

The amp functions well overall except for the lack of treble. I'll be adjusting values and voltages still to get more headroom as that is what the customer likes, but the missing treble is the big one that I can figure out at the moment, and it affects everything else I do for voicing....incidentally the speaker in the amp was doing some popping and cutting out so it may be brand new and bad...I still have to check the cable wiring for that, since someone else wired it up before the customer brought the amp to me, but since I've been testing in my Celestion speaker and it doesn't have a problem like that, I was going to get to the speaker problem later.

I recall reading somewhere where a cap can be added in parallel with the resistor to ground in the feedback path, in this case the 47 ohm, but for the life of me I can't remember where I saw it, and I didn't save it, and don't remember the cap values to start with....I thought I saw it here at Hoffman but I couldn't find it.....

I plan to work on the amp more tomorrow and see if I can track some of this down. I'll probably start by verifying the part values and checking connections again, but if you guys have any other suggestions I'm all ears. Thanks!

Greg

[spacelabstudio]

FWIW, sometimes this happens to me if I have a leaky/bad cap somewhere.  It's usually the silver micas that do it to me.  The cap checks out on a meter because it has the right value, but is leaky.  Any silver micas in your tone stack?

(I've just about resolved to purge my shop of silver micas, if anyone wants my stash...)

[jjasilli]

1.  I would first triple check the tone stack.  A mistake or bad component here could eliminate the famous Fender mid-cut, which translates to the human ear as a lack of treble.  Possible culprits:  slope resistor is really less than 100K; mid resistor is really more than 6.8K; bad ground connection.  Also wrong cap values.

In these cases the tonestack will still retain some function, but full treble cannot be achieved.  Sounds like your problem.

2.  Signal generator & scope.  I would do the following. Dime the amp's vol & tone controls. Input signal voltage at a comfortable listening vol.  You may wish to use use a DVM to measure the signal generator's output voltage. Put lo freq signal into the amp, say about 100Hz.  Increase the input freq until you here a volume drop off.  The problem is at that frequency and above.  Pick a problem frequency.  Use the scope set that frequency to measure it's voltage at each stage of the amp including the input jack. 

3.  A schematic must be seen.  Talking about an unseen schematic is not good enough.

[eleventeen]

Scope & sig gen:

The overbassy response is, we think, a drastic falloff in higher-end freq response somewhere. So, if we give the amp an input signal and get a scope trace that covers several vertical divisions --- we don't really have to measure anything, we're just looking for a scope trace we can see and can see if it shrinks as we crank up the input freq. We're going to manually sweep the input freq, in essence, and see if the height of the trace shrinks as we do so. You want to have a clip lead on your scope probe so your hands are under control with the voltages, of course.


Pick out several places on the schematic and find the corresponding places in the actual circuit. Typical would be on the cold sides of the caps coming off plates. Those are going be the grids of the following stage, in general. Or, you could install a blocking cap on your scope probe and go to town = probe anywhere. Point being, generally you don't want to touch your scope probe to 400 volt places...and maybe not even 200 volt places...unless the scope can take it, but even if so, most would want to take a 600 volt-rated capacitor, say .001 uf, and wire it in series with the probe if you question the voltage capability of your scope. When moving to a subsequent stage, you are going to have to adjust the input sensitivity/gain on the scope so that the scope displays a 4 or 6 div-high trace of your input signal. Doesn't have to be the same, it just has to show the whole sine wave. Use AC coupling on the scope so that if you grab some DC, it doesn't throw the trace up to the third floor.

All you are doing is to probe several places in the amp, deeper and deeper into the amp. Get a trace. Start with 100 Hz and diddle the freq of the input signal up to 4Khz-5Khz. As you do so, you are looking for that stage where the height of trace shrinks with the rise in freq. Capiche?

That's it. Unless there is some macro characteristic that is robbing your treble, and it could be, you want to eliminate any single stage based treble-robbery.

This is a crude "measurement" if you can even call it that. You're not measuring anything, you're just adjusting the scope so that a visible trace 4-6 divs high shows, then sweep the freq higher, 100 Hz > 5Khz. The stage that is robbing your treble will show a crush of the overall height of the display as you move up in freq. That's your problem child.

[stevehoover]

Sounds like you have already double checked component values, solder joints and circuit paths...
If it  was on my bench, I would break out my "amp stethoscope".  (You can find directions on how to make a "listening amp" in the cool amp tools section elsewhere on the Hoffman site, I made the separate box version
And plug it into a working combo.)
Start at the first triode plate then cathode and so on, quickly working your way through the amp.   
My guess is that it will take you less than 5 minutes to locate the offending circuit area.

