Two '77 Fender Twin issues

[DrGonz78]

I only did it when I was absolutely sure it was fine. The amp hadn't actually been turned on for a few days! I have read a lot and watched a lot on the subject, so I am now extremely cautious when it comes to these things!

Wow!! Yes always confirm that voltage has drained from your power supply filter caps before touching anything in the amp. Hopefully "I am now extremely cautious when it comes to these things!" means that you are verifying with a meter that the high voltage has drained out of the caps. Just remember not to touch anything that you can't outrun. In other words, shoot the tranquillizer into the wounded wild blood thirsty Tiger before you go up to offer it any medical relief.

[Minty]

I appreciate all your concerns! I actually have received a fairly nasty shock in the past from being a big gung-ho inside a different amp, and I'm very keen to not repeat that any time soon. From now on I am using my multimeter to check for any remaining charge in the caps before proceeding to touch anything!

[Minty]

I fixed the reverb hum issue! Checked inside the tank and low and behold, one of the springs is missing. For now I've swapped the dead head's reverb tank in and it works a treat! However, the dead head is still indeed dead. There's no power getting to the power tubes....The pre's are hot and glowing but the powers stay cold. So what's the next step?

[Leo_Gnardo]

However, the dead head is still indeed dead. There's no power getting to the power tubes....The pre's are hot and glowing but the powers stay cold. So what's the next step?

An odd symptom because factory wiring typically brings filament current to the power tube array then the pre tubes. Perhaps your amp is connected differently, shouldn't be a problem. Might be a good idea to do a quick continuity check on those power tube filaments, takes just a minute. Double check they're plugged in correctly. If no joy then search the filament wiring closely, with a bright lamp and maybe a magnifying glass. More than likely you'll find a faulty filament wire connection.

[52 Bill]

Can you identify pins 2 and 7 on the output tube sockets? The hint would be that the locator tab sits between pins 1 and 8.

Set your meter to read ac volts. Place the chassis open side down on your bench and pull out one of the power tubes. Now insert one meter lead into pin 2 hole and the other into pin 7 hole. Turn the power switch on and see if you get about 6-7 ac volts. Do not turn on the standby switch for this test.

You may need to poke around to get the meter leads to make contact with the tube socket connectors.

Test all four power tube sockets.

[Minty]

Okay so after re-flowing every pin on the power tube sockets, and with a bit of wiggling...I've got all 4 power tubes up and running! There was no sound coming from any of the inputs, only a loud invasive hum that turned into more of a buzz when I turned the master volume up. I noticed the first pre tube wasn't lighting up...swapped it for one I know to work, and sure enough, it's up and running! The next issue is there is quite a loud humming at all times. I guess I should be replacing some of the caps because these amps are getting to be pretty old. Which are the first caps I should be swapping out?

Thanks!

[The Dude]

The hum could be any number of things- caps, poorly matched tubes, socket pin not making connection, etc., etc. I would start by checking to see how much AC is on the B+ line.

[Enzo]

You should be finding out what is causing the hum and correcting that. DOn;t start just replacing caps until all the problems are solved.

[Minty]

Fair enough, I'll try and find out the problem before trying to fix it! That makes sense to be honest...

[Steve A.]

You should be finding out what is causing the hum and correcting that. DOn;t start just replacing caps until all the problems are solved.

With a 39 year old Fender tube amp it might not be a bad idea to replace the big filter caps inside the "doghouse" if they are still the originals. The same goes for the electrolytic cathode resistor bypass caps, although with less urgency. That is at least if Minty is planning to keep the amps and not just resell them hopefully for a profit.
But other than those parts (and the tubes) I agree that the "shotgun" approach is not advisible.

Steve

[Richard]

I think SFTR values, especially the master volume and 135 watt ones are pretty flat, there seems to be a good supply around $1000. Don't know if things are different across the pond, particularly in light of the Brexit.

[Enzo]

Steve, I am all for updating the caps, but if we start doing that before we fix the thing, we run the rick of complicating the repair or perhaps hiding the symptoms. I always vote for replacing the doghouse parts AFTER we find out what all the amp has issues with.

Like going in for an appendectomy and telling the surgeon, "As long as you are already in there, go ahead and do some liposuction and maybe a nose job."

[Steve A.]

Steve, I am all for updating the caps, but if we start doing that before we fix the thing, we run the rick of complicating the repair or perhaps hiding the symptoms. I always vote for replacing the doghouse parts AFTER we find out what all the amp has issues with.

Good point- if the amp has major problems the owner might want to sell it "as is" without spending money for any repairs.

I guess I minimize hum issues with old tube amps unless I hear an audio sample and say to myself "Now THAT is bad!" For minor hum issues I do think that changing at least some of the electrolytic caps is a good place to start (after experimenting with different tubes first, of course.) Like if the hum is only an issue when recording an amp and not when playing gigs.

Steve A.

[Steve A.]

I appreciate all your concerns! I actually have received a fairly nasty shock in the past from being a big gung-ho inside a different amp, and I'm very keen to not repeat that any time soon. From now on I am using my multimeter to check for any remaining charge in the caps before proceeding to touch anything!

I have ruined cheapie DMMs just making voltage tests inside a tube amp. I'll drain the big filter caps by putting a special jumper cable I made with a 10k resistor in the middle connected between the positive terminal of the first cap and the chassis ground. I'll make sure that they have been drained with a long blade slotted screwdriver between the positive terminals and the chassis and then put a regular jumper cable between the + terminal on the first cap and ground.

It is important to do this when changing the big filter caps on an old Fender as once you have removed the bleed resistor I have seen the caps regain enough voltage to make you flinch and knock over your beer...

Steve Ahola

[Minty]

With a 39 year old Fender tube amp it might not be a bad idea to replace the big filter caps inside the "doghouse" if they are still the originals. The same goes for the electrolytic cathode resistor bypass caps, although with less urgency. That is at least if Minty is planning to keep the amps and not just resell them hopefully for a profit.
But other than those parts (and the tubes) I agree that the "shotgun" approach is not advisible.

Steve

Yeah I was just thinking that the caps in here are close to 40 years old and could probably just do with being replaced anyway...but I'll find the humming culprit first! Oh and I'm definitely not going to be selling either of these on for a profit, I'm going for a double Twin rig! The 135's aren't exactly rare over here but paying £100 each for them, even in this condition, is not an every day occurrence!