PlasticIsGood, measured heater cathode resistance increases as the voltage differential increases away from zero. Different preamp tubes show different levels of 'zero voltage differential' resistance, and different increasing resistance characteristics as the voltage difference is increased (and with both symetric and asymmetric behaviour for positive and negative polarities).
For the small number of 12AX7's that I have measured, there has been no obviously generic characteristic or magnitude of resistance other than a minima around zero voltage differential. But I also note that the measured resistance levels observed at 0V have been very high (much much higher than some spec sheet 'minimum' levels), and so in context I couldn't see how dc elevation would provide any improvement at all to hum - and so I would suggest that dc elevation is not a good first in design use unless there is specific evidence that it is the root cause of any discernable hum.
In those situations where cathode-heater resistance contributes to a discernable hum, then introducing a voltage difference will per se increase the effective resistance, and so can help lower hum. Given that the pk heater AC voltage may be say 9V, then applying a dc elevation higher than 9V will start to move all parts of the heater-cathode voltage differential waveform away from 0V. General recommendations I've seen are to use at least 20-30V elevation at least. I've got a few old amps that use the bypassed cathode bias of the output stage as an elevation voltage.
For the small number of 12AX7's that I have measured, there has been no obviously generic characteristic or magnitude of resistance other than a minima around zero voltage differential. But I also note that the measured resistance levels observed at 0V have been very high (much much higher than some spec sheet 'minimum' levels), and so in context I couldn't see how dc elevation would provide any improvement at all to hum - and so I would suggest that dc elevation is not a good first in design use unless there is specific evidence that it is the root cause of any discernable hum.
In those situations where cathode-heater resistance contributes to a discernable hum, then introducing a voltage difference will per se increase the effective resistance, and so can help lower hum. Given that the pk heater AC voltage may be say 9V, then applying a dc elevation higher than 9V will start to move all parts of the heater-cathode voltage differential waveform away from 0V. General recommendations I've seen are to use at least 20-30V elevation at least. I've got a few old amps that use the bypassed cathode bias of the output stage as an elevation voltage.
1. it starts with the tube(s) you want to use, and the heater cathode voltage ratings for that tube, this is found in datasheets....
2. next is the topology you want to use the tube(s) in....
knowing these 2 things, you can then proceed to calculate voltage dividers for heater reference....this voltage reference need not draw large currents but may also serve as bleeders to discharge psu caps at shutdown...
in my tube builds, i have resisted using dc supplies to the heaters, having a dc reference helped to combat hum....
I think the quick answer is that you can't. However, in his book "Getting the Most Out of Vacuum Tubes" the author says that life tests have shown that there is a broad curve with best behaviour at +40V wrt cathode. The more typical 'slightly negative wrt cathode' is almost as good, because it is a broad curve. Most datasheets say +-90V or something like that.PlasticIsGood said:
This isn't quite true. I did a project where the heaters float due to the use of a SS/HS hybrid power supply. The SS negative rail comes up much faster than heaters can warm, and would bust the Vhk spec of the 6FQ7 I used as a grid driver.
Floating these heaters didn't make for excessive noise at all. "Buzz" is more likely a grounding issue.
i use DC filament supply for both the input - AX7 and invertor - AU7 stages but AC for the output stage - 6550. so do i need to ground or reference the DC filament supply cct? its floating right now as it has its own line transfo and the whole thing does not tie to any part of the main HT supply cct.
cheers
cheers
they maybe referring to AC filaments heaters. if its so also for DC, do i just tie the negative leg of the heater to the star ground?
Yes floating is not good. It can chrage up to any un-known voltage. 'd at least use a high value resistor to ground. But if being extra carful why bias it it up? It only takes one more resistor
Thanks, all. Just been peering into my amps for the first time in years.
When I made them, some years ago, I agonised for some time over heater grounding. Small valve heater supply is current source, and heaters are in series. Eventually I decided to ground the middle of the chain, probably on the basis that any fluctuation in current would cause least common mode voltage variation. Actually current is as constant as I've ever seen a constant thing be.
When I get round to it, I'll ground the negative end of the chain, so the valves with greatest gain are least positive, and the elevated drivers will see 20-odd volts. I guess the ultimate would be to connect the centre of the chain to an elevated ground, but I don't care enough.
