preamp tube voicing?

[dmartn149]

Hi, I'm new to this forum. I am hoping someone here has already figured how to fix my problem. I just got an old Acoustic Control Corp. g60t amp. It is a 1-12 combo with 2 channels, but only one eq. The clean channel sounds great, but the over drive ch. sounds like poo. I thought maybe I could set the drive really low, and get ch2 to sound like ch1 only louder. It does sound better that way, but there is still way too much mid freq. (no bass or sparkle) in ch2. Is this normal with these "cascaiding gain " amps? An effect of one triode overdriving another? Would a low gain tube in this stage help. Say a 12ay7 or 12au7? or do I need to change a cap or resistor in the circuit? Is there any way to fix it, or should I just forget about ch2
Here is a link to the schematic.
http://www.ampix.org/albums/userpics...ice_Manual.pdf
Thanks, Dan

[dmartn149]

Well, quite a few readers, but no answers. I borrowed a couple of 12dw7's from my Eico st70 hi-fi amp and found that they moved the sound in the right direction, so I ordered a pair of Chinese 12at7's. I'll post my results when they get here. In case anyone is interested.
Dan

[stokes]

I am not a fan of 12AT7's anywhere but the PI or reverbs.I am especially not a fan of any chinese preamp tubes at all.Not knowing what you have in the amp as far as brand of tubes,I would tell you to get a NOS 5751 and replace V1 for starters.I wouldnt recommend any current production 5751's,to me they sound like weak chinese AX7's.Experimenting with preamp tubes can get quite expensive and is pretty time consuming,but I think the 5751 will get you in the direction you want.

[dmartn149]

Thanks for the reply Stokes. The amp has Groove Tubes in it now. The Chinese tubes I ordered are from an oem manufacturer that (supposedly) supplies tubes to lots of amp builders. I know that NOS tubes can get really expensive, but I was thinking that in a guitar amp it wouldn't be as critical as a hi-fi amp. After all, we do expect a guitar amp to color the sound. Any way the Chinese tubes were super cheap and couldn't possibly make the overdrive channel any more unusable than it is now. If the Chinese tubes don't work out then I guess I'll start looking for some old 5751's.
Thanks, Dan

[Steve Conner]

It's not that there's too much midrange. More likely just not enough bass and treble boost. The amp was probably voiced to sound good at big-hair-and-spandex levels of gain.

You can't change voicing by swapping out the tubes, except in so far as lower-gain tubes let you turn up the gain controls further, which reduces loss of treble due to the control's resistance, and they also have less Miller effect which amounts to the same thing.

To make a radical difference you have to change other components, for instance adding bright caps to any volume/gain controls that don't have them, and beefing up a few of the coupling capacitors (there's a 0.0068uF in there) to let more bass through. This would brighten up the lower gain settings, at the expense of maybe making the high-gain end too buzzy and farty.

[MWJB]

"You can't change voicing by swapping out the tubes, except in so far as lower-gain tubes let you turn up the gain controls further, which reduces loss of treble due to the control's resistance, and they also have less Miller effect which amounts to the same thing." I have to disagree here...as would many harp players who run lower mu tubes in 5F6As & the like, or guitar players loking for an airier, more Fendery sound out of DeVilles etc. You can substantially alter frequency response & harmonic content/character by subbing preamp tubes, as well as effecting changes in dynamics. I have done a LOT of this, as have others.

There's more going on that simply changes in pot settings - operating voltages change, tube bias changes, crunchy harmonics are typically reduced the lower you go...these things ultimately affect voicing.

That's not to say that tweaking bright caps & coupling caps isn't a good plan, or even the first port of call, but I certainly wouldn't rule out preamp tube swaps.

[Steve Conner]

There's more going on that simply changes in pot settings - operating voltages change, tube bias changes, crunchy harmonics are typically reduced the lower you go...these things ultimately affect voicing.

Sure, but I'm thinking that swapping a tube out for a lower-gain one would reduce those crunchy harmonics and make the sound even duller than it was. I don't see how it can make the voicing brighter, other than the effects I mentioned.

Still I agree, it can't do any harm to try different tubes. But at the end of the day, tubes aren't filters: it's the resistors and capacitors in the circuit that mainly determine the voicing, and the tubes have only second-order effects.

