70's Fender DR Red plate fuse blown

[alien]

Since the rectifier is the source of all DC voltage, we know that either the rectifier itself is bad, or the problem is ANYTHING that uses the voltage it supplies.

Oh no Enzo

Cant you just leave that last part out.

I tell ya..you guys are great. I've asked around the internet for a while now with no progress.

Im not assuming anything but can this change of tubes blow the SS rectifier?--or is it more likely something in the "anything" category. The inside of the recto doesnt look fried although there some golden goo on it.

Im gonna buy a meter, but I wont do anything till I know I'll be safe and maybe I should get another recto. The one I have is a GT s55u4 and I think it says 7m on it.

[Bruce / Mission Amps]

I think you should pull the chassis out of the cabinet and remove that funny looking, small, upside down, cake pan thing next to the preamp tubes so you can see which filter cap is now exploded. Don't be alarmed if there is more then one.

Then while the chassis is out of the cabinet, spend $45.00 on parts and do a complete electrolytic capacitor refurb on this poor old timer... that means all the high voltage filter caps under the little cake pan (commonly called a dog house)... but don't forget the bias supply filter cap over by the pilot light assembly (a new one goes back in with the positive end grounded) ...and please do replace all the smaller E-caps on the eyelet board at the same time.
After that you can decide if the power tubes are shot. And as far as that SS rectifier plug goes, IMHO, it should NEVER EVER EVER be used in silver face Deluxe reverb with 6V6s.
Especially one that has only a bias voltage balancing pot.
In other words; dump that, filter cap and power tube killing SS rectifier and get a real NOS 5U4GB rectifier in this amp.

[alien]

Hey thanks

I talked to a guy from eurotubes today by email and he said he doesnt think Deluxe reverbs had balance bias pots----fender silverfaces did but not silverface deluxes to his knowledge--I even showed him the picture. Now I dont know what to think


I will check the cakepan. The ss recto was in for 24 years without blowing a tube but I dont care which I use--I just dont want to blow it as soon as it goes in. As I said the inside of the SS had a little golden goo on it but I dont know what that means(nothing was cracked or burnt looking)

I just cant afford to do major work. If there is a simple route to its prior operation... I have to take that route if possible. I also know I may be pipe dreaming but I have actually read over 20 hours of amp tech stuff in the few day to try and get this done

[Bruce / Mission Amps]

Look at the wire from the center lug of your bias pot. The pot with the slot.
If it goes to the junction of the two 220K (red-red-yellow) resistors on your eyelet board, then it is a true bias voltage level setting pot.
He maybe right about the Deluxe Reverb amps having adjustable bias vs balance.
I can't remember positively seeing one with a balance pot either now that that has come up.
Regardless... with modern line voltages, you might need to change the resistor on the bias pot to a larger value so the negative voltage can go deeper negative.

added:

I don't know if the s55u4 SS plug is made with a corrective voltage drop or not but the silver face amps were made with power transformers that had higher secondary voltages so a 5U4GB vacuum tube could be used... that tube drops more voltage then others so they compensated by making the tranny with a higher secondary voltage.
Using a SS plug negates all that and the B+ rail voltages will be dangerously high... possible over the rating of many filter caps... hopefully yours are all at least 475vdc, most were 16uF@450v.

You still need to open the dog house and see if one or more of the filter caps have been vented out from over heating.
When you replace them use 475v to 500v caps.

[52 Bill]

Alien:
First off, if the photo that you posted of the chassis is your amp, you do not have a bias balance control, you have a bias control. If you have played with the setting of this pot, do not run the amp with the power tubes installed until you adjust this pot back to where it was originally set, or re-set it using at least a volt meter.

If the amp blows the fuse anytime you have the rectifier "cap" plugged in, either the rectifier "cap" itself is shorted, or there is a short somewhere in the power supply circuit of the amp.

You need to test the rectifier "cap" or replace it in order to find out where the problem lies. Until you do this you will just continue to blow fuses, wasting time and money, as well as possibly doing more damage to the amp.

If you don't have access to a meter, you will not be able to track down the original problem with the amp. So either buy or borrow one, or take the amp in to a tech for service.

[alien]

I bought a tube rectifier from Guitar center--its a 5u4. They said it was for my amp but it only has 4 pins? My ss rect had 8 pins. Is this right?

Anyway...it blew the fuse as well as soon as I went from standby. The tube looked like it was gonna blow up

I made a stick to discharge the caps and only got like 3 tiny sparks on one chassis cap. The main caps underneath did not spark at all(and I used no resistor) and they look brand new(I do remember now they were changed in 1984). Is it possible they had no juice in them? Maybe blowing the fuse drained them or there is a safety mech in there? I am borrowing a meter tomorrow

When discharging chassis caps I could have sworn I saw a spark coming from........


