Howdy guys, need help with a Fender 65 SRRI.

[Ravie]

Hello everyone! I've read a lot through time on this forum...you've all been helping me without knowing it for a couple of years...

So, I'm working on a friends 65 Super Reverb Re-issue. It's a weird 2001 model. It's all the same as the schematic but for some reason it's got a potentiometer where the speaker out should be coupled to the external speaker out. Why that's there or who did it or if it came that way I do not know. However that is not my problem right now...

So, my friend brought me this amp and told me that the output transformer was bad. He had some friend in electronics tech school check out the amp and his analysis was that the OT was bad and he pulled it out. I tested it on the bench and it seemed to be fine, so I reinstalled it and buttoned it back up and stuck the chassis in to see what kind of sound I could get. He had told me that it would make sound but that something just wasn't right that he couldn't figure out.

Well, I get it back together, hit the switch, and it certainly isn't the OT. I can play music, turn it up, etc.. But there's a big problem - the V8 Power Tube arcs. I've switched the tubes back and forth and it does NOT follow the tube...which means it happens to whichever power tube is in the V8 position.

I'm really not sure what to check at this point. I don't see any obviously blown components...the ones I test, test out at the right value... I'm just not sure where to go next!

Thoughts?

[tubeswell]

Maybe your friend ran the old output tubes for too long and one of them went south, possibly affecting the OT, or at least leaving carbon deposits around the socket for that tube. Clean any carbon deposits off the socket thoroughly with a dremel tool, or replace that socket.

Also (carefully) test the impedance ratio of the OT properly with a Pr:Sec VAC ratio measurement. You should have even (and highish) VAC on each side of the OT primary centre tap, when applying a low voltage VAC across he OT secondary. (Hint - you can use the PT's rectifier winding or the heater winding to supply the low VAC to the OT secondary for this test). If you don't feel confident about doing this, then take it to a tech. If the OT is buggered, it'll need replacing (and probably along with the output tubes)

[Ravie]

I don't know that I'm going to be able to explain this well, so please bare with me.

When the OT was off the bench, I had a small wall wart hooked up to the B+ and the ground on the other side. I could measure equal voltage to any of the opposite side wires (any secondary to any primary). There were no shorts across the PR:sec. I was getting full voltage in every position. Also, I get roughly the same resistance from B+ to either secondary. in the 40's each (I don't remember the exact number but one is slightly higher than the other if I remember correctly)

Now that it is connected I tested continuity again with everything on the OT hooked up, output and rectifier tubes OUT, power disconnected. No shorts across the PR;Sec, and OL when testing continuity between PR and Sec just like http://www.geofex.com/ampdbug/outtrans.htm tells me should be correct.

BUT!

I just happened to notice that the pin 3 wire for V7 is connected to the V8 socket, and vice versa. Um, what? It looks to have come factory this way!

Also, I feel like the potential is there for that weird pot that has been installed where the stock output jack should be is causing there to be a slight short between the jack connections. I disconnected the pot.

Someone before me has been into this amp and it's all kinds of strange inside. The further I get into checking things out, the more I wonder what the guy before me was thinking.

Pin 3 is the plate output, isn't it? Would those wires on that pin that are switched for some reason cause any of what's going on?

I'm comfortable with just about anything as long as it can be explained in a manner that makes sense. I'm not new to electronics at all, just a bit of a newbie of sorts to the amp world. I understand schematics just fine, etc...

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[tubeswell]

RE; the Ot voltage I'm not talking about measuring DC idle voltage, I'm talking about measuring the AC voltage ratio of the primary to secondary winding. To do this you have to supply the OT's secondary winding with an AC voltage (supplied between/across the ends of the winding), and then measure the resulting (and higher) AC voltage that should then be present between each side of the OT primary's centre tap and each end of the OT primary winding. The AC voltage on each side of the OT primary's centre tap should be the same if the OT is working okay.

