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volume drop

5/14/2003 11:49 AM
volume drop
Finished my Mission 5e3 yesterday, and after a few false starts, Bruce got me going. One quirk, though. When plugging into either channel, high or low, sweeping the unused channel's volume control up to 11 or 12 causes a volume drop. Matters not where the used channel's volume is set..the unsed channel volume control makes the amp volume drop when it gets to 11 or 12. Any help is appreciated  
Thanks again. Bruce!!!  
5/14/2003 1:26 PM
One quirk.  
But a normal quirk.  
Someone else can probably give a scientific explanation, but it is basically an interaction between the 2 volume pots.  
You'll also notice that the sound cleans up at the same time (less gain).  
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5/14/2003 4:09 PM
Bruce /Mission Amps
Glossed over description...
What Ed said.  
First of all, the two triodes of the 12AY7 share a common cathode resistor and bypass cap plus the two volume controls on their respective outputs are in parallel.  
That means the cathodes and the plates of the two channels are in common in a strange way since the two triode's plate loads are connected to the same filter cap, they share the effects of the same AC and DC cathode current, the B+ node, etc., ... and thus, a very low AC (zed) path to ground once past the plate loads.  
With one unused channel's volume pot moved around, it exposes the other 12AY7 triode's output signal, and the grid of the following 12AX7 triode, to a slight different set of impedances!  
Besides the actual parallel zed of the unused triode, that new impedance offers a different audio path to ground through the plate load resistor of the unused triode and the last filter cap.  
Look at the schematic and imagine what the 12AX7 grid is doing when one the unused volume pot has it's wiper (with the big .1uF coupling cap) set to 12, 6 and then to 0.  
Imagine that the actual plate of the unused triode is not there but it's 100K plate load is, and any audio (or AC), is grounded through the 16uF filter cap.  
Now move that .1uF cap connected to the wiper up towards the max volume position of the unused channel's volume pot and you should notice it's right at the grid of the 12AX7 now, forming an AC voltage divider that is slightly frequency selective.  
This is a form of a weird tone control and also alters the absolute max signal strength to the 12AX7 too.  
Split the cathodes of the 12AY7 with two seperate biasing resistors and bypass caps and it's a whole new ballgame with no where near as much fun!  
5/14/2003 6:19 PM

Thanks for the explanation. I think I actually understand what's going on!  
Your help and service-after-the-sale goes way beyond what is reasonable to expect. It is a rarity in today's world, and I salute you. Thanks again for all your help and patience. My amp sounds WONDERFUL.  
5/14/2003 6:38 PM
Bruce /Mission Amps

Oh no problem Tony.  
This home brewing amp thing is worthless if it's too hard, doesn't sound good or no fun!  
5/14/2003 6:57 PM

Wow, being a little over-caffeinated, I couldn't quite follow all of Bruce's excellent explanation, but I'll read through it til I do.  
I still have the Guitar Player issue from several years ago that has a long article on Neil Young's trusty old '59 Deluxe he bought for $50 back in '67 or so. It explains the way he uses a gizmo called the "Whizzer" that sits on top of the Deluxe and basically turns those two volume pots around to get the few basic tones he uses. That is the generation of his sound.  
that's why I'm always surprised when people want to take one of the great all-time circuits and tweak it. the two volumes and one tone pot allow a very wide range of usable tone and gain structure!.. although, as I've stated here before, they don't always react in the way people expect from playing other amps
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