Inside The resistor directly above that brown ceramic disc (the left section of the V shape)

[Enzo]

The fact the amp worked for 20-30 years is irrelevant, it doesn't work now. Everythiing works until it doesn't, after all.

Most of the time, bad parts look just like good parts. You cannot tell by looking.

Your amp now blows the fuse when the high voltage (B+) is applied to the circuit - that is what the standby switch does. If it does it with the power tubes removed, then there are only so many places it can be.

The output transformer can be shorted, either to secondary or to the frame. You can test for that with an ohm meter. measure resistance to chassis from pin 3 of either power tube socket. There should NOT be a low resistance.

A tube socket or the transformer can be arcing - most likey would be from pin 3 to pin 2 on one of them.

A simple test for both those ideas is to disconnect the red wire of the output transformer from the parts board. if the fuse now still blows, both those ideas were not the problem. If the fuse now holds, then either the transformer is bad or there is an arc. (Or there is a physical wiring issue.)

The filter caps can be shorted - most likely one of the first ones. Up in the cake pan, look closely, you sure none of those caps have a little bulge in the insulated end? Or worse yet, leaking white stuff? Whatever they look like, are any shorted?

The choke could be shorted to the frame. Measure resistance from either wire of the choke to the chassis. There should NOT be a low resistance.

[alien]

Thats some helpful info Enzo..thanks. I will now try those things

About that resistor I saw sparks come from..could that be shorted, bad or the cause of this whole thing?

thanks guys
I dont know what I can do to thank you guys except maybe play some tasty sound clips when I get it running..that Im kinda good at. Amp troubleshooting--not so good

BTW..anyone know what value that resistor is

[alien]

Ok..I have a multimeter now

But I've never used one so I have to read up what to do. I got some info already but Im looking for procedures and readings information for my amp. Its really hysterical..I have a Degree in Applied sciences and in Recording engineering but the only thing I know about electricity is that Ben Franklin once flew a kite. It is very interesting consuming this information though.

Can anyone also recommend another forum and tech website to supplement this one?

thanks

[alien]

The fact the amp worked for 20-30 years is irrelevant, it doesn't work now.

The output transformer can be shorted, either to secondary or to the frame. You can test for that with an ohm meter. measure resistance to chassis from pin 3 of either power tube socket. There should NOT be a low resistance.

Hey

I know your the amp expert but I didnt fail Logic 101 and it is relevant if the amp worked for 30 years until the day I changed the tubes. All data that has changed is relevant in problem solving in my field and yours. But Im guessing you may be brash as me when dealing with those you teach.

So what number is considered low on the resistance? With the amp off it measures 220 ohms from pin 3 on either Power socket to chassis. It measures 154 ohms from the black choke wire(the wire that eventually hits pin 6) and .5 ohms on the other black choke wire(the one that eventually hits the speaker jack). I didnt know if the amp should be on or if I should disconnect the wires to tak ethe readings--so I did neither

Other data
There is no sign of goo on the filter caps
There was not 1 single drop of voltage in the caps
Im checking all the resistors now but so far none are open(3/4's so far).
thanks guys

[Enzo]

Sorry, not trying to be hard on you. What I was trying to do is steer us on a course of solving the problem instead of wondering why it occurred. Whether we caused it through our actions or if it was just a coincidence, the problem exists. We have to solve the problem we face. You might well have caused the problem somehow while changing tubes, but that doesn;t tell us WHAT is now wrong with it. If we just approach the problem by itself, we can determine the bad part or parts. Once we do that and repair it, we can reflect on putting together the way the failure was caused.

The amp must be powered off to take any resistance readings in it. SInce it blows fuses when on, we are forced to take power off readings anyway. WHat is too low? A judgement call. With the power off the pin 3 ends of the transformer have no current path. SO any resistance to ground is either within the tranny - not good - or on the B+ line. SInce B+ is on the whole output transformer primary, we can look at the 420v of B+ with a 220 ohm across it. Ohm's Law says that would be drawing almost 2 amps from the B+ supply. Way more than it can handle, especially without the circuits running.

So the center tap wire of the transformer is the red one - blue and brown going to the pins 3. Disconnect the red wire from the parts board, and remeasure resistance. If there is any resistance to ground, the transformer is bad. The primary winding should have NO continuity to the frame or chassis. It also should have no continuity to the secondary winding, the speaker side. But since that side is grounded already, a chassis measurement is sufficient.

