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Champ 5C1 with EL34 Vibrato Problem??


 
6/10/2005 10:13 PM
Spiderdog
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Champ 5C1 with EL34 Vibrato Problem??
I recently built a 5C1 Champ clone with a 5Y3GT, 6SJ7 but used an EL34 in place of the 6V6GT. I scavenged a power transformer that fed a 6973 P-P (plus 5Y3GT & two 12AX7 tubes) amp and seems very beefy. The amp sound very loud and clean upto about 1/2 volume and then grinds and fries (in a good way up to 9-1/2). When I peg the volume the last little tick to full (10) the amp starts cutting in and out like a vibrato but there is no vibrato circuit.  
 
Could I being overloading the demand on the 5Y3 to supply the EL34 at max volume? Should I change to a 6L6 or 5881 and rewire the OT to 5,000 ohm?  
 
I am using three 20 mf filter caps in place of the 8 mf caps called for in the original Fender schematic. The OT is a Hammond 125ESE set at 2,500 ohms into an 8 ohm speaker jack. The rest of the circuit except for the 500 ohm cathode resistor (for a 6V6) is exactly like the schematic. I used a 330 ohm resistor for the EL34 cathode.  
 
Thanks in advance to all the helpful people at this site! I still have a lot to learn.
 
6/11/2005 9:34 AM
Chuck
Nothing wrong with the primary impedance.  
 
Have you measured the plate voltage while the problem is happening? Be careful while doing this. Do not touch anything with your bare hands.  
 
It sounds like you might have a phase shift oscillation. You can try moving the input jack lead and circuitry as far from everything else as possible. You also might try a 10uf filter for the the B+ on the 6SJ7.  
 
Chuck
 
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