Howdy guys, need help with a Fender 65 SRRI.

[Ravie]

From Center tap to each side? 49 on V7 and 46 on V8. That's tubes in, everything still connected.

[martin manning]

And does the high current follow the tube if you swap them?

[Ravie]

I could go test but I'm going to say probably not since the tube arcing only happens to whichever tube is in the V8 position.

[martin manning]

Why would you think that the arcing and the high current have to be related? Both tubes should have the same applied voltages, so if the current doesn't match the first thing I would suspect is that tubes are not matched. How about this: measure the voltage on all pins on both output tubes and see if that explains the unbalanced current.

[Ravie]

Because they are both conditions that are present on only V8. (aside from a bit too much voltage)

I'll go test them all.

[Ravie]

Made a mistake tonight that is going to cost me some more work.

I'll get back to you after I replace some things....

[martin manning]

Oh, man... Sorry to hear that.

[Ravie]

Yeah. I was retesting (I have two meters and they like to give me different answers so I was retesting a few things) and suddenly my voltage on Pin 3 just up and went away. I couldn't figure out why. Suddenly my test probe arced, a big flash happened, it burned up the heater resistors and instantaneously ruined the socket.

Needless to say THAT was exciting....

Somehow when I was moving the Pin 3 leads from the primary on the OT, I didn't make sure the solder was happy on the wire that goes from Pin 3 to the circuit board. I'm guessing somewhere in there I got 2&3 touched together and created catastophe.

So I'm building an order from AES... new socket, resistors..a couple of other items...

I'll probably need to get a matched set of tubes for this amp, too. I don't know what the other ones have been through besides the abuse I have given them and I don't own a tester, so it's probably best.

[Ravie]

So, I was thinking.. If I can't lower the bias voltage to the best number(at rock bottom I had .44...that number only got bigger...), Can I change the resistor to a lower resistance value to get more positive? (bias voltage speaks in negatives, right? So I would be letting more positive voltage in to get a lower negative number?)

It looks like there's a 27k there. Maybe a 15k?

[martin manning]

Can I change the resistor to a lower resistance value to get more positive? (bias voltage speaks in negatives, right? So I would be letting more positive voltage in to get a lower negative number?)

It looks like there's a 27k there. Maybe a 15k?

You need to go the other way... It's a voltage divider. It happens to be below ground, but same as usual you move the output further from ground by increasing the ground leg. You could lower the 470 ohm, but it limits current through the diode on start-up so I'd leave it as-is.

[Ravie]

So you're saying I would need to increase that to maybe a 33k or 39k, rather than going down to a 15k?

[martin manning]

Yes. Get a known-good pair of tubes in it that are reasonably well matched, then see if you need to adjust the value. Another possibility is that the coupling cap C28 is leaking DC and causing both the imbalance (high current in V8) and the inability to get the idle current down. You'd see unequal voltages on the grid pins if that were the case.

[renshen1957]

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?

Hi,

You received great advice, which I found very informative. Here is my solution to a Fender re-issues and for the Hot Rod and Blues amp:

Check to see if the PT and OT are in good condition.
If so, get rid of Fender's POS PCB and build a real amp with either circuit cards or turret boards. Build an original circuit with new parts (if a reissue), or a new amp (loads of ideas on TAG)

This isn't a slight at anyone's skills or helpful advice. Just my personal experience.

Fender PCB, especially the blues and hotrod boards, are barely adequate in both the copper traces and some components are undersized. This may work okay for an amateur musicians or infrequent players. However if these amps were used frequently by a working/gigging players the heat build up will lift the traces or some component that is merely adequate will go south in time. After this happens the semi-pro gets the amp repaired and sells it quick.

I have yet to start a repair on a Fender PCB amp that hadn't had a previous issue (issues) that some techs before had to deal with.

The reason for the above is simple economics, these amps were built with economy in mind, the MBAs have run fender for a long time. Also money is worth less through inflation and living wages have not kept pace. The amps are built to a price point and there's are always compromises galore. Quick assembly and if an amps returned replace the PCB.

To me PCBs in consumer electronics equals future land fill material only now it's electronic waste. To be dealt with.

Many Super Reverbs from the 60's and 70's are still going strong. They're simple to repair and repairs/maintenance is a breeze by comparison.

But I have another reason or two.

I found I could build a new amp faster than dealing with the PCB being fixed only to have another issue crop up during the repair or later (I warranty the amp repairs) an unrelated issue crop up and the amp comes back to haunt me.

A new amp with a vintage or original circuit sounds better and will last longer. And my stress levels are lower. The owner is happier.

That's my 200 cents (adjusted for inflation).

Best Regards,

Steve

PS Yes Ampegs and Selmer and Marshall et all use PCB for quite some time. I can only think of a handful of amps thses by boutique builders that have their traces at military or Nasa standards and correctly designed for audio.

[Ravie]

Bringing this thread back from the dead as I've just now finally had some time to mess with this amp.

I replaced:

- Bias resistor with a 33k in place of it's 27k.

- V8 Power Tube Socket with brand new

- Capacitors at positions 27, 28 (104's)

- Resistors (which I blew messing around like a frustrated bull months ago)70, 71 (They feed pins 2 and 7 off the PT)

- New Sovtek 5881's (I was going cheap in case I mucked it up again)

I've gone over the schematic with a fine tooth comb and all of the wiring is now as it should be.

I went to bias the beast, with the bias pot turned all the way down as it should be before beginning, and when I flip the power switch it's NOISE CITY even with every potentiometer on the front turned down to zero.

The reading at the bias resistors with the bias pot turned all the way to the left (down) is .044 and .043mV which is still WAY too high.

I don't get it.

Here's a video. Pardon my rambling.

http://youtu.be/G1eQPlac-sM

[ampdoc1]

Try removing your preamp tubes 1 at a time, starting with the PI and working toward the front of the amp: If you still get the noise with the PI removed, the problem is in the power amp section. If the noise is gone, you know the problems is somewhere toward the front. You ought to be able to narrow down the root of the problem pretty quickly this way.

And, with noise problems I always recommend using a coolant spray on the components to see if any parts are noisy due to thermal problems. If you don't have a coolant spray, just buy a can of "dust remover" made for computers. Turn the can upside down, and when you spray only the coolant comes out.

adoc1