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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

9/24/1997 2:40 PM
Craig	...I almost forgot... I almost forgot to mention the main trouble I'm having with this amp; low volume. I was having some pretty severe oscillation problems, but shielding seems to have taken care of most of that. But, I am still having trouble getting the amp to put out volume beyond a whisper. What happens is this: From 0 on both pre and master volumes... - bringing up the preamp all the way, then bringing up the master results in no sound at all - bringing up the master all the way, then bringing up the pre-gain results in low volume, but you hear the signal. As the pre-gain is brought up, the distortion increases, which seems natural enough, but at around 6-7, the volume gets squelched. At the moment, the plate voltages are as follows: 5U4G 675v AC 6L6GC 260v DC (seems pretty low, huh?) 12AX7 170/119v DC (inverter) 12AX7 130/116v DC (2nd preamp) 12AX7 113/70v DC (70v ?) (1st preamp) I verified that the neg. feedback is passed back through the 82K side of the inverter circuit (self-balancing paraphrase inverter). The other question I have is regarding the low DC voltages being produced by the rectifier. I rigged up a tube socket mounting bracket that I salvaged somewhere (original amp had diode rect). What may or may not be a problem is that due to the placement of the socket mounting screws on the bracket, the tube won't seat all the way into the socket - it sits up about 3/64 in. above the bracket surface. Could this be enough of a gap to affect the performance of the tube? The connections appear to be being made, because it's basically doing it's job; but I wonder if it's doing it WELL?

9/24/1997 2:40 PM

Craig

...I almost forgot...

I almost forgot to mention the main trouble I'm having with this amp; low volume. I was having some pretty severe oscillation problems, but shielding seems to have taken care of most of that. But, I am still having trouble getting the amp to put out volume beyond a whisper. What happens is this:

From 0 on both pre and master volumes...

- bringing up the preamp all the way, then bringing up the master results in no sound at all
- bringing up the master all the way, then bringing up the pre-gain results in low volume, but you hear the signal. As the pre-gain is brought up, the distortion increases, which seems natural enough, but at around 6-7, the volume gets squelched.

At the moment, the plate voltages are as follows:

5U4G 675v AC
6L6GC 260v DC (seems pretty low, huh?)
12AX7 170/119v DC (inverter)
12AX7 130/116v DC (2nd preamp)
12AX7 113/70v DC (70v ?) (1st preamp)

I verified that the neg. feedback is passed back through the 82K side of the inverter circuit (self-balancing paraphrase inverter).

The other question I have is regarding the low DC voltages being produced by the rectifier. I rigged up a tube socket mounting bracket that I salvaged somewhere (original amp had diode rect). What may or may not be a problem is that due to the placement of the socket mounting screws on the bracket, the tube won't seat all the way into the socket - it sits up about 3/64 in. above the bracket surface. Could this be enough of a gap to affect the performance of the tube? The connections appear to be being made, because it's basically doing it's job; but I wonder if it's doing it WELL?

9/24/1997 8:12 PM
Gary	You might want to put a scope on the output of the amp and monitor for an ultrasonic oscillation when you get the volume up to 6 or 7 or more. You may or may not need an input signal depending on whether the circuit needs a signal to excite it into oscillation. Also, you may be able to see it on the input to the master volume. I've experienced this sort of problem but never to the severity that all audible sound was gone. If that's what it is you may need to check your layout again--maybe some more shielding (concentrate on the grid circuits). Does the level of the master have any effect on the sound loss(other than when it's all the way down)? Gary

9/24/1997 8:12 PM

Gary

You might want to put a scope on the output of the amp and monitor for an ultrasonic oscillation when you get the volume up to 6 or 7 or more. You may or may not need an input signal depending on whether the circuit needs a signal to excite it into oscillation. Also, you may be able to see it on the input to the master volume. I've experienced this sort of problem but never to the severity that all audible sound was gone. If that's what it is you may need to check your layout again--maybe some more shielding (concentrate on the grid circuits). Does the level of the master have any effect on the sound loss(other than when it's all the way down)?
Gary

9/24/1997 8:43 PM
gary	Those voltages are low WRT the 675 VAC across the 5U4 anodes. I would have expected 370 - 400V on the 1st filter cap. You might want to check voltage at the cathode of the 5U4 with the other tubes removed. See if it rises a lot. Maybe something is shorted to gnd thru a resistor or a bad tube, filter cap? I didn't notice what power xfmr you used. What current is the HV winding rated for? Perhaps you could sub in a couple of 1n4007's in place of the 5U4 to check the integrity of that. I don't recall if you said you tried another rectifier tube. Gary

