71 Fender Pro Motorboating

[Thorny]

It came in needing to be capped. I recapped it. I did put 100mfd/350v instead of the stock 70mfd/350v caps. I changed a few weak tubes, new power tubes, biased. Since it was a 71 I changed the bias from a "balance" like the blackface circuit.

If you turn it up on 10 and turn the bass up on 10 (either channel) I can get it to motorboat. Well, rather than a straight "putt, putt, putt" sound it is more like a galloping horse. It will usually continue until you turn the bass down or the volume down - sometimes eventually stopping on its own after a while.

I did some "chopsticking" and never could make it start or stop by moving anything - at least with any consistency. Nothing seems to be getting overly hot, nor does cold spray isolate anything. I have tried different tubes.

This one is driving me nuts! Am I crazy to worry about it because no one would play an amp this way? The customer brought it back this way - so maybe not.

Any ideas? I will go to sleep now and look at it over the weekend.

[Tom Phillips]

Thorny,
Re: “Am I crazy to worry about it because no one would play an amp this way? The customer brought it back this way - so maybe not.”

If it were me I’d worry because the amp is not behaving normally for one of it’s kind so it’s telling you that something is wrong. I understand your frustration. If by “The customer brought it back this way” you mean that it was motorboating before you re-capped, then it’s probably not the caps. Consider the possibility that the amp has been doing this all it’s life due to a mistake made at the factory when it was built.

I’d do the following:
1) Check the integrity of the grounding of each electrolytic looking for a bad ground. I thought of this because it appears that the amplifier stages are talking to each other. This could be because they are not properly decoupled. Could be a broken solder joint or that factory mistake. Maybe a missing wire under the eyelet board. You can check each filter cap ground with an ohm meter with the amp off and the caps discharged or get the amp motorboating then jumper each filter cap negative lead to ground one at a time and see if the motorboating stops. Be sure NOT to ground the midpoint of the two first stage filter caps that are in series.
2) Check all the other grounds too. Especially mechanical ground points like at the speaker output jack.
3) Also check that the voltage supply to each stage is connected to the proper filter stage and that two stages that are supposed to be connected to different points are not connected together.
4) With the amp off and the filters discharged. Measure the value of the decoupling resistors in the power supply between each filter stage. You are looking for one or more that’s WAY low or maybe an incorrect value installed like a 470 ohm where it is usually a 4,700 Ohm.

Sounds like a lot to check but it doesn’t take much time.
That’s my idea. Good luck.
Tom

[Rick Erickson]

You might want to look at the input side of the PI. If it has the .01uF (SF value) cap on the input and the 1M (BF value) load resistors you could be getting excessive bass response, which could cause a motorboating problem. You might try reducing the size of the PI input cap anyway to see if this tames it. Since both channels are doing the motorboating I doubt it's a problem in the channels, unless it's a bad filter cap or ground in the preamp supply.

RE

[Enzo]

Did the amp work when it came in? ALways always always make sure an amp works before modifying it. ANd that includes pre-emptory things like changing all the caps. Once an amp works, THEN start changing things. If ther is a bad cap, change it, but make things work before replacing the ones that are still working. The reason is that in case of trouble, you know whether it was your work or a pre-existing condition.

I too would be looking for soft filters in the preamp - loss of ground to decoupler, bad cap, other bad connection or missing one. ANy chance one of those points where a component ground side lead is extended to the eyelet next to it, but when replacing hte cap, that extra part was not replaced and the cap was simply soldered to the eyelet and no further?

[Amp Kat]

I had one like that and it was one that I redid the whole circuit to a Pro. Try putting a 500pf cap from the treble wiper to ground. Also move some wires some more. I had to bring the PI to the grid resistors wire around the first tube socket and it stopped the sqeal I was getting but it was motorboating too. It does sound like a connection thing but all of the solder joints were fine.