Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AmberB on September 12, 2020, 10:35:53 pm
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Is there a way to make a reference voltafe point for the heater center tap if your amp is push-pull and not cathode biased? I saw one plan for using the bypass resistor on the first stage B+ filter, but if you have 2 caps in series for that, I'm guessing it's a bit trickier to build it there. I've also read that you could use the cathode resistor in a cathode follower circuit, but not all amps have a cathode follower stage in their circuits.
I figure even 20 volts on the reference point is better than ground as a reference point.
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See heater elevation...
http://www.valvewizard.co.uk/heater.html
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Thanks for the link!
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Two caps in series should give you additional filtering, so you could connect your voltage divider for heater elevation to the junction of the two in series reservoir caps giving you better filtering and a reference voltage that is not switched off with standby. If your standby switch is after the reservoir filter. But actually i had been wondering if there are any sideeffects when you elevate heaters on a fixed bias amp lately as i had never done that before and i am glad the question came up.
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Two caps in series should give you additional filtering, so you could connect your voltage divider for heater elevation to the junction of the two in series reservoir caps ...
You really don't want to do that. Instead, stick to what Merlin drew under "Heater Elevation" and have only C1 across R2.
The reason is caps in series form a voltage divider for alternating current, just as resistors in series. Rather than resistance, the capacitive reactance (https://en.wikipedia.org/wiki/Electrical_reactance#Capacitive_reactance) of the caps will be the impedances making up the divider. The smaller-value cap will have the higher capacitive reactance for a given frequency, and the higher impedance.
The problem is at least one of your series caps (maybe both) will be electrolytic. Tolerances are wide, and exact value is not sure to be stable over time. Your series resistors will be trying to set one value of voltage-division, and the series caps will be trying to set another.
- We accept any extra bleeder current in a power supply filter with its series caps & bleeder resistors.
Where a voltage divider can be made with caps of stable value & close tolerance (like film or mica capacitors), you will see their use either alone, or to tune the behavior of a resistive divider.
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I built a voltage divider circuit on 3 unused grommets on the circuit board. I tapped into the B+ voltage for the preamp section for the starting point. The article said not to use a resistance higher than 100K ohms for the lower resistor in the voltage divider, so I chose to use a 47K ohm for that. The B+ voltage at the spot where I tapped into it is 410 volts, so I figured a 10 to 1 dropping ratio should give be about 41 volts, so I used a 470K ohm resistor for the upper resistor in the voltage divider. I ended up with 42 volts. The article recommends 30 to 60 volts for the reference voltage, so that worked out pretty well. I didn't want to use a 50 volt smoothing cap, to close to the voltage for my comfort. I had a 10uf at 160 volt electrolytic cap, so I used that. Perhaps higher voltage than necessary, but I had it, so I used it.
The voltage divider works quite nicely, and I have the center tap of the heaters tied to that point now. This is the first time I've built a heater center tap reference voltage circuit in an amp.
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I built a voltage divider circuit ...
Nice job reasoning through all the choices! Sounds like your divider will work well for you for a long time to come.