By popular demand, and because....

[tedmich]

were there clips? I missed the clips!

Important because while many a schem looks good, one man's "sounds awesome" is often another's "blech!"

[Chuck H]

were there clips? I missed the clips!

Important because while many a schem looks good, one man's "sounds awesome" is often another's "blech!"

A little off topic, but it illustrates your point... I like the guitar solo in Sweet Home Alabama. I like how Ronnie utilizes the unusual nature of the tone to great effect. But if "my" amp sounded like that I'd think it was broken.

[daz]

A little off topic, but it illustrates your point... I like the guitar solo in Sweet Home Alabama. I like how Ronnie utilizes the unusual nature of the tone to great effect. But if "my" amp sounded like that I'd think it was broken.

Clips are worthless because they don't give you a clue how the amp feels, only the tone and even then only a ballpark idea of it. Then you gotta capture on mic what you hear and have it translate when played over a PC. It's ridiculous to even compare the result to what the player hears and feels in the room with the amp and his hands on the guitar. You see people on guitar forums all the time posting clips on forums and asking what guitar is on a track and people who are anal about the difference in saddle material can't decide if it's a LP or strat ! As to tone, If you ever played a jcm 800 it's right there in that same ballpark but more gain than the master model and very similar to the channel switcher. (yes, i know it uses diode clipping)

[Chuck H]

It's not bad if a tube amp sounds a little like amp with diode clipping! Some of the best tones ever used are a dirt box in front of a tube amp with both clipping. In fact, I try to get something closer to the harder (more symmetrical and flat) clipping that is more typical of silicon than tubes. Of course I never get all the way there so the effect is not dissimilar to what you describe. Two words that have been used repeatedly to describe one of my designs are "crunchy" and "saturated". Not always in the same sentence. Crunchy AND saturated is what I was aiming for. So I guess it worked out.

EDIT: Incidentally, on that amp the cascade circuit from V1b to V2a is a .022u cap, a 220k/100k voltage divider and a 68k grid stop. So, you see, I have no loyalties to one ideology. I do whatever sounds best for a given design.

[daz]

It's not bad if a tube amp sounds a little like amp with diode clipping! Some of the best tones ever used are a dirt box in front of a tube amp with both clipping. In fact, I try to get something closer to the harder (more symmetrical and flat) clipping that is more typical of silicon than tubes. Of course I never get all the way there so the effect is not dissimilar to what you describe. Two words that have been used repeatedly to describe one of my designs are "crunchy" and "saturated". Not always in the same sentence. Crunchy AND saturated is what I was aiming for. So I guess it worked out.

I'm not saying theres anything wrong with it at all.When done well it can be great and i had a few of those 800;s with diode clipping and a silver jubilee which also has it. The thing with the channel switching jcm800's that i thought was a great is that they not only made the diode clipping sound very good, but at stage volumes the output would compress some and a feedback loop was easily established and you needed to turn the gain lower and use less diode clipping and got a good balance between the two. I think they nailed the perfect blend and when playing at low volume you could use more SS clipping to get enough gain and it still sounded quite good. But they changed those circuits a few times and not all of those amps were great. I had one early one that was so good i still kick myself now and then for selling it.

I think i know what you mean by crunchy and saturated. More or less great clarity/articulation and more dynamics than usually available at a given level of OD. Thats what i strive for anyways.

[SoulFetish]

Despite the Trainwreck thing, to my ears, a (next stage eg grid leak / vol pot) load of much less than 100k on a regular CC 12AX7 stage tends to make the tone thin and constricted.
I like to replace the trem pot with a 100k type, it just seems to sound better, even when the gain difference is compensated for.
I've not tinkered much with Trainwrecks, so not tried increasing that 3rd stage grid leak yet.
Maybe someone could try an AB test on one, keeping that corner frequency and gain compensated both ways?

Trainwrecks third stage is really interesting in that, depending on how you set up the frequency/gain structure of the stage(ie. Grid leak resistor value vs. coupling capacitor value), it can make significant differences on how hard the phase inverter and ouput stage gets pushed as well as overall base response of the amplifier. I incorporate this in the amp im building right now actually. Lets say you settle on a coupling cap value of .002 and a minimum grid lesk of 39k. By adding a 100k potentiometer in series with the grid leak, it goes a long way in finding one's sweet spot in touch dynamics, bass response, and overall volume at the output. In my experience this is really usefull when playing through different speaker setups and can make a huge difference. But it's a good job for a RV4 pot with a locking bushing so you can set it and keep it fairly consistant.

