Stray capacitance???

[MikeT]

I have a champ that I rebuilt to a 6G2 with cathode bias and ss rect. Around the 8th fret up on my strat (Area pickups) on the high strings I get a subtle tremolo like oscillation. It's not cone cry because I've eliminated that possibility. I thought it may be stray capacitance so I was sure all leads to plates on the first 2 triode stages and the PI and also pin 3 on the 6V6s hug the chassis. It's still there. Any ideas?

[Robert M. Martinelli]

check pickups' distance

Hi,
maybe I' m wrong, but it' s worth to check the distance between the pickups' magnets and the strings, I have witnessed such a thing on a strat, the magnets' attraction creates a sort of "fake fret" in their area and the trem-like effect you hear could be the result of a "beating frequency" which is the difference between the real note and the fake one created by the pickup.

Loosen the pickups' screws so that you have at least a 3-4 mm gap on the high E and 6-7 mm on the low E side with the 21st ( or 22nd for newer strats ) fret depressed, then check if the problem' s gone, if so readjust the pickups but don' t go too close to the strings, because, instead of getting more signal , you' re killing sustain and creating "false" notes. Fender strat pickups and the like have quite strong magnets, so they "try" to stop string vibration if they' re too close.

Hope this helps

Best regards

Bob

[MikeT]

Hi,
maybe I' m wrong, but it' s worth to check the distance between the pickups' magnets and the strings, I have witnessed such a thing on a strat, the magnets' attraction creates a sort of "fake fret" in their area and the trem-like effect you hear could be the result of a "beating frequency" which is the difference between the real note and the fake one created by the pickup.

Loosen the pickups' screws so that you have at least a 3-4 mm gap on the high E and 6-7 mm on the low E side with the 21st ( or 22nd for newer strats ) fret depressed, then check if the problem' s gone, if so readjust the pickups but don' t go too close to the strings, because, instead of getting more signal , you' re killing sustain and creating "false" notes. Fender strat pickups and the like have quite strong magnets, so they "try" to stop string vibration if they' re too close.

Hope this helps

Best regards

Bob

I tried putting a 22@500 cap in parallel with the first leg of the filter caps and it did help. I have dimarzio Area's with Alico II magnets, which has less string pull than the standard Alnico V. I did lower them down pretty good on your suggestion but there is still some of that funky vibrato/oscillation on the full range of the high e and less prominant but still present on the b. I'm completely baffled. Thanks man....

Mike

[Robert M. Martinelli]

further suggestions

Hi,
have you tried the same amp with a different guitar and cord? This would help to sort out if the problem really resides inside or outside the amp....

I don 't know how much you' re into electronics, so I don' t know if you have a signal generator and an oscilloscope. If you don' t, it 's pretty easy to build a little battery-powered signal generator ( signal tracer ), there are tons of schematics on the internet, all you need is a chip and few caps/resistors. with this tracer and a small probe, it' s possible to inject an audio signal in various points of a circuit, which is very helpful. A good starting point is a sine wave generator with a frequency between 500- 1000 Hz and an amplitude of, say, 0 to 1000 mV p-p ( adjustable with a pot to be able to drive inputs ( low mV p-p ) as well as poweramps sections (1 V p-p ).

If the problem vanishes using another guitar/cord, I' d suggest checking the wiring and components inside your strat, if the problem' s still there when trying with another guitar, consider the tracer option to generate a stable and constant amplitude signal, then, inject it starting from the inputs and going towards the final stage to see if the problem persists - if the problem disappears at a certain stage this means the stage immediately before the one under test is responsible for your problem. If you do have a scope check the output waveshape for integrity and likeliness with the one you' re injecting, it will make your life a lot easier.

Hope this helps

Regards

Bob

[Steve Conner]

Some Strats have funky resonances between the strings and the springs on the whammy bar, if you set them up with the bar floating. I think mine does it around that area of the fretboard, too, but it's only on a few spots, not a whole range of notes.

