Fender Twin Turn On Thump

[52 Bill]

Here's one for you guys that are much smarted than I am.

1970 Silver Face Fender Twin no Master Volume. When switching from Standby to Play mode the amp makes a very loud thump, almost like an arcing sound. After a few hours of trying everything from switches, filter caps, tubes, etc. finally isolated it to the non-vibrato channel, first preamp tube.

To make a really long story short, I discovered that when the Volume control is turned down to 1 (input grid grounded) the amp makes this noise. Turn up the Volume control to 4, the amp turns on with almost no noise at all. When the stage powers up the tube oscillates or something that causes a loud noise in the output.

I've moved wires around and checked every connection in the stage with no change. When I get back to the amp, I'll try a few things to see if they will change the symptom, but I would have thought that with the input grounded, the tube couldn't oscillate.

Any thoughts?

[Chuck H]

Funky cathode bypass cap? If something funny is happening as it attempts to charge it would be very audible as the first gain stage.

[nevetslab]

The Turn-on Thump, or lack of it has always puzzled me. I never had tried playing with the volume control settings of either channel in chasing that thump. The 65 re-issue power supply circuit is different than the older Twin Reverbs, all which have the first stacked-filter supply charged up, with the Standby switch following it, energizing the C/T of the Output Xfmr & the choke feeding the rest of the supplies. The re-issue Grounds the bottom of the diode bridge, charging up all of the supplies in that one closing. I've given up on trying to understand it, being puzzled by hearing some having a little bit, others seemingly none at all, others a healthy whallop. I never got any results in changing the supply caps when I had pursued it. I always wondered if it was related to the power transformer.

What 52 Bill has reported makes me want to check this out further, as there's a good 20 or more Fender Twins in our rental inventory, and I'm still doing preventative maintenance on our Fender inventory. Nice find!!

[cerrem]

Try a cap across the Stand-By switch... Use 600V or larger film cap... like .047uF to .1uF ...
Should silence thing down...

[Leo_Gnardo]

Try a cap across the Stand-By switch... Use 600V or larger film cap... like .047uF to .1uF ...
Should silence thing down...

What cerrem said ^^^. And if it doesn't work well enough, remember when switching standby to operate mode, there's a bunch of caps that have to charge up. Both in the power supply and all those EQ and interstage caps, there's bound to be some noisemaking. Which brings us back to the old argument, why do we need standby switches anyway? That one's been kicked around several times on MEF, also on other amp tech websites.

IF, instead of using the standby switch to disable/enable high voltage, instead it's used to mute audio say by shorting output tube grids to each other, then all those caps are fully charged when you switch into go mode. Some old amps do it this way, no modern ones that I know of.

Also we kicked around the idea of removing hi voltage from the screen grids as another standby method. Again, since all other circuits are charged up, that should also result in minimum noisemaking when switching into play mode. Just gotta remember high voltage is ON if you go fiddling inside whilst powered up - don't find out the hard way - OWWWW! DAM' I just zapped myself again...

[52 Bill]

Try a cap across the Stand-By switch... Use 600V or larger film cap... like .047uF to .1uF ...
Should silence thing down...

If fact that was one of the first things that I tried, didn't work. This is not a power arc noise, as I noted earlier this is some sort of start up oscillation that happens only when the volume control is either set to 1 or 10. Absolutely no noise when the pot is set to mid-rotation.

This problem is definitely being generated by the plain channel preamp tube right out of the volume control. This is the stage that shares the cathode circuit with the other preamp tube. I will try the cathode cap and resistor and even try separate caps and resistors to see what happens.

[Chuck H]

The HV rail caps would affect both channels. The volume control follows the second gain stage. The second gain stage shares it's cathode bypass cap with the reverb channel. and the problem is gain dependent.?. High or low? I'm still considering the normal channel first stage cathode bypass cap. Though I've never attacked this problem that seems like the most vulnerable charging element with your criteria.

[pdf64]

How about Merlin’s suggested standby, ie 47k across the switch terminals?
Although the second stage cathodes are shared, likewise the HT node, and both plate resistors are 100k, they’re charging different value coupling caps. I wonder if making them the same value may help?

