Fender Blues Deville Issues

[greenmeanh1]

This amp had tons of problems. Sitting in a cold storage locker for the last 10 years didn't help. So far i have changed all the jacks and pots out and a dozen or so caps. I have all the power supply voltages sort of back to normal but they seem higher than spec. So searching around I found a FP 470 ohm resistor feeding one of the 6L6 tubes that is burnt through with a bad reading so i think that may be the issue. It is the 415 volt Y feed to pin 4 on the 6L6's and the resistor sits in between. Given my limited knowledge on this stuff,,,
Question 1,,Is there a name for this feed? eg bias, grid, collector??
Question 2,,I know the FP stands for fire proof but i don't have any of these in my collection. Can i use a standard 1 watt 470 ohm temporarily for testing purposes??
Question 3,,. The tube PCB requires removal to get at the parts. When i put it back together Is there any issue mounting the resistors on the visible side/trace side of the PCB?? Simply easier to change out later without removing the PCB.
So there is my 3 questions.
Thanks

[The Dude]

That resistor is a screen resistor (feeding voltage to the screen grid). It doesn't necessarily need to be FP. I'd just recommend you stand it off the board a bit so if it does flame out it doesn't burn the board. I don't usually remove the board. You can unsolder it and shake the old part out. Then mount the new part on the solder side. It makes no difference and it's a lot less work. With an open screen resistor, that tube won't be conducting like it should, so B+ may lower, but I suspect not substantially at idle.

[greenmeanh1]

Sorry, another question if i may. Would a bad tube cause this resistor to blow??

[greenmeanh1]

Wow. I have to say this is the fastest forum i have ever been on. Thank you so much for the ultra quick response. Advise noted!! Awesome!!

[The Dude]

Sometimes caused by a bad tube. Other times they just open over a period of time.

[Leo_Gnardo]

Sorry, another question if i may. Would a bad tube cause this resistor to blow??

An internal short in the 6L6 served by that resistor would be the #1 reason for it to burn. So, yes.

FWIW I usually use 5W 5% wirewound resistor to replace these. It's a debate that goes on & on - some techs like to see this resistor as sort of a fuse. If that's the notion then 1 watt is a good power value. Note all the classic Fenders used a carbon composition 1 or 2 watt resistor here. Those are getting hard to find and expensive.

[The Dude]

Agreed, Leo. I forgot to mention wattage in my post. My experience is that the 1 watters will open on their own over time, so IMO not necessarily a tube short. That tells me that 1W might be on the edge of large enough. If a 5W goes open, more likely a tube issue. I usually use 2W and I'll buy a stack of 'em at a time so they're always on hand and cheaper. As you say, the debate goes on & on.

[greenmeanh1]

Great info Thanks. I will go with the 5w 470ohm replacements. I can get them down the street easy as well.
If i recall the amp was used as a tube tester many years ago by way of plugging power tubes and so forth into it to confirm their operation. At some point the amp failed, probably as a result of bad power tubes being swapped in and out.
I recently went through my whole collection of tubes and tested them with my well maintained Stark Tester. I had many shorted out tubes in that pile i have since thrown out. I have a few tube amps in the repair pile from years back that likely suffered the same fate while swapping unknown tubes in an attempt to get the amps working. At least i have a good clue on where to look now in my efforts to repair them.
Anyway i will report back results and thanks again for your replies.

[Leo_Gnardo]

Agreed, Leo. I forgot to mention wattage in my post. My experience is that the 1 watters will open on their own over time, so IMO not necessarily a tube short. That tells me that 1W might be on the edge of large enough. If a 5W goes open, more likely a tube issue. I usually use 2W and I'll buy a stack of 'em at a time so they're always on hand and cheaper. As you say, the debate goes on & on.

In any case, the sg resistors are one of the first things I check when looking around inside an amp that's been brought in with complaints of "sounds weak" or similar. If one is popped, the associated tube automatically becomes a prime suspect, unless fresh outputs were installed in an attempt to make the amp work better.

