Fender BM rebuild sound question

[mlp-mx6]

Folks,

I have just completed a rebuild of a Bandmaster Reverb chassis. The rebuild went very well and it fired up on the first try. However there is a nastiness in the tone that I wanted to ask about.

I largely followed "blackface Fender" wiring. A few departures - I sent some wires under the board rather than floating them along above the board. I put the 68K grid stoppers right on the grids and used low-pf shielded cable for all jack runs as well as from the volume to the 2nd triode stage. I used a ground buss rather than the brass plate for ground "plane" approach. Finally, I have an extra filtering stage that goes to the "Normal" channel. I was able to re-commission several of the "blue molded" caps from another amp that I rescued several years ago.

At low volumes it sounds excellent. When I turn it up the pick attack has this NASTY sounding distortion. It gets all blatty and ugly if I turn it up further.

This behavior does not sound like what I've read described as parasitic oscillation. And the tubes bias up just fine. Honestly, it sounds like I remember it sounding when I first got it - which was over 15 years ago. I'm wondering if the OT is not well from a sound/tone perspective. So I wanted to show the chassis and ask if anyone has any suggestions. Is this what it's supposed to sound like? I hope not.

I welcome your input. Thanks!

[Jana]

Just guessing--that sounds like either an intermittent solder joint/connection or maybe a coupling cap that is leaking.

What happens if you take the amp and isolate it from any vibrations from the speaker cabinet? Like, put it in room or place separate from the speakers. Does this change anything? If it does, it is probably a mechanical problem/connection that gets vibrated to make intermittent contact.

If there is no change from isolation to not, then I would start looking at coupling caps. Maybe a leaky one that with more volume is allowing the following grid to shift in bias.

I had a similar problem in the past and narrowed it down to a coupling cap (orange drop) whose lead was making intermittent contact inside the cap. With volume, the chassis would vibrate a tiny bit and the cap would vibrate as well. Really nasty, blatty sound that is kind of like a highly amplified version of ripping a piece of cloth.

I eventually found that by turning the amp on, turning it up and chopsticking parts. Pinging them hard.

[mlp-mx6]

Jana,

Thanks for the reply. I checked all the coupling caps and did not find any leakage. I'm going through it now checking all the joints to be sure things are good, but I was very careful when soldering to get the good flow on each joint.

One other observation, it is much worse (on both "normal" and "vibrato" channels) when the bright is on and treble is turned way up. If I turn the treble down around 4 or less it is much less apparent. Does that point you to any particular thing?

Thanks again,
Michael

[rfgordon]

What you describe is common to front-panel Fenders. It's a parasitic oscillation caused by too much grid wire.

I've fixed this in over a dozen Fenders. You need to move the tone stack caps from the board to the backs of the pots. If you want to, you can mount tag strips behind the tone pots for each channel, then mount the cap leads and the 100k ohm slope resistor to the tags and the pot tabs, running the leads (what are now plate wires) back to the attachment points on the board.

Gerald Weber of Kendrick amps turned me on to this fix, and it's never failed.

You might want to try to shorten the reverb leads, too.

Remember, short grid wires equal happy, stable amp!

[mlp-mx6]

Well, I tried the "tone stack right at the pots" on the vib channel and there was zero improvement. I can try it on the other as well, but I would've thought there would be *some* improvement in the vib channel at least.

Frustrating.

[Jana]

Do you have a scope, signal generator and dummy load? If so, set it up and start probing around to see if you can find where it is originating.

Maybe open up the amp, turn off all the lights and look for any arcing?

[mlp-mx6]

Jana,

Thanks again. I have recently been given a Tektronix scope. I need to learn how to use it!

[rfgordon]

To help you locate the gremlin, you can unsolder each channel, in turn, where they join the mixing resistor. If the noise is there for each, then the problem is downstream from that point.

[jjman]

I had a similar problem with my '71 SFDR recently and resolved it with shielded grid wires on the output tubes. Although your lead dress looks good in that regard. I had a similar problem on my 1st scratch build and it was a poor connection on the cathode ground for the output tube.

[mlp-mx6]

Thanks for the feedback and suggestions so far. I hope to get a chance to work on it again later this week. "Life" in the way of progress on this amp for the moment.

[John C]

To see where the "attack-splat" begins, tap and send the signal after each of your stages to a different (clean) amp. If, by chance, you've mistakenly installed a cathode resistor value that is too large or a plate resistor value that is too small, you could get a splat attack from a preamp stage that is biased too close to cutoff.

Please keep us posted!

[Cliff Schecht]

Those big blue capacitors are old oil in paper types and are also suspect. Tap test all of these too, it's pretty obvious (usually) when one of them has gone.

[mlp-mx6]

Those big blue capacitors are old oil in paper types and are also suspect. Tap test all of these too, it's pretty obvious (usually) when one of them has gone.

I thought they were film/foil polyester. Regardless, I have checked them and have no problems from any of them.

[Cliff Schecht]

Hrmm, you may be right. I'm not 100% sure (should have stated that initially) so let's see what others say.

And if they check out fine then I wouldn't worry about them.

[Structo]

I'm pretty positive those blue molded caps are film caps.
And highly prized as well.