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HELP with odd observation...

12/17/2004 12:58 AM
Bruce /Mission Amps
HELP with odd observation...
So I'm getting ready to leave this evening and I have a SE EL34 amp on the bench in preamp checking test mode at 330vdc (plate to cathode), idling at +64ma with a 15 watt 4K SE OT.  
Volume control is set so the amp is delivering a whopping 5 watts output into a dummy load and one channel of my dual trace O'Scope is hooked across the dummy load so I can view the output the other channel is on the output of the first stage.  
Signal gen input is 100mvac at 150Hz.  
Nothing odd here.. no fuzzy oscillations on the scope CR tube.  
I play with the preamp for an hour... couple tweaks here and there... clean as a pin when I want it, flattish tops when dimed.  
Set the preamp stages back to clean clean.... and now it's time to go home....  
To drain out the power supply, I just flip the power switch off on the amp while it's amplifying... but I'm still watching the O'Scope and as the B+ drains away in just a few seconds, the amp puts out MORE power into the dummy load momentarily as the B+ drops and then it just dies off.  
The scope pattern stays completely clean (sine wave like) during this increase in power.  
This is repeatable over and over again.  
I hook it up to a speaker and sure enough, it gets louder when this happens too... plus, it's real clean audio power!  
So, I set my peak reading vac meter to the speaker leads and after doing the math, as the power supply unwinds, the amp momentarily puts out close to 11 watts... CLEAN!!! Twice the output power level!  
Was getting too tired to think about it anymore and I went home.  
Somebody figure this out for me so I can sleep.  
12/17/2004 1:31 AM
Chris @ CMW amps

Hi Bruce  
Just checking Ampage while opening my shop.... ( where's my coffee? :D )  
How's the G2 circuit? The only thing I can think of right now is that the G2 or pre voltage does change compared to the platevoltage.  
Sleep well!!!  
12/17/2004 6:01 AM

Is the power amp cathode biased? If not, maybe it's the fixed bias going down faster than B+? I've noticed that some Marshalls like to do something similar to this.
12/17/2004 9:39 AM
Bruce /Mission Amps

Yes it is cathode biased.  
This is a very simple amp with two preamp tubes and a single SE El34.  
The tube is sitting around 19-20 watts at idle.  
12/17/2004 6:11 AM
Le Basseur
If it's not A2,then it must be voodoo!:D
12/17/2004 9:37 AM
Bruce /Mission Amps

Drink few for me so you'll be sharp.  
G2 was seperated from the plate node by a 2200 ohm 5w resistor.  
32uF... 2200 ohms... 32uF.  
Now it has a 510 ohm 2w resistor between the two caps and other then an increase in G2 voltage, there is no change in this behaviour.  
I did watch the plate voltage and it drops in a constant degradation when this happens.  
I didn't think to monitor the G2 vdc.  
12/17/2004 5:59 AM

Bruce, sounds I've had couple of times similar kind of happening goin on, like the amp really comes alive immediately when the power is turned off (but stby is still on) and right after turning the power off. Also the volume raised a lot and the overall tone somehow cleaned very much including od sounds. I always thought it was just a bad grounding thing or something like that because I got once same "amp coming alive and loud" effect when just moved a bad soldered wire in front panel pot. Of course what you have there could be completely different animal.
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