I resisted building one of these for years!.......If I could only have the time back that I wasted substituting parts and dittzing around and guessing "what it might be causing the problem".

Trust me, for less than 20 bucks worth of parts, (or less, I had most of the parts laying around already)
you will be amazed at the time and grief this little goodie will save you.

Happy Hunting

Steve

[jjasilli]

The overbassy response is, we think, a drastic falloff in higher-end freq response somewhere.   I'm not yet ready to concede that.  Because there might be a failure to achieve the mid-cut which gives the mere appearance of loss of treble.

[SoundmasterG]

FWIW, sometimes this happens to me if I have a leaky/bad cap somewhere.  It's usually the silver micas that do it to me.  The cap checks out on a meter because it has the right value, but is leaky.  Any silver micas in your tone stack?

(I've just about resolved to purge my shop of silver micas, if anyone wants my stash...)

You called it....it was a bad 250pF silver mica that read fine but wasn't....but it was also a bad 6.8k resistor to ground on the tone stack. It would read 6.8k most of the time, and then if you moved it just right, it would read 15k....strange! Anyway, I replaced the silver mica 250pF with another one, and replaced the 6.8k with a 5.6k because I am currently out of 6.8k's and it actually sounds fine...so I will leave it. Now I need to add power scaling to it and revoice after that possibly, and then it should be done.

Thanks for the help everyone!

Greg

[jojokeo]

FWIW, sometimes this happens to me if I have a leaky/bad cap somewhere.  It's usually the silver micas that do it to me.  The cap checks out on a meter because it has the right value, but is leaky.  Any silver micas in your tone stack?

(I've just about resolved to purge my shop of silver micas, if anyone wants my stash...)

You called it....it was a bad 250pF silver mica that read fine but wasn't....

Wow, glad I came across this info? I'm wondering if the silver mica's having this issue could be attributed to being over-heated from soldering? Or if they're just not that good of quality or if that type of cap's structure is just precariously delicate & failure-prone to lead bending (like a glass diode if not careful)???

[spacelabstudio]

FWIW, sometimes this happens to me if I have a leaky/bad cap somewhere.  It's usually the silver micas that do it to me.  The cap checks out on a meter because it has the right value, but is leaky.  Any silver micas in your tone stack?

(I've just about resolved to purge my shop of silver micas, if anyone wants my stash...)

You called it....it was a bad 250pF silver mica that read fine but wasn't....

Wow, glad I came across this info? I'm wondering if the silver mica's having this issue could be attributed to being over-heated from soldering? Or if they're just not that good of quality or if that type of cap's structure is just precariously delicate & failure-prone to lead bending (like a glass diode if not careful)???

What I've been told is they're very sensitive to soldering iron heat.  Except that for me they usually fail after they've been in the circuit a while, so that doesn't necessarily explain it.  But, if handling is some component in it, then apparently I lack the magic touch.

Chris

[Ed_Chambley]

Never seen a resistor read differently like you mentioned, but I can see the possibility.

I have had silver mica problems which all lead to the type.  I call them blobs cause they are not very symmetrical.  Never had a problem with the ones Doug sells or the brown Cornell's from Mouser.  Never bought any from Weber, but they are blobs.  Had a friend with a weber kit he was having problems with.  None of them were correct in his amp, but he said he checked before installing.

[jojokeo]

Anytime I read of a person building from a kit I have to consider the source and "take things with a grain of salt" if there are problematic reports such as this. Kit builders by nature a not very experienced, tend to make rookie mistakes galore, and definitely do not understand how to even solder correctly, not to mention how to handle various parts that may have a sensitivity nature. Not their faults, they just need experience but it is what it is...

[SoundmasterG]

Never seen a resistor read differently like you mentioned, but I can see the possibility.

I have had silver mica problems which all lead to the type.  I call them blobs cause they are not very symmetrical.  Never had a problem with the ones Doug sells or the brown Cornell's from Mouser.  Never bought any from Weber, but they are blobs.  Had a friend with a weber kit he was having problems with.  None of them were correct in his amp, but he said he checked before installing.

Me either....pretty strange...it was a metal film too...an NTE I think.

I haven't had silver mica problems in the past myself, though I've heard about them....this is a first for me with that being bad. I had a hunch that might be part of the problem but wanted to see what people thought since I wasn't able to work on it for a bit due to school. I'm currently waiting for some parts and should be able to get the amp down the road soon happily.