[52 Bill]

I give up!

I tried:
Changing the shared cathode cap.
Changing the shared cathode resistor.
Separating the cathodes and replacing with two separate resistors and caps.
Changing the plate resistor.
Changing the coupling cap.
Changing the grid wire from the volume control to the grid.
Heating the fiber board.
Changing the tube socket.

At this point I am doing more damage than fixing, so I have put it all back together for the customer to pick up. Maybe the next guy will be able to figure it out.

[christarak]

I give up!
so I have put it all back together for the customer to pick up. Maybe the next guy will be able to figure it out.

Hi Bill. This a business question, more than a repair question. When I have dumped hours chasing a rabbit through a frustrating maze of dead-end ideas, like you just did, what do I charge the customer? Do I write my hours off as "my stupidity" so No-Charge to the customer? Or some compensation, eg my 1hr quote charge?

[52 Bill]

Hi Bill. This a business question, more than a repair question. When I have dumped hours chasing a rabbit through a frustrating maze of dead-end ideas, like you just did, what do I charge the customer? Do I write my hours off as "my stupidity" so No-Charge to the customer? Or some compensation, eg my 1hr quote charge?

This was more a matter of me trying to find an answer to the problem, so I wrote the whole thing off to experience and did not charge the customer. I will often spend more time than I should trying to make something right, just because I feel better about making it right not just working.

That is unless it is a warranty situation, where it is my job to fix it not re-engineer it. That doesn't mean that I haven't contacted the factory to let them know of something that seems really wrong or troublesome. They usually are grateful for the field information when there is a problem that can be fixed on future production runs, etc.

As for compensation, was the time spent fixing the problem or fixing your mistake? If you fixed the problem and it just took longer that you thought, then perhaps it's a shared experience, you write off some of the time and the customer pays a little more than quoted. If it was you just looking in the wrong places until you hit upon the problem, then you might want to eat the cost and learn from the experience.

[christarak]

As for compensation, was the time spent fixing the problem or fixing your mistake? If you fixed the problem and it just took longer that you thought, then perhaps it's a shared experience, you write off some of the time and the customer pays a little more than quoted. If it was you just looking in the wrong places until you hit upon the problem, then you might want to eat the cost and learn from the experience.

Thanks Bill.
Most of my under-quoted repairs is thinking I know the problem (eg thinking the crackling noise was a tube or plate resistor. But it's not ...and then the search continues for coupling caps, dry joints, other components. 3 hours later I still have a crackle and no solution).

I'm hoping to cut the umbilical cord to my part time job and do repairs exclusively this year, but lost hours never come back.

Uncommon for me to cause a problem, and where I have, I repair it quietly and at my own cost.

[Justin Thomas]

I don't wanna get a toaster thrown at me for this, but, new tube?

Not asking for theories as to why or why not, but is it POSSIBLE that sone tubes just do this? Try either removing it entirely first & with a different tube...

Justin

[christarak]

I don't wanna get a toaster thrown at me for this, but, new tube?

Not asking for theories as to why or why not, but is it POSSIBLE that sone tubes just do this? Try either removing it entirely first & with a different tube...

Justin

Hi Justin,
i really appreciate your suggestion. I should start another thread for this fault, but I have to set it aside while I catch up on my other repairs. But to answer your question, I always start with the easiest fix first and then eventually work myself up to the ridiculous ideas of a desperate man. In this case, I changed each tube in succession, and then the whole lot in one batch, and then back to original. I would never throw a toaster at a gentlemen who lends his expertise at no cost to a total stranger on the other side of this planet.
My question to Bill was more one of how do my Superiors (that's you guys) charge for chasing dead-end clues in solving a problem....and still have a commercially viable business?

[Enzo]

.and then the search continues for coupling caps, dry joints, other components.

The thing to do is learn good troubleshooting technique, rather than hunting. MAybe it's this, maybe it's that is not an efficient way to look. The key is to isolate the problem. A systematic narrowing down the source of the problem. In the case of a crackle, say, we want to know if it is in the preamp or the power amp. If preamp, is it before or after the volume control? Before tone controls? Whcih tubes kill the noise when pulled, and which do not affect it. And so on.