I was given power and output transformer, along with an EZ80 rectifier and EL84, and looking to build a small guitar amp.
The heater secondary is measuring 7.8v, and has no center tap.
If I plan on using with the EZ80, EL84, EF86 and a 12AX7, what would be a good voltage measurement without the load? And how can I lower to that voltage?
Ive seen other posts where adding resistors is suggested, but wonder if this would work for dropping 1.5v.
Is it possible to disassemble these transformers and remove windings?
Thank you
The heater secondary is measuring 7.8v, and has no center tap.
If I plan on using with the EZ80, EL84, EF86 and a 12AX7, what would be a good voltage measurement without the load? And how can I lower to that voltage?
Ive seen other posts where adding resistors is suggested, but wonder if this would work for dropping 1.5v.
Is it possible to disassemble these transformers and remove windings?
Thank you
A series resistance is probably the best way to go.
I suggest you do in this order :
- build your amp, this includes connecting all tubesockets with proper connections to the filament winding.
- insert all tubes and power on. Measure the AC across the filament.
( it's entirely possible that the filament is acceptable here , anything between 6 and 6.5 volt is ok. If it exceeeds 6.5 volt then you will add a series resistance
to the filament)
Needed series resistance might be one ohm or 0.5 ohm, but measure voltage first.
I suggest you do in this order :
- build your amp, this includes connecting all tubesockets with proper connections to the filament winding.
- insert all tubes and power on. Measure the AC across the filament.
( it's entirely possible that the filament is acceptable here , anything between 6 and 6.5 volt is ok. If it exceeeds 6.5 volt then you will add a series resistance
to the filament)
Needed series resistance might be one ohm or 0.5 ohm, but measure voltage first.
If you measure the filament winding without a load or connected to the actual tubes, you will get a higher reading. Another possibility is that the power transformer is rated for 110VAC mains input, typical for vintage units, and modern household AC is typically 120VAC. That happens all the time using vintage amps in modern era. Either use a Variac to lower voltage or add thermistors to the input to knock it down to 110VAC. In bigger amps, sometimes people use a low voltage bucking transformer connect it in series with the primary of power transformer to eat up the extra voltage. Most likely your measurement requires a load.
6.3V tubes will run months on 7.8V (which as said will drop under load).
So hook it up and *then* worry if it may be too high and how to drop it.
So hook it up and *then* worry if it may be too high and how to drop it.
110v: this is what I have
-AC 6.3v for 2-6sn7 and 1-12ax7
-Without load 6.9v
110v use variac setting at 115v
-AC 6.6v for 2-6sn7 and 1-12ax7
-Without load 7.2
110v use variac setting at 120v
-AC 6.8v for 2-6sn7 and 1-12ax7
-Without load 7.5v
-AC 6.3v for 2-6sn7 and 1-12ax7
-Without load 6.9v
110v use variac setting at 115v
-AC 6.6v for 2-6sn7 and 1-12ax7
-Without load 7.2
110v use variac setting at 120v
-AC 6.8v for 2-6sn7 and 1-12ax7
-Without load 7.5v
Open circuit voltage doesn't tell you much. Hook up the transformer to a power resistor that will pass as much current as all the filaments and then see what the voltage is.
The Secondary that drives the B+ current, also loads the Primary.
The Primary drops voltage due to the DCR.
Build the amplifier.
Measure what you have to set your variac to, so that you get the right filament and B+ voltages.
Compare the Variac voltage to your power mains voltage.
Then build a 'Buck' transformer.
Example, if your 110V power transformer is driven by your 117V mains power.
Use a 6.3V filament transformer, wire the primary across mains, wire the 6.3V secondary in series (out of phase), and use the series of those two windings to get 110.7 Volts to run your amplifier transformer.
If your mains are 120V, 123V, etc, use a different 'buck' transformer. 8V, 10V, 12V secondary.
Put it in a metal box, or get a big enough chassis for your amplifier to mount the buck transformer on it.
