Scope Traces?

[MikeKCA]

Does anyone have Oscilloscope signal traces of various stages in their clone? Alternately, peak-to-peak signal readings at various points would be helpful.

I'm using a Bassman head as a prototyping platform and initial results show much lower gain than expected. At 10, there's a nice crunchy tone for chords but nothing approaching the overdrive on any of the clips I've heard.

I know that my power supply voltages from the 3rd stage on are too high, but I can't imagine that lowering them (which is my next step) will help that much.

Thanks in advance.

[toddyjoe]

Paul Ruby did some scope traces on a Wreck at one time. I remember reading his posts over at the AX84 Forum regarding his results. Maybe drop Mr. Ruby an e-mail and see what he might have. I do not have an e-mail for him in front of me but he is a regular over at the AX84.

[doctord02]

Mike, shoot Paul an email and ask him - the maple wreck featured on his site is mine, and I know he's got the skinny on what you are trying to find out. He's got an email link from his site - www.paulrubyamps.com

[ampdoc1]

Mike,
Which schematic of the Express did you use? I've built a couple from my own drawings of Kelly '90, but the TW schematics I've seen on the web are all pretty close. I can't imagine if you've stuck close to these that you don't have enough gain. I've substituted a 12AU7 for V1 and a 12AT7 for V2 in my amps to lower the noise, and they still have ridculous gain.

Good luck,

DEH

[MikeKCA]

Thanks guys.

I started with version A1 with higher plate voltages in the preamp (using a Bassman AB165 as a proto bed). I got the power supply voltages very close (mid 200s) and the amp still didn't break up significantly until dimed.

I increased the stage III input resistor to 150K and I'm getting closer but the amp is still much cleaner than any of the clips I've heard. At 5 on the volume control it's still clean.

I'm assuming that most of the grind is supposed to come from the preamp, right? With the volume on 10, chords are beyond crunchy but single notes are just in the blues range...think moderately overdriven ZZ Top.

Can't seem to figure out what's wrong.

[Moose]

I don't have traces, but the distortion isn't just preamp. It actually breaks up sorta all at once. The poweramp should break up first (with EL34s -- maybe not as much with 6L6 which will take a larger grid swing) and cascade back to the PI, clipping stage, and tonestack recovery. When I say first, I don't mean by much, it happens pretty much all at the same time. I can get some pretty rock and roll distortion with everything set to 12:00, so even with 6L6 something's not quite right.

I'm wondering if there's something being attenuated before the clipping stage that you're not seeing. Have you checked values at the tonestack and before? Is that an 820K resistor instead of an 820R as the grid stopper? Is the volume pot really a 1Meg?

These are all just ideas, here. I don't know for sure. I am just guessing that something like that must be happening if you're not getting clipping at stage 3 until that high in the volume range.

[MikeKCA]

Thanks Moose. I've measured most resistors and pots to be sure but I'll check again. I actually replaced the treble pot with a 1 Meg with pull switch (for bright or semi bright switch). Not sure which 820 you're talking about as a grid stopper. The only ones I have are the 1.5Ks on the power tubes.

I did realize last night that I still have 3 original carbon comps from the Bassman in the preamp circuit. The 100K plate load resistors for the tone stack stages and the slope resistor. Pretty sure I checked these too, but from past experience, these can test fine with a meter but do strange things with power applied.

Does anyone have plate voltage readings? This is a pretty basic piece of data which is lacking from all schematics that I've seen.

[toddyjoe]

Mike, you indicated that your third stage power supply voltage is high. How high? The third stage is a cold-biased clipping stage that plays is a part of the Trainwreck sound. I assume you are using a 10K bias resistor for that stage. Perhaps you need to adjust the cathode resistor value due to the higher voltages? Anyway, I am just thinking out loud. Good luck!

[MikeKCA]

Thanks Toddy Joe. I have since lowered the voltages in the preamp...not sure of the plate voltage but the power supply side of the plate load resistor is about 260- to 270 if I recall. I'm sure that increasing the value of the 10K would add more gain and make it clip, but I think I must have something else wrong.

[MikeKCA]

Thanks all. Problem solved. Turned out to be the negative feedback circuit in the amp I was converting (Bassman AB165). It was loading down the output from the 3rd stage.

I disconnected the NFB temporarily and the amp is MUCH louder and starts breaking up at about 3. I'll have to reconfigure the NFB circuit and switch the OT phase and should be good to go.

[Moose]

TOTALLY missed that bassman model. I had the 5F6A in my head, which has a regular feedback/presence circuit, not one that feedsback before the PI.

Now you justhave to tweak to taste. You told me elsewhere you want to run from relatively clean to Duane Allman at the Filmore tones.

Since you're looking for a more old-school marshall sound, and are more blues oriented, you might consider messing with the slope resistor to tame that treble control a touch, roll out a little crunch in the third stage, and experiment with the tonestack set below 12:00 so they attenuate a little more signal going into the second triode. Between 6V6 and running most of the tonestack at 9:00, I get a pretty good plexi-style tone at very usable volume. Remember, just because you have the gain, doesn't mean you automatically have to use it all!

You might also want to try a 12AT7 in the first triode. Less drive through the tonestack, but it might be more controllable if you want more range on the gain control and never use the full extend of this amp's gain. I'd be wary of screwing with the gain too much, though, as I'm in love with the touch sensetivity in this circuit.

Anyone else have tweaking ideas for a more bluesy version of this amp?

[MikeKCA]

Thanks Moose. The big issue for me now is I'm getting quite a bit of hum (120 hz?) from somewhere after the volume control. Haven't had a chance to dig in yet.

Man, this thing is LOUD with the 6L6 power section! No way I'd ever be able to use it above 3 or 4 at a gig. Can't wait to try a master volume control and to put the ciruit in a Traynor Guitar Mate (18 watts with 2 EL84s). I'll bet even this will be too loud.