Fender twin HISSSSSSSSSS

Darvin Willhoite · Post by **Darvin Willhoite** » 4 Jun 2002 2:12 pm

I hadn't used my Twin for a couple of months, and when I fired it up a couple of weeks ago, the lead channel has a bad hiss. If I turn the treble control to 1, it virtually goes away, but I can't play that way. I suspected a tube, so I changed all of them, no difference. Any of you amp guru's have any ideas, or should I send it to the shop?

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Darvin Willhoite
Riva Ridge Recording

Jerry Erickson · Post by **Jerry Erickson** » 4 Jun 2002 2:41 pm

Could be the plate load resistors. 100K ohm
resistors that go to pin 1 and 6 of your preamp tubes. You might also remove one preamp tube at a time,starting at the phase inverter tube to see which section is making the noise.

Ken Fox · Post by **Ken Fox** » 4 Jun 2002 8:52 pm

Darvin, swap tube #1 (channel 1 preamp tube) with tube #2 (channel 2 preamp tube). If the problem goes to channel # 1 you have found the culprit is the tube. Turning the volume down eliminates the problem, correct? That tells you the hiss is prior to the volume control. That places the problem as the first 1/2 of the 12AX7 on that channel. Plate load resistors (100K) will give you a lot of crackling noises. They are quite easy to replace as well.

Darvin Willhoite · Post by **Darvin Willhoite** » 5 Jun 2002 7:47 am

You're right Ken, the hiss goes away with the volume all the way down. I switched the tubes and the hiss is still on the lead channel. I do hear some crackling noise so it's probably the resistor. I'll check that out this weekend.

Thanks Guy's

Donny Hinson · Post by **Donny Hinson** » 5 Jun 2002 7:10 pm

If it's not a tube causing the problem, it's the negative-feedback (820-ohm) resistor at the speaker jack. The sole purpose of this resistor is to lower gain, and reduce the high frequency response...which reduces hiss substantially
.

Darvin Willhoite · Post by **Darvin Willhoite** » 6 Jun 2002 8:23 am

Donny,

If it was the resistor at the speaker jack, wouldn't it have the hiss on both channels? The normal channel is quiet.

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Darvin Willhoite
Riva Ridge Recording

Donny Hinson · Post by **Donny Hinson** » 9 Jun 2002 5:51 am

Quite right, Darvin. If you're only getting the noise in the reverb channel, then it's not the negative feedback resistor. As Ken said, check the plate load resistor in the first stage, also try replacing the small bias electrolytic (25uf at 25v) in the first stage.

thurlon hopper · Post by **thurlon hopper** » 14 Jun 2002 4:28 am

HAD A PROBLEM WITH MY 1965 ORIGINAL Twin in
Okinawa and took it to alocal s

Had a problem with my 1965 Twin while living in Okinawa and took it to a local
shop where the Electician did a transformer
tap and never had any more problems with
hiss or hum. Geuss there are lots of solutions to fixing those great sounding
amps. Wish i had mine back. TJH

1972 P/P Nashville 1000 2001 Carter S-10
Peavey Bandit Transtube Fender 1965 (R) Deluxe

Michael Brebes · Post by **Michael Brebes** » 14 Jun 2002 7:09 am

Half of one of the preamp tubes is being used for the circuit before the volume/tone controls. What you describe means that the noise is happening in that circuit. I would start off by replacing the associated resistors on that circuit. They are old carbon composite resistors. You can replace them with metal film resistors of the same wattage rating, usually 1/2 watt, which are much quieter. If you follow the lead from the input connector to the board, you should be able to figure out which tube and tube half is being used. I believe 1,2,&3 are used for the first half of the tube. Except for the heater connections, the rest should be for the other half. I agree that the prime suspect will be the plate resistor.

Bob Metzger · Post by **Bob Metzger** » 23 Jun 2002 12:37 am

If you're gonna go to all the trouble to replace the 100K plate load resistors (there are 6 in all, in every Fender 2 channel amp with reverb), Michael Brebes is correct in recommending metal film resistors, but use 1 watt units.

Also, the offending channel should have shielded grid wires from the input to Pin 2 of the preamp tube for its corresponding channel and from the wiper of the volume control to pin 7 of the same preamp tube. Further noise reduction can be realized with star grounding and de-coupling of the preamp stages from one another.

Of course, you could just take it in and get it fixed and forget all this other junk.

Bob M.

P.S. If you already have shielded grids, check the shield wires at their ground point and heat it just enough to let the solder flow. That could also be the culprit![This message was edited by Bob Metzger on 23 June 2002 at 01:43 AM.][This message was edited by Bob Metzger on 23 June 2002 at 01:44 AM.]