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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 19th December 2017, 04:42 AM   #11
w5jag is offline w5jag  United States
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What is the Neon lamp doing?

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Old 19th December 2017, 12:38 PM   #12
Tubelab_com is offline Tubelab_com  United States
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The neon lamp does absolutely NOTHING (OK, it's a 1 pF cap)......once the tube heaters are hot.

When the board is first powered up the CCS will pull V1's cathode to the negative rail, which could be several hundred volts. Who knows where the heater windings are biased, but usually at some positive voltage, and V1 could be any one of several possible tubes. The neon bulb clamps the cathode voltage at about -75 volts during warm up so that the HK rating doesn't get seriously violated. If the negative supply is less than 75 volts the neon bulb can be safely omitted.

My usual application of this board develops +150 and -150 volts from a cheap isolation transformer like a Triad N-68X. This works for all output tubes except screen driven sweep tubes and big transmitting tubes, but puts the negative voltage in the blown HK range for many common input tubes.
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Old 19th December 2017, 06:49 PM   #13
mkane77g is offline mkane77g  United States
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Tubelab Universal Driver Board, 2015 version
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Old 19th December 2017, 10:32 PM   #14
woynar is offline woynar  Poland
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I am dreaming about KT88 amp, so just waiting for the possibility of PC board purchase :-)
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Old 20th December 2017, 12:17 AM   #15
mchrismag is offline mchrismag  United States
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I'm assuming there will be a way to build the driver board so that several different output tubes can be used without any changes to the driver board? For example an el34 or kt88? I realize the bias on the output tubes would need to be adjusted but other than that no changes? This is what I'm looking to do with it. 50wpc, triode, no feedback. Thanks. I'm hoping these boards sell very well for you, George! I know I want a pair along with the the power supply board once everything is sorted out.
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Old 20th December 2017, 02:12 AM   #16
Tubelab_com is offline Tubelab_com  United States
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I spent a few hours today tweaking out the board. I made some serious headway, and developed enough confidence in the boards to drop in some tubes that I have been saving for about 30 years.....Lets just say that I bought these for $15 each!

I had been chasing my tail every time I fired these up thinking that there was something wrong. The distortion would rise as the power was increased then drop a bit, then rise again. I dug up one of the boards from the 6L6GC in AB2 thread that uses 6SN7's, and it indeed does the same thing......I dug deeper into my "box of broken dreams" and found the original universal driver board from 2007, but it's pretty dead, so I didn't get to test it.

I separated the two stages, tested and tweaked each individually, then put them back together......Still happens. OK, I got it. You have a triode gain stage, it has a built in nonlinearity due to non constant Mu. You have an identical gain stage, which has an identical nonlinearity being fed by the first stage, but is operated out of phase with the first stage. Part of the nonlinearities will cancel each other. The cancellation will be almost perfect IF the stages are operated under IDENTICAL conditions, but they are not. The second stage sees a higher signal voltage, and runs at a higher voltage and current. This is why the cancellation is best at about 2/3 of maximum voltage swing. It is also why the cancellation is strongest when the same type (identical Sylvania 6CG7's) of tubes are used in each stage, and non existent when a 12AX7 is driving a 12BH7.

I agreed that the first test for this board would be with KT88's in triode, so that's where I went today. I started out with the same 20 year old pair of Chinese KT88's that I always use for experiments where I am not sure of the outcome. If one blows up, no big deal. I bought about 50 of these when I made guitar amps in the 80's. The quality was so bad that one sparked out so violently that the glass shattered. Others would red plate even when operated well within the ratings. I never trusted any of them beyond 35 watts per pair in pentode mode, and most were used in single ended Fender Champ clones. There were however 4 or 5 survivors that have followed me for all of these years without exploding. This pair has cranked out over 100 watts in pentode mode on 525 volts, so I figured that they should deal with a full on attack in triode on 450 volts, and they survived all testing.

The breadboard was tested with the Chinese KT88's in TRIODE mode. The B+ power supply was a Fluke 407D bench supply that I got on Ebay for $45 (non working). I fixed a broken switch with some JB weld epoxy, but other than that, it's all original including the 807's and bumble bee caps. The +/- 150 volt MOS (for the MOSfet PowerDrive) is a simple bridge rectifier and cap connected to a Triad N-68X transformer(seen to the left of the breadboard). An old HP 204C audio oscillator provides the 1KHz at about 0.12%THD, so it sets the lower limit of measurement. Clip lead interconnects and Walmart LED shop lights also raise the noise floor.

