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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 23rd April 2013, 05:54 PM   #1
acceler8 is offline acceler8  United States
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Default TSE Testing and Bias

Over the weekend I tried to run some tests against my Tubelab SE (300b, mostly stock) amp. I was using Arta (ARTA Home) with the Pulsed Pink Noise in Spectrum Analyzer mode at what had to be close to maximum amplifier output. I also happen to have dual panel meters for my output tubes to monitor the bias. While performing the test, I noticed that the bias meters were dancing vigorously in time with the pink noise pulses. I panicked and stopped the test immediately. I didn't get a firm idea of the range, b/c I stopped the test so quickly. I would guess it was at least 15ma of difference (with the idle bias set to 65ma @ 388V).

I'm still learning, so I don't understand the schematic for the TSE well enough to know if this is normal or not. During normal music, the bias seems to drift back and forth a very wee bit but nothing like they did during the test. I played some music through the amp after, and it played well (so no damage was done - it seems). For reference, I've attached the result from Arta. The peaks below 300Hz are something in my test chain (there for any device/loopback).

Is this bias response to stress normal? If not, where do I need to look for improvements?


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Old 23rd April 2013, 06:34 PM   #2
rknize is offline rknize  United States
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The primary feature of the TSE that sets it apart from other DHT SE amps is George's PowerDrive circuit. When the output tube strays out of class A, the MOSFET happily feeds the grid whatever current it needs to enable fast recovery afterwards. This avoids the so-called "blocking distortion" problem.

When reproducing music, these peaks don't happen often. The amount of grid current drawn in practice is not very much and not a concern. On a test bench with a signal generator, it can happen on every cycle. Perhaps this is what you saw.

DHTs typically lack radiating fins for the grid wires, so I suppose there is a danger of overheating the grid if you kept at it. George would know better. If it is possible to melt the grids on a TSE, he certainly would have done it long ago. I don't remember him ever mentioning it.
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Old 24th April 2013, 04:52 PM   #3
acceler8 is offline acceler8  United States
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Thanks for reminding me about "Power Drive". I had read the original web page about it when I was researching what amp to build. Admittedly, I didn't understand most of it at the time. I just re-read it, and it now makes more sense (and sounds exactly like what I experienced).

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Old 25th April 2013, 06:45 PM   #4
Tubelab_com is offline Tubelab_com  United States
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Theory states that the AVERAGE current over time remains constant in a class A amplifier. In practice the current will vary a few mA as the power is increased. Most people understand this.

What most people fail to realize is that any class A amp does not remain in class A once it hits clipping. You will probably notice that the current will remain within a narrow range as the volume is advanced to a point where it changes rather abruptly. This is the point where the amp is clipping on peaks. It may not be audible yet, but it will be obvious on a scope.

Pink noise has about a 20 db peak to average ratio, maybe more. The amps response to clipping is probably normal. Every amp is different, and dependant on the output tubes and their condition. A TSE will usually increase current in response to an overload, while an SSE will show an increase on a mild overload, then the current will actually drop when you really hammer it due to grid blocking.

George would know better. If it is possible to melt the grids on a TSE, he certainly would have done it long ago.
I have hammered a TSE with my guitar preamp and not blown a tube, but I wouldn't recommend long term abuse especially if you are running 300B's with a fairly high B+. The continued operation in the grid current region can heat up the mosfets and might lead to thermal failure. My original Lexan TSE still works great and I have abused it a zillion different ways, but it only runs 325 volts of B+.
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