Help With Spitfire Build?!

[guitback1]

I have a kit I bought years ago (when Ceriatone was affordable!). Ceriatone has gone through several versions of the Spitfire clone thing. They all have a 180pf cap on the volume pot for a little added brightness. I have used several values through the years. I even had them switchable using a 2-position (a-la-Fender) switch to be able to switch between 90pf and 220 pf. For some reason though, now if I put a cap like that between the terminals on my volume knob...I get an odd distortion artifact. The larger the capacity -the more obvious the effect is. The amp does not do this whenever I have no bright cap on there. However, the minute I insert one...boom...I get a weird and ugly distortion on top of the signal coming out of the speaker. I'm at a loss. Any suggestions???

[loudthud]

I suspect an ultrasonic oscillation, easy to see if you have an oscilloscope. Connect a meter set to AC Volts across the speaker. Normally there will be a low reading like 0.010 V (10mV). Adjust the Volume control up and down. Does it suddenly jump up to a reading of several Volts?

Check that the nuts holding the pots are all good and tight. Check that the ground connections to the input and speaker jacks are good. Try moving and wires that go to the pots or tubes with a chopstick. Keep an eye on the meter connected to the speaker.

[guitback1]

Sadly, I don't have an oscilloscope. This is such an aggravating problem. The amp works fine -as long as I have nothing in the way of a bright cap on the volume knob! It'd be fine if I only gigged with my Tele. However, I have several guitars that are dark enough to need the bright cap. The stock bright cap is 180pf, but I have used all sorts of values in there through the years. (42pf all the way to 220pf) At this point though, use of ANY value bright cap on the volume pot comes with an annoying (and unacceptable) distortion on top of the notes!

[Justin Thomas]

Without a scope, you probably will have to wiggle wires around to find the right lead dress... I had the same issue in my Spitfire build, but I used an untraditional chassis, layout, etc. I just did trial & error; took four months of wire routing to get it to quit. But, WOW! It was worth it, and everybody wanted to buy it. Oddly, I play a Tele too, and I want it even brighter... especially in the "higher gain" settings.

Justin

[guitback1]

So Justin, do you think that maybe something has just devolved into a loose connection or something? This amp worked well for a couple of years. I just hadn't used it for quite a while because I acquired an AC15c1. The problem wasn't there a few months ago (the last time I used this amp). As with yours, mine is a bit different in execution. In my case it is in a Fender-style chassis, but has a "rail" style ground that functions much like a star ground.

[Justin Thomas]

Seems I misunderstood! I thought you bought the kit then sat on it for a few years, unopened! I'd still question tubes, though. One thought - do you put in the amp back together fully (in the cabinet) when you add the cap & test? Sometimes you can lose out on shielding, etc. if you don't. There is a possibility as loudthud said of a ground connection going bad. Check the pots, jacks, switches, grounds, etc. And try wiggling some wires with a chopstick while the cap is installed and the amp is malfunctioning. It's possible something shifted. Or maybe a filter cap went bad. Hard to tell.

Justin