blowing fuses

[patm]

Hi all,

The Iron upgrade didn't go as well as I had hoped. For clarity, I will refer to L and R Paramounts. Did R first. Plugged in with tubes installed and all seemed well, tubes lit up and all seemed well. I didn't play music as I was anxious to move on to L. When I picked L up, I noticed ceramic bits on the floor. Sure enough the 4 pin socket was cracked. I did the iron upgrade and tried to carefully reinstall the tubes. Blew a fuse on startup. Ordered some nice teflon tube sockets and installed one in L. Plugged it in and tubes light up and all seemed well. I plugged L into the system (FPIII, 300b Paramounts, & Regga Apollo) and turned on L and R. Now R is blowing fuses. L and R were never on the workbench at the same time so no parts were interchanged. Resistance checks on R are all within limits. I have looked long and hard for stray bits shorting across something and can find nothing. Removed and reinstalled iron noting position of insulation washers, double checked Iron wiring. After reading some posts about blown fuses, tried without tubes, still blowing fuses. I need some direction to trouble shoot further.  (forgot to say amps played fine for two years before iron upgrade. L plays fine)

Pat McSween

[Grainger49]

Pat,

Booger!  If the amp blows a fuse without tubes it might be in the diode chain.  Look very carefully at the diodes around Terminal 11-15.  There might be some indication that there was a problem.

I can't imagine your circuit boards going bad after 2 years.  But DO check the connections from the transformer to the power supply circuit board (dead center of the chassis).

[Paul Joppa]

So, the Right amp did not blow fuses on the bench, it only started after the amp was moved to the listening room. I conclude something mechanical happened during that move. Can't be a wiring error, so it must be some connection. The most likely to cause a blown fuse is detaching the 300B grid from ground, which would cause excessive plate current.

Three possibilities from my first reaction are 1) internal short, grid to filament, in the 300B - swap tubes to check this; 2) the 220 ohm carbon composition grid stopper at A3; 3) cold solder connection at A3 - T7 - T10.

[patm]

Granger,

The diode chain looks normal no discoloration tight connections, no stray bits shorting across. I rewet all connections from xfmr to board. Still blowing fuses.

Paul,

Would the tube be suspect if the amp blows fuses with the tubes out? I get 223 ohms from pin socket 3 to T7. I will have to remove some stuff to rewet A3,T7, & T10 a job for tomorrow.

[Doc B.]

Granger,

The diode chain looks normal no discoloration tight connections, no stray bits shorting across. I rewet all connections from xfmr to board. Still blowing fuses.

Paul,

Would the tube be suspect if the amp blows fuses with the tubes out? I get 223 ohms from pin socket 3 to T7. I will have to remove some stuff to rewet A3,T7, & T10 a job for tomorrow.

Can't tell if a rectifier diode is good just by looking at it. Measure across each one with your ohmmeter. You should see some resistance in one direction, maybe 500 to 1500 ohms. In the other direction you should see infinity. If instead you see a very low reading, like 50 ohms or less each way the diode is blown and needs to be replaced.

[patm]

Thanks for the lesson Doc, I get 2 or 3 thousand one way and inf the other.

Pat

[Paul Joppa]

Pat, you're right, no tube blowing fuses means it's not in the amplifier circuit, so grid shorts etc. are NOT the problem!

[patm]

This morning I removed the plate choke and output xfmr and had a good look under the power xfmr for stray bits. I can see nothing amiss under there. While I had it apart I replaced the 4 pin socket with a teflon unit I got when the other Paramount had a broken one. I reinstalled the iron. Still blowing fuses. With my lack of knowledge and trouble shooting ability, I think it's time to throw in the towel and contact Ed Fallon about a fix. Thank you all for the help.

Pat

[Lee Hankins]

Pat, in the past I have had similiar problems with Bottlehead gear.  It was found is several cases (IIRC) that the fuse was actually underspecified.  It happen to me again with the Crack, solution is go to the next larger fuse.  In all cases installing the next larger fuse solved the problem for me.

If you have a direct short, using a larger fuse (within reason) will not hurt the amp.

Cordially,
Lee Hankins

[Paul Joppa]

Often a slow-blow fuse of the same rating can replace a fast-blow fuse that went out, and live to tell about it. The fuse can blow on start-up as the power supply capacitors charge up. Sadly, fuses are not precision components, and a fuse from one batch might survive while another batch will all blow right away. It's worth a few bucks to be sure before shipping your amp!   

Slow-blow fuses are less widely stocked, and more expensive when you do find them, because they are a little more complicated inside. But they will often allow a smaller rating, which (slightly) increases safety.

[patm]

YOU GUYS ROCK! went to the shack and got an assortment of larger rated fast blow and slow blow. The first one I tried was the 1amp slow blow, success!!! Listening as we speak. Once again thanks to Paul, Doc, Lee, & Granger. This is the intangible that you receive when you enter the Bottlehead family, The support through the Forum is unmatched anywhere.

Pat