9/24/1997 8:43 PM

gary

Those voltages are low WRT the 675 VAC across the 5U4 anodes. I would have expected 370 - 400V on the 1st filter cap. You might want to check voltage at the cathode of the 5U4 with the other tubes removed. See if it rises a lot. Maybe something is shorted to gnd thru a resistor or a bad tube, filter cap? I didn't notice what power xfmr you used. What current is the HV winding rated for? Perhaps you could sub in a couple of 1n4007's in place of the 5U4 to check the integrity of that. I don't recall if you said you tried another rectifier tube.
Gary

9/25/1997 2:29 PM
Bruce C	The fact that you get no sound when you start out with preamp vol all the way up tells me that the problem is in the preamp. I bet if you set the preamp vol at 5 (just below the problem area at 6-7) then adjust the master vol, you will get no change in tone, only volume. Try this: run the preamp independantly from the power amp section, feeding it unto another amp off of the master vol control. If you get the same problem, you know it's in the preamp. If it is, try putting a 100k to 470k resistor feeding into the grid of one or more gain stages AFTER the peamp vol control. This solved a farting-out problem I had in a preamp I built. I put a 470k resistor between the preamp vol and the next grid - no more farting and no signal loss (due to the high impedence of the grid). Good luck!

9/25/1997 2:29 PM

Bruce C

The fact that you get no sound when you start out with preamp vol all the way up tells me that the problem is in the preamp. I bet if you set the preamp vol at 5 (just below the problem area at 6-7) then adjust the master vol, you will get no change in tone, only volume.

Try this: run the preamp independantly from the power amp section, feeding it unto another amp off of the master vol control. If you get the same problem, you know it's in the preamp. If it is, try putting a 100k to 470k resistor feeding into the grid of one or more gain stages AFTER the peamp vol control. This solved a farting-out problem I had in a preamp I built. I put a 470k resistor between the preamp vol and the next grid - no more farting and no signal loss (due to the high impedence of the grid).

Good luck!

9/25/1997 5:10 PM
John Greene	Gary has the right idea of where to start, remove all the tubes and then check the voltages. If they are at the normal higher level then the problem is with a tube(s). If the voltage is still low, there is something drawing an excessive amount of current from the B+. A shorted filter cap, a solder blob, etc. If the amp has a filter choke, measure the voltage drop across it. With the amp powered off measure the DC resistance through it. From this you should be able to get a rough idea of how much current is being drawn from the B+. If there is no voltage drop at all (meaning no current draw), the problem would appear to be with the rectifier tube. If the problem is with the tubes, then the place to start is with the 6L6's. 260V is really low and the tubes are either biased way too hot or one of the tubes is bad. The voltage drop down the chain of the preamps doesn't seem to be too far from normal so I would say they are probably OK. Check the bias voltage on the grids of the output tubes. You may find that one of them is shorted to ground although I would suspect it's plate would be glowing if it was. If you solve the problem of the low B+ and you still have low or no volume trouble, you'll need to trace through the signal path. Sometimes you can get aways with just touching the grids of the tubes with a lead from a voltage meter. It will tick or hum if the signal path is good from there to the output. Start at the power tubes and work your way forward. --john

9/25/1997 5:10 PM

John Greene

Gary has the right idea of where to start, remove all the tubes and then check the voltages. If they are at the normal higher level then the problem is with a tube(s). If the voltage is still low, there is something drawing an excessive amount of current from the B+. A shorted filter cap, a solder blob, etc. If the amp has a filter choke, measure the voltage drop across it. With the amp powered off measure the DC resistance through it. From this you should be able to get a rough idea of how much current is being drawn from the B+. If there is no voltage drop at all (meaning no current draw), the problem would appear to be with the rectifier tube.

If the problem is with the tubes, then the place to start is with the 6L6's. 260V is really low and the tubes are either biased way too hot or one of the tubes is bad. The voltage drop down the chain of the preamps doesn't seem to be too far from normal so I would say they are probably OK.

Check the bias voltage on the grids of the output tubes. You may find that one of them is shorted to ground although I would suspect it's plate would be glowing if it was.

If you solve the problem of the low B+ and you still have low or no volume trouble, you'll need to trace through the signal path. Sometimes you can get aways with just touching the grids of the tubes with a lead from a voltage meter. It will tick or hum if the signal path is good from there to the output. Start at the power tubes and work your way forward.

--john

9/25/1997 5:25 PM
John Greene	Just thought of another question...When you measured the AC voltage, was it winding to winding or winding to ground (CT)? I see from the schematic there isn't a filter choke so my previous suggestion won't do much for you. --john

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