The amps have been happy for years as they are. Heaters are left at half power when not in use, so they've been on for years. There is no apparent difference in the condition of the valves with negative heaters.
I've been a bit lazy about bias I notice...
When I made them, some years ago, I agonised for some time over heater grounding. Small valve heater supply is current source, and heaters are in series. Eventually I decided to ground the middle of the chain, probably on the basis that any fluctuation in current would cause least common mode voltage variation. Actually current is as constant as I've ever seen a constant thing be.
When I get round to it, I'll ground the negative end of the chain, so the valves with greatest gain are least positive, and the elevated drivers will see 20-odd volts. I guess the ultimate would be to connect the centre of the chain to an elevated ground, but I don't care enough.
The amps have been happy for years as they are. Heaters are left at half power when not in use, so they've been on for years. There is no apparent difference in the condition of the valves with negative heaters.
I've been a bit lazy about bias I notice...
While I am still on a pretty steep learing curve about all this it occurs to me that some confusion may come from the way this technique is
explained, as I recall I read that referencing a DC voltage allowed the AC componet to "Float" on top of the DC stablizing the leakage current from heater to cathode...or something close to that. I do know it helped to reduce hum in my 12au7 preamp.
explained, as I recall I read that referencing a DC voltage allowed the AC componet to "Float" on top of the DC stablizing the leakage current from heater to cathode...or something close to that. I do know it helped to reduce hum in my 12au7 preamp.
Is it possible to see A schematic diagram of franks cathode follower preamp circuit showing how to elevate the heater supply? I have a NOS Chicago Standard PC8406 which has a center tapped 6.3V heater winding. I understand the artificial center tapped methodology using the 100ohm resistors and the hum dinger concept which is not needed because I have a center tap. The elevation of the heater's is what I am having difficulty implementing. Also, why is the recommended transformer for this preamp 100ma. I have single ended power amps that utilize transformers less than 100ma.
A potential divider coming from the power supply works. Use large values so you don't pull too much current and set the divider to around 50V.
For a B+ of 300V
Two resistors from B+ to ground in series. A 220k on the B+ side and 47k on the ground side. Tie your your heater center tap to the junction of these two resistors and decouple with a cap. Should only draw a milliamp and give you 52V.
For a B+ of 300V
Two resistors from B+ to ground in series. A 220k on the B+ side and 47k on the ground side. Tie your your heater center tap to the junction of these two resistors and decouple with a cap. Should only draw a milliamp and give you 52V.
famousmockingbird, THANK YOU!! just to be certain that I fully understand, use two resistors tied from my 300v B+ to ground. At the junction of the 220K and 47K to ground resistors, solder a capacitor to that junction. Solder the 6.3 v heater center tap to the other end of the capacitor. Is the connection placement to the B+ at the power supply output?
Could you please help me to understand why the rating of the transformer is recommended at 100 ma.
I have a Magnavox AM/FM STEREO console That utilizes 2 EL-84 output tubes and additional tubes for the tuner and the transformer rating is 80ma.
The Stancor transformer rating I would like to use is 650v c.t. at 50ma and I am not using the 5v 2A winding, can' that drive two 6sn7? Thanks to all responses.
Could you please help me to understand why the rating of the transformer is recommended at 100 ma.
I have a Magnavox AM/FM STEREO console That utilizes 2 EL-84 output tubes and additional tubes for the tuner and the transformer rating is 80ma.
The Stancor transformer rating I would like to use is 650v c.t. at 50ma and I am not using the 5v 2A winding, can' that drive two 6sn7? Thanks to all responses.
AJT, thank you. You clarified my understanding of the decoupling cap also !! I mistakenly thought the cap was used as a coupling cap. Your drawing states Fil. Ref. Just to be sure what you mean is filament center tap?
The 6sn7 DC cathode current draws 20ma. so two tubes would draw 40ma. Won't the stancor 50ma transformer handle the load of two preamp tubes?
The 6sn7 DC cathode current draws 20ma. so two tubes would draw 40ma. Won't the stancor 50ma transformer handle the load of two preamp tubes?
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