The real McTube II : how to increase gain?

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Hi everyone,
first off, tube noob here!:eek:

i have constructed this diy guitar tube overdrive http://dogstar.dantimax.dk/tubestuf/graphics/mctube2s.gif, which works really nice. I have enclosed it in a small box, and even added a small power amp (tda2030) to directly drive speaker cabs. It sounds really good as is, it breaks up real nice and the noise level is minimal/nonexistant. With reverb added, it trully sounds amazing! I used a separate transformer for the HV side, which gives me roughly the same voltage as per the diagram (150 volts B+). I used way bigger filtering for B+ (old pc-psu caps of 200v/680μF) and i used a 7812 regulator for the heater.

Only problem is, i don't know how to go about increasing the gain (distortion). Some times you need more...:D but the circuit will only give so much.

One idea is to reduce B+. Another is to maybe increase the value of R4. But i don't really know/understand tube circuits.
Also, i would like to add a little more bite in the trebble region. As is, the circuit sounds a bit dark, like a blanket over the speakers.... how would you go about that? Maybe a small (pF) cap between first anode and second grid?

Any ideas are welcome...
Thanks in advance!
 
Ugh!!!
That preamp looks poorly biased.
They seem to have tried to get the maximum gain possible, no matter what, so they increased plate resistors as much as they could, way outside normal so throwing operating point out of whack, then experimented wild cathode resistor values to sort of correct that.

Problem is, they are operating tubes in so callled "starved mode", the name is self descriptive, and one of the characteristics is heavy loss of treble ... ring a bell? :rolleyes:
In my book, a poor choice to be avoided.

Personally I suggest using the Datasheet suggested values, in this case the General Electric 1953 one, which "should" :rolleyes: know what they are talking about.

They supply a quick use stage gain and bias table, which by the way everybody follows, for guaranteed no Math results.
One such user was Leo Fender :eek: and last time I checked his amps were well liked.
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They suggest 3 operating ranges: 90V (typical of AC DC operated radios and record plyers of the time) , general purpose 180V and maximum performance 300V , typical of large Guitar amps such as Twin and others same class.

You use the 180V values, which are close to your 160V (raw) supply.

So rebuild your amp using 2200 ohm cathode resistors , 220k plate resistors, your load is already 500k (the potentiometer) and that will give you the best results.

Treble should improve to normal, gain will be maximum "good" gain.
Just look at the table, there is clearly a point where going beyond it increases gain very little (as going from 59 to 62 or even 66) but kills everything else.

As of further tweaking:
* you might add a "bright" capacitor, 220pF between wiper and hot lug at the volume pot.
* you may replace both R9 100k and RV2 500k Audio by a single 100k Audio pot.
Gain and everything else will stay the same, but it will drive the output cable (pedal to amplifier) way better.
* You may reduce cathode caps to slightly cut boominess and make distortion tighter. Use .47uF electrolytic caps.
 
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Thanks,
I did just try it, but it did not alter the distortion significantly, it just increased by a tiny amount with a lot of (anticipated) reduction in ouput volume. I went down to even 33k, but not the difference i'm after...

Any other ideas?
You did ask for an increase in gain, but what you actually want is higher distortion... Follow JM's advise, and if you still want more, then you can always try diode/LED clippers between the two stages a la Marshall and others.
 
Only problem is, i don't know how to go about increasing the gain (distortion). .... But i don't really know/understand tube circuits.
To get more gain you need really more stages. Either valve or solid state (FET or op amp). Got an overdrive/booster pedal handy?

I'm going to save myself some typing: Read the two links below before messing with the working-as-the-designer-intended circuit you have.

Rob Robinette has written a very good explanation of how the JCM800 works.

Modern hi-gain amps go much further - the Soldano Super Lead Overdrive runs a 39K cathode resistor in a similar cold clipper stage


Also, i would like to add a little more bite in the trebble region. As is, the circuit sounds a bit dark,

Reduce the cathode bypass capacitors to raise F3min. As described in the links above. See also Grant Will's Lamington SOLO for a practical example where this low-cut is implemented with a switch.
 
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In mid 1980's there was a circuit in Wireless World for a soft limiter.

That's neat. I'm filing that away for later reference. It looks good as a penultimate soft-clip circuit before a conventional high negative feedback power amp. For guitar use it would be interesting to mismatch R4/R9 and/or the two transistors to get some more even harmonics.
 
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Hmmm metal maybe...

lower R4 and R6 to the fender values suggested in a previous post...
Change r8 to 330k, r7 to3.3k, (german roar) and you might want to bypass c8 for now.:redhot:
R8 to 220 and r7 to 2.2 for less distortion (American growl).:devilr:
If it's not hot enough, test it with a boost or eq pedal to push it up higher to determine if you need another gain stage or don't like the quality of the distortion generated by a single tube. You might have to lower the decoupling caps to cut out some low end if you are trying to go from cleanish to metal.
 
Thanks JMFahey for the info. The default fender values definately brought back the missing trebble. Now it's much better! But still the distortion does not reach death metal levels! I think that is as much a single tube will give. For more distortion, more stages should be needed, or a booster in front, as suggested.

nigelwright7557, the circuit you propose seems really interesting. Soft clipping without the hassle of tubes, what more does one wish? But could you please share a more complete schematic of it (with the input buffer and everything)?
 
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