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Discussion Starter #1
Bill with regards to the 71 Traynor YGM-3; do you recommend adding screen grid resistors on the power tubes? Also could you explain how these amps are biased. Mucho appreciado. bRian
 

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bRian said:
Bill with regards to the 71 Traynor YGM-3; do you recommend adding screen grid resistors on the power tubes? Also could you explain how these amps are biased. Mucho appreciado. bRian
bRian, EL84/6BQ5 tubes don't normally call for screen resistors. These resistors have a voltage drop as the screen current increases. The idea is that if the voltage drop reduces the screen voltage the tube gain will be throttled back if things are running too hot. With some types of modern-manufactured tubes they offer a bit of protection but the idea can be over-rated. Whatever, screen resistors ALWAYS change the tone! If you do want to add resistors I would use maybe 100 ohm for these tubes, nothing more. If the screen voltage swings too much you can get troubles!

The real killer to the output tubes in this amp is the high plate voltage. The schematic calls for 398 vdc on the plates and this is really pushing it for modern EL84s. The Sovtek EL84M is a very rugged version that can handle the high voltage but I don't know of any other brands. If you have trouble with short tube life you can try the Ms or spend the money on NOS.

These amps were not designed with bias adjustment. They have a fixed voltage from the bias supply with no trimmer pot. Pete Traynor choose a voltage that would always be safe but may not be at the "sweet spot" for any pair of tubes. I think the Viva Analog site might have a schematic for making the bias adjustable. If not, check back and we'll figure something out if you've really gotta have adjustable bias.:food-smiley-004:
 

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Discussion Starter #3
Bill thanks for clearing those points up for me. This is the first EL84 amp I've owned. When I got it I was getting an intermittent sizzle sound so I swapped the original Phillips tubes for GT's but found out the power tube sockets needed replacing. Now the original tubes are back in and I have just a little hum in the amp even with Kinman's. I'm hoping new filter caps will clean that up. As far as the bias goes, for now I'll leave it as it is.
 
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