For a 6L6, Pin 3 is the plate, Pin 4 the screen, pin 5 the grid, pin 8 the cathode (with an internal connection to the suppressor grid), and pins 2 and 7 are each end of the filament. Pins 1 and 6 have no connection.

[Ravie]

Using the 6.3V heater wire (actually it measures 6.5V) for testing AC, to Secondary tap:

6.5V from either Secondary to Chassis

5.4V from any Primary to Chassis

.5V from any Primary to any Secondary


This is with no tubes in and the secondary wire and B+ removed from the circuit

[tubeswell]

No you still didn't get it.

[img1240]http://farm6.staticflickr.com/5550/9475 ... 2528_o.png[/img]

[Ravie]

With each heater wire hooked to each of the secondary leads, I measure 6.74 volts between those wires. From the center tap on the primary to either end is 151 volts. 302 volts testing from each end of the primary.

[Ravie]

So, using this math...

302v/6.74v=44.8 turns ratio.

44.8*44.8 (squared)=2007.04

2007.04*2ohms= 4000 ohms impedance. (well, ok 4014.08)

That's spec for this transformer.

[martin manning]

Looks good to me!

[Ravie]

Yep.

But now I'm back to square one in a way. None of the components inspected visually look wrong/blown/buggered to me. The pin 3 wires for v7 and v8 were switched (which I have since fixed) But I don't think that would have made a difference since all those are getting is plate voltage anyway. Also, the potentiometer someone installed on the speaker output was pretty strange but I've since unhooked that as well. It's like someone was trying to lessen the load going to the speakers or something by trying to take it out with a potentiometer. I don't know. I may put a new jack there so it is like it should be stock.

These tubes I have for this amp were tested in another 65 reverb I just fixed before I had the ones that were going into the other amp (brand new TAD matched 6L6GC's, my favorite power tube of late) and they weren't behaving this way in that amp. Anyway, the arcing is only happening in the V8 position anyway and does not follow the tubes, so I think that pretty much rules those out...

Next steps?

[martin manning]

The plate leads (pins 3) have to be connected correctly because the circuit has negative feedback taken from the output. Reversing the primary leads will flip the phase of the FB and you will have oscillation. Since you didn't report any squealing, maybe that has been disconnected by the previous owner or tech? The pot at the output jack could be for a line out or something.

Once there is arcing on a power tube socket it can be very hard to cure due to residual conductive carbon tracks. Replacing the socket is sometimes the only cure.

[tubeswell]

Yes next stop would be that socket. (Well done on the OT test by the way - top marks)

[Ravie]

Ha, Thanks TW. Like I said, I'm not really a newbie to electronics, but a little bit of a newbie to amps, so I'm still learning but I know my basics pretty well.

I bought some cleaner and cleaned out the sockets with a brush and cleaner. I switched those leads for Pin 3 where they belong and I also removed that weird potentiometer and put a jack back in it's place as it should be. Black/tap to ground and Green to tip. I'm going to stick the chassis in place and test once more. If I'm still getting arcing I'll look through my socket pile and see if I have any good ones.

I'll report back!

[Ravie]

Pin 8 of each tube...

So, V7 has about 40mV on Pin 8.

V8 hassignificantly more than that at 67mV. Spec on schematic says it should be more like 33mV, but even at the lowest bias setting they're getting 10 volts more than spec (schematic says 433VDC, lowest I can go on the bias pot still gives me 444VDC, so that's probably not far off on the 44mVDC, but it still seems high. I know 6L6's like something more like 34mVDC.

So, aside from the fact that they're getting too much voltage to begin with from the bias pot...why the heck am I getting a whopping 66mVDC on V8? With the bias resistors still connected they both test out right around 1 ohm like they should (Fender used 5% resistors instead of the 1% I would use)

Here's a schematic just in case you're all not familiar...

[tubeswell]

If the tube swap didn't change these results, then somehow there is more current on one side. What are the DC resistances of each side of the OT primary?