Taming a bright amp

[Souljunkie]

I own a Blackstar Artisan 15 combo. This amp has a 12AX7 channel and an EF86 channel. Both channels are overly bright and have little to no bass. I jumper the channels and turn the tone knobs to 0 and it is still too bright. I recently tried an eq pedal in front and that helped to add some bass  but I would like to dial the amp in to a more pleasing tone . I read other posts where they recommend changing values of the caps before the volume and tone pots but I would like to know which ones specifically I should change to let some of the bass through and cut highs. I've provided the link to the schematic as my attempts to reduce the size of the image and attach haven't worked. There is a 10n cap on channel 2's tone pot like channel 1 but isn't shown in the schematic. Any advise is much appreciated.

http://www.el34world.com/Forum/index.php?action=dlattach;topic=11923.0;attach=24873;image

[terminalgs]

i think those people are suggesting you change the 22nf's to 100nf's., but I don't know if that'll really do the trick.

I'd start with the single triode channel. Turn the ef86 channel to zero and forget it for now.

disconnect the 1nf on the tone control and see how it sounds.  If that doesn't do it, then Id try changing the 250K volume pot for a 500K or a 1M.

[Backwoods Joe]

Jumpering the channels is not recommended. Look at the schematic, one channel has one stage and the other channel has 2 stages. This will cause phase cancellation that will reduce the bass response drastically.

[The_Gaz]

First I would change the speaker. After that, you could try a zobel network on the output, or a conjunctive filter on the primary side, but that requires beefier components. If it were me, I would try a Vox style cut control, then hardwire it where I liked it using a resistor and cap in series. That's assuming you don't want to add an extra control, of course.

[tubenit]

I'll offer a little different perspective on this.  Looking at the schematic, I am guessing some of what you are referring to as too bright may be what I would refer to as upper mids harshness. So the goal is to change the harshness to something more smooth sounding in tone.

First thing I would try is to change the 470uf cathode cap for the power tubes to something closer to 47uf  (try anything between 10uf -100uf).   I find anything above 100uf with EL84's to sound very harsh to me and lacking smoothness.

I would add the "enhance" caps across the LTPI plate resistors.  Try anything between 82p- 220p with the larger smoothing more highs.  This has been a very positive mod I add to my amps and I like the results a lot. And it's very easy to do.

I'd also change the .047 coupling caps between the LTPI to power tubes to .02  and use 56k grid resistors.  Typically,  you will find .01 or .02 coupling caps between the LTPI and EL84 power tubes.   I think .047 tubes will add harshness to the tone also with EL84's.  The 56k grid resistors should add smoothness also.

The approach I would take is to find ways to smooth out the tone of the amp and remove harshness to the upper mid tones.

This may or may not be what you are wanting, but I thought I'd offer this approach as a consideration.

Changing the EF86 cathode cap from 22uf to 2.2uf is just a personal preference to reduce some of the gain.

with respect, Tubenit

[terminalgs]

i think those people are suggesting you change the 22nf's to 100nf's., but I don't know if that'll really do the trick.

I'd start with the single triode channel. Turn the ef86 channel to zero and forget it for now.

disconnect the 1nf on the tone control and see how it sounds.  If that doesn't do it, then Id try changing the 250K volume pot for a 500K or a 1M.

I should add  my advice is based on looking at low-frequency roll off of an inline cap followed by a resistance to ground.  If the resistance is too low -or the cap is too high- low frequencies will be attenuated, highs will not be.   

The 22nf coupling caps followed by a 220K resistance to ground (via the volume pot) sets up a 33Hz low freq. roll off.

The 47nf caps followed by the 200K grid leak resistors set up a 17Hz low freq. roll off.

these seem reasonable to me,  at least compared to the RC circuits that the vol/tone controls form:

if the volume is at 50%:
-if tone control is at the upper wiper, it looks like everything below 1447Hz is att. 50%, and everything above goes though the 10nf cap 100% (C=1nf R=110K)
-if tone control is at bottom wiper,   it looks like everything below 842Hz is att. 50%,  and everything above goes though the 10nf cap 100% (C=1nf R=110K+220K) *except* what high freq's see the 10nf as ground.

