Hotrod Deluxe - Muffled sound

[dlkbs1]

Hi,

I am new to the forum and new to owning valve amps, specifically a Fender hotrod Deluxe, bought second hand a few weeks ago.

The amp has a problem and I have done a thorough search on here and have found similar problems but nothing exactly the same.

Anyway here goes. After using the clean channel for about 15 - 20 minutes (from cold) it suddenly goes muffled with corresponding slight drop in volume.

I had no idea how old the valves were so swapped those out following advice from an amp tech. This has not made any difference. I have had a look at R78 and R79 which are 330ohm in this particular amp. both are discoloured or actually the bits of gloopy stuff on them are discoloured. Before I take the Circuit board out to have a look at the joints I just wanted to ask advice if this is the likely area for this sort of muffling sound?

Any help greatly appreciated

Thanks
Barry

[52 Bill]

Welcome to the place.

I can't think of any specific thing that would cause your symptoms, but if the amp does this only after warming up there are a few things to check. The new tubes are always a good starting point.

When it does this, do any of the controls affect the sound? Does mechanically vibrating the amp make anything change? Have you tried jumpering between the preamp out and power amp in jacks? Try rotating/twisting the speaker plug in the jack, is the sound affected?

Now we need to know what your skill level is and what sorts of test equipment you have access to. There may be a problem with the power resistors that you have noted, and there could be other things that have either cold solder or broken solder joints. Removing the board is not difficult, but you really only want to do it once if possible.

[dlkbs1]

Thanks for the response 52 Bill.

All the controls still work, just not quite as effective when it goes to its muffled state, it's like someone has thrown a blanket over it. Letting the Amp cool down solves it for a while. My friend and I have a fair bit of electronics experience and have various kit between us. We have tried the obvious poking around with a plastic ruler to check for loose components, speaker jack is fine. Not tried the effects loop jump yet though.

[52 Bill]

We have tried the obvious poking around with a plastic ruler to check for loose components, speaker jack is fine. Not tried the effects loop jump yet though.

Start by checking the tube socket board along the bottom of the chassis for any bad connections and solder joints. While you are in the area check the extension speaker jack's switch connection.

When it goes into fault mode, try plugging your guitar into the power amp in socket. It won't be very loud, but see if the sound there is clear or muffled.

[Loudtubeamps]

hello. just popped by and saw you amp question.
On these amps, the capacitors rattle loose quite often.I would see if any of the Cathode cap.s (22mfd @25 volts) are loose on the board first.
cheers, doug

[dlkbs1]

Thanks for the suggestions, hopefully try those thongs in the next few days.

[Chuck H]

I might also check the bias on the power tubes. Also, IIRC from previous threads, the filters in those amps have a tendancy to fail prematurely. I don't know how old the amp is. But if the filter caps are original and the amp is over five years old I would just replace those, and any other electrolytic caps in the amp, with high quality replacements. The work isn't that hard and the expense isn't that great. Small sacrifice and great insurance, possibly gains.

[Enzo]

My first advice would be look REAL close at the solder on the pins of the power tube sockets. Look for tiny cracks around the pin itself. Also, the phase inverter tube is the small tube next to the power tubes. FInd pins 1 and 6. When this condition happens, check the voltage at pins 1 and 6 on that socket. Did either of them drop to close to zero instead of the couple hundred volts?

In my experience, the cracked solder of the power tubes, followed by the failure of the 82k plate resistor on the phase inverter are the most common repairs I do to these amps.

I could be wrong, but I tend to doubt a cap problem. FIlter caps may indeed need replacing, but if they fail, you generally hear a large increase in hum, or distorted low notes rather than a loss of highs or volume. FIlter caps later in the chain are for decoupling mostly, and if thise fail, you get oscillation or other odd things.

[dlkbs1]

Ok, fingers crossed, it was OK last night following surgery and I had it switched on for over an hour. I did two things after taking the board out and inspecting. The solder under R78 and 79 was a bit of a mess a big lump on one end, scorched and didn't look to be that well adhered to the trace, so re-did that. Also re-did all the Power tube solder points. One of those things seems to have done the trick, needs a bit more testing yet before I am completely convinced.