Fuse Blows When I Max The OD Pot

[flcmcya]

Have a D'lite 44 Pretty stock layout except I added OD entrance, Skyliner mod and a 360 choke with 470R/3k3. Twice while toying around I have maxed the OD pot and immediately fried the fuse....obviously it could be the pot but is there another area I should be checking that would cause this to happen?......Thanks

Randall

Thanks for the tips on the voltages Structo !

[Funkalicousgroove]

Sounds like a parasitic oscillation, do the tubes redplate when you turn up the OD Knob? Sometimes you get an oscillation at a frequency you can't hear, it causes the tubes to work super hard, and you get diminished output . IS your lead dress clean? Are your OT Primary leads reversed?

[glasman]

I agree with Brandon, do you own or have access to a scope?

[ChrisM]

I'd check your lead dress, as already mentioned.

Especially wires going to the grids. Watch out for the output tube's grid wires they are particularly long.

[flcmcya]

I was told this was Moss built when I bought it and the wiring looks real clean.......I copied a stock layout with voltages and added mine to it but mine is looking more like the tweaked layout right now.....so transpose my numbers to a 6L6 tweaked layout...........

I will check the red plating out as well......and no scope and I am pretty new with this...did some Valve jr tweaking and am building a couple of Fireflies at the moment ............What exactly is "lead dress".....I am getting that it is clean wiring?........also when I dropped the choke and related screens and raised the grids.... the bass seemed to get fartier rather than tighter which I thought was the result of this change.

Appreciate the info you all put out as it sure helps 49 year old greenhorns like myself pick up on a really cool hobby.

[IMG:617:799]http://i72.photobucket.com/albums/i199/ ... tage-1.jpg[/img]

[butwhatif]

a scope will easily show, but does it do it w the eq and pre vol off?

[Funkalicousgroove]

Just a thought, Is your Screen Voltage higher than your plate voltage? I know some Brown Notes use an OT that for some reason puts the plate voltage below the screens when you swap the resistor with a choke, it could be that during the duty cycle the screen is becoming the conductor, which would certainly fry Screen Resistors, Fuses, Tubes, etc.

[flcmcya]

Sorry about the terminology but actually the 360R was a resistor and I believe the voltage was the same on the screens and plates but will definitely look there !
Thanks for the hookup guys !

[glasman]

Pay close attention to the position of the wiring into the OD entrance and the output from the second OD section. These are in phase and if closely coupled the stage could become an oscillator.

Also adding a 47pf between the plates on the PI will reduce the high frequency gain, of course this is only bandaiding the problem.

Can you upload some pictures of the amp, this might help in isolating where it my be occuring.

Gary

[bluesfendermanblues]

I agree with Brandon, do you own or have access to a scope?

How do you actually 'see' ocillation on a scope. How do you go about it?

[glasman]

The parasitic will appear as a sine wave. So it will be easy to "see" on the scope. Given this problem I would set up the testing like this.

If the amp has a built in loop, turn the return control off. If no loop insert a jack with the tip and shield shorted.

Set the scope up to measure the signal at the send jack. The time based will have to be relatively fast as I would guess the parasitic is in the 40 to 50 kHZ range (it is drawing power and taking out the fuse). I have seen parasitics in guitar amps as high as 1Mhz.

See if the parasitic exists as you twist the OD trimmer and then if it does work backwards in the circuit.

If you find a point where the parasitic exists, leave the scope where it is visable and "chopstick" around the wires and see if there are any that cause it to increase (or decrease) in amplitude.

Another quick test to see if stage is oscillating is to measure the DC grid to cathode voltage with a high impedance voltmeter and see if the grid is positive with respect to the cathode, if it is the stage is an oscillator.

Gary

[bluesfendermanblues]

Glasman, Thanks for thorough explanation.

Do you always/rutinely check amps with the scope, even if they sound good?

[flcmcya]

A couple of leads in the OD area look pretty close together.

I will try the meter check for Osc.

[butwhatif]

Have a D'lite 44 Pretty stock layout except I added OD entrance, Skyliner mod and a 360 choke with 470R/3k3. Twice while toying around I have maxed the OD pot and immediately fried the fuse....obviously it could be the pot but is there another area I should be checking that would cause this to happen?......Thanks

Randall

Thanks for the tips on the voltages Structo !

Post a hi quality pic of the layout and let the garage team have at it---can u get some macro clear images? Do you have the shielded runs grounded on both ends--they shouldn't be.

[glasman]

Glasman, Thanks for thorough explanation.

Do you always/rutinely check amps with the scope, even if they sound good?

Yes, every amp is on the scope anytime it is connected to the dummy load.

I use the following test equipment and why. Every amp goes through the same testing.

1- Scope - watch for parasitics, check supplies for supply ripple and checking cap polarization.
2 - DMM - measure all DC voltage and measure AC on speaker to tewst output power into dummy load, sorting resistors and pots.
3 - Spectrum analyzer - Test each stage for proper frequency response, gain setting and creating amp log files.
4 - LCR Meter - Measuring all signal caps for tolerance and dissipation, filter caps for value and ESR, measuring the inductance, resistance and turns ratio of transformers and chokes.