Soft start with EML 45 report

[Dr. Beaker]

I just converted my soft start Paramounts from 2A3 to 45 like others have done.  I was very happy with the 2A3 configuration but I had a pair of lightly used EML 45 solid plates begging to be used.  I run bi-amped speakers with the Paramounts driving 110 dB horns from 600 Hz up so an easy task for the 45 tubes.

I added an extra 500 ohm resistor between the two 3k resistors as my house voltage runs about 122 most times.  With 6500 ohms on the cathodes I measure:

Plates 492 / 490
Cathodes 217-220 / 215-218
Grid 164 / 163

V p-k 273.5 / 273.5
i 33.6mA / 33.3mA

9.2 / 9.1 watts disapation


After initially testing them on the bench without any issue, I let them cool for over 4 hours before the first turn on hooked up to my speakers.  The next time I powered them on there was a freakishly loud high voltage kawap noise from one of the 45 tubes.  My heart skipped a beat as I was sure it just blew.  Moments later music started playing from both channels and sounded very good.  I have since powered them on about 6-8 times and have not had any further occurance of the pop.  I don't know if it is possible that the driver circuit failed to reset or not even after a long turn off period.

The EML tubes sound great.  Very detailed and dynamic.  It's a great combination with the soft start direct coupled amps.

[Paul Joppa]

I can't explain the arc the first time. If the speakers are silent for the first 5-10 seconds now, the soft start must be resetting properly. It "resets" when the 10000uF cap on the board discharges through the 33K resistor adjacent to it, so there is no mechanical reset function.

If it ever arc or pops again, immediately replace the 45s with the stock 2A3s and don't put them back until you have identified and solved the problem.

[Dr. Beaker]

Glad to report that after about 20 more power on cycles, I have had no further arcing episodes on either amp.  I pay close attention everytime I turn them on.