Ever build an amp that just doesn't have it?

[mobisimo]

Hey all,

This topic isn't so much a cry for help as a conversation starter.

About a year ago I finished my first amp, which was an 18 watt TMB. I eventually added feedback/boost mods, cleaned up the lead dress, etc. and it turned out to be a great amp.

Recently, I completed something of a hybrid: 4xEL84 tubes - cathode biased, traditional trainwreck power supply, ~rocket preamp with a chieftain mid control between gain stages instead of a T(M)B tonestack.

When I first fired the thing up it was a monster, which I attribute mostly to the way I had the first gain stage set up - first stage of the TB channel of a DC/30. At this point, that stage is 100k/1.5k-25uF; this definitely calms things down a bit. I've also tried just about every combination of coupling caps I can think of. And for all the addition re-tooling I've done, from tweaking the power supply to alterations in the PI and second gain stage, this amp still doesn't do what I'd like it to. Granted, I am a relatively inexperienced builder, but I was hoping that the inputs of time and thought would do something to offset this.

That being said, have any of you experienced the same?

[Normster]

Yup, the second Bassman I converted to a Hybrid-A always sounded lifeless to me. Nothing really wrong with it, but it just didn't have anything special going for it. I recently converted it to an HRM circuit and now it's my favorite amp.

[Darkbluemurder]

I agree with Normster. Some amps just don't seem to like a particular circuit put into them whereas other amps don't seem to care. My 100W JMP Marshall sounded good with any circuit in it. My 50W Lead & Bass Model did not like the non MV set up but sounds a lot better with a high gain preamp. And finally my Sundown A-50 seemed to say "no" to a high gain preamp by squealing. It sounds good for medium gain blues tones. I believe somewhere that part of Ken Fisher's tweaking abilities was to listen to the amp what it wants to sound like. I understood this as the amp will not sound as intended if you try to mod it into something it doesn't want to be.

[Funkalicousgroove]

I built a Rocket that didn't sound at all 3-D, at this point it has had every single component changed except the trannies, is now a totally different design, and still lacks in the magic category. I'm 99% sure that changing the OT to one of higher quality and better fidelity will help my quest. I hate it when they don't come out!!

[mobisimo]

I believe somewhere that part of Ken Fisher's tweaking abilities was to listen to the amp what it wants to sound like. I understood this as the amp will not sound as intended if you try to mod it into something it doesn't want to be.

Agreed. Seems like the type of characteristic that is akin to the mysterious faculties possessed by a gifted writer or poet.

Since the first post, I've swapped the PI values for those of an 18 watt. This change seems to balance things out a little. And simply as a stroke of luck, I traded a couple resistors for vintage CC types, which appear to relieve some of the graininess.

Still more changes to come. Hopefully this amp will ultimately sound like what it wants to sound like.

-M

[rfgordon]

It's funny you bring this up, cuz I was out at a guy's house yesterday to show him some amps. He tried one, liked it--tried the second (the Two Nob Trainwreck) and really liked it, then tried the third (kind of a cathode biased bassman with attitude) and bought it on the spot. So, what's yer point, you ask? He handed me a Holland "Little Jimi" amp and told me to "make it sound better so it'll sell." He played it (as I did when I got it home), and it's just anemic until it's pretty much wide open. by this point the EL34s are roaring--way to loud for most situations.

So, what to do? I opened it up and, lo and behold, there was an unused triode on the reverb recovery 12AX7, and it faced the first preamp triode. Woohoo! An extra gain stage just waiting to be brought to life, and located with no lead dress issues! It was just like Christmas.

Now, this amp has a very complicated tone network sandwiched between two triodes (860ohm/25mF w/ 81K plate resistors). So they're working hard, but they're terribly loaded down. So I moved the input to the new triode, adding a 22K grid stopper, biased it with a 10K resistor and a 100K plate resistor (hmm, sounds familiar...a little touch of Ken Fisher). Coupled it to the next stage with a .022mF cap, ran a 68K load resistor to ground, and put a Dumble-style .047mF/20Mohm LNF on it.

So now it sounds rich and fat and nicely compressed, kinda like it's been goosed with a boost pedal. (Just for hoots, though, I first put a 2k2/25mF bias resistor on the new gain stage--Talk about a ragin monster!! Dudes, this thing was huge. But, alas, it wasn't what the client wanted, so out came the fun-time mod, but not after I'd played every Metallica and Judas Priest song I knew.)

The only thing I don't like is that the amp's volume control (even pre-mod) never completely killed the signal. So I replaced the 1MA pot--it got a little better. Odd. So I'm thinking I might put a pull switch pot on it to mute it to ground. Any ideas out there.

So that was my cure today for the dull amp.

[mobisimo]

Good stuff, man. Maybe this job needs another gain stage to get things kicking.

I'm not familiar with the two knob wreck. Have you got a schematic?

[rfgordon]

mobisimo,

The Two Nob Trainwreck is kind of an mixture of things. It's got preamp from the Blues Express versions of the wreck, except that, instead of the full tone stack, it runs the vol/tone combo from the Z Carmen Ghia. It goes into a pair of 6V6s cathode biased. Starts out really sparkling clean, but with great punch. As you crank it past halfway, it starts to really grind in a marvelous way, particularly with humbuckers. Oh, and I put a traditional hi/lo pair of input jacks on it, and they make it very flexible. I stuffed the whole thing into a Tweed Deluxe chassis and cab with a Celestion V30. Mucho tone-o. Since it was in a pre-made Tweed Deluxe chassis that was already marked, I covered the face with carbon fiber-looking automotive/motorcycle styling vinyl and hand letterd it.

I don't have a way of making schematics in the computer, but I can draw it out and snail mail it to ya if'n ya really want it.

[mobisimo]

Thanks! I actually looked up the TNT immediately after I posted last night. Great concept! I think that maybe adding that third gain stage is the way to go here, having done a couple more mods.

Yesterday, I changed the 500K volume pot to 1M and what a difference that made. Although the tone control became much more useful, I was surprised to find that it cut the volume completely in the counter-clockwise position. Is this normal?