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EL34 to 6550, or 6L6GC?


 
12/10/2005 3:06 AM
Frank Russo
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EL34 to 6550, or 6L6GC?
Hi there,  
 
I'm fixing up what remains of a Dean Markley "Signature Series 60" for my 14 year old brother whom is beginning to outgorw his "Peavey" phase. The amp currently uses EL34's and sounds a bit like a JCM-800. The current output tubes are beat and I am going to replace them. The amp lacks bass response, and I would like to beef it up a bit.  
 
Throwing in a couple 6L6GC's shouldn't be a problem. There is a nice little bias pot and a 100 ohm resistor from Cat to Ground. I was wondering if the transformer had enough juice to handle 6550's (hopefully, someone is familiar with this amp)?  
 
I have two matched 6550's (GE 6550A) on a shelf. Is there any harm in throwing them in, setting the bias to 45ma, and giving it a test drive (pausing to check the transformer temp)? Should I just 'play it safe' and pick up some 6L6GC's? Is there a better/easier way of telling whether an unmarked transformer has what it takes?  
 
Thank you for your time,  
Frank Russo
 
12/10/2005 8:59 AM
steve
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Frank,  
 
Just as a heads up, I owned a Dean Markley Signature 60 amp, and without a doubt it was the worst, most unreliable amp I've ever owned.  
 
When I got mine, I noticed *both* the PT and OT had been replaced. The bias supply promptly died, taking out the power tubes. I had that fixed, and then some SS parts in the preamp died. I ended up selling the carcass as scap/parts.  
 
I'd be really hesitant to put decent tubes in that amp for fear they would be toasted.  
 
I really liked the sound of that amp, but I've heard from others that those amps are poorly constructed and just plain unreliable.  
 
FWIW,  
 
steve
 
 
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12/10/2005 12:44 PM
Steve Dallman
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Check the heater voltage with EL34's then put in the 6550's. If the voltage holds, you should be fine, but if it drops significantly, you'd need an auxillary heater transformer.  
 
I just worked on a solid state combo and it was a strange mix of good parts, and cheesy connectors from one board to the other. It must have been stored in a damp basement because all the screws were rusty, the cabinet was covered with mildew and it smelled like an abandoned old cottage in northern Wisconsin.
 
12/10/2005 1:40 PM
Frank Russo
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Steve D,  
 
Dumb Question (I guess)....  
 
You want me to measure the heater voltage with a Dummy speaker load and no input signal, or should I measure/watch while I'm actually putting a signal through the amp? If the latter, what would an acceptable variance in voltage be?  
 
Other Steve,  
 
Yup, The amp does look like a factory built POS. Some of the parts look as if they have been replaced on the power/cap PCB. Fortunately, it is a good sounding amp. I'm hoping the change of tubes will give it a JMP50 sound, rather than the JCM800 sound it currently has. At least it beats the TnT-100 that it's replacing.  
 
Thanx Again,  
Frank Russo
 
12/11/2005 4:09 PM
Steve Dallman
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You can just measure the heater supply while idling. Chances are if it can handle EL34's, it will handle 6550's. 6L6's draw less current so they'd be OK without checking.  
 
Kicking up the B+ filtering will usually increase headroom.
 
12/14/2005 1:59 AM
Frank Russo
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Done, and Done. Amp plays well. Bias'd at 44mv, heaters read 6.3v. With the EL34's it was 6.35v.  
 
Funny thing about this amp. There is a little pot on a PCB that looks like a bias pot. It's not. It's nothing... By that, I mean "it is connected to absofrickinglutely nothing". Odd.  
 
Last question. It appears pin a 1 and 8 are jumpered (Suppresor grid on the EL34 to Cathode?). On a 6550, where does pin 1 'go'? I distinctly remember an experience with a 6L6 where pin 1 was connected to the metal shell at the base of the tube (an experience that I won't soon forget). Since the 6550's I have are plastic based, I guess it doesn't matter (untill someone replaces them).  
 
Thanx Again,  
Frank
 
12/15/2005 9:09 AM
Steve Dallman
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You just have to make sure pin 1 isn't carrying the bias voltage. If the sockets aren't rewired from standard Fender 6L6 wiring, where pin 1 is just holding the grid resistor (6L6's don't use pin 1) you'll get the bias voltage on the metal bases. 1 & 8 wired together for 6550's and EL34's. There are other ways of wiring the suppressor grids but 1 to ground is most common.
 
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