Hiwatt DR103 blowing fuse

[Oddvar R]

I have now gotten to the point where the voltage seems to be present, but now, using the light bulb limiter, get low light when turned on, but as soon as the standby comes on, the light rise to full. Why could that be?

[sluckey]

Most likely wiring error. Maybe bad filter cap.

[martin manning]

With standby on standby, check the negative bias voltage on output tube pin 5's.

[Oddvar R]

With standby on standby, check the negative bias voltage on output tube pin 5's.

It's 138v when in standby, not using the lamp limiter.

[Oddvar R]

With standby on standby, check the negative bias voltage on output tube pin 5's.

Hi would you suggest checking that?

[martin manning]

Positive 138V?

What does you bias supply circuit look like? Do you have a dedicated bias winding like the original, or something else, like a tap on the high voltage winding, or is it taken from one of the HV leads?

[Oddvar R]

It's like the original.

[martin manning]

Should look like this then. Must be some wiring error if you are getting positive 138V.

[Oddvar R]

On the red circle I get aprox 500v. 0n the blue 250ish ant the other two close to zero, so maybe the the green circled ones are dea? Either or both?

[martin manning]

I think you need to go through the whole power supply and check it against the schematic.

[Oddvar R]

I think you need to go through the whole power supply and check it against the schematic.

You are probably right. Ordered new filtercaps.

[Oddvar R]

The reason why the amp is blowing the main fuse is probably because of som bias issue. All the power tubes are redplating after a little while, then the fuse goes. I am using a 290HX PT from hammond and the bias is said to be 0/98v, is there a problem with that? I have changed the OT to Tube Town Hiwatt DR103 and it seems to be doing better than the previous one, that is I get voltage on the secondaries, round 500v before the redplating begins.

Any suggestions on how to proceed?.

[martin manning]

The reason why the amp is blowing the main fuse is probably because of som bias issue. All the power tubes are redplating after a little while, then the fuse goes.

What is the idle current on the power tubes when you start up, and did you calculate the plate dissipation?

[Oddvar R]

It seems to be 3.6mA?

[martin manning]

It seems to be 3.6mA?

That doesn't sound right. How are you measuring it? If you don't have them already, I would add 1Ω resistors to the cathode grounds on all the power tubes so you can easily measure mV (= mA) for each one.