Motorboating and NFB

[ajeffcote]

Last year I rewired a Deluxe Reverb RI with a Hoffman turret board.When I fired it up it started motorboating and would not quit until it was turned off.Fiddling with the tremolo controls(with the tremolo turned on)did vary the rate and intensity of the boat motor.I removed the NFB wireand the motorboating stopped.There were a few other problems that I worked out and after that I just played the amp.Sounds pretty good.
Now I am becoming more curios and want to experiment with different mods.But I want to cure the motorboating with the NFB hooked up first.The NFB resistor,per the Hoffman plan is 820ohm.What is causing the problem only when the NFB is connected?Maybe a different resistor value?Smaller?
I've read that improper grounding can cause the problem,but the amp sounds good otherwise.
Any and all suggestions will be appreciated. Thanks

[Enzo]

None of that.

Your NFB has become PFB because you have the leads reversed on the output transformer. Try swapping the plate leads on the output tube sockets. That is typically the blue and brown wires going to pins 3 of those sockets.

[ajeffcote]

I will certainly try that.While poking around tonight I found a bad ground in the first pre-amp stage,and I can see where I can improve the lead dress around the output tubes and phase inverter.
Occasionally,when first turned on,I could hear static when playing hard.This would go away after a while and the amp would play good the rest of the night.Not static when the pots are turned but when emphasizing a note or phrase.Possibly the ground problem.
I will try to get on it tomorrow.Thank you.

[Chuck H]

Working on a problem with another member I suggested something similar. Steve Conner was quick to point out that lower fequency oscillations like motor boating were more likely to be power supply filtering and/or ground issues. Where the high pitched whine is more likely when the NFB loop is a PFB loop. But I would try the lead swap first anyway. Then I would triple check the power supply filter and grounding circuits.

Chuck

[Enzo]

I wouldn;t disagree. I never know what someone means when they say motorboating. To me that means a freq low enough it sounds like a series of pulses. But in hard wired feedback like wrong phased OT wiring, the resulting oscillation can easily be a low note sounding remotely like a steam boat horn. Someone might refer to that as motorboating, and as one of the most common home build problems, I went that route. Especially as disconecting the NFB stops it.

[ajeffcote]

The noise is a quick paced sputter,not a squeal.As a matter of fact it sounds quite a lot like an outboard motor depicted in,maybe,a cartoon.

[Amp Kat]

It is usually caused by either a filter cap that seems to be grounded but isn't and it doesn't take much. Just a tad off and it can motorboat like crazy. Also a wrong value cap in the signal chain can do it also. Try pushing down on the caps on the main board and see if you can get it to fluctuate. Check your connections under the hood and make sure none of the dropping resistors are open are not making good contact and verify the 1st caps are the right polarity. Some are series and some are parrallel so check the schemo good and make sure they are right. A wire going to the wrong place will do it also. And yes it sounds like a very low frequency oscillation like a motorboat because that's what it is.

[ajeffcote]

I will check all the grounds tonight.Which filter caps would be the most likely culprit?
The cap values are all according to Doug Hoffman's layout and schematic for the AB763 board(I bought the board predrilled and with the turrets installed,but populated the board myself).I will double check.Press down on which caps,coupling caps?
Will check the dropping resistors.
First caps?You mean the filter mains?

[Amp Kat]

Yes the E-caps and any will do it even the bypass caps on the preamp cathodes. Carefull though I know you know but theres high voltage there so the sharpie may come in handy.

[ajeffcote]

OK,now I'm reconsidering my main filter cap scheme.I have 2 100uf 350V caps in series,with 270ohm bleeder resistors.I'm thinking of replacing them with 1 40uf 500V cap.Will that be enough filtering?The screen supply,phase inverter and pre-amp filters are all single 20uf 500V caps.With the big 100uf caps,there wasn't enough room in the cap can under the chassis for all the filters so I put the big ones inside against the front and outside wall near the light.They eat up a lot of room.I never was really happy with that so I'm going to start over again on the power and ground scheme.
Thanks,Aubrey

[ajeffcote]

I know it's been a while since I started this thread but I haven't been able to work on the amp like I want to.I am here to report some degree of success.After opening the amp again I went over the Hoffman AB763 layout and checked and rechecked everything.It all looked right.I started checking resistor values and one that I had triple checked visually was supposed to be a 470K and I had put a 470 in by mistake.That was in the trem circuit and did nothing for the motorboating,but did help with the tremelo intensity.
Upon further inspection I found the cause of the motorboating.I had wired the output jacks wrong.Easy fix and could have been avoided if I had paid more attention to what I was doing in the first place.
Now I have another problem.The amp sounds good until pushed into distortion,then I am getting some farting that starts at about 7 on the volume pot dial,and some intermittant different distortion when digging into the notes hard.At max volume I get some sizzly sounding distortion in front of the power tube distortion that I can hear after striking a bar chord,as the chord is decaying.Sometimes there is also some snap,crackle and pop in it too.Bacon frying and rice crispies with milk,only uglier.Any thoughts on this development?
Thank you,Aubrey