Why would.....

[daz]

...two amps with the same preamp design sound so different? And i mean with the PA out of the equation. In other words, say i take amp #1's preamp out into amp #2's PA and it still sounds like amp #1. A little different of course, but very close to amp #1 with it's own PA. Both amps as i said have the same pre, and both even have the same PSU caps. The dropping resistors are different only because i was trying to get the preamp voltages the same on both amp's preamps. One is a el34 amp and the other a 18 watt converted to 6V6. Could it be that the PT current rating when much higher than needed could cause a sonic difference over a PT who's rating is safely above the requirements of the amp but not by a lot? I'm trying to determine how to get the sound from amp who's pre i prefer to the other amp, but both pres are pretty much the same right now. the voltages are close too.

[Chuck H]

If they sound substantially different then something is substantially different between them. If they sound a little different you could chalk it up to the +/- tolerance of the componants.

Some non obvious things that could effect the tone are:

1)Tubes (try swapping the tubes in either preamp)

2)Voltage sag in the power supply. If one amp is drawing more current (yes, due to resistances in the power supply among other things) relative to the PT rating this can cause a greater voltage drop in the preamp under certain playing conditions. Even if the preamp voltages are similar now, if one amp is saging 20 volts on peaks and the other only 10 this will greatly effect the tone AND feel of either preamp.

3)Layout can effect the tone in the preamp due to small parasitic introductions of positive and negative feedback in various componants. Lead length, location and how every lead and componants radiant field is relating changes with layout. This also applies to how each preamp is oriented relative to the power amp componants.

4)Grounding. Leads of different lengths or grounded in different locations can change how much signal is shared and what other signals they are sharing with along each lead and at ground points. This includes the ground point locations on the chassis. I know it seems like too fractional an amount to matter but consider how many thousand times some of those tiny interactions are amplified by the time they get to the speaker.

5)Componant type. Carbon composition resistors do interesting things in some applications that metal films don't. Polyester caps sound different than polypropylene, etc. Alot of guys will poo this, but my own experience has shown (to me at least) that it's true.

6)Even chassis material is said to have an effect on tone. Steel and aluminum have different resistances. This would be similar to the grounding issue discussed above. Some builders and players say they can tell the difference. I'm still on the fence on this one.

When you put all the variables together it can add up to more difference that you might expect. I've copied a few circuits and they didn't sound like their intended product in the strictest sense. I've also "cloned" a couple of circuits (match layout and componants exactly) and they sounded exactly right. Consider that Fender, Marshall, etc. can build thousands of amps and they all sound pretty much the same. If you want to build an amp that sounds exactly like one of those then you would have to build it exactly like one of those. Change any of the above and you start going in a different direction.

HTH

Chuck

[tubeswell]

...two amps with the same preamp design sound so different? One is a el34 amp and the other a 18 watt converted to 6V6.

OK that's one significant difference. These tubes are designed run on different voltages and to handle differing amounts of current with different gain, transconductance and load resistance. If you run them at the same voltage, you still would have a lot higher current draw in the EL34 than the 6V6 (even if the tubes were running cathode-biased off the same value cathode resistor - because of the different current draw of either tube type), and you would need to run them at different load resistances - both of which affect the transconductance (how much the signal grid-to-cathode voltage varies in relation to how much the plate current varies) Pretty significant impact on sound.

Could it be that the PT current rating when much higher than needed could cause a sonic difference over a PT who's rating is safely above the requirements of the amp but not by a lot?

I'd say less of an impact that what I said above - altho' the ability of the PT to handle the current drawn by the tubes would affect the sagginess when it was getting close to the respective winding's upper design limit (up to the point where the secondaries are so overloaded that they burn through).

[daz]

Thanks guys.

1)Tubes (try swapping the tubes in either preamp)

I did that already. they bopth have 1st gen chinese, but i swapped them anyways just to be sure. No diff.

6)Even chassis material is said to have an effect on tone.

One IS steel and on aluminum, and the layouts are different. No way to test this tho unfortunatly, at least no w/o a whole new build.