The choke is not connected to the speaker jack, so we may have confused a couple wires. IN any case, by pin 6 I assume you mean the screen supply - from each pin 6 there is a 470 ohm resistor going to pin 4, right? 154 ohms there is also bad. VOltage there is 415, so Ohm's Law says 2.7 amps. (Those amp reading I calculate would be potential current, in reality, the supply could not provide that much current at that voltage.) SO that is further evidence of a problem.

SInce the choke and output transformer are connected together, either one could be shorted to frame and cause low readings on the other.

If that .5 ohm wire is in fact the choke's lead, then that is a dead short to ground on the B+, and that explains blowing fuses. If the black wire is really the secondary of the OT, then the reading is normal.

In the case of either piece of iron reading a short to ground, we then need to know if the part is bad itself or if the places the wires connect are somehow shorted to ground. SO disconnect the OT red wire, and disconnect both ends of the choke. Now take new resistance readings from the wires we lifted to chassis. Disconnected like that, those wires should all measure open to chassis - infinite resistance. Any resistance is bad. OH, we might allow a several meg ohm reading due to humidity or something. We are looking to cure the 220 ohm thing.

Now here is a curve. Measure resistance from the standby switch to ground - do both terminals to be sure. Since the standby switch is directly wired to the choke and OT center tap, you might get a shorted to ground reaqding at the. If it is .5 ohm, then that agrees with your initial choke assesment. If you get 150-200 ohms or somethiing that is consistent with your other readings. The switch itself could have crumbled inside shorting the contacts to the switch frame, which is grounded to chassis. That wouold be extremely rare, but still possible.

There is zero voltage in your filter caps, right? SO measure resistance across each one. That would be further evidence to localize the B+ short.

To me sitting here remotely, it is looking like you have a bad piece of iron.

[alien]

`Thanks for that detailed post

With the red wire disconnected from the board measured no reisitance to chassis

The black wire from the choke that goes to board and eventually to the standby was .5 ohms(and that switch to ground was the same but on the other standy terminal it was none.

The other choke black wire that goes to board and eventually hits the power tube pin(with the 470 on it) reads 154 to chassis. the actual resistor from pin to pin it reads 480 both ways,

when the choke is desoldered from the board both wires read nothing to chassis.

Im gonna check the other stuff you mentioned now

Also, with that red wire and choke wires disconnected the pin3 to chassis on both sockets reads nothing--Im gonna check again with the wires connected again to see if I messed up on that 220 ohm reading

Ok..I checked..it does read 220ohms with the wires back on--. When I disconnect the choke only it still reads 220 but when I connect the choke BUT remove the red wire it reads nothing. I dont if this means anything but I'd thought I'd try
thanks so much

[Enzo]

It sounds like the choke is OK, but the output transformer is not. I think.

With the red wire disconnected, the low resistance is gone? Measured at pins 3? SO before you reconnect the red wire, measure resistance from that red wire dangling free to chassis. If you get the 200 ohms, the transformer is shot. If pin 3 no longer reads resistance to ground with the red wire off, then maybe it is OK.

I am not completely clear just where you measured.

With the red wire off, is there 200 ohms to chassis from the point where the red wire used to connect? In fact leave both the choke and the red wire disconnected - three wires in mid-air - Now measure resistance to chassis from each point where a wire used to be connected. ANy of those showing the 150-200 ohms? If so we can eliminate the iron.

[alien]

Hey
The power tubes are not in BTW--if that means anything


the picture above shows the section but Im measuring at the point where those wires meet the board.

None of the 3 wires read anything when off and measured to ground and nothing reads on pin3 and about .5 to 1 on the spot the red was connected. If I put the red wire back on both pin 3's read 220. if you look at the pic at the far left of the blackboard--the red wire attaches to the point just below that center hole

BTW..that point where the red wire hits the board it goes 2 ways --one to standby and one through a hole in the chassis into the filter caps

OK..I checked the filter cap resist. From Right to left(with + being on top),,the first 2 measure .2 or 3 ohms. The other 3 measure OL on my meter--I cant get any reading.

Also those first 2 had no juice whatsoever in them but the others had under a volt in them. The 2 reisitors both were at their correct reading.

[Bruce / Mission Amps]

Never mind I missed the picture with out my glasses on.

Make sure the ceramic caps on the tubes sockets are not shorted to chassis ground on the grid side or have not just gone bad. Not sure if you said you checked for a deep negative voltage on the
grid lugs of the power tubes... with no tubes installed.