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Layout & Oscillation

9/24/1997 1:02 PM
Craig	Layout & Oscillation So I've fixed a few amps now, done a few mods, and figured it was time to try building an amp from the ground up. I decided to jump right into the deep end, so I'd learn more. So, I decided on combining a 5D4 Super power amp with a Mesa-Boogie Mk IV preamp. In the process, I've come up with some questions: 1) With the power transformer feeding a 5U4G 675v AC, what DC voltage should I expect the rectifier to produce? Right now, I'm seeing 260v DC on pins 4 of the 6L6's. I'm looking to get that up around 375-450v. I have another transformer rated at 800v AC - what kind of DC rating will the same rectifier produce? 2) In trying to avoid oscillation, I know I need to be keep the input lines either well-shielded, or far enough away from the coupling caps (or both). Are there any other trouble areas in the signal chain that I should be aware of? I've studied Fender layouts, and mine doesn't seem that different, but maybe I'm not understanding what to look for... ...and what about Master Vol circuits? Any specific layout concerns with Vol controls? 3) WIRE: Which guages? Which guages where? Where to typically use shielded wire? What about the AC handling wire? 4) What is a good source that details the (practical) range for cathode and plate resistane/capacitances for tubes? I have a Sylvania tube reference, but it mostly only lists max. voltages, (which Fender and others regularly exceeded), and recommended (typical use) values. Is there a real world, comprehensive guide that lists these ranges? I can deduce in a general way by looking at a lot of schematics of popular amps, but I notice enough variance that I'm wanting to know what's possible, and why. 5) I may ditch the MB preamp in favor of a fender design - it's pretty complicated, (for instance, I left out the switchable lead channel). The preamp has 4 gain stages - I may find that this is just plain too much gain, but then again MB pulled it off somehow (with 4 EL34's). Does anyone see any obvious inherent problems with this hybrid plan? Basically, I reproduced the 5D4 power circuit exactly, with the MB circuit taking over at pin 2 of the inverter (which is where the master vol is). Any thoughts? Thanks! Craig

9/24/1997 1:02 PM

Craig

Layout & Oscillation

So I've fixed a few amps now, done a few mods, and figured it was time to try building an amp from the ground up. I decided to jump right into the deep end, so I'd learn more. So, I decided on combining a 5D4 Super power amp with a Mesa-Boogie Mk IV preamp. In the process, I've come up with some questions:

1) With the power transformer feeding a 5U4G 675v AC, what DC voltage should I expect the rectifier to produce? Right now, I'm seeing 260v DC on pins 4 of the 6L6's. I'm looking to get that up around 375-450v. I have another transformer rated at 800v AC - what kind of DC rating will the same rectifier produce?

2) In trying to avoid oscillation, I know I need to be keep the input lines either well-shielded, or far enough away from the coupling caps (or both). Are there any other trouble areas in the signal chain that I should be aware of? I've studied Fender layouts, and mine doesn't seem that different, but maybe I'm not understanding what to look for...

...and what about Master Vol circuits? Any specific layout concerns with Vol controls?

3) WIRE: Which guages? Which guages where? Where to typically use shielded wire? What about the AC handling wire?

4) What is a good source that details the (practical) range for cathode and plate resistane/capacitances for tubes? I have a Sylvania tube reference, but it mostly only lists max. voltages, (which Fender and others regularly exceeded), and recommended (typical use) values. Is there a real world, comprehensive guide that lists these ranges? I can deduce in a general way by looking at a lot of schematics of popular amps, but I notice enough variance that I'm wanting to know what's possible, and why.

5) I may ditch the MB preamp in favor of a fender design - it's pretty complicated, (for instance, I left out the switchable lead channel). The preamp has 4 gain stages - I may find that this is just plain too much gain, but then again MB pulled it off somehow (with 4 EL34's). Does anyone see any obvious inherent problems with this hybrid plan? Basically, I reproduced the 5D4 power circuit exactly, with the MB circuit taking over at pin 2 of the inverter (which is where the master vol is). Any thoughts?

Thanks!
Craig

Replies:

Craig ...I almost forgot... -- 9/24/1997 2:40 PM