Fender Hot Rod Deluxe - weird noise on Drive channel

[RichardLodge]

Hi, Hoping someone can shed a bit of light on a problem I have with my fender Hot Rod deluxe.

Bought the amp second hand and it's been totally fine for the last 3 months.

Turned it on the other day and got a very weird loud noise when the amps on the drive channel, the clean is totally fine.

The noise is a low oscillating hum, weirdly when I increase the gain knob the pitch of the oscillations decreases. I'm guessing there's a loose connection somewhere and was hoping that this noise points to a specific fault.

You can still hear the guitar through the noise (if that makes sense) so the channels not 'disconnected' totally.

As a note the previous owner put two groove tubes in the amp.. maybe this has had some effect on it?

I'd be so grateful if anyone can help at all.

Thanks

Richard

[RichardLodge]

Also when I tried it today it was fine for about a minute before the noise kicked back in.

[nickb]

My first guess would be one or more bad power supply caps. Try tacking a 22uf (or more) 500V in parallel with C33, C35, and C36 in turn.

[RichardLodge]

Hi Nick.. thanks for your response. Have to confess to being very new to doing any amp repairs. Could you explain a bit more as to what you mean.. I assume you're suggesting testing each power supply cap to see if it's faulty?

[52 Bill]

Hi Nick.. thanks for your response. Have to confess to being very new to doing any amp repairs. Could you explain a bit more as to what you mean.. I assume you're suggesting testing each power supply cap to see if it's faulty?

This test will require you to have a known good test filter cap and the ability to work on the amp with the back off and the power on. Do you feel qualified to do so?

As a first test, just swap the two right hand preamp tubes. Any difference?

[nickb]

Hi Nick.. thanks for your response. Have to confess to being very new to doing any amp repairs. Could you explain a bit more as to what you mean.. I assume you're suggesting testing each power supply cap to see if it's faulty?

Like Bill said, be sure you feel able to do this. The principle is to test the filter capacitors one at a time by soldering a known good one across the suspect one. Very, very , very important that
(1) Power is REMOVED (don't just switch off, pull the plug).
(2) ALL Capacitors are discharged - check with meter!
(3) New cap is attached with the +ve and -ve polarity correct.

[Mick Bailey]

Preamp tubes would be my first check - I've just had a Deville with similar problems and that's all it was.

Soldering a filter cap across a suspect one only works if the original cap you're checking already has a good solder joint. I've seen plenty of Hot Rod Deluxes where these joints have failed and this method leads you to a false conclusion if you don't re-solder the joints first. It only needs a microscopic ring-crack around the joint to cause problems. Check especially C36.

I would re-solder the main fiilter cap joints first and then test the amp. While you're at it check the tube bases where they're soldered to the PCB. This is a common failure point. I always re-work these anyhow as you can't always see they're cracked. Then move on to attaching a new cap across the suspect ones if the amp still doesn't work correctly.

[RichardLodge]

thanks for your replies guys.. really appreciated.

Ok so did a few tests with swapping the preamp tubes round.

1. First.. swapped the two pre amp tubes on the right around... same problem.

2. Then out of those two tubes removed the one on the (furthest) right.. same problem although louder and without the pitch change when moving the drive knob.

3. Then out of those two tubes removed the one on the left.. problem gone. (although obviously amp running very quietly with no distortion.

4. Then tried the right hand tube in the left socket.. right socket empty... same result as 2

This makes me think its something to do with the wiring around the left hand socket. I assume this very much points towards the filter caps?

Again thanks for your help.

[RichardLodge]

Also another weird thing... the amp only makes the humming noise on the 'more drive' setting new...

[Jazz P Bass]

Capacitor C36 is the most likely culprit.
Take that info & find a good tech who will do the work.

[RichardLodge]

Thanks.. yeh going to get a tech to do it for me.. good the narrow down the problem first though I guess.

[Mick Bailey]

For some reason C36 fails first, or often is the only failed cap in these amps.

[pdf64]

There's a lot of 'gain' fed from that B+ node; I guess if the cap gets just a bit marginal on its ESR performance then such problems are likely to occur.
Pete

[blindboybenton]

Hi
I would look at the soldering on the pre amp tube bases. had a Blues deville doing the very same. If in doubt re solder all the preamp tube bases with fresh solder