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unexplainable distortion in DIY Deluxe

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5/4/2000 6:58 PM

unexplainable distortion in DIY Deluxe

Hey,
I have built a Blackface deluxe amp from the schematics on Ampage. I've followed everything except the output tubes are cathode-biased, there is only a single tone control and a lower plate voltage. My problem is this: After 1 on the dial, I get this very Marshally distortion. Its prewtty high gain and is very musical. In fact, I like it a LOT...but, I need a good clean amp. With this in mind, I wanna find out what I did to cause this so that I can do it again and so I can get rid of it in this amp. I'm thinking its the lower playe voltages. The Schemo says that theres over 300VDC on the preamp but I have around 120 on mine. I'm using a 12AX7 in the preamp and a 12AU7 in the PS.

5/4/2000 7:39 PM

quote:
"Hey,
I have built a Blackface deluxe amp from the schematics on Ampage. I've followed everything except the output tubes are cathode-biased, there is only a single tone control and a lower plate voltage. My problem is this: After 1 on the dial, I get this very Marshally distortion. Its prewtty high gain and is very musical. In fact, I like it a LOT...but, I need a good clean amp. With this in mind, I wanna find out what I did to cause this so that I can do it again and so I can get rid of it in this amp. I'm thinking its the lower playe voltages. The Schemo says that theres over 300VDC on the preamp but I have around 120 on mine. I'm using a 12AX7 in the preamp and a 12AU7 in the PS."

Dave, our amp is not really a blackface given your description above, so I wouldn't expect it to sound much like one. But, regardless of that:

You voltage specification is confusing to me: are you referring to the DC voltage AT THE PLATES or at tthe POWER SUPPLY TAP that feeds the plates of the preamp tubes? The DR (I have a '68) have about 180V at the plates themselves, so that means on the order of 300VDC (give or take) at the B+ tap. If your B+ tap is 120V... it means your plates aro probably voltage starved. If, however, you mean your plates are at 120V... that is a little low, but 12AX7s should work just fine in spite of the lower-than-usual voltage.

Maybe a little clarification would help me to understand exactly what it is we're dealing with.

Also, you mentioned the use of a single tone control.... where? If you bypassed the Fender tone stack all together, expect the amp to crunch a lot. Simple enough to try in any blackface Fender... lift the gorund of the middle tone control and you get a Marshall.

5/5/2000 3:05 PM
Dave	Well, I reduced the plate resistors to 47k and that gives me around 120VDC on the plates. The distortiopn has been reduced but its still there and lots of it. I guess I'll try a few more mods and see what happens. Otherwise, I guess I'll find a x-former w/ higher voltage... --dave

5/5/2000 7:10 PM

Dave-
You shouldn't have to bring the plate load resistors in the preamp down to 47K - like Gil said, what are your supply voltages like? Also, changing to a higher voltage trans. probably isn't neccesary either, but what are you seeing at the plates of the 6V6's? Perhaps you have a power supply resistor that's too big somewhere downstream of the power tubes. Check out the voltages at each stage of the supply and get back with some more info.
Tom

5/5/2000 11:36 PM
SpeedRacer	Scope each stage and see where it's happening.. Like Gil noted, swapping a traditional tone stack for a tone knob will boost pre-amp gain. You may just need to lose some (via resistive divider or whatevere).. also, cathode bias is going to have less headroom typically (or feel that way at least). If you need clean, try fixed bias.

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