|2nd November 2014, 11:19 PM||#1|
Join Date: Feb 2007
6146 in the SSE Part II - triode mode
DISCLAIMER: The SSE was not designed for these shenanigans, so this is a your mileage may vary, you accept all the risk of undesired kerblamos, type of thing.
The original thread on use of the 6146 (and 2E26) in the SSE can be found here:
6146 in the Simple SE
That thread addressed use of the 6146 in pentode mode in the SSE. The original RCA 6146 data sheet from 1952 had some limited information and a curve sheet for use of the 6146 in triode service. Maxmum voltage in triode mode was stated to be 400. That limited information seems to be absent from data sheets published subsequent to the original.
In the original thread, the 6146 proved itself to be a very satisfactory performer in pentode mode over many thousands of hours in the SSE. Pentode mode worked well enough that I never felt the need to set it up for triode operation.
Luck smiled on me today, the wife and baby were out of town at our second house, no office or business issues needed immediate attention, and I had time to get out a soldering iron and sniff a little solder. The result is the old warhorse SSE is now set up for triode operation of the 6146, and back in service in my ham shack.
Details of installation of the 6146 in the SSE and pictures of the amp are set forth in the above thread and are not repeated here. Minimal changes were needed to set up for triode operation. The Schade feedback loop used to tame the pentode brilliance was removed. The screen voltage supply board was disconnected from its power transformer taps, but the board and wiring was otherwise left intact. A 1.2 K resistor was connected between the base of the anode connector (plate cap) and pin 3 of the power tube socket to tie Grid 2 to the plate and establish triode onfiguration. A 180k resistor was paralled across the 220K grid resistor to set the grid at 100K. I chose a 5K plate tap on the Hammond 125ESE output transformer for the plate load and selected the 10k tap to maximize cathode feedback. Initially I chose a 470 ohm cathode resistor.
The 6146 will, in fact, work in triode mode, and in the SSE, albeit with some issues.
With the original Rk of 470, and a 5AR4 rectifier tube, these are my recorded values:
B+ at anode cap 406 volts; grid 2 voltage 393, cathode voltage 57, cathode current 119 ma, and 41 watts dissipation.
With a 5931 (5U4) or 5R4 rectifier, results were:
B+ at anode cap 355 volts; grid 2 voltage 343, cathode voltage 49, cathode current 104 ma, and 32 watts dissipation.
These are obviously the same issues that one sees when running the high perveance TV sweep tubes in triode mode, at plate voltages beyond what the screen would normally see - very high power dissipation and high cathode voltages.
Unfortunately, I couldn't explore this setup very far. Although the 6146 seems perfectly happy at 41 watts, my output transformers, rated at 80 ma, were not, and were audibly saturating. The amount of Rk likely needed to get the current down below 80 ma would have pushed the cathode voltage past the rating on my bypass caps, and i didn't feel like changing them to a higher value part, or cleaning up a mess, so reluctantly I pullled the 6146's and grabbed a pair of 2E26.
2E26 seems to have quite a bit different behavior than I expected as a triode and quite a bit different from the 6146, and seems to work quite well in triode configuration in the SSE.
Without making any other changes, other than rectifier tubes to adjust B+ down, I recorded the following:
With a 6087 (5Y3WGTB) rectifier:
B+ at anode cap 347 volts; grid 2 voltage 343, cathode voltage 30, cathode current 64 ma, and 20 watts dissipation.
With a 5W4 rectifier;
B+ at anode cap 331 volts; grid 2 voltage 328, cathode voltage 28, cathode current 59 ma, and 18 watts dissipation.
The 2E26 is quite well behaved in this configuration, and at 20 watts sounds quite good as a triode. It has probably 10 dB more gain in this configuration than 6146. I'm in my ham shack listening to "Live at Leeds" on it as I write this.
Hopefully this will add a little bit of extra data to the use of these tubes in audio service. My gut is that the 6146 needs to be around 275 ish on the plate to work well as a triode, and as luck would have it, I can set the child resistant SSE to right about that point. Hopefully, I will find some time to pursue that in the near future.
|10th November 2014, 11:53 PM||#2|
Join Date: Feb 2007
I had a small bit of time to play with this setup yesterday.
Using the lower voltage secondaries on my power transformer and with a solid state rectifier dropped in the tube socket, I recorded these values with the 6146 tubes:
Rk 470 ohms, B+ at anode cap, 239, Grid 2 226 volts, cathode voltage 28, 59 ma cathode current, 12.5 watts.
Rk 390 ohms, B+ at anode cap, 237, Grid 2 225 volts, cathode voltage 31, 80 ma cathode current, 16.5 watts.
80 ma is the upper limit of my output transformers. I wanted to run them around 275-300 volts, but just didn't have time to pull the SSE PCB and reconfigure the power supply to LC input.
While these work well as pentodes, using them as triodes seems like a solution in search of a problem. With a transformer that can supply a lot of current but only at a modest voltage, and suitable output transformers, they might be worth considering, but even then a 6BQ6 or 6DQ6 will work as well and is much less expensive, if the tubes are not already on hand.
They do look nice, and are appropriate in a radio room, so I'll leave them as is for awhile.
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