I took basic data on the test amp with a 3300 ohm load, and a 6600 ohm load. The same $16 guitar amp quality OPT (I bought 1000 pounds of these in the early 90's) was used for each case, I just changed the output tap. Bias for each output tube was set at 45 mA which is 20 watts idle dissipation (arbitrarily about 60% of where I run KT88's in my SSE amps 80 ma on 430 volts, 35 watts).

The "experimental feedback resistors" in the parts list were used, and worked great. There is a total of 240K on the driver board, and 300 K off board, so each path has a total of 540K working against a plate load resistor of 43K, for about 4 db of local feedback.

The results for a 3300 ohm OPT:

power THD B+ current output stage efficiency

0.1 W 0.14% 90 mA 0.02 %
0.5 W 0.19% 95 mA 1.1 %
1.0 W 0.25% 100 mA 2.2 %
2.0 W 0.34% 102 mA 4.4 %
5.0 W 0.58% 115 mA 9.7 %
10 W 0.99 % 140 mA 15.9 %
20 W 1.66 % 175 mA 25.4 %
50 W 1.76 % 260 mA 42.7 %
60 W 2.72 % 285 mA 46.8 %
70 W 5.22 % 305 mA 51.0 %

Maximum tube dissipation is reached at 60 watts output, and is 34.125 watts. It drops to 33.6 watts at 70 watts output. Maximum tube current is reached at maximum power (70 watts) and is 152 mA. Both of these are within the specs for KT88 and 6550.


The results for a 6600 ohm OPT:

power THD B+ current output stage efficiency

0.1 W 0.15% 90 mA 0.02 %
0.5 W 0.22% 92 mA 1.2 %
1.0 W 0.28% 94 mA 2.3 %
2.0 W 0.39% 96 mA 4.6 %
5.0 W 0.62% 104 mA 10.7%
10 W 0.94 % 118mA 18.8 %
20 W 1.44 % 143 mA 31.1 %
40 W 1.93 % 182 mA 48.8 %
50 W 5.01 % 199 mA 55.8 %

Maximum tube dissipation never reaches 23 watts. Maximum tube current is reached at maximum power (50 watts) and is 99.5 mA. Both of these are very conservative for KT88 and 6550.

Perhaps 5000 ohms would be a good compromise.......I just happen to have two excellent sets of 5K OPT's. That's for another test day.........

But wait, there's more!!!!!!

After seeing how well some crusty old Chinese KT88's worked, I plugged in a "matched pair" of Electro Harmonix KT88's. I tested these at a few random points and at 3300 ohms only. The distortion dropped slightly across the entire power range without any significant changes in efficiency. Perhaps the matching done by the Tube Store over 10 years ago helped.

Still more.......

I dug deep into my secret stash and pulled out a quad of NOS JAN GE 6550A's still in their military packaging with 10/83 dates. I got them in the late 80's. They have never seen power......these things are stupid money today! Were they still good? How good?

I did a quickie matching job....leave the bias pot where it was with the EH tubes, plug in each tube, give it 5 minutes warm up, then turn on the B+ and read the current. pick two pairs from 4 tubes.

Surprise! The expensive tubes really do work better than the others. The distortion at 1 watt was 0.157% for one pair and 0.160% for the other. This compares to 0.22% for the EH tubes, and 0.25% for the unbranded Chinese tubes.

The distortion was lower across the board for the GE. The next surprise comes at 70 watts. While the other tubes have ran out of steam and started to clip, the GE's still had a bit left. the distortion at 70 watts was 3.95% and 3.98% VS 4.54% for the EH and 5.22% for the Chinese tubes.

I changed a few resistor values to make these boards sing. I will update the BOM when things aren't changing every few minutes. I plan to make the second board like the one I tested today, and maybe make a few more......I got some big sweep tubes to test, and some 300B's and some 7403's and ......
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Old 20th December 2017, 04:43 AM   #17
deadpool is offline deadpool  Australia
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oh looks like a new project for me next year
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Old 20th December 2017, 05:18 AM   #18
w5jag is offline w5jag  United States
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.... large transmitting tubes like the 811A, 813, 211 845 ...
I'll bet this board and a good modulation transformer would make a really clean AM phone rig.

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Old 20th December 2017, 12:26 PM   #19
Tubelab_com is offline Tubelab_com  United States
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Remember the Globe King? I had one. It used a pair of 5514's (like an 811A) as the modulator for a 4-250 RF amp. 500 watts plate input AM, all band HF TX.

Globe King 500
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Old 21st December 2017, 02:11 AM   #20
w5jag is offline w5jag  United States
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Yep, seems like I saw one at a hamfest as recently as five or six years ago.

I have never had a real plate modulated AM transmitter .....

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