Hint: not all power mains voltages are constant. My mains power varies from 117V to 123V. I design my amplifiers to work off of 120V, but with parts that can take the 123V safely and that do not get hot.
The Primary drops voltage due to the DCR.
Build the amplifier.
Measure what you have to set your variac to, so that you get the right filament and B+ voltages.
Compare the Variac voltage to your power mains voltage.
Then build a 'Buck' transformer.
Example, if your 110V power transformer is driven by your 117V mains power.
Use a 6.3V filament transformer, wire the primary across mains, wire the 6.3V secondary in series (out of phase), and use the series of those two windings to get 110.7 Volts to run your amplifier transformer.
If your mains are 120V, 123V, etc, use a different 'buck' transformer. 8V, 10V, 12V secondary.
Put it in a metal box, or get a big enough chassis for your amplifier to mount the buck transformer on it.
Hint: not all power mains voltages are constant. My mains power varies from 117V to 123V. I design my amplifiers to work off of 120V, but with parts that can take the 123V safely and that do not get hot.
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Hint #2:
A 110V rated transformer that runs on 123V is going to get hot.
Use a "Buck" transformer with a 12.6V secondary to get about 110V to run the 110V transformer.
Safety First!
A 110V rated transformer that runs on 123V is going to get hot.
Use a "Buck" transformer with a 12.6V secondary to get about 110V to run the 110V transformer.
Safety First!
110V rated transformers should be banned in the US and Canada.
110V grid voltage only existed in ancient times, nowadays the grid voltages are normally around 120V, so a 110V rated transformer will bing wrong voltages.
110V grid voltage only existed in ancient times, nowadays the grid voltages are normally around 120V, so a 110V rated transformer will bing wrong voltages.
if at the sockets you are getting higher than 6.8vac, then you can also use an ntc to lose a little bit of voltage..
I noticed once that when putting more than 10 Volts accidently on a 6.3V 6SN7 tube the noise the tube generated was huge. It was immediately clear something was very, very wrong! Again, my tube survived.
Regards, Gerrit
Regards, Gerrit
A short-term excess voltage is rarely a problem. Tubes are surprisingly robust. But a long-term excessive heater voltage can shorten tube lifetimes significantly. A number of years ago, I had a pair of Cary 40M mono amps which were a simple PP EL-34 design. I found my output tubes burned out very quickly, only lasting a couple hundred hours. A little checking under the hood revealed the heaters had 7.4v. Adding a resistor in series dropped the voltage down to a little under 6.3, and the premature tube failure problem was solved. I generally shoot for about 5% less than the rated heater voltage.
The series resistor has another advantage.
Most filaments are extremely low resistance when cold.
The series resistor provides an intrinsic soft start voltage to the filament.
+
Most filaments are extremely low resistance when cold.
The series resistor provides an intrinsic soft start voltage to the filament.
+
I used bridge rectifier to drop 1.4V from AC filament voltage. Added benefit is DC for heaters. But measure under load before making decision.
Jim
Jim
a pair of 3 amp diodes wired in parallel with cathodes opposite directions will drop about 0.7 volts with little heat. (diode cathodes not tube cathodes)
mike567,
If the RMS voltage to the filament is 6.3V, then the RMS current is equal to the rating of the tube RMS current (within the tubes % rating tolerance).
The heat of the antiparallel diodes in series between the filament winding and the filament, is exactly equal to the heat of a series resistor that gives exactly the same RMS heating to the tube filament.
I mean RMS voltage, RMS current, and Heat integrated over time.
The Conservation of Energy and Laws of Thermodynamics are not violated or broken here.
If the RMS voltage to the filament is 6.3V, then the RMS current is equal to the rating of the tube RMS current (within the tubes % rating tolerance).
The heat of the antiparallel diodes in series between the filament winding and the filament, is exactly equal to the heat of a series resistor that gives exactly the same RMS heating to the tube filament.
I mean RMS voltage, RMS current, and Heat integrated over time.
The Conservation of Energy and Laws of Thermodynamics are not violated or broken here.
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