Heater voltages.......

[Bernardduur]

Hey all!

I was just starting a thread on a recently received HiWatt High Gain 100 with issues.......

Now when I measure her I noticed that the power tubes run on 5.5V AC (unloaded 6.1V); preamp tubes have another dedicated heater supply (24V DC which powers 2 preamp tubes).

How much is too low? How much is too high, on heater voltages......

And I guess this could be why this amp is popping fuses.......

Any idea on correcting it? Unit is set on 240V (no 220V though)......

[R.G.]

Now when I measure her I noticed that the power tubes run on 5.5V AC (unloaded 6.1V)...
How much is too low? How much is too high, on heater voltages......

The datasheets on most tubes state +/- 10% on heater voltages. So a nominal 6.3V winding would be 5.67 to 6.93. 5.5 is really close, and before I worried about it, I'd measure with a couple of meters, and make sure the meters were on scales that measured to another digit of display (i.e. "5.58" or "5.53") because the accuracy of a digital meter is always +/- one count on the least significant digit.

And I guess this could be why this amp is popping fuses...

I don't think so. Maybe, but I would be sure that something else is not a problem first. If there was a problem which pulled a lot of current out of the transformer, but was unrelated to the heater voltages, it could account for the low heater voltages. That is, the low heater voltages could be a symptom, not a cause. Low heater voltages lead to low cathode emissions, and I would think that would make power drain less, not more. It acts in the direction to not pop fuses.

Something like leaky rectifiers, leaky filter caps, etc. would be a good place to suspect.

[Bernardduur]

Thanks! I'll check the rest too.....

Amp is quite new...... would guess that filter caps SHOULD be good...... but I was surprised before . And I noticed that one of the heaters wasn't working so I resoldered most of the power tube connections......was quite needed!!

[g1]

A few questions for you. From what I understand, the voltage in the Netherlands is 230V? And the amp needs 240V? This would explain your low voltages.
Was the amp working ok in the Netherlands before the fuse went? It is the heater fuses that blow? Are they slow blow or fast? Have you tried other power tubes?

[Bernardduur]

The voltages fluctuate; at night at approx 3 o clock the voltage is approx 235V; normally it is around 230

The amp still works fine but after some play the fuses that are on the cathode of the power tube blows; always one fuse. It is a 500mA fuse, 250V, slow.

Other powertubes give the same results......

I've checked the sockets and more...... found the solder on the power sockets of bad quality (on time on of the tubes didn't even power up) so I redid that and now all works fine........ no blown fuses yet. Hope the resolder did the job.....
still the problem remains of the low voltage;

[Steve Conner]

Where did you measure the voltage? Can you access the heater wires right where they come from the PT and measure it there?

[Bernardduur]

Where did you measure the voltage? Can you access the heater wires right where they come from the PT and measure it there?

Right on where they enter the PCB; I also disconnected em and measured the same (no load) voltage

[Steve Conner]

Is the tube whose heater didn't work, the same one that is popping cathode fuses?

[Bernardduur]

Is the tube whose heater didn't work, the same one that is popping cathode fuses?

Yep! Weird thing is that before it did light up.........

[Steve Conner]

Well then, sounds like that tube has a partly shorted heater. If a couple of turns were shorted out, that would explain the low heater voltage, and if they were also shorted to the cathode, it would explain the popping cathode fuse.

Try replacing the tube and see if it fixes the problems.

[Bernardduur]

Already tried that; doesn't work.......

Heater voltage creeps up a tad with lesser tubes but the voltage stays low......

Since I resoldered the power tube sockets I had no problem with popping fuses....... Maybe that cured it

[jmaf]

Measure mains voltage with the same multimeter you're using to measure the heater secondary, at the same time of day. Report back with those two voltages - that'll help diagnose the power transformer.

[Bernardduur]

Mains are 234V AC; heaters (powertubes) are 5.58V AC; preamp tubes heaters measure 23.9V DC

[jmaf]

Mains are 234V AC; heaters (powertubes) are 5.58V AC; preamp tubes heaters measure 23.9V DC

You'd need 263 VAC mains to get 6.3 VAC at the heaters, and giving it some slack, still you'd need over 250 VAC to get 6 VAC at the heaters. Both unattainable.

Is the mains transformer heating up quickly without you playing?

Edit: the 24 V at the preamp tubes looks ok, so you've got an issue with the 6.3 VAC secondary. Maybe this transformer was rewound by a drunken sailor?

[Bernardduur]

Guess we never get to that point........ not even on a good night

Only heat comes from the tubes! Mains transformer is a no named version; no markings or whatever on it.....

I asked HiWatt a reaction to this; I would imagine they would deliver some decent products, not products that are working rather bad......