Tube Amps / Music Electronics
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|5/21/2000 2:15 PM|
||Master in 5F6A circuit (YBA-1)|
A little while ago I installed a master volume in my YBA-1 with mediocre results. I love the amp's tone but wanted more flexibility to play at lower volumes.
The master I installed is Ken Fischer's #3 mod from Webbers first book. It simply uses a 500K audio-taper pot between pin 5 of both output tubes.
The problem is that at the lowest range, the volume is really muffled and distorted...meaning you can barely hear anything but what you can is very spongy and broken up. If you increase the pot a little the volume jumps WAY up and the tone is great. But, I'm actually unable to run the channel volume at max even with the master at the lowest useable setting. I was expecting whisper quiet operation with the master at lowest setting.
Does this sound familiar to anyone? I think the MV pot is ok. Are there better MV circuits for this amp? FYI, I have yet to check the bias or any other output section components.
Thanks very much!
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|5/21/2000 2:55 PM|
I'm using this circuit on my AX84 November and it works OK but pretty much all the master volume effect is in the first 20% of rotation. There is a definite deterioration of the distortion quality at the very lowest settings. Since my amp only puts out 15 watts or so, I can run it a little higher but it does get loud pretty quick.
I could be wrong (again!) but I thinks it's very difficult to get quality distortion at "whisper quiet" volumes with any master volume circuit. That's one reason people are build 1 watt amplifiers.
|5/21/2000 7:02 PM|
What Lee has to say is true, but many people with 5F6A clones have been pleased with the results of the KF MV that uses the 1M or 500k dual ganged pot and the extra pair of coupling caps. Here is a drawing that Bruce Collins made:
In general, master volume circuits for vintage design amps are best suited for cutting the stage volume a bit (like if you are playing in a smaller club and want to use the same preamp settings). To play at the "whisper quiet" volumes you mentioned there will be some degradation of the signal— but not as much as with the "cross-line" MV you used (which will exaggerate the distortion created by the PI).
|5/22/2000 12:59 PM|
Thanks Lee and Steve!
Originally I was going to try using a dual ganged 500K pot for the MV you describe Steve, but the one I have is from Mouser and seems to be inconsistent, meaning the values are not the same for both pots during the swing.
Hey, I just thought...I could use a small resistor to make up the difference and match the two sides.
Are there better sources for quality dual-ganged audio taper pots? I thought I remembered a recent thread but can't find it in the archives...
|5/22/2000 8:49 PM|
I don't think it matters much that the pot is not quite balanced. Your transformer probably isn't either. For that matter your tubes probably don't have matched gains. How far off are the values? Have you tried the circuit using the Mouse pot? Just curious. I use these and have not had any problems related to their lack of tracking.
|5/24/2000 10:05 PM|
That's the same master I used on my 5E3 Deluxe copy, and it sounds incredible. I used .47uf coupling caps though to reduce fartiness.
|5/25/2000 1:58 AM|
I used .047uf coupling caps though to reduce fartiness.
Good point! A pair of 0.047uF caps in series adds up to ~0.0235uF, which is close to the 0.022uF value that most/many people use these days. The pair of 0.1uF caps that Bruce drew in would add up to 0.05uF...
I believe that TUT suggested that the second cap (the one that goes to the power tube grid) should be 10x the value of the first cap, which might not be that feasible. And when Ken Fischer drew it up he recommended that a 0.1uF cap be added after the stock cap (if you do the math you will see that a 0.1uF cap will not change the effective capacitance with a 0.022uF cap very much at all).
Bruce has been telling us to use smaller coupling caps to reduce the "Fender farts" for many years; for some amps he will use a 500pF silver mica cap for the input to the PI...
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