Blue flash on V9/V10 of Fender Twin Reverb leaving S/B

[nickb]

The high current persists for less then 100mS so i would not have thought that would cause a slow fuse to blow.

Seems this is just a quirk of the design. I can think of a few different ways to fix this, of varying complexities. You could rewire the standby so that it's in the cathodes of the 6L6's and leave the HT on. Or easier, just bypass that cathode interface resistance inducing horror altogether.

[nevetslab]

65 Twin Reverb RI sch attached

The high current persists for less then 100mS so i would not have thought that would cause a slow fuse to blow.

Seems this is just a quirk of the design. I can think of a few different ways to fix this, of varying complexities. You could rewire the standby so that it's in the cathodes of the 6L6's and leave the HT on. Or easier, just bypass that cathode interface resistance inducing horror altogether.

65_Twin_Reverb_037627 Sch.pdf

Referring to the 65 Twin RI schematic attached, are you referring to the cathode resistor R41 470 ohm between the two cathodes of V6 and the junction of R41/R36/R37/R38? Bypassing just that, or perhaps bypassing R36 22k?

Now, good fortune arrived....another steady client of mine just delivered another 65 RI Twin Reverb for service, so this exercise with the Turn-on anomaly has peaked my interest. Wonder if this amp behaves the same way.

While the engineer side of me want's to fix design quirks that sometimes cause field failures, I have to remember that there are around 20 of these in our inventory, just here in Burbank, and another chunk of them in our New York rental depot.

[nickb]

[ATTACH]50621[/ATTACH]

Referring to the 65 Twin RI schematic attached, are you referring to the cathode resistor R41 470 ohm between the two cathodes of V6 and the junction of R41/R36/R37/R38? Bypassing just that, or perhaps bypassing R36 22k?

Now, good fortune arrived....another steady client of mine just delivered another 65 RI Twin Reverb for service, so this exercise with the Turn-on anomaly has peaked my interest. Wonder if this amp behaves the same way.

While the engineer side of me want's to fix design quirks that sometimes cause field failures, I have to remember that there are around 20 of these in our inventory, just here in Burbank, and another chunk of them in our New York rental depot.

Sorry. I guess I was being I little too oblique. By "cathode interface resistance inducing horror" I was referring the the standby switch, that was all.

I look forward to what you learn from the new arrival and perhaps the rental stock too. Always enjoy reading your posts.

[nevetslab]

Sorry. I guess I was being I little too oblique. By "cathode interface resistance inducing horror" I was referring the the standby switch, that was all.

I look forward to what you learn from the new arrival and perhaps the rental stock too. Always enjoy reading your posts.

Oh.........S/B switch. OK. I've pulled that back off anyway, as it wasn't any solution to the problem on this amp. I've never tried switching the cathode connections of the output tubes to ground on any of the Fender amps. Or, for that matter, switching across the power tube side of the PI feed caps as a mute.

I likewise enjoy reading your posts as well.......you go quite deep into the subject matter, and it is great education material, as is so much of what flows from the brilliant minds contributing on this forum!

[nevetslab]

Another Fender Twin Reverb's Turn-on Transients @V7-V10 input grids

Another Fender Twin Reverb (65 Reissue) came in with a different issue...intermittent Rt Angle spkr plug, but I needed to pull the chassis to address a loose fuse post, so I took the opportunity to measure this amp's power tube input grids. I noticed V9 & V10 on this amp ALSO showed the same amount of blue flash inside the glass. After setting up the storage scope to burn the turn-on transient onto the storage phospher, it looked very similar to that of the amp that began this thread. V7 & V8 on this one, though, ALSO hit 0VDC at turn-on from Standby, though for much less time, though still higher than the first amp. Here's the images of this amp:



The next images are of V6 driver tube, first showing the Positive Driver's plate & grid, then the Negative Driver's plate and grid, and the 'tail' where the cathodes' resistor, grid resistors and tail resistor where it joins the feedback:



I'm now curious to look at the same stages on one of Fender's amps, with similar circuits, that DOESN'T have a turn-on Thump or Pop. Is it this set of transients on the four tubes that is causing the turn-on transient?

[nickb]

I'm now curious to look at the same stages on one of Fender's amps, with similar circuits, that DOESN'T have a turn-on Thump or Pop. Is it this set of transients on the four tubes that is causing the turn-on transient?

