atomic 18

[Andy Le Blanc]

Here's a bias manual for the 50w Atomic pod bay thing.

that Soldano looks neat, a marshal based , lead/master vol. with a small power side.

those couple extra resistors in the signal path can make a difference.

[Snapcase]

that Soldano looks neat, a marshal based , lead/master vol. with a small power side.

Not bad, but too tight, harsh and weak in the low-end. The mods helped with the lo-end somewhat though still stiff and harsh. I'm planning a full modification with ideas borrowed here and there from similar amps to smooth it out.

[Darkbluemurder]

I once had a Soldano Atomic 16 and noticed the exact same things about that amp - little headroom, medium gain at most and harsh/gritty when played loud - my modified Fender Deluxe Reverb II ate it for lunch. I would increase the screen resistors to at least 1k, preferably 2.2k and the grid stopper resistors to the power tubes to at least 10k. Both changes would take out a bit of the harsh edge the amp has.

There are further things that can be done but first we should find out where the harsh sound is coming from.

Cheers Stephan

[Andy Le Blanc]

Marshall has had a fairly well accepted pre/tone

replace the cathode resistors and bypass in the pre..

1K8/25uF with 2K7/.68

2k2 ...... 10k

1K8/1uf ...... 820r

the rest of the gain structure is so close, you can see what he was after.
I think a different master vol might be good to look at, there are choices.

But the real issue is how best to stick a marshall pre on that power tube type.

If your getting the dirt/gain before the master and PI, it might be worth
looking at a lower gain tube, maybe optimize the power side around a
12AT7 or even a 12AU7 PI, or reduce the plate resistors with the 12AX7.
The peak AF grid to grid voltage for EL84/6BQ5 is half that of the 6L6
and 3/4 that of a 6CA7/EL34, in a push pull amp.

Most people design the amp backwards, the big feature here is the high
gain pre, but amps are designed from the load, you choose a tube type
and you have to respect its design needs, including the drive voltage,
before you choose the inverter, not the other way round.

[Snapcase]

Besides the preamp gain I think the major problem is the PI. I'll go the 18Watt (or Vox or Fischer way). Amps designed for EL84 tubes. This amp has a PI designed for power tubes with much lower gain such as 6L6 or EL34, and hits the power tubes too hard too fast. From that point I'll tweak a few things more for better headroom and to tame the harshness.

[Darkbluemurder]

Here are my suggestions for a smoother yet harmonically richer tone:

Preamp:
1. Add a 1000pf cap over the 1Meg after the 0.022uf cap after the first gain stage. To compensate for the increased highs and mids of this change, remove the 470pf bypass cap in parallel to the 1Meg after the 0.022uf cap after the second gain stage.

What this does is to get a better balance of highs and lows early in the signal chain that can be tailored to taste later.

2. Definitely keep the 1.8k/1uf combination at the third gain stage. I tried it without the cap and the sound got hollow and lost richness and harmonics.

PI:
1. Change the tail resistor from 10k to 47k and the bias resistor from 470R to 820R. If you want you can decrease the 1Meg grid loads to 470k each.
2. Do you want to overdrive the output stage really hard? If not, you can increase the PI output coupling caps for more low end.

PA: see my post above.

Good luck
Stephan

[Snapcase]

Thanks Stefan. This sound interesting, but I think I'll go the 18Watt and Badger ways to beef up the tone and reducing some gain in the preamp for a more progressive transition into full distortion and making it more responsive to the guitar volume pot and right hand playing. I won't use the master volume. Maybe I'll install a VVR instead.

[Darkbluemurder]

Thanks Stefan. This sound interesting, but I think I'll go the 18Watt and Badger ways to beef up the tone and reducing some gain in the preamp for a more progressive transition into full distortion and making it more responsive to the guitar volume pot and right hand playing. I won't use the master volume. Maybe I'll install a VVR instead.

Hi,

In that case Andy's suggestions are spot on. That in addition to the PI tweaks from 18watt should get you what you want.

Cheers Stephan

[ampdoc1]

I hate to dredge this up again, but I've got one of those Atomic Reactor amps (for the pods) with some minor damage. One of the 6L6 tubes shorted and took out a screen resistor (47 ohm, 1W) and the R21 (looks like 1W). This resistor is badly burned and open, but it runs from the final 47uF cap (+) to the standby wire going to the standby switch.

Anybody still got one of those to look at that could give me the value here?

Thanks,

ampdoc

[BnATX]

Don't think i can be much help. Mine is the 18watt version and doesn't have a standby switch. My R21 measures 8k2 on two of my meters but the color bands say it is 11k. have no clue on that one. Mine runs between two caps in the power supply and then to two 100k resistors (R12, R13) and to the 12ax7 , uh some kind of way. I am not to great at tracing PCB's as of yet, and this one confuses me.
It has(had) some solid state type stuff in there and i tried to do a schematic and i just got lost.

[ampdoc1]

Bn


Thanks anyway! Your info was as good as I got from the manufacturer. My guess is these were re-engineered a number of times, and no one knows what is in any particular unit. Typical of offshore electronics.


ampdoc1