I'm not too impressed with many of the parts on the Weber kits, but since they are bottom feeders in the market, that isn't surprising. Some of the parts they sell are great though.

Greg

[SoundmasterG]

Fender used a bright switch on some of the twin amps.  basically, a 0.0001 micro f, cap between the one and two tabs on the volume pot. 

Yah I know about that thanks. This amp has a hardwired 100pF. I tried a 47pF and a 120pF before I settled on this one. What was causing me grief is that despite the circuit being the same as some of the BF Fenders, it was lacking in top end...but I've figured it out now as I noted above.

Greg

[12AX7]

P.S. I remember reading somewhere that you can parallel a cap with the 47 ohm or 100 ohm resistor in the feedback network to ground and control the treble response of the whole amp....I can't recall where I saw it however and don't know what cap values to start with.

Someone please correct me if i'm wrong, but doesn't that ATTENUATE brightness?

[sluckey]

Someone please correct me if i'm wrong, but doesn't that ATTENUATE brightness?

Nope. Gotta use negative logic when talking about negative feedback.

[12AX7]

Ok, then this is something i would like to try because i feel the need for some more brightness in the PA, mainly just the higher stuff in the say 3k and up range. What would you suggest as a cap to bypass the 47k?

[jjasilli]

12ax7: I think you are referring to a Presence control.  This will vary the brightness of the amp, but NFB also affects the PA's relationship the speaker.  This is the damping factor, and will vary with different speaker drivers and enclosures.  You can temporarily disconnect the NFB loop and see how you like the tone and performance of the amp.  You can vary the NFB with a presence control; there are schematics on the Forum.  Or you can boost treble in the preamp.

[12AX7]

12ax7: I think you are referring to a Presence control.  This will vary the brightness of the amp, but NFB also affects the PA's relationship the speaker.  This is the damping factor, and will vary with different speaker drivers and enclosures.  You can temporarily disconnect the NFB loop and see how you like the tone and performance of the amp.  You can vary the NFB with a presence control; there are schematics on the Forum.  Or you can boost treble in the preamp.

No, it's not that. i have a variable NFB and a pull switch that disconnects it and a presence control. What i'm asking about is a cap in parallel with the feedback resistor. (i have a resistor and pot in series for variable nfb) granted, when the variable NFB is turned all the way to minimum NFB or the pull switch pulled, the cap across the resistor won't do much or anything at all if disconnected. But when i'm using a normal amount of nfb with the pot at zero resistance i want to know what value of cap across the 47k set resistor would get me the kind of highs i described.

[sluckey]

Just get a handful of various caps and try it.

[SoundmasterG]

P.S. I remember reading somewhere that you can parallel a cap with the 47 ohm or 100 ohm resistor in the feedback network to ground and control the treble response of the whole amp....I can't recall where I saw it however and don't know what cap values to start with.

Someone please correct me if i'm wrong, but doesn't that ATTENUATE brightness?

If you look at say a Deluxe Reverb circuit, you will see the 47 ohm resistor to ground by the phase inverter. If you parallel that with a cap it will make the amp brighter. I have no idea what values to try though. I read about it somewhere awhile back where values were mentioned but don't recall where and haven't been able to find it again. The other resistor in the feedback loop is the 820 ohm that connects to the output transformer. If you parallel this one with a cap, then it attenuates high frequency content. I've seen this particular application of the technique in some old organ amps from Conn and others, which makes sense for those, but not for a guitar amp. If you already have a presence pot in there, you have a variable increase in brightness, so I don't think adding another cap in parallel with any of that will help you.

Greg

[jjasilli]

If you parallel that with a cap it will make the amp brighter.

Yes, because:  NFB reduces highs; and the 47Ω resistor is lifting the NFB's AC signal from ground.  Bypassing that resistor with a cap defeats the NFB loop by allowing the NFB signal to bleed to ground through the cap and around the resistor.  No more NFB means more treble.

If that bypass cap were itself lifted from ground with a pot in series with it to ground, you would have a form of variable Presence control.  Lots of schematics are posted on the Forum and various stock schematics of production amps.  For theory on how to design NFB / Prsence controls, checkout the website recently posted by Kagliostro in the Amp Tools Section.

[SoundmasterG]

During my research on class B amps, I found a discussion where an amp designer, converted his class B amp using el34's to KT88's and switched from a full bridge rectification, to half wave rectification, to address reducing bass.   

That seems kind of dumb and complete overkill to me....

Greg