A nice solid short bottle 6L6 that looks more like a 6L6WGC but has 6L6GC performance ratings. The plate structure is very similar to a Svetlana with the extra cooling wings and the bottles are very similar to the Tung-Sol 5881. One could be led to believe that the parent company used bits and pieces from current production and came up with the Tung-Sol 6L6GC-STR. This is partly true but inaccurate. These are a very rugged tubes with a unique, “dusty-black” plate coating. The sound is closer to a 5881 than a 6L6GC but they have a higher output rating. Biasing in my sample tubes was very particular. If you bias them hot they lose some definition and take on that tweed amp tone. With these tube hot could be anything above the 70% mark. During the test I found that idle settings between 15 watts and 20 watts (50-60%) provided a clean, warm sound that would overdrive nicely. A lot of techs go for the 70% mark, or about 21 watts at idle. The Tung Sol 6L6GC-STR starts to change character at this point. The result is not unpleasing and they are rated for more output than a 5881 or 6L6WGC so by all means experiment. This is one of the few tubes I’ve tested where I would start at 18 watts idle (60%) and then slightly adjust up and down to find the spot your ears like best. Check again with your test equipment to make sure you’re in spec and you should be good to go.
Hello, I want to install these in an ab763 circuit with two 6l6, how does 18 watt translate, 60% bias translate to ma measured when i bias them? I now have the bias set at 38 ma which i assume is too hot. Want to go for a more typical fender blackface clean. Best, anne
Anne,
To calculate the ideal bias range for any amplifier a tech will first measure the amp’s plate voltage. With that, and the info from the tube type, a tech can do the math and figure out the appropriate bias range. A formula example is this: (0.7 * 30/450) = 47mA. The “0.7” represents 70% of the “30” watts max rating, in an amp with “450V” plate voltage. The current setting would be 47mA per tube (or 94mA combined measurement for a pair). If you want to go with a classic Fender “clean” setting it would be at the 50% range, which works out to about 33mA per tube. Many techs would aim for 60mA for a pair or within a few mA of that is close enough.
If you don’t intend to measure plate voltage you can estimate it and cross your fingers. If you’re sticking to the cooler bias like 50% anyway, you don’t have risk of overheating the tubes and aiming for 60mA/pair is going to be fine.