Also, does your amp hum? The 60Hz (or 120Hz) hum can intermodulate with the guitar signal and make weird ghost notes. If the 22uF capacitor you added helped, that suggests that this is your problem. The hum on stock Champ-type amps is known to be an issue, and it always bothers me: I've ended up modding them with a Pi filter, which is to say 47uF, a big-ass choke, and another 47uF. You have a solid-state rectifier, so you can use as big a capacitor as you want: maybe try 100uF.

[Robert M. Martinelli]

good point!

Steve, you got a good point,
this is one of the reasons why I advised Mike to try another guitar; if trem springs are under suspicion I' d also try to block-off the trem ( ala Clapton ) setting it flush with the body, increasing springs tension; If trying another guitar, it could be good if it had a fixed bridge ( e.g. Les Paul and the like ). In the beginning I thought about string pull, but the magnets are AlNiCo II, so their string pull is lower than AlNiCo V, anyway Mike has lowered the pickups already to be sure this is not an issue.
The trem-like effect Mike complains about makes me seriously thing about "beating frequencies", but I' m still unsure if they come from the amp or the guitar.

I would advise Mike to go through all the tests we suggested so far, and come back with the results...Good luck Mike!

Best Regards

Bob

[MikeT]

To A/B with another guitar is something I haven't done yet.....good idea. BTW, it is a hardtail Strat, no whammy bar.. I just assumed it was the amp, especially since I have Alnico IIs and I did lower them anyway. As far as a signal generator and scope go, well, I use a transistor radio and feed the signal to the input. Then I use a probe on the end of a guitar cable, the shielding of the cable grounded to the amp chassis and the hot going into a little amp. I can then trace the radio signal through the circuit and find where it gets funky. Got the idea from a good friend who got his training in the Navy. So I don't know if this could help. I do use a choke but no pi filter. The circuit is not a single-ended Champ, but push-pull sort of like a Deluxe - same tone stack & long-tailed driver but SS rectifier and cathode bias. Hum is minimal. The amp absolutely smokes. At volume the oscillation effect isn't so obvious. But I'll try my Tele and 135 and get back to you. Thanks, guys!!!

Mike

[Robert M. Martinelli]

well, I use a transistor radio and feed the signal to the input. Then I use a probe on the end of a guitar cable, the shielding of the cable grounded to the amp chassis and the hot going into a little amp. I can then trace the radio signal through the circuit and find where it gets funky. Got the idea from a good friend who got his training in the Navy.

Mike

Hi,
using a radio is good if you don' t have anything better, but it' s much better to have a signal with a "known" waweshape ( such as a sinewave ) because distortion it' s quite easy to spot "by the ear", so, especially if you plan to go on tinkering with amps I strongly recommend you to build a small sinewave generator with variable frequency ( but anything around 1 Khz will suffice ) and amplitude ( well, a pot at the output it' s not so difficult to add.... )

Looking forward to hearing the results of your testing...

Best regards

Bob

[MikeT]

Hi,
using a radio is good if you don' t have anything better, but it' s much better to have a signal with a "known" waweshape ( such as a sinewave ) because distortion it' s quite easy to spot "by the ear", so, especially if you plan to go on tinkering with amps I strongly recommend you to build a small sinewave generator with variable frequency ( but anything around 1 Khz will suffice ) and amplitude ( well, a pot at the output it' s not so difficult to add.... )

Looking forward to hearing the results of your testing...

Best regards

Bob

It's very strange....it comes and goes. It is there and then the next day it is gone....I tried another guitar and it was faint, but it was there. Today it is very subtle with my strat but yesterday it was totally clear....go figure??? I've been tinkering with amps for 15 years, and there have been a few times when I wish I had proper training and tools. The work I have done has been for myself and friends, and never in a professional situation, although if push came to shove I probably could. I have a gig tomorrow and I'll use the amp for it and see how it goes....

Thanks!

Mike