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Old 31st July 2011, 10:19 PM   #1
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Default Power supply overload

I'm building phono amp. First I wanted to use two 12AX7 tubes with heaters rated 6.3V @ 300mA. My transformer is 6.3V @ 600mA, so I'm safe here. What if I change one 12AX7 to 6922, which heater is rated 6.3V @ 365mA. I'm going to be 65mA over limit. How much trouble I'm going to get into? Another transformer I have is 6.3V 1.2Amp, which is overkill. What is my best option? Go back to two 12AX7s or bigger transformer.
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Old 31st July 2011, 10:26 PM   #2
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Quote:
Originally Posted by voycieck View Post
I'm building phono amp. First I wanted to use two 12AX7 tubes with heaters rated 6.3V @ 300mA. My transformer is 6.3V @ 600mA, so I'm safe here. What if I change one 12AX7 to 6922, which heater is rated 6.3V @ 365mA. I'm going to be 65mA over limit. How much trouble I'm going to get into? Another transformer I have is 6.3V 1.2Amp, which is overkill. What is my best option? Go back to two 12AX7s or bigger transformer.
For any reasonable design, 65mA is nothing. Try it and see what it does to the output voltage. It should still fall within spec for heater voltage.
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Old 31st July 2011, 10:41 PM   #3
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Try it. The additional load is only 8.3% above the transformer rating.
Most transformers can deliver more current than their specs indicate.
If the transformer gets too warm, replace it with the larger one.
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Old 31st July 2011, 11:14 PM   #4
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Sorry but I must disagree, running any component at the design limit is a bad idea, running it over is a terrible idea. Have you considered what the consequences would be if the AC line voltage was 10% high (which is still within specs as far as the power company is concerned by the way)? At 10% over voltage those tubes wouldn't be drawing 665mA as expected (which is already more than 10% overload remember) but would be drawing more like 720 mA, or 20% overload. Do yourself a favor and be safe, use the larger transformer.

Mike
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Old 1st August 2011, 01:19 AM   #5
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I'd use the 1.2A transformer in either case - it'll run cooler. 0.6A transformer probably won't overheat at 12% over rating, but output voltage may be low.
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Old 1st August 2011, 01:32 AM   #6
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by voycieck View Post
I'm building phono amp. First I wanted to use two 12AX7 tubes with heaters rated 6.3V @ 300mA. My transformer is 6.3V @ 600mA, so I'm safe here. What if I change one 12AX7 to 6922, which heater is rated 6.3V @ 365mA. I'm going to be 65mA over limit. How much trouble I'm going to get into? Another transformer I have is 6.3V 1.2Amp, which is overkill. What is my best option? Go back to two 12AX7s or bigger transformer.
The limit is defined by temperature. They might have set it at "20C over ambient. If so this means when the transformer is running right at the limit the core reaches "room temp+20". Drawing more current will make it hotter. But as other have pointed out, only a little hotter. We don't know what limit they set.

What you could do is run it with a resistor as a load for an hour or two and see if you are happy with the temperature. You will need load resistors for each secondary. What I do is first run at 1/2 the load, then keep reducing the resistance by a small amount every 30 minutes or so. At some point you say "this is hot enough". This depends on your level of conservatism. Some people are fine with going 10% over the nominal limit, some want to stay well clear of it.

One other thing. You can safely push the heater secondary of the B+ it not running at full load. The total heat load from all the windings (primary and secondaries) is what matters.

People here are saying you can do it. But I don't like running transformers near their limit, they get hot.
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Old 1st August 2011, 03:28 AM   #7
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I must agree with everybody. I think I could push transformer extra 65mA and that running it near the limit is not a good idea. Well, it started as two stage SRPP 12AX7 amplifier. That is right on the limits of the transformers. The choke is rated at 8mA, so I may not have enough current to run 6922 tubes, anyway, unless I can run SRPP 6922s at 3mA. Most likely I stay with all 12AX7 tubes and 600mA heater transformers. If someone has some idea here it is:
Power transformer=450V CT @85ma
tube=EZ80 - heater powered by main transformer
choke=60H, 8mA
I know it is good for two stage SRPP 12AX7 two channel phono amp. Can I make it first SRPP 12AX7 and second SRPP 6922?
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Old 1st August 2011, 04:50 AM   #8
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I'd recommend replacing the EZ80 with 2 diodes. More current for your heaters, less heat, and more voltage to play with. You can't lose!
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Old 1st August 2011, 06:36 AM   #9
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Michael Bean View Post
Sorry but I must disagree, running any component at the design limit is a bad idea, running it over is a terrible idea. Have you considered what the consequences would be if the AC line voltage was 10% high (which is still within specs as far as the power company is concerned by the way)?
I have to agree. If you want to try it out to hear/measure the difference between the two tubes, it's probably OK. But don't make it a permanent solution.

I made the mistake once (many moons ago) to rig a 4 A transformer to a string of 12 V, 60 W (= 5 A) exterior lights. I figured, meh... what's a little over-current between friends. It'll be alright. It worked just fine until one day when I forgot to turn off the lights. I came home to a blown fuse in the fusebox inside the house (fuses on the primaries of transformers are for pussies!). The tranny had managed to get a 10 A, 230 V fuse to blow when its windings fused together. The shed where the transformer was mounted stunk of burnt enamel. I took the supply apart the next day. The transformer core was still hot to the touch...!! I could easily have caught the shed - if not the house - on fire.

~Tom
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Old 1st August 2011, 05:39 PM   #10
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by voycieck View Post
....
someone has some idea here it is:
Power transformer=450V CT @85ma
tube=EZ80 - heater powered by main transformer
choke=60H, 8mA
One way to get more power from the transformer is to replace the vacuum tube rectifier with solid state diodes. This frees up whatever current the rectifier heater uses. It will also raise the B+ a few volts. It will also remove a bit of heat from the amp.

It does change the sound. If you have a push-pull amp (this does not apply to single ended designs) and you push it by turning the volume control up the drop in the voltage across the rectifier provides a degree of "compression" and reduces the dynamic range. (the common term for this is "sag") Most HiFi enthusiasts do not think of this as a good thing but many guitar players do. If you want to preserve the sound of the tube rectifier and also use solid state diodes then place resistors in series with each diode (between the diode and transformer) such that the B+ voltage remains the same as with the tube. Roughly 50R might be a staring point. (but as I said, "Most HiFi enthusiasts do not think of this as a good thing.")

Compression is an interesting effect. It reduces the loudness of the peaks but this has the counter intuitive effect of making the music sound louder. Compression is added intentionally by recording engineers to most modern pop music for this reason.
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