My suspicion is that the difference is in the time constant of the power node that feeds the PI. The 65 Twin has 22uF and 1K i.e. about 50mS. The Delux Reverb has 10K and 16uF making 350mS. Try a big cap say 100uF or even 220uF in parallel with C33 and see what happens.

Or.... paint the tubes black and no one will ever see the problem again

[nevetslab]

While the two amps I had characterized are back together, a third Twin Reverb just arrived that needs to get pulled apart, so that will be a good opportunity to try what you suggested. I also forgot to capture the output transient from the speaker jack. Something tells me the slight blue flash has been happening with these all along, and I just happened to spot it, looking for clues on why that amp periodically blows a 4A Slow Blow fuse.

[Chuck H]

looking for clues on why that amp periodically blows a 4A Slow Blow fuse.

Since the flash phenomenon and the asymmetrical transient is common to at least two amps (wouldn't it be interesting if the one that's in now doesn't do it?), but one is blowing fuses and the other is not I think the one blowing fuses must have an intermittent fault or short in a contact or component. The sort of thing you can only "fix" when the problem goes full time. Or, if that particular amp is typically in the care of one user there may be some misuse going on that may continue until something more telling goes kaput. In both cases it's out of your control as a repair tech until that time. So I would try not to sweat it.

EDIT: If the fuse eater get's back to your bench and still isn't showing the cause maybe give it the ol Enzo whack with the UUT to see if anything remarkable happens.

[nevetslab]

........ maybe give it the ol Enzo whack with the UUT to see if anything remarkable happens.

The UUT??? lessee....I have sandbags, wooden mallets, small children.........

[nevetslab]

ADDING 200uF/450V caps across C33, which feeds V6

My suspicion is that the difference is in the time constant of the power node that feeds the PI. The 65 Twin has 22uF and 1K i.e. about 50mS. The Delux Reverb has 10K and 16uF making 350mS. Try a big cap say 100uF or even 220uF in parallel with C33 and see what happens.

Or.... paint the tubes black and no one will ever see the problem again

Good call, Nick! This is Twin Reverb # 3, which is behaving the same way as the other two have been, so we have a consistent characteristic being inflicted on the power tubes from V6 @ turn-on switching out of Standby. Before I added 100uF, then 200uF across C33 (22uF/500V) in the dog-house, I measured the output turn-on pop/thump, as well as the turn-on transients at V7 & V10.



Now, adding 200uF across C33, being the filter sourcing V6 plates:



Listening to the amp with those caps in place, I'm not hearing the pop/thump at turn-on, and not seeing same degree of blue flash at turn-on (a tiny bit), switching out of standby. I'd really have to have the room dark to see it....one of the overhead florescent lights 12 ft up has it's switch behind a shelf. Some day they'll burn out and I can go dark as needed. I do periodically get a snap switching back INTO Standby, but not out of it with this change. Interesting!

[Chuck H]

Unit Under Test. Please don't harm children because we would miss you here at the forum

[nevetslab]

Another Fender Twin Reverb with substantial turn-on pop, though no 'Blue Flash' of any significance on the power tubes. When I had my bias probes plugged in, using the Compu-Bias probe amp, each time I switched out of S/B, it would reset the instrument. I remember having gone thru this exercise and investigation so dug thru my archives of posts, finally finding this, along with my service notes from that period.

This particular Twin Reverb wasn't yet in our inventory, but from the transient I was seeing on the scope, looked much like the others. It seems the most effective change made was increasing the filter capacitance of C33 (22uF/500V) which feeds the plates of the driver tube V6. I had tried both 100uF/450V and 200uF/450V across it, which dropped the turn-on transient coming out of S/B by 4:1. I looked to see what I had on hand, which was nothing but 22uF/500V caps and a couple 47uF/450V caps. Getting that much capacitance into the doghouse would be a challenge, and my records showed I lived with the transient.

This amp has somewhat microphonic power tubes. I pulled them, and re-tensioned the forked terminals within the ceramic sockets to tighten them up, as they all felt a bit loose. I'm not getting the transient noises that was causing the complaint, but, I can still tap on any of the four tubes and definitely hear it. Removing the driver tube V6 makes no difference with that. I've just used up the last matched set of 6L6GC's on an older Twin Reverb this past week, so I guess I'll put this back into play for now to see what happens. Pounding on the top of the cabinet doesn't cause any issues, and having hit the input with high level short noise bursts (200mSec on, 2 sec off), I wasn't able to get the amp to produce scary noises as it was doing. We'll see what happens.