Hi,

I have a Fender Hot Rod Deluxe guitar amplifier. I bought it as second hand like 3 years ago, and never had any problem with it. Until now. The amp has been made after 2002, as this a the date written on the PCB.

The Clean "channel" seems OK, but the Drive one has a problem. I wrote "channel" with "" because in this amp Clean and Drive aren't independant, as you can see on the schematic here :

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/hotrod_deluxe.pdf

All tubes seem OK, filament glowing. I swapped every 12AX7 with new ones but the problem remains. I checked the voltage on both preamp 12AX7's grid for an eventual leaky coupling cap, they all read 2-3mV, that seems pretty ok. I also listened to the preamp output with a jack plugged in the Power Amp In socket (that cuts the connection between preamp and power amp) : noise still here, so the problem is already here in the preamp.

I recorded the sound I get on the speaker :

http://www.wuala.com/ricothetroll/public/HotRod_Motorboat.MP3/ : you can hear the pitch of the noise (motorboating ?) changing as I turn the Drive knob.

http://www.wuala.com/ricothetroll/public/HotRod_Motorboat+instr.MP3/ : how it sounds with an instrument plugged in, in that case the higher string of a bass

http://www.wuala.com/ricothetroll/public/HotRod_Acoustic_Noise.MP3/ : with a jack plugged in the "Power Amp In" socket, to cut the sound on the speaker. Here is the acoustic noise I hear inside the amp (actually pretty weak, I had to push the gain of the microphone very high for it to be heard).

Looking on the web for clues, I found that motorboating can be caused by faulty decoupling caps. I the called a guy I know in Antwerp that has a hi-fi tube amplifiers shop and sells those high voltage rated caps but he told me that he doubts that this is the problem, as the amp is quite young, caps generally don't get dry in 10 years. I also watched a whole video on tube amps servicing (Tom Mitchell - How To Service Your Own Tube Amplifier) but found no similar case in it.

Any clue would be appreciated. It's the first time I try to service a tube amp so I have a lot to learn about it !

Happily a friend of mine has the same amp, so he'll lend it to me as I have a concert tomorrow night (those issues always occurs at the worst moment....)

Thanx in advance.

Best regards.

Eric

what are your voltages on the cathode (pins 3 and 8 of the 12ax7)?

the first sounds like oscillation
The schematic is weird, why is there only 180mv on the plate of the preamp tubes.. or am I miss reading this schematic?

But I would look at the relays.. They can cause noise when going bad

 Did you suspect output tubes ?

Quote from: iccaros on June 03, 2011, 03:03:01 PM
why is there only 180mv on the plate of the preamp tubes.. or am I miss reading this schematic?

Those voltages are the signal voltages you should be getting if you feed it a 4mv test signal, see TP1 connected to the input jack.

Quote from: slacker on June 03, 2011, 03:08:12 PM

Quote from: iccaros on June 03, 2011, 03:03:01 PM
why is there only 180mv on the plate of the preamp tubes.. or am I miss reading this schematic?

Those voltages are the signal voltages you should be getting if you feed it a 4mv test signal, see TP1 connected to the input jack.

missed that..

Thanks

As for the original, Tubes heaters working means nothing.. you can break a tube and as longs as the heater coil does not break it will still light up..
Petemore brings ups a good point about the power tube, but they are using diodes and not caps on the cathodes. I have not seen diodes go bad, but its possible.

Hi,

Thanx a lot for answering !

I might be wrong but if the power tubes were the problem, it should also appear in Clean mode, isn't it ? And I shouldn't hear the noise when listening to the preamp only at the Preamp Out outlet ? Or could the noise in the output stage couple to the preamp ?

Best regards.

Eric

Hi,

I changed all filter/decoupling capacitors, and the problem is gone !

While googling, I found this post on TheGearPage that confirmed my suspicion on the filter/dec. caps :
http://www.thegearpage.net/board/showthread.php?t=831196
That guy had exactly the same problem as me and solved it like that. Apparently some later series has crappy caps :

QuoteI've seen two versions of the IC caps. The older ones were uniformly a darker gray with clearly marked dark lettering. Recently we're seeing a lighter gray body with less dark and less clear legends. The latter have proven much less reliable. The older IC caps were generally okay.

The ones I found to replace the faulty ones are Illinois Caps but a bit darker than the former ones. I hope those will last longer than the previous ones !

Best regards.

Eric