Heater Voltage 500C - What Is Too Low?

I had another look at the Fisher 500C schematic today. I noticed that the bias voltage is 22VDC. This supply also powers the filaments for 4 of the preamp tubes. Fisher's design intended for each filament to get 11 VDC rather than the normal 12.6VAC.

My amp has been biased at 19VDC which means that I'm only powering these tubes with 9.5VDC!

I've read different opinions on the internet about extending tube life via filament starvation, but others claim it may actually be damaging to the tube. I believe that the safe thing to do is to stay within +/- 5% of the rated 12.6V. Both values above are well below this percentage. Does anyone else here have any concerns about lower than normal filament voltages on their precious telefunken 12ax7s?

Chappy

Chappy -- It depends on the type of tube you are talking about.

For power output tubes, it is important to keep the heater voltage as close to design center as possible. Operating them with significantly less voltage reduces the peak current handling abilities of the cathode such that trying to actually draw rated peak current from a cathode that is not up to temp will in fact damage it. Operating the heater at significantly higher voltages causes cathode material to be boiled off the cathode sleeve, which usually then find its way to the grid, causing problems with grid emissions and the resulting unstable bias that produces.

For small signal tubes, the heater voltage is of much less concern. Operating a 12.6 volt heater at 9.5 volts will certainly cause extended warmup times, and limit amplification as well. Operating these tubes slightly under rated voltage can result in lower noise, which was typically what Fisher was trying to do. Operating these tubes at higher voltages simply stresses the heater and promotes burn out.

I hope this helps!

Dave

dcgillespie said:

Chappy -- It depends on the type of tube you are talking about.

For power output tubes, it is important to keep the heater voltage as close to design center as possible. Operating them with significantly less voltage reduces the peak current handling abilities of the cathode such that trying to actually draw rated peak current from a cathode that is not up to temp will in fact damage it. Operating the heater at significantly higher voltages causes cathode material to be boiled off the cathode sleeve, which usually then find its way to the grid, causing problems with grid emissions and the resulting unstable bias that produces.

For small signal tubes, the heater voltage is of much less concern. Operating a 12.6 volt heater at 9.5 volts will certainly cause extended warmup times, and limit amplification as well. Operating these tubes slightly under rated voltage can result in lower noise, which was typically what Fisher was trying to do. Operating these tubes at higher voltages simply stresses the heater and promotes burn out.

I hope this helps!

Dave

Click to expand...

That certainly helps. I think that I will modify my bias supply circuitry to provide the original 22V supply to the heaters and then use a pot as a resistor divider to provide a lower adjustable voltage for the bias delivered to the output tubes. Should be an easy to implement fix.

I also have the unit hooked into the isolated side of an RCA Isotap transformer. I use the switch on the Isotap to turn the amp on and off and then set the switch position to give the correct AC line voltage to the unit to ensure close to ideal AC filament voltages for the other tubes.


Once again thanks for your input.

Chappy

That is an excellent way to operate your amp. If you are ever in doubt of what the design center AC line voltage is supposed to be for a given amplifier, use a variac to set the heater voltage to the output tubes at 6.30 vac while the amplifier is operating normally, and then measure the voltage at the primary that supplies that heater voltage. Then, any appropriate adjustments can be made based on knowing an accurate target to shoot for.

Good luck with your amp!

Dave

dcgillespie said:

That is an excellent way to operate your amp. If you are ever in doubt of what the design center AC line voltage is supposed to be for a given amplifier, use a variac to set the heater voltage to the output tubes at 6.30 vac while the amplifier is operating normally, and then measure the voltage at the primary that supplies that heater voltage. Then, any appropriate adjustments can be made based on knowing an accurate target to shoot for.
Dave

Click to expand...

Great tip, Chappy and Dave :thmbsp:

dcgillespie said:

That is an excellent way to operate your amp. If you are ever in doubt of what the design center AC line voltage is supposed to be for a given amplifier, use a variac to set the heater voltage to the output tubes at 6.30 vac while the amplifier is operating normally, and then measure the voltage at the primary that supplies that heater voltage. Then, any appropriate adjustments can be made based on knowing an accurate target to shoot for.

Good luck with your amp!

Dave

Click to expand...

Dave,
Thats exactly what I do as you described above. I'll be replacing the Isotap with a variac once I get it boxed up appropriately.

Cheers
Steve

Aaargh! Didn't meant to do thread necromancy!

I just found a link here from another AK thread as pointing to this as a solution, and didn't notice the dates! Oh, well, too late to fix...

This is the conclusion at which I arrived while trying to find a general solution to over-voltage issues using 115 VAC equipment on 120 VAC mains.

Operating a variable autotransformer at a fixed point of operation so as to form a constant step-up or step-down function is problematic, as many units will fail when used in this fashion. (The name "Variac", by the way, was a trademarked name of General Radio for many decades until the registration lapsed. The technical term is "variable autotransformer" or "variable autoformer".)

Two problems arises because of the interaction between brush and windings:
Both of these problems may cause the brush to fail, and even the autotransformer itself to fail.

Back in the 1960s General Radio solved this problem using special silver-impregnated brushes called "Duratrak". It reduced the corrosion so that the autotransformer might be continuously operated in the same spot. This was a short-lived product, and only applied to less than a decade's worth of product.

The better solution is to either build a fixed autotransformer, add a bucking transformer to the equipment, or to build an isolation/bucking transformer.