Fender hot rod deluxe loud hum issue

[ogeecheeman]

Well here we go...had one of these hot rod dogs on the bench last year.same problem.went through all of the usual fixes,re-cap power supply,rectifier diodes,tubes,reflow joints,check all grounding points. in the first amp the re-cap and rectifier fix solved it.this one not so lucky.I noticed in another post someone said they could kill it by switching on drive channel then turning the drive pot off. on mine, the only way to kill it is to switch to drive channel then turn master off.also if I turn up drive channel and turn on more drive the hum is unbearable.on the scope it looks like AC riding on the DC.checked cap c49 (22uf / 25v) checked good. replaced B+ caps.bias is stable.If I insert a plug into power amp input jack the noise is gone-completely.if I run signal from pre amp out to another amp noise is there.even if I remove all tubes except V1 & V2. heater AC reads normal although I haven't checked 100ohm resistors.interesting to note from the customer, when the problem first showed up it was intermittent.comes and goes. customer re-tubed.ok for a couple of days then he said it got to sounding terribly distorted. he brought it to me and one output tube was red plating. I subbed a good pair and no more red plating but still distorted and one tube sounded microphonic. swapped tubes and trouble followed the tube.after replacing all tubes,no longer microphonic but still distorted.suspected output tranny. customer insisted on trying to use it.a few weeks later he brings it in,very low power very distorted. open 100k plate resistor-pin 6 phase inverter tube. after replacing,clean channel now loud and clean,usable but with annoying hum. turn on drive channel not usable-unbearable hum. shorting pin 2 or pin 7 of V1 no change. shorting pin 2 of V2 kills it which is the same as turning down master volume.it's in the background in clean mode but in drive mode it's just wacked.any insight appreciated.

[ogeecheeman]

o.k. fellas. have a good laugh.I deserve it. just reminded myself of a long forgotten lesson.never,ever under any circumstance, allow the customer to figure into the diagnosis of a tube amp because if I never learned anything I learned that brand new matched set of tubes couldn't possibly be the problem.they're brand new.guess what customer...your brand new tubes suck. dug a pair of 12ax7's out of the trash to sub for V1 & V2...quiet as a mouse.no hum,no buzz just quiet.

[g1]

Yep, ruling out tubes as suspects just because they are new is not valid.
In this case, it was a side issue caused by your customer doing a re-tube in an attempt to fix another issue.
Just had the same thing happen. The owner had taken the amp to a store for an unrelated issue. They tried a retube but it did not help and I guess no one noticed the extra hum that I found objectionable (after fixing the complaint). The hum was from one of the new preamp tubes.
The arguement of not throwing parts at something because it can cause more issues may be doubly true for tubes.

[ogeecheeman]

my customer had replaced his pre amp tubes with chinese tung sol 12ax7's. I'm wondering if their quality control has gotten that bad.a friend of mine that works at the music store that sold them says they have had trouble with these before

[jmaf]

I pushed a recent made tungsol 5881 into the socket and my thumb just went straight into the top of the tube where it had the 5881 printed. The glass had a fault and it was just waiting for someone to push hard enough. I've had other issues with new tungsols, apart from not really being impressed with the tone of their new EL34's and 5881's.

[52 Bill]

A few years back, I had a few problems with Tungsol reissue 12AT7, but nothing recently.

I thought that they were all made in Russia, with the rest of the Sovtek, EH, etc, tubes.

[Leo_Gnardo]

I thought that they were all made in Russia, with the rest of the Sovtek, EH, etc, tubes.

Within the last couple years New Sensor has started to distribute Chinese tubes, of course with a variety of their ersatz legacy brand names painted on. WHY? I can think of only one main reason: it's more profitable! Quality, who gives a flying ####. And of course it adds yet more confusement to the tube distro game.

[ogeecheeman]

just read in another forum,warning not to use tung sols in cathode follower stage.that they would fry. many post recommend 12at7 for V1-V2-V3. any thoughts on this?

[The Dude]

I have a thought, but I'll reserve comment until you link the forum page where you read that.

[jmaf]

just read in another forum,warning not to use tung sols in cathode follower stage.that they would fry. many post recommend 12at7 for V1-V2-V3. any thoughts on this?

Well the cathode followers found in Fender tone stacks do go over the maximum rated volts between cathode-heaters. Not that Leo ever cared about maximum tube ratings.

But I'm not at all convinced this is a problem with just the new Russian Tung Sol's. I've never seen a 12A*7 fail those Fender style direct coupled cathode followers because of the excessive voltage at the cathode.

[diydidi]

I had a marshall haze 40 here that was humming like crazy. Swapped V1 for something else, problem solved. I hate to ask, but can someone explain to me how a 12AX7 can cause hum in an amp? Does it have to to with the heater?

[Enzo]

One way is for the heater to make contact with the cathode inside the tube, that will connect the 60Hz (or 50Hz) to the signal path.