Motorboating Silvertone 1484Motorboating Silvertone 1484 Try

[Micajah]

Motorboating Silvertone 1484
Trying to finish my 1484 refurb. When I got it it was missing the output transformer. I replaced it with a new Weber 1484 direct replacement transformer. I then replaced all of the electrolytic caps including the multi can (re-stuffedit to save space) and the drifted resistors as well as a couple of the caps that looked suspicious. I added a grounded 3-pin AC cord and disconnected the unnecessary ground switch. Replaced a bad 8 pin socket. Deoxed the other sockets as well as all the pots. Replaced the input jacks with switched and isolated jacks - grounding them back to the preamp tube with shielded wire. Also added a switchable 4/8 ohm output jack.
Now I have a motorboat oscillation that varies from sub-audible to treble depending on the tone controls. This occurs on both channels when I turn them up. I can hear the amplified input but the motor boating is very strong overwhelming. Is it possible that the choke is bad? - it's the one thing I didn't check so far.
Im going to go over the work I did one more time to make sure I didn't get something wired incorrectly.
Any suggestions where to start?
Thanks in advance.

[David Root]

Assuming you used the same or higher value PS caps than the specified originals, something else is doing this.

I would suspect parasitic oscillation(s) somewhere. Let us know if anything comes up after you check your wiring.

If you are using 6FQ7s in V3 & V5, try 6CG7 as per the schematic. 6CG7 has an internal screen pin 9 that should be grounded to the chassis. 6FQ7 is a 6CG7 without the screen. If you already have 6CG7s in there make sure the screens are grounded.

[Stevem]

Swap the blue and brown OT leads on pin 3 of each output tube and see if it gets better or worse.
You can also swap the two OT primary leads if thats easyer.

[David Root]

Good point Stevem. I forgot the OT was replaced - 50% chance of wiring it wrong!

[Micajah]

Swap the blue and brown OT leads on pin 3 of each output tube and see if it gets better or worse.
You can also swap the two OT primary leads if thats easyer.

Thanks for the responses. I'll try the OT Lead swap but this amp doesn't have a feedback loop so I don't think it will make a difference. But I'm game.

If you are using 6FQ7s in V3 & V5, try 6CG7 as per the schematic. 6CG7 has an internal screen pin 9 that should be grounded to the chassis. 6FQ7 is a 6CG7 without the screen. If you already have 6CG7s in there make sure the screens are grounded.

I am using 6FQ7 in one position and a 6FQ7/6CG7 in the other. Will make sure the screens are grounded or switch both to the 6CG7's.

[Structo]

Does this amp have grid stoppers on the power tubes?

[Micajah]

Does this amp have grid stoppers on the power tubes?

I don't think so - here's a re-drawn schematic that i found elsewhere.

[Structo]

Might be worth a try to install grid resistors, start with 1K5.

You can go up to around 3K3 before it being to detrimental.

[David Root]

The original schem is on Schematic Heaven. It definitely does not have power tube gridstoppers, or screen resistors.

Real 6CG7s show the screen clearly, located between the plates. In a 6FQ7 it's not there. In my experience, most if not all tubes where the box is labeled 6FQ7/6CG7 are actually 6FQ7. You might want to check that.

They are usually later manufacture, 1970 or so and later--as tubes were being driven out of business, manufacturers did all they could to cut costs, and losing the screen in the 6CG7 was part of that effort.

[Micajah]

Swap the blue and brown OT leads on pin 3 of each output tube and see if it gets better or worse.
You can also swap the two OT primary leads if thats easyer.

Yeah - didn't seem to make a difference. on the volume knob If 7:00 is 0 then it starts oscillating at 9:00. Als o noticed something else - If volume is down and I tap on any of the pre-amp tubes (1-5) with a chopstick I get a lot of snap crackle & popping noise that dies off after a few seconds. Have to keep digging - couldn't have been that easy.

[Stevem]

Thats a sign high resistance from tube pin to socket, so you have a grid acting like a antenna and creating oscilation.
Sorry I forgot the amp had no feedback, so the lead swap will change nothing.
I use a dremel tool with a round wire wheel on it to clean tube pins and then a use a cut off section of round wound D or G string(depending on the gauge) and run it down loaded with cleaner into each reciver in the tube(s) sockets to clean and polish things out, then I Q tip swab some cleaner lube on the tube pins to stop tarnish and corrision.

[Micajah]

That's what I was thinking. I tried retensiioning and cleaning the sockets and pins. I had to replace one of the 9 pin sockets already because of bad solder pins. I may have to start replacing the others. I'll try the procedure you suggested to see if it will help. Thanks

[Micajah]

Ok- retensioned and cleaned the 9 pin sockets - got rid of the snap crackle pop but Motorboating is still there. Moving on.

[Stevem]

Start pulling preamp tubes from V1 (12ax7) on back to the PI tube and see where the motor boating stops and at least we might oin it down to a stage or two.

[Micajah]

OK - tracked the motor boating problem down - When I bought this amp it was missing the OT. And it was obvious that someone else had done some fixin' - some of the caps were already replaced etc. I ASSumed that they were all replaced correctly. Well on V1, the electrolytic between the cathode pins 8 & 3 was hooked up not to a ground via a terminal strip, but to the strip connector next to the grounding connector which was a leg of the power supply. Wont ASSume anything is correct again (maybe that was why the OT want bad.)
Switched it to the correct connector and now I have no more motor boating.
In fact the amp is quiet. No hum or buzz. Too quiet. Another problem to track down.
Input one can just barely be heard turned all the way up with the guitar all the way up. This is obviously not right.
Input two is much louder - but not what I expect when I turn it all the way up. Seems like it should be louder.
Also seems to breakup real easy. But on the positive side - I can hear and adjust tremolo. But no reverb. Though before when it was motor boating I could swear if I turned up the reverb knob and knocked the tank I could hear it.
I switched tubes thinking it may be the prob. No change.
So back to the circuit tracing.