70's Fender DR Red plate fuse blown

[alien]

Hi guys

I have a 70's Deluxe Reverb. A tube's glass broke in the 3rd socket without me knowing and I turned it on. I then noticed it so I bought new matched groovetubes(gt6v6r). The same socket that broken tube was in started giving me a red plate after a few minutes. I switched them and it was still coming from the same socket.(oh yeah..this amp has a cap in the first socket)

While trying it a week later the fuse blew and I smelled smoke. I dont see anything burnt on the board and I looked real close. Below is a pic of the bias panel. None of the caps look like they breaks.

Any ideas?
Thanks so much

[Garydean]

If you replaced the output tubes without biasing then that needs to be done no matter what. The fuse blowing could be one of many things. What tools do you have to continue and repair. DMM, soldering iron etc.. Do you know how to properly drain caps before you start probing and poking? Sorry but I have to assume this is your first time? Safety first. The voltage level is really up there in those amps even with the amp turned off and unplugged

When you say "there's a plug in the 1st socket", is it using a solid state rectifier? If so, I suggest going back to the GZ34 glass tube.
Be sure you're using 1amp slo blo fuses (this is not the problem, I just wanted to be sure you're using the right value).

Gary

[alien]

If you replaced the output tubes without biasing then that needs to be done no matter what. The fuse blowing could be one of many things. What tools do you have to continue and repair. DMM, soldering iron etc.. Do you know how to properly drain caps before you start probing and poking? Sorry but I have to assume this is your first time? Safety first. The voltage level is really up there in those amps even with the amp turned off and unplugged

When you say "there's a plug in the 1st socket", is it using a solid state rectifier? If so, I suggest going back to the GZ34 glass tube.
Be sure you're using 1amp slo blo fuses (this is not the problem, I just wanted to be sure you're using the right value).

Gary

Hey thanks

I dont know how to discharge caps but this was a year ago and I havnt even plugged it in since then--although I still did not touch anything inside when I opened it today---
How long do caps hold a nasty charge?


It says 2 amp sloblow fuse on the back panel--should I still use 1 amp?

About biasing..Im guessing the bias screw on the bottom is a balance bias--I have never turned it in over 30 years--haha...is this how its biased?

I have some tools and will buy a meter but I truly respect the expertise you guys have so my main goal is to know whats wrong so I dont get ripped off or if its easy I will do it myself if possible(Im on Long Island and people charge a lot here)

thanks

[Garydean]

2amp slo blo is good. That pot is the balance pot and not used for biasing the amp. It basically gets unmatched tubes a bit closer to being matched. Modifying that pot so you can bias your amp from there is a great idea. Not knowing if your amp has been worked on in the past, I personally would suggest bringing it to a tech as this could save you money in the long run, or your life.

[alien]

Anyone else?

Plus does anyone know if their is still juice in my caps if I have had this amp unplugged for more than a year?

[Garydean]

The best way to tell is if you have a DMM. I've never seen lethal voltages after a cap has rested for a year. Maybe someone else has? Be sure no one else has flipped the switch without your knowledge. I never assume anything.

Start reading, for your safety.

http://studentweb.eku.edu/justin_holton/caps.html


Gary

[alien]

Thanks Gary

I bought some fuses and thought that maybe the balance bias was so off it was making one tube red plate--so I turned it on and the speaker is making crackling noises(without even plugging my guitar in)

I didnt even get to the point to adjust the balance bias before the new fuse blew

Oh yeah and the fuse blew with both power tubes out.

[Garydean]

pull out the rectifier tube or plug and see if the fuse blows. If it does, it's not the rectifier and you need to look at...........

Your filter caps could be bad.
you could have a bad screen grid resistor on the output tube that was red plating.
I would lift one end of those resistors to see if it's in spec.

You're at the point where you need to drain the caps... do you know how to do that? A good digital multimeter would be handy right now too.

Gary

[mikeboone]

There could me several things that cause a power tube to behave this way.

Since the fuse is blowing without tubes, you need to find where voltage is finding its way to ground either through a short or low resistance. Take the preamp tubes out and see if it still blows fuses.

Its possible that the amp needs a cap job. Do you know how long it has been since it was re-capped?


If the tube still red plates, look at the following:

-Check the tube socket to see if there was any arc'ing. If so, change it.
-Check to make sure the bias voltage is present on Pin 5. No bias voltage present will make tube red-plate.
-Check the 470 ohm screen grid resistor on the tube socket.


By no means are the the only things to check, just some things that have given me trouble.

Be patient and thorough and these guys will help you through your issues.

Mike

[alien]

Yeah..its a rectifier cap..a guy put in on for me in like 1984. Its funny..that was the only time I ever had a problem with this amp..Im the original owner and it had all the original tubes since the day I bought until now(except the cap)

I'll try that. I wonder why the speaker was crackling.

Its just my luck,,if that original tube didnt break when I was moving the amp I probably would have no problems. Everything went south when I plugged in those new tubes. But of course Im am ass because I was told I probably didnt have to bias because these tubes were real close to the originals(and I listened)

Im no engineer but Im thinking because I didnt bias when I put the tubes in and played it for a while it blew something? But what is most likely to go?

wow..thanks for the help guys

ps..about draining the caps....I have read so many ways to do this I dont know what to believe...one guy said you just play it while unpluggin it and it will drain at least to a safe level.

OH..can I safely pull out that rectifier cap?????

[mikeboone]

You can drain the caps initially by doing the following...

While playing your guitar, switch the standby switch off and play it down til it quiets,then kill the power. Then turn the SB switch back on and you should hear it come back up, then drain down.

THEN - you can short the + side of your filter caps to ground. You may see a small spark when you do this. I have a wire with alligator clips on each end with a 10 ohm resistor in series.

Then BEFORE YOU DO ANYTHING measure the voltage on the + sides of a you filter caps. It will go to zero when you short it, but climb back up to a few volts but this is normal.

Never assume that your caps are drained, always measure. It will ruin your day and possibly kill you.

[alien]

I wonder why the speaker was crackling.


Im no engineer but Im thinking because I didnt bias when I put the tubes in and played it for a while it blew something? But what is most likely to go?

Regarding what I said above

I read that carbon plate resistors can cause a crackling sound. I wonder if these would go when I put the new tubes in

[txstrat]

Fix one thing at a time.
I wouldn't care for the crackling sound right now.
I'd start finding out why the amp blows fuses on the basis of the things Mike and Gray told you.
After that you can still go and troubleshoot other problems.

[alien]

Fix one thing at a time.
I wouldn't care for the crackling sound right now.
I'd start finding out why the amp blows fuses on the basis of the things Mike and Gray told you.
After that you can still go and troubleshoot other problems.

It sounds like bacon is frying. It has to be related because this amp has never made that sound. Remember, this amp had no problems in 30 years until I changed the power tubes

-----------------
anyway I did what Gary said

I took out the Solid state rectifier and the fuse didnt blow and no crackling. I also tried many other combination's including removing 1 power tube, both, and all smaller tubes. The fuse blew every time the recifier was plugged in and never blew when it was out.

So, what does this mean?

thanks guys

[Enzo]

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.