[Leo_Gnardo]

If i recall the amp was used as a tube tester many years ago by way of plugging power tubes and so forth into it to confirm their operation. At some point the amp failed, probably as a result of bad power tubes being swapped in and out.
I recently went through my whole collection of tubes and tested them with my well maintained Stark Tester. I had many shorted out tubes in that pile i have since thrown out. I have a few tube amps in the repair pile from years back that likely suffered the same fate while swapping unknown tubes in an attempt to get the amps working. At least i have a good clue on where to look now in my efforts to repair them.

Good story. And it points out that it's a good idea to test tubes first, even if it's just a pinball machine go/no go emissions & shorts tester, before trying them out in an amp. I have both, an old Knight tester plus a Heath W4 power amp I use to test output tubes under real world conditions. Seems a waste to unknowingly plug shorted 6L6's into an otherwise good amp until one day resistors start to burn, ow. Well in the end it put a good amp into your possession, I'm sure you'll be taking better care of it.

[greenmeanh1]

Lessons learned over the years. LOL
I have been running my rehearsal/recording studio (amusikzone) for just over 30 years. Its unbelievable the amps i have gone through or junked. Its only been the last few years i started fixing stuff myself and so far rescued a lot of nice amps that were sitting around broken in the storage closet. A lot of help from guys on this forum and some tools go a long way and its amazing the money saved and the skills gained. Takes me longer than most to study schematics but im getting better. Thanks for the tips

[greenmeanh1]

A little progress. I put in the 470 ohm resistors and got the amp to fire up but still have issues.
I have good volts on X,Y,Z, B+ C- and also the 16v feed. The power section seems to work as i can get sound going into the preamp in jack although its a bit farty. The pre out is not working and if i plug into either of the channel inputs i get very little volume. Less than a watt for sure. I can hear the eq's working, the channel switch seems to be functioning but its not getting signal into the power section of the amp. A bit stumped at this point. I tried numerous 12ax7's with the same result.
There is a TL072 (U1) that appears to buss the preamp line signal to the pre in and power amp out jacks. Is this part ever known to be an issue or blow? Not sure how to test other than to see incoming voltages on pin 4 and 8 which both U1 and U2 have.
Thoughts if any??

[52 Bill]

There is a TL072 (U1) that appears to buss the preamp line signal to the pre in and power amp out jacks. Is this part ever known to be an issue or blow? Not sure how to test other than to see incoming voltages on pin 4 and 8 which both U1 and U2 have.
Thoughts if any??

That chip is completely out of circuit as long as the loop jacks are not being used. If it was dead the amp would still work.

Have you taken plate and cathode voltage readings on the preamp tubes?

Have you checked the input jacks for broken solder joints?

[greenmeanh1]

OK Im not that smart yet. I do not know the difference between the two or what pin numbers are assigned to them in total.
The best i can figure is based on what i think i might know LOL.
Here are the readings i have with all the tubes out with standby on. Voltages are high because there is no load i suspect.
Power supplies.
X 498vdc measured at C46+ 0v on neg
Y 492vdc measured at C45+ 0v on neg
Z 498vdc measured at SP2 note negative side of cap shows -188v ???
B+ 500vdc measured at SP1 note Negative side of cap shows -435v ???
C- 50vdc measured between R53 and R54
16vdc from the z diodes
Pretube V1 has 492vdc on pins 1 and 6
Pretube V2 has 492vdc on pins 1
Driver V3 has 492 on pins 1
6l6 V4 Pins 3 and 4 has 498vdc pin 5 has -48
6l6 V5 Pins 3 and 4 has 498vdc pin 5 has -49
Best i have confirmed thus far.

[Jazz P Bass]

When measuring the preamp tube voltages, it is not enough to simply state the Plate voltage.
You Must include the Cathode voltage.