Max. heater voltage for 6V6GT / ECC83?

[Michael Tibes]

Hi all,

I'm building this kit here http://www.musikding.de/Kit-Madamp-A15Mk2-deluxe, it is an 18W Fenderish Amp. I've just inserted the tubes, gave the heater wiring a test and realized that the heater voltage is about 7,3V AC with all tubes in. This seems a bit too much, or is it still ok?

Thanks for the input,

Michael

 

[ruffrecords]

It is a little high. Most ubes are designed to work with a heater supply that is within 10% of the nominal voltage for for 6.3VAC the maximum would be 6.93VAC. The7.3VAC you measure is about 15% up. Judging from the tubes used it looks like your total heater consumption will be about 2.1 Amps and unless the mains transformer is grossly over rated, the heater voltage ought not to be this high with that sort of load. Are you sure all the tubes are lighting up?

Cheers

Ian

 

[Michael Tibes]

Are you sure all the tubes are lighting up?

Cheers

Ian

Yes, they all light up - fairly bright, not the typical dark orange but more whitish. I had checked without tubes before and it was about the same voltage. The power transformer is labeled to be rated for 6,3V 4A on the heater voltage. The heater current doesn't  go up significantly once the anode voltage is on (this measurement was without the high voltage supply), does it ? I believe not, it would make the 'standby'-switch a really bad thing...

Thanx,

Michael

 

[ruffrecords]

OK, so part of the increase could be due to the 6.3VAC winding being rated at 4A. If the regulation of the transformer is 10% (which would be quite good) it means that the no load voltage would be 10% higher than 6.3VAC. Your load is about half the rated current so you might expect the voltage to be about 5% higher ~6.6VAC but not much higher than that. The fact you say the filaments look bright (white) also indicates your measurmenent is correct. I see you are in germany so the maisn voltages should be about 230V and I would expect the primary voltage of the transformer is rated at 230V also so it is unlikely to be a mains voltage problem (the UK mains meets the European 230V specification but it is usually close to 240V in practice - whithin the spec but on the high side - and I often get slightly brighter tube filaments when using 230V primary mains transformers)

It seems odd that a manufacturer would over specify a transformer and at 7.3VAC you are likely to shorten the life of the tubes. I think you need to take teps to reduce the voltage. You need to drop about 1V at 2 amps which is a resistance of 0.5 ohms and the power dissipation will be 2 watts. I suggest you get a couple of 0.22 ohm resistors rated at 5W each and connect one in series with each of the 6.3VAC wires from the transformer. I suggest doing it this way in case the 63.V winding is centre tapped.

Cheers

ian

 

[emrr]

part of the increase could be due to the 6.3VAC winding being rated at 4A.

Guaranteed.

 

[CJ]

you can kill 1 volt at 1 amp with a 1 ohm resistor, power dissipated by the resistor will be I^2R which is

1^2 * 1 = 1

can it get any easier?

5 watt resistor isgood here

 

[Michael Tibes]

Thank you guys for the input! When I wanted to check the actual mains voltage here I got a reading of about 270V AC which surprised me quite a bit and I realized that the battery in my DMM was running low  Always expect the unexpected!

Now, with a new battery, heater reads 6,7 V, which is still higher than expected, but probably 'in spec' ? Mains is 225V, so not even 230.

This little amp seems to sound quite good at first glance 

Cheers,

Michael

 

[CJ]

6.7 is good for a sunday,

watch it drop to 6.57 volts after everybody goes back to the steel mill,