Fender Hot Rod Deluxe Issue

[gbono]

The owner of this HRD is a working musician who gigs with this amp as a back up. While playing a show we was using the HRD for several hours until it started to lose volume, making noises, the pilot light flickered and the amp started to smell like it was getting hot.

I get the amp set up with all the chords, stomp pedal and guitar used by the customer. He proceeds to play the amp without issues and leaves it with me. I check all cables and jacks no issue found.

I pulled all the tubes (they look like original Fender issue) and tested them on a Hickok tester. All tubes pass without issue though one of the 6L6's has much more faded lettering (looks like it ran hotter) and one of the 12AX7's had a shorted T1 which mysteriously healed itself when retested. I realize that the Hickok is not a true test of the integrity/large signal performance.

Dis-assembled the amp and have done a visual/DMM test on all the tubes sockets/PCB, output tube grid and plate resistors also checked, and PI plate resistors but no issues. I haven't removed the main board yet but the cement resistors for the +-15 supply test out okay with a DMM.

I'm going to finish doing a visual on the main board and then replace the tubes, check all supply voltages, check bias and apply a 1000Hz input signal and monitor/scope the output into a resistive 8 ohm load, baseline/monitor component temperatures with an IR thermometer - probably will need to burn in the amp for hours to see if problem surfaces.

Not sure what other testing needs to be done to replicate the problem. I'm not much of a tube expert and would like to get some opinions on intermittent failure modes that "self heal"???

[52 Bill]

Not sure what other testing needs to be done to replicate the problem. I'm not much of a tube expert and would like to get some opinions on intermittent failure modes that "self heal"???

I'd clean and service the extension speaker jack's normally closed contact. It will cause the signal to fade in and out just like a dirty FX loop jack will. Test this by sending a signal through the amp and then push on the tip contact with your finger. If the jack is dirty, you will hear the signal fade in and out.

If one of the power tubes shows signs of overheating check the soldering of the power tube sockets and see if there is any loss of bias to that socket. Cold solder joints on the tube socket board are fairly common.

As for the self healing failure, there can be any number of reasons for false tests results, that correct themselves in a second test. I'd retest a third time and see what happens.

[Enzo]

The first thing I do whenever I open a Hot Rod or any other similarly constructed recent fender is to look closely at the solder on the power tube socket pins. In fact I usually just resolder them anyway. Bias coming and going can overheat a tube, and screens coming and going can fade your sound in and out.

Look at the pilot light itself - the holder really. If the solder tabs rotate around, they can touch the frame of the holder and short the 6v line. That would make the light blink and could easily load down the transformer.

[Billy R aka DynaFreak]

I always check the bias circuit caps to see if they are loose - a common problem in these amp. If you loose bias voltage then you get the symptoms you described in your OP.