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?
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?
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.
Then a grounded filament supply is fine. Certainly it should not be elevated that high, perhaps +50V at most.
More
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.
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.
Can you post the schematic? This sounds very unusual. Is there a tube in the circuit with a cathode voltage well above ground?
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!
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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.
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.
The original system had no regulation for the power supplies, so while I was at it I just included the soft start section to the added regulator. The original design still applied the plate voltage of the 7025 to elevate the filament voltages. It has always been well above the recommended 40=50 volts. What would be the advantage/disadvantage to changing the original design/purpose in doing this? The schematic has been posted some time ago and I'll see if I can find it here.
Thanks for the help, this is above my full understanding.
Thanks for the help, this is above my full understanding.
schematic
Rayma; Here is the original schematic. I have removed the connection at "point X" of V1(pin 1 and 6) and connected them to the soft start.
Schematic on post 7 on http://www.diyaudio.com/forums/tubes-valves/245754-very-strange.html
Rayma; Here is the original schematic. I have removed the connection at "point X" of V1(pin 1 and 6) and connected them to the soft start.
Schematic on post 7 on http://www.diyaudio.com/forums/tubes-valves/245754-very-strange.html
trying to learn
DF96; Yes, this is a strange tube amp(my first and only), thus I am confused about some of the unusual goings on here. I did find that some of the older 12ax7 allowed up to 200vdc between the heater/cathode.
I have modded the bias circuit such that each side of the output has it's own bias pot. I did buy two sets of Svetlana Flying Cs a while back that seem to have held up well. I now have two sets of Mullard XF2s in the amps. It took a bit of jugging to find 8 each that I could get to balance out to a range of 28-34 ma bias at 645v.
Still learning these things and there are many more questions to be answered!
DF96; Yes, this is a strange tube amp(my first and only), thus I am confused about some of the unusual goings on here. I did find that some of the older 12ax7 allowed up to 200vdc between the heater/cathode.
I have modded the bias circuit such that each side of the output has it's own bias pot. I did buy two sets of Svetlana Flying Cs a while back that seem to have held up well. I now have two sets of Mullard XF2s in the amps. It took a bit of jugging to find 8 each that I could get to balance out to a range of 28-34 ma bias at 645v.
Still learning these things and there are many more questions to be answered!
Looking at the schematic only the heaters of V1, V2, V3 and V4 are elevated. All the output tubes are ground referenced. The only reason I can see for such a high heater elevation is that the cathodes of V2 and V3 are at 225V. The cathodes of V1 and V4 are just a few volts positive.
Cheers
Ian
Cheers
Ian
Ok, if there is only one filament winding, you have to elevate it to about +115 VDC to meet the requirements for all the tubes.
Preferably, there would be a separate winding for the tubes with high cathode voltages.
ruffrecords; Just to clarify, on V2/3, it is the plate of the 12ax7 section that is at 222vdc, and the cathode of those tubes sections is at 4v. I haven't measure the 4v to confirm this as the access is quite difficult. I see the issue with the pentode section. I haven't even looked into that section of this part to confirm if it is within spec, and I will do that. Also, I have removed V4 as it was only the driver/comparator for a distortion light. This may not be applicable, but somewhere in my memory I recall that the guitar/marshall guys said something about positive/neg grids with respect to the heater affect sound/breakup. I need to review this thought.
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Can you post a schematic? If all tubes have their cathodes near ground potential, then the filaments should be grounded as well.
From his post:
V1 shares filament winding with V2 and V3 have 225vdc at their cathodes so, yes, the filaments have to be elevated.
Images of the physical amp available here.
V1 shares filament winding with V2 and V3 have 225vdc at their cathodes so, yes, the filaments have to be elevated.
An externally hosted image should be here but it was not working when we last tested it.
Images of the physical amp available here.
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rayma; see post 12 for the link to the schematic, then go to post 7.
ruffrecords; Just looked up ECL86/6gw8 spec sheet. Both Vhk for both section should be a max of 100v. Looks like to hit the best spot as rayma indicated would be right around 115v for the dc elevation with only one source. Does this affect the operation/sound of the tube? Haven't lost a tube in these positions.
ruffrecords; Just looked up ECL86/6gw8 spec sheet. Both Vhk for both section should be a max of 100v. Looks like to hit the best spot as rayma indicated would be right around 115v for the dc elevation with only one source. Does this affect the operation/sound of the tube? Haven't lost a tube in these positions.
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