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Old 21st October 2014, 09:40 PM   #1
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Default elevated filament voltages

The stock system had both the 7025 and 6gw8 filaments elevated up to 150vdc(voltage divider) with the 6.3vac on top. I installed a soft start B+ for both tubes. Now I realized that there is now at the start up a great difference between the voltages of the components of these tubes. The final plate voltage of the 7025 is 120vdc, the 12ax7 part of the 6gw8 finals at 220vdc. The ramp up on the soft starts takes about 70 secs.

Is this hard on the tubes? Should I connect up a similar soft start for this elevated filament supply?
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Old 21st October 2014, 09:58 PM   #2
rayma is offline rayma  United States
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Originally Posted by optimationman View Post
The stock system had both the 7025 and 6gw8 filaments elevated up to 150vdc(voltage divider) with the 6.3vac on top. I installed a soft start B+ for both tubes. Now I realized that there is now at the start up a great difference between the voltages of the components of these tubes. The final plate voltage of the 7025 is 120vdc, the 12ax7 part of the 6gw8 finals at 220vdc. The ramp up on the soft starts takes about 70 secs.

Is this hard on the tubes? Should I connect up a similar soft start for this elevated filament supply?
You worry about the cathode voltage of each tube, not the plate. That's what is near the filament.
What are the two cathode voltages after warmup?
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Old 21st October 2014, 10:03 PM   #3
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Slow or soft starting of heaters may damage the cathodes as they will tend to strip the electrons whist warming up. What should really happen is quick start heaters and then after 30Seconds or so allow the HT. That is why we have a standby switch on commercial valve amplifiers; to allow HT when hot only.
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Old 21st October 2014, 10:19 PM   #4
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I was reading that the max voltage between heater and cathode was 100vdc for 12ax7s. Is this not a problem at start up?
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Old 21st October 2014, 10:29 PM   #5
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Default cathode voltages

Rayma; I have not measured the cathode voltages. The schematic indicates that the cathode of the 7025 is 1.1v and the cathode of the 12ax7 section of the 6gw8 is 4vdc.
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Old 21st October 2014, 10:45 PM   #6
rayma is offline rayma  United States
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Originally Posted by optimationman View Post
Rayma; I have not measured the cathode voltages. The schematic indicates that the cathode of the 7025 is 1.1v and the cathode of the 12ax7 section of the 6gw8 is 4vdc.
Then a grounded filament supply is fine. Certainly it should not be elevated that high, perhaps +50V at most.
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Old 22nd October 2014, 03:45 AM   #7
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The reason for the soft start was to avoid any possible stripping(what ever the cause) by ramping the plate voltage after the filament had time to heat the cathode. The original system tied the elevating dc filament voltage to the plate voltage of the 7025. Thus instant plate voltage with a cold cathode. I then installed a soft start for the plate voltage of both tubes to address that issue. However, I now see that the filament voltage has exceeded the cathode voltage by a wide margin. The data sheet says the maximum difference is 100vdc. However, they have been that way since new?? I was confused as to why this hadn't cause problems and if I should address this issue.
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Old 22nd October 2014, 03:56 AM   #8
rayma is offline rayma  United States
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Originally Posted by optimationman View Post
However, I now see that the filament voltage has exceeded the cathode voltage by a wide margin. The data sheet says the maximum difference is 100vdc. However, they have been that way since new?? I was confused as to why this hadn't cause problems and if I should address this issue.
Can you post the schematic? This sounds very unusual. Is there a tube in the circuit with a cathode voltage well above ground?
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Old 22nd October 2014, 08:44 AM   #9
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by optimationman View Post
However, they have been that way since new?? I was confused as to why this hadn't cause problems and if I should address this issue.
The heater-cathode voltage limit is not a 'hard limit' but a recommendation. The tube won't instantly explode if Vhk goes over 100V, you simply get a little more leakage between heater and cathode. If both elements are still cold then there is even less of a problem. It sounds like you're over thinking it. Ordinary receiving valves do not require a soft start of any kind (in fact it can cause more problems than it solves), unless you're doing something really weird...

As Rayma said, if your cathode voltages are basically grounded, don't eleveate the heater to 150V!

Last edited by Merlinb; 22nd October 2014 at 08:46 AM.
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Old 22nd October 2014, 10:12 AM   #10
DF96 is offline DF96  England
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You need power sequencing for a multi-kW PA (or similar radio transmitter). You don't need power sequencing for small signal valves. 'Soft start' should be reserved for protecting reservoir caps from inrush currents.

Similarly, people these days get far too excited about heater elevation. For most circuits, using decent valves, it is unnecessary. For some circuits it may be helpful, but don't go much beyond +40V with respect to the cathode.
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