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Adam's Champ sounding wierd

7/14/2005 5:15 AM
Jesper Andersen
Adam's Champ sounding wierd
Hi there!  
I've just finished building Adam Alperns microchamp (  
It' sounds fine when played at low volume, but when I crank it, it starts to distort quite ugly and untube'ish. When I strike a loud note or chord the amp kicks in loud, but the volume drops emidiately to very low and distorted. It is as if the amp haven't got the power required to play and gives up. Sometimes I have to turn the amp off to get normal sound again, normaly I just turn the gain down again and everything is fine.  
Could it be my output transformer? I use a Hammond 125BSE connected to a 8 ohm jensen speaker. How many ohms should there approximately be between the two wires at the transformers primary side? I meassure about 400 and thinks this is way to low.  
Does it matter what wire (blue and brown) I connect to V1 and leg 5 of the output tube?  
I read somewhere that the problem could be due to the filament powering and a bad reference to ground. I've wired the filament power exactly as in the Adams diagram, so it should be allright. My powertransformer has a filament C. T., but I don't use it, is that okay? I wonder why the wire coming from the two 100 ohms resistors is not connected directly to ground?  
Questions, questions... I hope someone recognise the problem and is able to help me :)  
7/14/2005 7:31 AM
Wild Bill

Jesper, it sounds like a wiring error.  
I know you've probably re-checked it a hundred times but it's just human nature to keep missing the same mistake. Having a friend check your work would be a good idea. Not meaning any disrespect but the nature of your questions shows that you are just starting to gain experience. No shame in being new - the fact that you want to learn how to build tube amps shows that you are a fine and intelligent person! :)  
The output transformer is probably fine. 400 ohms is the DC resistance of the wire in the primary winding. TThis is NOT the primary impedance of the transformer! Impedance is an AC form of "resistance", consisting of the DC wire resistance combined with the reactance value of the winding. It is many times higher than the DC resistance. You can't measure a primary impedance with a simple multimeter resistance check - the answer is meaningless. You need to put an AC voltage through the transformer, do some measurements and then some math. Do double check which speaker taps you're using to give the desired impedance. I'm guessing pins 1&4 or 5. There should be a table with the transformer that tells you this. The wrong pins could cause trouble by giving a plate impedance value way far off from the proper one.  
In an SE circuit it doesn't really matter which of the brown or blue wires goes to the plate. Standard practice is for the blue one to be the plate lead, however.  
The two 100 ohm resistors could go to ground (by themselves - nothing else connected to their common point!) and things would be fine. Many amps do just that! With your circuit the designer added an extra hum reducing trick. If you put a little positive bias voltage on the filaments it can often reduce the last little bit of AC hum. With a cathode biased output stage the top of the cathode resistor is positive with respect to ground. So the common point of the 100 ohm resistors is run over to the cathode to steal some "free" positive bias. This is not done instead with high plate voltage because we only need 10-40 volts or so. Anything higher on the filaments could injure tubes.  
I realise all I'm doing is explaining what's right and not pointing out what's wrong but as I said at the start, you need a fresh pair of eyes to check your wiring.  
Good Luck!  
---Wild Bill
7/15/2005 3:35 AM
Jesper Andersen

Thanks! I'll check it again. It's strange because there's defenately a point on the gain-pot. When I go beyond that point, the amp starts to oscillate and go crazy. Could it just be bad wiring. I'm not using shielded wire in the signalpath...  
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