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Friday, September 25, 2015

Tube of the Month : The 76

Hi!

After the presentation of the types 27, 37 and 56 of the early indirectly heated triodes, the logical next candidate for the tube of the month is the 76.





The 76 is basically an equivalent to the 56 only with different heater specs. It needs 6.3V at 0.3A.

All other parameters are identical to those of the 56 which of course includes the same UX5 base and pinout and also the same dimensions. See the RCA data sheet for a complete set of technical data. So unlike the 37 which received slight bumps in amplification factor and transconductance over the 27, the 76 is exactly a 56 with 6,3V heater. If these kind of triodes are appealing to you but the 2,6V of the 27 or 56 are inconvenient, this tube might be the right choice. I have not used it myself yet so cannot share any actual experience. But that might change some time in the future. As always lets start by looking at the plate curves of the tube:




And actual curves from the tracer:





Another shot with different spacing for the grid voltage steps:




As expected these look identical to the curves of the 56. It is also very similar visually:





The tube above is from the Wards brand







RCA 76 still sealed in the box:






Different style RCA box:




Insert with floating tube removed:




The tube:




Another view:





Like the 27, 37 and 56 the 76 has circular grids and plates which are arranged in a concentric manner around the cathode:




The top:




Another RCA 76 with 'meat ball' logo on the base:



Triad 76:



The top:



Plate structure:




76 made by Sylvania:






The top:



The 76 shows a similar glow as the 56 when in operation.




A close up to the heater as it enters the cathode:




This post concludes this mini series about the early indirectly heated triodes. I hope you enjoyed it.

Best regards

Thomas




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