Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on March 24, 2017, 09:42:59 am

Title: Fender pro 300 farting
Post by: pbman1953 on March 24, 2017, 09:42:59 am: I have a Fender Pro 300 - 300 watt tube amp. Recently after using, for about 10-15 minutes, it starts to fart/sizzle with each note. The sound goes up and down with the volume control. I've even swapped caps to eliminate speakers. The sound went to the other cab swapped with o it's the amp.

I have a newer set of JJ -6550's which have been fine. When this happens there is a significant loss of power.

I made some some tests because each pair of tubes has a fuse next to it and the fuse can be pulled to take them out of the circuit. I've run each set alone and could not duplicate the sound.

I know I'm the early stage of detection but was wondering if my story rings any bells.

Here's the manual- It shows Sunn but all Fender did was change the nameplate.

http://www.sunnamps.com/pdfs/manuals/bass/300T.pdf (http://www.sunnamps.com/pdfs/manuals/bass/300T.pdf)

Thanks
Title: Re: Fender pro 300 farting
Post by: shooter on March 24, 2017, 11:46:39 am: Quote
I have a newer set of JJ -6550's which have been fine.

Need some clarification, do you mean with the JJ's in the problem is gone?
Title: Re: Fender pro 300 farting
Post by: pbman1953 on March 24, 2017, 11:49:47 am: I really don't know if it is the JJ's. Using in pairs I couldn't get the amp to exhibit the issue, only when running all 6. I thought maybe I would see some flickering in any of them but I saw nothing.
Title: Re: Fender pro 300 farting
Post by: shooter on March 24, 2017, 04:25:05 pm: You can probably use the send return to split the amp in half, then you only have 1000 things to check!
Title: Re: Fender pro 300 farting
Post by: jjasilli on March 24, 2017, 07:29:39 pm: Quote from: pbman1953 on March 24, 2017, 11:49:47 am
I really don't know if it is the JJ's. Using in pairs I couldn't get the amp to exhibit the issue, only when running all 6. I thought maybe I would see some flickering in any of them but I saw nothing.

Exactly how are you splitting the pairs?
Title: Re: Fender pro 300 farting
Post by: Tony Bones on March 25, 2017, 11:33:27 am: Quote from: pbman1953 on March 24, 2017, 09:42:59 am
Recently after using, for about 10-15 minutes, it starts to fart/sizzle with each note.

As it seems to be a thermal issue I would check the resistors and look for bad solder joints. With the amp on and warmed up, gently tap each resistor with a chopstick or something nonconductive. Listen for pops from the speaker.
Title: Re: Fender pro 300 farting
Post by: kagliostro on March 25, 2017, 05:24:21 pm: The Service Manual

http://music-electronics-forum.com/attachments/6721d1256437284-300t-11x17.pdf (http://music-electronics-forum.com/attachments/6721d1256437284-300t-11x17.pdf)

Franco