Hiwatt DR103 blowing fuse

[Oddvar R]

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.

From tube pin to ground? I will mount the 1R resistors.

[martin manning]

From tube pin to ground?

Correct.

[martin manning]

If you have 500V on the plates, I would adjust the pin 5 bias voltage to get about 30 mV on the power tube cathodes, which would be 60% of the 25W EL34 max plate dissipation at 500V: 25W*0.6/500V = 0.030A.

[Oddvar R]

If you have 500V on the plates, I would adjust the pin 5 bias voltage to get about 30 mV on the power tube cathodes, which would be 60% of the 25W EL34 max plate dissipation at 500V: 25W*0.6/500V = 0.030A.

I have bought a new PT that's made for Hiwatt. I have never experienced fuse blowing like this, so I hope it is because of the Hammond PT.

[Oddvar R]

I have now changed the PT also, bought one from Tueb Town- Now the situation is that the PT's 3 and 4 are redplating, but not the PT's 1 and 3. If I change the 1/2 tubes to the 3/4 position, they also redplate and the fuse is going.
The redplating only happens when the standby switch is turned on. Everything seem normal with just the power switch engaged.

Someone pointed out that it might be because of the Coupling Caps, but I am not sure where they are for the PT tubes? Could someone help me with that? (Or possibly other parts?) Strange that it only happens on one side of the power tubes?

[martin manning]

With no power tubes installed, what are the voltages on pins 3 and 5 for all four output sockets?
Do you have 1Ω resistors installed between the cathodes and ground?
Do you have a bias trimmer installed or did you just use the fixed resistors as shown in the layout?

The PI coupling caps are the 47n marked Output 1 and Output 2 just to the right of center on the preamp tag strips.

[Oddvar R]

With no power tubes installed, what are the voltages on pins 3 and 5 for all four output sockets?
Do you have 1Ω resistors installed between the cathodes and ground?
Do you have a bias trimmer installed or did you just use the fixed resistors as shown in the layout?

The PI coupling caps are the 47n marked Output 1 and Output 2 just to the right of center on the preamp tag strips.

Pin 3 is around 500 v on all four and -69.7 pin 5 on all 4, with standby off, 500v on the middle board, close to zero on the lower.
I didn't have 1Ω resistors so I have ordered them.
I have fixed bias as shown in the layout.

[martin manning]

Pin 3 is around 500 v on all four and -69.7 pin 5 on all 4, with standby off, 500v on the middle board, close to zero on the lower.
I didn't have 1Ω resistors so I have ordered them.
I have fixed bias as shown in the layout.

An oscillation can cause red plating, but the -69.7V on the grids should be enough to put the output tubes into cut-off. When you get the 1Ω resistors installed, put the output tubes in and power up using a current limiter. Measure pin 3, 4, 5, and 8 voltages.

[Oddvar R]

I've changed the coupling capacitors, so finally the redplating is gone for now.

The strange thing now is that when I turn it on, also standby, the amp is silent, but as soon as I touch 3 or 5, an incredibley loud sound is turned on. When I turn off the standby and then turn it on again, the sound is gone. I am in the dark here, totally.

[martin manning]

Sounds like you have an oscillation that is started when you probe pins 3 or 5. What kind of sound do you get?

[Oddvar R]

I reversed the polarity of the OT, now it's a bit more normal. But then the redplating changed to th second of the power tubes. What could this be?

[Oddvar R]

So now the tubes is not redplating, but still no sound, except the master volume and the presence, They react when I touch pin 7 on the V3.

There is also a very low, but audible sound in the speaker which seems to indicate some kind of connection.

The power tubes have 69.7v on pin 5, 492v on pin 3 and 6.

All the plate pins make sound when touched with the meter pin. V3 being the loudest by far.

And if I touch the tip of the jack cable, I can hear a very weak buzz.

It's a puzzle?

[sluckey]

The power tubes have 69.7v on pin 5, 492v on pin 3 and 6.

EL34s will be cutoff with -69.7v on the grids. You need to change the bias circuit to put about -40v on the grids.

[Oddvar R]

Hi Sluckey, how can I do that?

[sluckey]

Replace this 1K resistor with a larger resistor to lower that -69.7V bias on pin 5 of the EL34s. I'd start with a 2.2K, then increase/decrease as needed to get a good bias voltage.

***PULL THE EL34s WHILE EXPERIMENTING WITH THIS RESISTOR***

When you have about -40v to -45v on pin 5, put the tubes back in and re-evaluate.