Heaters question

[keaster]

Hey all I have a question about heater current draw, right now I have 4 JJ EL-34 2 JJ 12AU7 and 1 JJ 12AX7 reading the data sheets and doing the math of all the tubes heater would put close to 7 amps but with a meter the actual draw is about 6.2A when all tubes have warmed up. the transformer I'm using is 6.3v @ 7A is this reading to close to the max draw, also when first turned on the draw peaks at about 10A and then drops to the 6.2, I'm running AC heaters hum free and don't really want to change to another supply like a SMPS

also after 15 minutes of running the transformer is still stone cold

[piano3]

Have you actually measured the secondary voltage? Naturally, the current draw is higher when the tubes are cold as filament resistance is a function of temperature.

[tomchr]

I think there are a couple of things going on here:

- The heater current of the tubes is not that tightly controlled. It wouldn't surprise me if the current draw of various heaters varied at least +/-10 % from the datasheet values.

- The transformer output voltage should droop slightly under load.

- The internal resistance of the ammeter (as well as the leads connecting the ammeter to the circuit) could affect the measurements. With a resistive load (think tube filaments here), the internal resistance of the ammeter will cause a voltage drop across the ammeter, which results in the filaments drawing less current. If you have two meters, you can measure the voltage across the filaments with and without the ammeter in place and calculate the error.

We don't normally think of the effects the test equipment has on the circuit, but we are reminded of this fact when we start pushing the limits of things. Recently, I tried to measure a reference voltage through a 1 Mohm resistor. Even with my fancy 6-digit bench-top meter, I couldn't do it without loading the circuit causing my 2.5 V reference to read 2.25 V on the meter...

~Tom

[richwalters]

Originally Posted by piano3

Have you actually measured the secondary voltage? Naturally, the current draw is higher when the tubes are cold as filament resistance is a function of temperature.

This is one of those underestimated current draws. Several years use of a stage amp with a 7805 + 2diodes in the front end heater supply, I noticed it taking gradually longer to come up. Answer may be obvious, the IC has an internal current limit which responded to each switch-on as the cold tungsten heater current exceeded the rating. Replacing the IC solved the problem but repeated start up peak currents appears to tire out SS devices. Temporarily replacing the IC regulator with a discrete version with similiar current limiting properties indictated a peak switch on current of 10A for a tube heater rated at 0.6A. This implies the pass transistor must be able to
jerk a current way higher before the fold-back characteristic limiting starts.
Some ic reg's aren't up to the nasty job.

richy

[simon7000]

Heater currents are a maximum in normal operation, so you can select your transformer properly. You are fine.

Warm up should take under a minute and the transformer will limit maximum current draw from the ac line.

[keaster]

Thanks for all the info but still a bit confused, is my transformer going to be OK for the job?

[nigelwright7557]

Originally Posted by keaster

Thanks for all the info but still a bit confused, is my transformer going to be OK for the job?

Yes.

The transformer self limits the max current on power up which is good.

[trobbins]

Yes you should be fine, given your winding current rating is 7A, and you calculate just less than 7A from the datasheet levels. However, if you normally have a high mains voltage, or the primary side tapping is not correct, then you could draw more than the transformer is rated for, although your measurement seems to indicate you are fine - however your don't indicate what the meassured voltage was when the heaters had settled.

The only other aspect of operating close to full spec, assuming the 6.3V winding is the only output of the transformer, is that the transformer should not sit in an excessively high temperature environment.

Ciao, Tim

[DavesNotHere]

there could be alot of different possibilities why your transformer don't heat up.

1. core size
2. core material
3. underrated by manufacturer to pass military specifications and/or over-sized transformer that is marketing to d.i.y.-ers that will try to run them at 100%.

what transformer is this?

[keaster]

Sorry for the long delay been away.
the transformer is an Edcor XPWR002-120
I have another heater question different project

2 jj 12ax7 and 2 VOSKHOD 6dj8 I tried using a smps I had kicking around 12v@3.5a and when I connect it the output reads 0v but if I have a CL-190 on each tube, boom! 12v is the rush current too much for the smps? Has anyone tried something like this before?

also the CL-190 on the 6dj8's are hot!