Thank you. We aren't the first to have struggled with the voltages. I'm okay though with the idea that there's more than one way to skin a cat here. I think it was Charlie Wilson who said that the original ran hot and knowing how these things can cook eggs, I take that to mean both literally and figuratively. I don't particularly advocate for everything Rob Robinette says about the 5E3 though he has done a great job of collecting a vast amount of research and development on the circuit and it may be an outlier, but in the first schematic (circa. '57) on this page https://robrobinette.com/How_The_5E3_Deluxe_Works.htm he writes the power tubes dissipated 18W. Current production hand-wired Deluxes according to the data, should test at 12.5W with hardly any changes in the circuit in the intervening years. I got something like 18W as well but the B+ voltages were something like 30V higher with a Hammond 290BEX (330-0-330 @ 138mA) and though I didn't like the tone so much at that level, the tubes didn't show any sign of red plating. I also have screen 470 Ohm screen resistors which neither the original 5E3 or the TD had and I think you can "probably" get away with 18W dissipation if you spend most of your time in the southern end of the dial and are willing to replace power tubes more frequently. Josip (bepone) used a 300 Ohm bias resistor in his El Kabongo as well.cowboyblues wrote: ↑Fri Apr 12, 2024 5:49 pm Amazing job documenting this iconic yet mystic amp. I built a similar one last year and really struggled with voltages as you have. I started with a Modulus clone of the Triad OT and PT. The amp sounded good but had about 18 watts on the JJ6V6S. Also, noticed that regardless of how good my heater line dress was there was low hum at idle that drove me nuts. I decided to swap in a Hammond 290BEX PT and a 1750E OT. Unfortunately, the voltages went even higher with 19watts on the 6V6S. I swapped back to the Modulus transformers. The Modulus Triad clone is running 325-0-325 @ 120ma. I also changed the cathode resistor from 250 to 300 ohms. This put me spot on 14watts for the 6v6S. As for the hum, I finally gave in and installed a 250ohm humdinger pot to equalize the heater voltage while elevated. My experience was this is a very cool amp that has awesome gain and more headroom than the standard 5e3. But definitely a headscratcher to build to get voltages in spec and quiet at idle.
The "dilemma" we have is that reducing this one resistor reduces the B+ voltages but it increases the dissipation of the tube. I think it's okay to get in the ballpark and not worry too much about datasheet values. On the one hand you want higher B+ voltages because they are what are giving you the headroom, too much though and the tone starts to (for lack of a better word) fossilize. In my latest iteration of the build I have aimed for a match between mine and Dumble's voltage on the first stage plates, that seems to be a happy balance between headroom and power. The PT is back to the Hammond 290BBEX (355-0-355 @ 115 mA). I'm using a 5Y3, and a 270 Ohm bias resistor which puts me spot on with a dissipation of just under 14 Watts (rms). Trying these two transformers in the circuit, it's now clear to me that while the voltage rating matters it is actually the power rating that has a more profound effect on the secondary B+ voltages. Even though the 290BBEX is rated for a higher voltage its lower mA rating results in a lower B+ voltage. The best explanation I have for this is that the lower power rated BBEX cannot charge the filter caps as fast as the lower voltage but higher power rated 290BEX. And it can't charge the filters as fast as the circuit drains them. The power rating throttles the capacity of the transformer to charge the caps to their full capacity and so the voltages are the product of a leaner charge in the filters. It would make sag a little more obvious as well but Dumble deals with that in some of his mods to the circuit. If you are in the market for a transformer then I would suggest looking for one around 355-0-355 and 90 mA to 100 mA on the secondaries. It will run hotter literally, but it promises to get you more in line with the voltages Dumble's TD produced.
Regarding the sound floor I'm lucky to have a dead quiet amp. Unlike the last time I built it, this time I followed Dumble's practice of soldering the short length grounds directly to the chassis though I didn't bother with the brass plate which sounds like more trouble than its worth if I am being honest. There is a guy on YT who seems to be making it his mission to demystify the 5E3 and this is one of his videos I found interesting.
As for the 50 Hz / 60 Hz hum, these days, I only use solid core heater wires and in this amp they are tucked solidly into the lip of the chassis, away from all the signal wires. Solid core throughout really, paying attention to good lead dress. Unless it's really humming though I wouldn't worry about it, you probably don't notice it when you start to play. Using a humdinger pot is a way to get perfectly balanced heater voltages, I tried it once and liked it, for about 10 minutes which is all it lasted before fusing in a puff of smoke. I fuse my heater supply separately in this amp, so a pair of 100 Ohm 1W or higher Wattage are absolutely fine in that position though I'm currently using 1/2W resistors in there and no problems. I've never used a heater CT tap and though there is probably nothing intrinsically wrong with them, with the exception of the humdinger experiment, I've always used artificial CTs with no problems.