One thing to note is, I suspect the unbypassed 10K bias resistor of the following stage also play a major part in why this coupling circuit functions the way it does, but I'll know more when i can put it under a scope.

[daz]

Rather than start another thread, and since the schematic is posted here, i have a question. My tone stack is typical marshall just like the example in duncans tone stack tool and the bass control is 1M audio. What i'm wondering is why the bass control's range from low to max bass is from full counterclockwise to about 1/4 of the way up and then nothing else for the last 3/4 of the travel. I measured the pot and it's very high, about 1.3M. Could that be it?

[loudthud]

I've seen the same thing in my amp. I've thought about changing the way the Mid pot sort of encroaches on the Bass pot's setting by shorting the top portion of the pot, the disadvantage being no sound when all the pots are all the way down. A 250K linear pot might be a better choice for the Bass pot.

[daz]

A 250K linear pot might be a better choice for the Bass pot.

Thats exactly what i was thinking after trying 250k in the duncan TS sim. Guess i'll give it a try.

[Chuck H]

It's true enough that you only gain about .5dB as the pot advances from 500k to 1M. Taper not withstanding. So there's no advantage to using a 1M pot for this circuit. Further... The Alpha 1M pots (the ones we all use except...NOTE*) have a 15% log taper. Soooo use a 500k Bourne's "guitar" pot (NOTE***). They have a 10% taper. The combination of increasing travel over the effective resistance and the smaller taper should make the bass knob setting more aesthetic. I deal with this sort of thing every time I build a custom design. I have to fuss over it because they're not usually for me, so the knob settings need to seem "normal".

If it still seems cumbersome after these changes then you can go ahead and try the "Fender" type wiring with the mid pot lugs shorted (the one I use, incidentally). The truth is that shorting the mid pot lugs won't make much difference because if the mid pot is set at, say, 10k then that's only 10k difference to a pot with a scale of 1M (or 500k if you go that way). Mouse nuts.

* CTS pots typically have a 25%log taper. That's much worse for this circuit.

*** There are a couple of models available through mouser and they have different "torque" specifications. You want the one with the higher torque spec. The lighter torque spec. is a really light touch so you can rotate the knob with your pinky while playing. Not ideal for the front panel of an amp.

[daz]

It's true enough that you only gain about .5dB as the pot advances from 500k to 1M. Taper not withstanding. So there's no advantage to using a 1M pot for this circuit. Further... The Alpha 1M pots (the ones we all use except...NOTE*) have a 15% log taper. Soooo use a 500k Bourne's "guitar" pot (NOTE***). They have a 10% taper. The combination of increasing travel over the effective resistance and the smaller taper should make the bass knob setting more aesthetic. I deal with this sort of thing every time I build a custom design. I have to fuss over it because they're not usually for me, so the knob settings need to seem "normal".

If it still seems cumbersome after these changes then you can go ahead and try the "Fender" type wiring with the mid pot lugs shorted (the one I use, incidentally). The truth is that shorting the mid pot lugs won't make much difference because if the mid pot is set at, say, 10k then that's only 10k difference to a pot with a scale of 1M (or 500k if you go that way). Mouse nuts.

* CTS pots typically have a 25%log taper. That's much worse for this circuit.

*** There are a couple of models available through mouser and they have different "torque" specifications. You want the one with the higher torque spec. The lighter torque spec. is a really light touch so you can rotate the knob with your pinky while playing. Not ideal for the front panel of an amp.

I've tried shorting the mid lugs several times on other amps and on this one and to me the tone is different and not in a good way. Anyways, can i determine a 10% pot with a meter? Would it be 10% one one side with the pot set in the middle ? I ask because i have a bunch of 500k's already.

[loudthud]

I usually have to install a knob on the shaft and carefully find the center of rotation. Then measure each side of the pot. On some pots the "break" (the point where the resistance starts to change more rapidly) is not right in the center.

[nosaj]

Wow, thanks ! Because i go strictly by ear and don't understand much of what you described. I just try use what i DO know as far as tonal results that certain components do in particular spots to try to visualize what is needed to change a given aspect to what i want. Much of it doesn't work but after stupid amounts of trial and error i usually find it.

Hey it worked for Edison.. lots of trial and error off others designs sometimes.

nosaj

[Richard]

I find the old Fender schematics easier to read than the new ones.

[GainFreak]

Anyways, can i determine a 10% pot with a meter? Would it be 10% one one side with the pot set in the middle ?

Yes. Set it in the middle and measure. One half should give you the taper percentage.

By the way what's all that fuzz about this amp? I don't see anything exotic or unusual.