[MWJB]

"I don't see how it can make the voicing brighter, other than the effects I mentioned." Brighter...? Maybe, you're right, perhaps not brighter per se, but low end & woofiness can be reduced, which to some folks can give a similar sounding overall effect. As you go lower mu the very high end can also be reduced, but as low end is reduced too, you can end up with a chimey-er, wiry-ier fundamental note...but perhaps with less "crispness " and "front" than with higher mu tubes? Really it depends on what your biggest problem/most immediate concern is.

[stokes]

Since he says he is trying to get the lead channel to sound more like the clean channel,"only louder".Well,we all know that aint gonna happen,but to get closer to this,it would seem lowering the gain on the lead channel is the only way to go.To do that,without getting into changing bias points of your preamp tubes and or voltages,it would seem the only way to go would be to use a lower gain tube,no?I gotta disagree with Steve on the "cant change voicings by swapping out tubes" thing.Nothing could be farther from the truth.If you do comparisons,you will see that even subbing,say,an RCA AX7 with a Bugle Boy AX7 can drastically change the voicing.Even two of the same brand/type can make a big difference.

[dmartn149]

Thanks for all the input guys. I have found that the eq of ch2 is okay when the gain is set to "big-hair-and-spandex levels of gain". The problem seems to be that the gain gets like that when the pot is just barely cracked open. If I set it low enough to just get just a bit of dirt or mostly clean, then I have to turn the bass all the way up, and the mid to about 9:00. I still can't get the very high freq. "sparkle" that is present in ch1, or at higher gain in ch2. I figured that this is just a result of having the pot set so low. I thought that I would go with lower gain tubes so I could open the gain pot more. If that takes care of the high freq. then I'll start looking at the bass.

I am new to amp tweeking. and am not really an "electronics guy" so please forgive me if I say, or ask something stupid. I have done some guitar rewiring, built fx kits, and replaced pots in amps. I have been reading/re-reading The Tube Amp Handbook, and am fully aware of the lethal voltages present in an unplugged tube amp.

Thanks again for all the help. I'll post my results with the new lower gain tubes as soon as they get here.

[reverendbk]

Using the lower gain tube will let you open up the gain control more, but so would replacing the 1M pot with a 500K. Make sure it is audio taper, not linear taper.
There's a lot more you can do though. Steve noted that there are some small coupling caps. C20 appears to be .002uf. This is really cutting the bass out. If you use lower gain tube, or change the gain pot value, you will probably want to beef things up a bit.
C22 appears to roll off treble after the clipping circuit. Removing this or reducing the value would allow more highs to come out of the clipping circuit.

One of the best mods for giving an amp more "sparkle" came to me in a Carvin X-amp. The second gain stage used a relatively tiny .022 cathode bypass cap.

[dmartn149]

Okay, I got the new tubes (12at7) They reduced the gain to more usable levels in ch2, and I can still get more than enough distortion when I want to. As you all knew it would, the frequency response stayed about the same. Ch. 1 has lush deep lows, and beautiful shimmering highs, while ch2 sounds like a cardboard box.

So should I remove C22, and change C20 to a .047 like C1 and C8 in ch1?

[dmartn149]

Can I get anyone to weigh in on my plan? I have never done any amp mods before and I don't want to screw it up, or engage in months of trial and error.

What I was thinking I would do is change C17 & C20 to .047 for more bass, and remove C22 for more treble.
Does this sound like a reasonable start?

Here is another link to the schematic
http://www.ampix.org/albums/userpics...ice_Manual.pdf

Thanks, Dan

[reverendbk]

That sounds like a good start.

The first thing to do is disconnect the grounded leg of C22 and see if that clears up the treble to your liking. If not, then do the same to C19 (I missed it before). It performs the same function as C22, but rolls off the treble before the gain circuit.

After you play with the treble roll-off caps, try the amp out and see. Don't do everything at once.

Then worry about the bass. Start with C20. Try anywhere from .01 to .047uf here. If you have several caps of the same type, start small. That way if the lows aren't full enough, you can temporarily solder in another cap in parallel. Once you get it to your liking, take note of the total cap value and find a single cap that best matches it.

C20 will control how much bass comes out of the drive circuit. If you start playing with C17 it may drive too much bass into the next stage, and sound "farty".

Happy tweaking.

[dmartn149]

Thanks Rev.
I don't have a supply of caps on hand, so I'll be buying components specifically for this project. I noticed that the coupling caps specified for 1 & 8 are 400v poly while 17 & 20 are 500v discs. What kind do I need to get? (to replace 20)
Thanks, Dan