Modded phase inverter is motorboating

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bluesbear
Hey get your own solder
Posts: 222
(7/7/03 1:13 pm)
24.165.123.219
Reply | Edit | Del All   Modded phase inverter is motorboating
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 Here we go again. This afternoon, I changed a non-reverb Princeton from a split load to a long tail phase inverter. Now it's motorboating (putt-putt-putt..about 3 times per second). When I turn the volume to about 6, it stops and at about 9 it starts again. It works but sounds crappy. I used Gerald Weber's instructions for this and it all looks right. Anyone have any ideas what I should be looking at? This is a new one on me (maybe I'm lucky!).
Thanks!!!
Dave
 
Tiny Daddy
Junior tube assistant
Posts: 25
(7/7/03 5:14 pm)
67.30.33.92
Reply | Edit | Del   Re: Modded phase inverter is motorboating
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 Are you using negative feedback? Maybe the feedback phase is wrong?
 
hmeewis
Senior tube assistant
Posts: 118
(7/7/03 6:02 pm)
208.132.19.140
Reply | Edit | Del   Re: Modded phase inverter is motorboating
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 A motor boating phase inverter is very common. Could it be that some of your wires are too long?
 
bluesbear
Hey get your own solder
Posts: 223
(7/7/03 7:36 pm)
24.165.123.219
Reply | Edit | Del   Re: Modded phase inverter is motorboating
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 I disconnected the negative feedback loop and the motorboating seems to have disappeared. Tiny Daddy, I don't know what you mean by "the feedback phase is wrong". The feedback loop is (was) wired exactly like a BF Deluxe. I'd like to hook it back up somehow, maybe it would help with the extreme farting on the low e string. It's really bad. The upper strings sound pretty good. So.......
1) What do I need to do to fix the feedback loop?
and......
2) What can I try to get rid of the explosive farting?
Thanks!!!!
Dave
 
Tiny Daddy
Junior tube assistant
Posts: 28
(7/7/03 8:28 pm)
67.30.17.60
Reply | Edit | Del   feedback
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 Hey Dave,

There are 2 possibilities here:

1. If the phase polarity is wrong (positive instead of negative feedback) your amp becomes a power square wave oscillator. With feedback connected, reverse the feedback phase by swapping the OT primary wires. If this fixes it, good… but if the motorboating becomes 10 times worse then put the wires back where they were. Make sure you are testing with a load or speaker that can take the full power output from your amp - things could get noisy.

2. There may be too much negative feedback. Adding negative feedback should REDUCE the gain slightly, 4dB or so. You may need to increase the size of the feedback resistor.

 
bluesbear
Hey get your own solder
Posts: 224
(7/8/03 8:59 am)
24.165.123.219
Reply | Edit | Del   Re: feedback
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 Okay, you meant the OT polarity. I checked that. It's correct. I'm using a 60 watt Mojo speaker. It's not the problem. I'll try raising the feedback resistance. Right now, however, there is NO negative feedback. Any idea what could be causing the SuperFarting at lower frequencies?
Thanks!!!
Dave
 
hmeewis
Senior tube assistant
Posts: 120
(7/8/03 9:36 am)
208.132.19.140
Reply | Edit | Del   Re: feedback
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 This superfarting might be a stability problem as well. By opening up the feedback loop you might not have completely fixed the problem yet. Maybe the band width of your phase splitter is to large and some small capacitors at the right spots could help you out. Doing that might require an oscilloscope and a signal generator.
 
Tiny Daddy
Junior tube assistant
Posts: 30
(7/8/03 5:25 pm)
67.30.31.67
Reply | Edit | Del   Re: feedback
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 Use a scope and signal generator and hand-plot a few points at different frequencies. Any gain irregularity or instability will show up on the plot.
 
bluesbear
Hey get your own solder
Posts: 225
(7/8/03 7:26 pm)
24.165.123.219
Reply | Edit | Del   Re: feedback
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 Got it.....Hoffman's Law, of course. It was a combination of stupidities and bad choices.
I decided the Weber PI mod was just too much high gain for my taste so I rewired back to the stock PI with the Paul C mod and reconnected the negative feedback. That got rid of the majority of the gain but the motorboating was back and the signal was real weak. I disconnected the negative feedback; the motorboating went away but the signal was still super wimpy. Then the shovel hit the back of my head....the last resistor that connects ground with the negative feedback and bias resistor from the second gain stage is supposed to be 47 ohm. Somehow, I put a 47K in there instead! That fixed everything but the strange noise when the low E string is hit. Without all the rest of the noise, I found that the cabinet was sitting on a pair of pliers and bouncing around.
Oh well, one good thing, somehow in all this, the last little bit of hum went away. That was what I actually started out to track down in the first place!!
Thanks for the help and suggestions. I'd never have had the balls to touch an iron to an amp if it wasn't for this place.
Dave
 
GroundhogKen
Moderator de forumus
Posts: 1678
(7/8/03 8:34 pm)
66.67.155.173
Reply | Edit | Del   Re: feedback
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 Please don't take this the wrong way--nothing personal
You know I like to joke around.

...But might we consider adding this story to the Hoffman Law Hall of Fame?


Ken
 
bluesbear
Hey get your own solder
Posts: 226
(7/8/03 10:00 pm)
24.165.123.219
Reply | Edit | Del   Re: feedback
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 Ken,
You can't hurt my feelings on this one! This amp has been my pet project for 6 or 8 months. It's totally my own design. I've tried every trick I've read, been told, or dreamed about. Some stayed, most were dumped. It's now perfect. Started as a Princeton with 25 watts, 12" speaker, vol-treb-mid-bass-raw (like Allens), feedback control, 5AR4, 6L6WGB, 6L6WGB, 12AT7, 12AT7. Now when turned on 10, you can't tell it's on. Weight - 15 lbs!! I've had firm orders for 5 as soon as I got the hum out. It's out! It's done! Yippie!
Use it in the Hall of Fame. I'm a happy guy.
Thanks to all!!
Dave
 
geoffreyreed
Senior tube assistant
Posts: 104
(8/7/03 4:38 pm)
152.163.253.69
Reply | Edit | Del   Re: Modded phase inverter is motorboating
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 Ground and high side of OT could be reversed,- out of phase. If the phase is wrong you've got positive feedback not negative. The bottom side of the inverter grid should see grnd thru a cap. Sounds like the inverter is not right. the farting on the low E might just be a different manifestation of the motorboating. geoff
 
 
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