No matter what, it looks (to me) that the tone control and the volume control can interact to heavily favor high freq's over lows.

Of course, that's if volume control is at 50%. if you run it wide open, the tone control loses some of its impact.

you can  go after coupling caps, but don't look at a value of .047 in a vacuum, analyze the cap value AND the resistance to ground following the cap to determine if its a 'bright' setup or not.

Also, regarding cathode by-pass caps, either the 22uf or the 470uf: lowering the values will promote higher gain for high frequencies, and lower gain for lower frequencies.   22uf on the first gain stage gives the stage maximum gain for all audible frequencies. As you lower it to 10uf or 1uf,  higher frequencies will still get maximum gain, but lower frequencies will operate at minimum gain.  The 470uf value is more common in hi-fi circuits, and a 22uf is more common in cathode biased guitar amps.  The 470uf probably makes the amp overdrive more like fixed bias amp than a typical cathode biased amp, but that's a lot of speculation.

a lot of factors can make  an amp bright.  speaker (The_Gaz) is huge, as is the type and size of wood, and the design of the speaker cab.  If you can, like The_Gaz suggested, plug the amp into a cabinet that you know to have good frequency response on the low end.

Jumpering the channels is not recommended.

+1.

BTW:, anyone know whats up with the .68 cap to ground on the 4ohm tap?  seems like that could be the culprit, too.

[tubenit]

Terminalgs has given you excellent advice regarding higher and lower frequencies. If you have determined that it is truly an issue of too high of frequencies and not enough bass, then I would advise following his advice because it is good advice.

If you think it may be more an issue of harshness in the highs, then I would at least try two things to see if you can get a smoother sounding tone. Reduce the 470uf cathode cap to much smaller value. And try the enhance cap on at least the plate resistor in the LTPI side fed by the EF86.

You can simply lift one end of the 470uf cap and then use insulated alligator clips and safely clip in something like a 47uf or less & see if you like that? I have found the higher value caps increase gain BUT they increase a more brittle and harsh/icepick tone to my ears when used with EL84's. You can also clip an 82p to 220p cap across the LTPI plate resistor to see if you like that and it moves the tone towards something that you enjoy more.

With respect, Tubenit

[Backwoods Joe]

. Reduce the 470uf cathode cap to much smaller value.

You can simply lift one end of the 470uf cap and then use insulated alligator clips and safely clip in something like a 47uf or less

+1 I'd try this first.

[Souljunkie]

I was hoping to get really good feedback but including changes on a schematic and full explanation of the caps and resistors relationship is way above and beyond. Thanks so much.

I've tried a few different speakers that I had, an eminence wizard, red fang, cannibas Rex  and the celestion heritage g12m that came with the amp. The red fang sounded best and that is what is currently in it. It's a 16ohm speaker and that's what the amp is set at by the way.

I will admit I'm guilty of thinking about the caps in vacuum and not considering what comes after them. I think an upper mid harshness might be what I'm describing. It's breakup is harsh an unpleasant IMO and it doesn't take to overdrive pedals very well. It does have a lack of bass though. I compared it to my buddies ceriatone lightning and there was no comparison in the low end.

I've had a love hate relationship with this amp and have considered selling it but the basic tone is great and it's built like a tank with a hand-wired chassis. I hope these ideas work and I'm totally happy. I'll report back.

[PRR]

> whats up with the .68 cap to ground on the 4ohm tap? 

It's at 61,000Hz so probably not our problem.

CH2 is supposed to be nasty.

Can't jack-jump this beast usefully.

Go into CH1. Center tone-pot. This should be a very flat channel, there's no trickery in it. If it isn't, there may be a weak coupling cap shaving away a lot of bass, OR a low-value load resistor hogging against a right-value coupling capacitor.

[darkbluemurder]

The Red Fang is a fine speaker but it has a bright top end.

If you regularily overdrive the amp I recommend to increase the screen grid resistors from 1k to 2k2 (5W flameproof). That should also take some of the edginess away. I also would increase the power tube grid stoppers to at least 10k. But you can go quite high here, 56k as tubenit suggested is no problem in a cathode biased EL84 amp.

I would not use a Zobel filter or a conjunctive filter on that amp - they work well on EL84 amps that have more preamp gain than this one.

Cheers Stephan