Fender Supersonic Output Tube Substitution

[CaptainKirk]

I just received a Supersonic for repair. The customer said the amp has low volume and distortion on the clean channel. After a preliminary inspection for obvious mechanical issues, I noticed EL34 tubes plugged in. The tube reference sticker in the cabinet says 6L6's as does the schematic. I checked the tube socket wiring and there is no connection to pin 1 which I believe is the suppressor grid. I also understand the EL34 draws 1.5 amp heater current while the 6L6 draws 0.9 amp. And I imagine the bias requirements are different.

I did a web search and turned up one reference on a forum from a user that said, "don't do this swap". Does anybody know if this is a valid substitution?

[Enzo]

No, it is not. Not unless the amp is modified for it. Try 6L6s in there and see if the amp wakes up sounding good.

[CaptainKirk]

Thanks

Thanks as always Enzo. Installing the correct tubes and biasing them to 38mA did wonders. Fender calls for 32mA but I think that's probably a little cold.

No one seems to know how EL34's got in there. I guess they thought one octal power tube is as good as another

[CaptainKirk]

The Rest Of the Story

Unfortunately this amp still has a problem. The customer took the amp home and played it for about 15 minutes at moderate volume. All of a sudden the amp started making a loud hum, even though he turned down the volume. He quickly turned it off and smelled something burned.

I looked the amp over and found the primary fuse blown. I removed the output tubes, replaced the fuse and turned it on. The fuse held and all voltages looked good. I specifically checked control and screen grid voltages on the output sockets and they looked good. The control grid was -50V.

Then I re-installed the tubes and turned it on again. When I closed the standby switch, a loud hum came from the speakers, the output tubes flashed blue and the fuse blew. Conclusion - bad output tubes?

The tubes are brand new Ruby 6L6GC MSTR, biased at 37mA. The factory setting is 32mA but I read this is a cold setting. I calculated 37mA by assuming 25 watt tube rating and used 70% dissipation. Plate voltage is 478-485V. What I don't know is if the tubes were red-plating when he was playing.

To make sure the amp still worked with new tubes, I installed a set of Svetlana 6L6GC's. Before switching on the B+, I set the grid bias back to the factory -52V for safety. The amp powered up just fine. I did notice during this test that when I disturbed the screen grid socket connection on one of the output tubes, I got some crackling in the speaker. I connected a voltmeter across the screen grid resistor and wiggled the tube connection again. The screen grid current dropped to zero and came back during the wiggling. Looks like a socket problem. Would an open screen grid connection cause a tube failure?

What I'm really wondering is why did a brand new set of tubes biased fairly conservatively fail? Bias too high? Infant mortality? Poor tube quality from Ruby? The customer said the amp sounded great until it blew up.

[52 Bill]

Would an open screen grid connection cause a tube failure?

Yes, it would cause the tube to red-plate and short out.

What I'm really wondering is why did a brand new set of tubes biased fairly conservatively fail? Bias too high? Infant mortality? Poor tube quality from Ruby?

Any of the above are possible, but I'd bet that the loss of bias was the real culprit here.

[Albert Kreuzer]

The customer said the amp sounded great until it blew up.

Quote of the day

[Enzo]

I have to differ with bill. if the CONTROL grid came disconnected, then yes the tube would red plate. If the SCREEN grid loses contact, the tube will go mute. Some amps go to standby by removing B+ from the screens.

My suspicion is that one of the new 6L6s was bad from the box. Tubes are ALWAYS suspects.

[CaptainKirk]

Thanks Bill and Enzo. I did some more testing last night and found one of the new power tubes appears to be arcing. I put it on the tube tester and saw a blue/pinkish "light show" among the elements and the gM was jumping all over the place.

So it sounds like a bad tube is the basic problem. Does anyone disagree with the bias setting I'm using? I want to make sure the tubes won't "redplate".

[ZippoTragedy]

Just to complete the story, does anyone have the tube layout - thought i saw this in a very vague section of the Fender Support website, but can't seem to locate it.

*Finally found it: http://www.tdpri.com/forum/amp-centr...onic-club.html


One last question: (first-timer) - where is the actual bias adjustment for the tubes? I've looked at the schematic, in a very ADHD way, but I'm not accustomed to reading engineering diagrams. Would be super helpful if someone could just say 1, 2, 3 (little hole next to transformer, or some such).

Thanks

zt

Thanks Bill and Enzo. I did some more testing last night and found one of the new power tubes appears to be arcing. I put it on the tube tester and saw a blue/pinkish "light show" among the elements and the gM was jumping all over the place.

So it sounds like a bad tube is the basic problem. Does anyone disagree with the bias setting I'm using? I want to make sure the tubes won't "redplate".

[Enzo]

As far as I know there is only one twiddle control inside the amp. That would be the bias control.

[Gtr_tech]

I have to differ with bill. if the CONTROL grid came disconnected, then yes the tube would red plate. If the SCREEN grid loses contact, the tube will go mute. Some amps go to standby by removing B+ from the screens.

My suspicion is that one of the new 6L6s was bad from the box. Tubes are ALWAYS suspects.

Maybe he meant that the opposite tube (the one w/screen supply) can redplate if the amp is used that way? Seen it happen.....I had a reissue AC50 in a few months back that had a socket pin fail on the screen of one tube. The other tube was cooked to death.

[Enzo]

Is that common cathode bias in that amp? Screen on one tube opens, so the tube stops conducting, so the overall current through the common cathode resistor drops, causing the cathode voltage to drop, thus reducing the bias to the remaining tubes, which are then free to overheat?

[Gtr_tech]

I don't remember what the bias scheme was. The service manual is on a hard disk I don't have immediate access to 'else I'd check it out.

[52 Bill]

I have to differ with bill.

And right you should! Once again I need to read these posts before typing. I just saw grid and to my vocabulary the control grid is always the grid, the screen grid is just the screen.

So just stupidity on my part. Sorry for any confusion.