OK that's one significant difference. These tubes are designed run on different voltages and to handle differing amounts of current with different gain, transconductance and load resistance. If you run them at the same voltage, you still would have a lot higher current draw in the EL34 than the 6V6 (even if the tubes were running cathode-biased off the same value cathode resistor - because of the different current draw of either tube type), and you would need to run them at different load resistances - both of which affect the transconductance (how much the signal grid-to-cathode voltage varies in relation to how much the plate current varies) Pretty significant impact on sound.

But the PA isn't in the equation like i said. See, i took the preamp signal from the amp who's pre i like and put it into the other amp's PA to see how much of that amps sound that i like was a result of the preamp and how much a result of the PA. Turns out the pre i like sounds relatively identical with the other amp's PA. So the PA is out of the equation except for how much current they pull, because i DO realize that if the PA is pulling a lot it will sag the pre along with it. But that shouldn't matter because i'm talking low volume playing and both PT's have plenty of current for thier circuits. The only thing i could see affecting them is if having a PT with LOT more current than the circuit needs can cause tonal differences. But the other way around i don;'t think should affect them unless the amps were being played loud or one has a slightly inefficient current rating. But then you guys know a heck of a lot more than I, so i may be blowing hot air. If it helps, the amp with the better sounding pre also has a more bright sound as tho the other amp has the proverbial blanket over it. I mean this to a slight degree tho, not a huge difference. But where that comes from i don't know because the preamps are identical in circuit design and component brand/value.

[Chuck H]

Are you using a shielded input lead from the jack to the first grid? If so then are both preamps input leads the same length? Are the shields tied to ground or the first plate?

Are you using any Marshallesque coupling circuits like the 470k resistor jumpered by a 470p cap? Is this done the same in both amps? It can make a difference sometimes even if the different circuit is on the channel your NOT using.

One the macro view many pre amps seem to be the same. Any difference to peripheral circuits can make a big difference though. Case in point: Fender used basically the same preamps in the 6g16 and ab763 versions of the Vibroverb. Each version uses a different reverb coupling and trem circuit. Even through the same cabinet each amp sounds different and has different gain characteristics. Because of the "load" placed on the preamps by the peripheral circuits, even if you plugged one preamp into the other amp it would still sound different. So another question: Is all else equal?

Chuck

[daz]

Are you using a shielded input lead from the jack to the first grid? If so then are both preamps input leads the same length? Are the shields tied to ground or the first plate?

Yes, yes, and to the main ground point.

So another question: Is all else equal?

Not really sure what you mean. But one thing i often wondered about is the PSU caps for the PI stage on forward. I built 3 amp all with the same pre and i think the two i used sprages there sound much more alike than the one where i used F&T. I know people say that doesn't matter, but i still wonder. The other amp that a friend now owns is exactly like my el34 amp except for the layout and those caps. It sounds very different than mine.

[Chuck H]

What I meant was do both amps use the same number of channels, mixing circuits, effects loop (or lack there of), coupling to the power amp, etc. Because the load these circuits put on the preamp can effect the tone. Caps do present an AC path to these peripheral circuits and resistance differences in these circuits effect the load at those frequencies passed by the caps. Even purely resistive circuits change loads on the preamp effecting tone.

And yes, different filters CAN sound different. But it's usually insignificant. I think with a high gain design using really low ESR caps in one amp and really poor caps in another would make an audible difference though because of the coupling in that circuit. Those "filter" caps are (among other things) supposed to send AC in the B+ rail to ground. The series resistance of the cap will cause a small interaction between any signals on it's + side. Like I said, this is usually so small a difference that it can be ignored. If you had enough ESR to cause a real tonal difference I think your amp would also have hum issues.

Chuck

[daz]

Actually both amps are very much the same ecxept the trannys. Same pre, same 32/32uf first can and 22uf node caps. Although the 6v6 amp has a resistor while the el34 amp has a choke. But i've tried a resistor in the el34 amp and it wasn't too good. i'll try it again tho because i have since found the place i was tweaking the amp in gave me much different results than how i'm doing it now, which is at home with music going so i can hear it in a mix. Also a more live room and closer to what most live environments are like