Blowing fuses

[Firestorm]

Which model Twin? Modified or not? Look closely at the output tube socket wiring to be sure that none of the pins can short together. Did you use a light bulb limiter to see what happens to brightness at the 15-20 second mark? Did it do this before the recap? I see it's now down to 5 seconds. And immediate red plating. Bias failure, maybe?

[martin manning]

Whatever resistor is on the bias supply board ahead of the bias rectifier (depends on which SF circuit), put another of the same value in parallel to drop the resistance there by half. Recheck the bias voltage, then maybe then you can get an idle current reading.

[gui_tarzan]

Here's the schem.

[gui_tarzan]

Which model Twin?

AA270

Modified or not?

No, all original except for the tubes and a couple of filter caps.

Look closely at the output tube socket wiring to be sure that none of the pins can short together.

Check.

Did you use a light bulb limiter to see what happens to brightness at the 15-20 second mark?

Yes, it never changes.

Did it do this before the recap?

Yes, it came to me with the fuse blowing symptom.

I see it's now down to 5 seconds. And immediate red plating. Bias failure, maybe?

I replaced all the bias components with new.

[Firestorm]

-37 volts at each pin 5? As Martin said, that is low for 467 plate volts. But I wouldn't think low enough to instantly red plate.

If the fuse holds with PT secondaries disconnected (or power tubes out) and -37V is getting to each control grid, that might leave an OT short as the possible problem.

[gui_tarzan]

Yes, but I don't have time to check more than one tube before it blows a fuse. After a few seconds it shoots way up (down?) to a couple hundred volts negative and the fuse blows. Could the PT have a short under load?

[Leo_Gnardo]

Yes, but I don't have time to check more than one tube before it blows a fuse. After a few seconds it shoots way up (down?) to a couple hundred volts negative and the fuse blows. Could the PT have a short under load?

-37V on control grids is way low, looking for trouble, if nothing else was amiss I'd say there's your problem, get that bias up (down) to -45 to -55

BUT that would be too easy...

"it shoots way up (down?) to a couple hundred volts negative"

I don't know what "it" is but it certainly is bizarre to encounter a couple hundred volts negative in an amp, there's something else amiss. All grounds good? Checked & double checked?

This one is a puzzler all right.

[martin manning]

An OT short wouldn't keep it from idling at reasonable plate current. The current is high enough to red-plate the tubes, however, so the question is why. The bias voltage could be too high (not negative enough), or it could be due to an ultrasonic oscillation (I'm assuming you have a speaker load connected, and you can't hear any screeching).

Try the experiment above (halving the resistor ahead of the bias rectifier) to lower the bias voltage and see if it will idle.

What voltage goes to hundreds of volts negative? This could be your meter going out of range.

[rp]

100R filament resistors?

[Phil_S]

You've got better experts than me helping you...

First, I'd follow what Martin says and get the amp running with a much colder bias point.

If that doesn't help, I'd be inclined to disconnect both the OT and the PT and do a simple check for ohms on each winding. In doing this, since we aren't sure what they should read, it is a simple reasonableness test. Maybe someone here has an idea of what the readings should be?

If you are (un)lucky, you'll find a dead short (zero ohms). I can't imagine an open winding would blow any fuses.

[gui_tarzan]

An OT short wouldn't keep it from idling at reasonable plate current. The current is high enough to red-plate the tubes, however, so the question is why. The bias voltage could be too high (not negative enough), or it could be due to an ultrasonic oscillation (I'm assuming you have a speaker load connected, and you can't hear any screeching).

Try the experiment above (halving the resistor ahead of the bias rectifier) to lower the bias voltage and see if it will idle.

What voltage goes to hundreds of volts negative? This could be your meter going out of range.

PT, not OT. I do have a speaker on it with the tubes in.

The voltage on pin 5 of any of the tubes goes way negative just before the fuse blows.

I have 61v coming out of the PT secondary bias lead, 48v after the 1K resistor, -36v after the diode. I wouldn't think that's too low to make it blow the fuse.

[gui_tarzan]

Disconnected the secondary leads.

R1 to CT = 11.6 ohms
R2 to CT = 10.6 ohms
R1 to R2 = 21.6 ohms

Bias lead to CT = 4 ohms

Without knowing what they should be I tested one from a Bandmaster and it reads 21 and 19 ohms R* to CT so I suppose it's possible that it's good. I wish I could test the PT under load outside the chassis to find out.

[gui_tarzan]

100R filament resistors?

Yes.

[martin manning]

An OT short wouldn't keep it from idling at reasonable plate current. The current is high enough to red-plate the tubes, however, so the question is why. The bias voltage could be too high (not negative enough), or it could be due to an ultrasonic oscillation (I'm assuming you have a speaker load connected, and you can't hear any screeching).

Try the experiment above (halving the resistor ahead of the bias rectifier) to lower the bias voltage and see if it will idle.

What voltage goes to hundreds of volts negative? This could be your meter going out of range.

PT, not OT. I do have a speaker on it with the tubes in.

The voltage on pin 5 of any of the tubes goes way negative just before the fuse blows.

I have 61v coming out of the PT secondary bias lead, 48v after the 1K resistor, -36v after the diode. I wouldn't think that's too low to make it blow the fuse.

Per the schematic you should have -52V after the bias rectifier, so you are quite a bit high there.

Very low bias voltage would seem to put the tubes into cut-off, but I'm wondering if there isn't a problem with the PT, where the HT shorts to the bias winding and creates an internal loop.

[Phil_S]

...a problem with the PT, where the HT shorts to the bias winding and creates an internal loop.

Time for this? http://www.premierguitar.com/articles/t ... r-tester-1