Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frank57 on October 03, 2011, 09:45:16 am
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What are the voltages supposed to be on the filament wires on a power transformer for a tube amp?
A test was suggested:
If I take out all the tubes and unhook them from the pcb what kind of reading should I get?
Between 3 and 4 ac on each one?
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Between 3 and 4 ac on each one?
That's about right, if you have a center tap connected to ground and are measuring from ground to each filament lead. It's a good idea to also measure BETWEEN the filament leads.
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If I left the Ct unhooked what would the readings be?Sky High?
Right now my readings make no sense so I'll check again.
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Okay I get 4.11 dead even on both and 8.2 or 8.4
between them.Is it good or bad?
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A truly no-load VAC reading on filaments could be as high as maybe 8.5 VAC with really high line volts. Presumably, you would be leaving the amp in standby, so no other load (as in B+) would be presented to the power transformer. No way "sky high". You wouldn't see 8.5 VAC if even a pilot lamp was lashed across the winding, and if there was a 5 V rectifier tube in-socket it would come down more. Anything higher than that kind of single-digit volts reading would lead me to believe something was pretty wrong.
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If I left the Ct unhooked what would the readings be?Sky High?
Right now my readings make no sense so I'll check again.
Connecting the CT to ground or leaving it disconnected has no bearing on your voltage readings. Where you connect your meter probes determines the readings. If you only connect one probe to something and leave the other probe hanging in the air, then you cannot measure any difference of potential (voltage). Any numbers on the meter would be totally bogus. Now, when you disconnect the CT from chassis, your filament winding no longer has a reference to chassis. If you try to measure voltage between chassis and either filament lead you may get some numbers on the meter, but they will be totally bogus and meaningless. It would be equivalent to leaving one meter lead hanging in the air.
If your filament voltage readings make no sense then you are not doing something right or your meter is stupid. Do the following and you will have meaningful readings...
1. Measure BETWEEN the filament windings. Put one meter lead directly on one filament lead and the other meter lead directly on the other filament lead. You should get 6.3VAC. (Probably be higher if no tubes in place.)
2. Measure between CT and each filament lead. Put one meter lead directly on the CT and the other directly on either filament lead. You should read 3.15VAC (half of what you measured in step 1. but probably higher if no tubes in place) Repeat for the other filament lead.
Steps 1 and 2 will give meaningful readings regardless of where the CT is connected or even if it's not connected.
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Okay I get 4.11 dead even on both and 8.2 or 8.4
between them.Is it good or bad?
That's good.
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About 7 - 7.2VAC unloaded is common. 8+ seems high. Still, loaded with 7VAC is OK.
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With the tubes in usually I get 3.8 on each and 7.7 on 9 to 4/5.
3.8 also on 9 to ground.It does make the tubes burn out quick, but other things cause other problems.
if I do the diode trick on the filaments to lower it things become unbalanced
With 3.4 on each but 2.9 from 9 to ground.6.8 on 9 to 4/5.
An artificial ct seems to correct that but seems to be generating a lot of hissy noise.
They seem to have done the heaters in a strange way on this amp so I'll double check what they did there.
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About 7 - 7.2VAC unloaded is common. 8+ seems high. Still, loaded with 7VAC is OK.
+1
Your measurements seem a tad high, even for vintage iron.
You can knock the voltage down with diodes.
Ray