Fender Blues Deluxe troubleshooting? FIXED! How to Dumblize?

[Leo_Gnardo]

Hmm good advise I'm sure.
I had to take a break but I'm still looking at this thing, I haven't tried everything suggested yet but thanks for all the comments!

I'll be back....I hear y'all thinking greaaaate.... Lol

Don't despair. I've been able to fix every single one of this series of Fenders that's come my way, Blues Deluxes, Not so Hot Rod Devils, etc. without ripping out the innards and building from scratch. And so can you. I don't much like 'em but they're here to stay so I gots to deal with 'em. Fender has guaranteed I'll have something to fix every day of my life.

[matt h]

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[eddie25]

It was mentioned, but did you double check the amp with the footswitch connected to rule out that jack?

My money on the original problem is a preamp tube socket. I had one of these fenders where every socket went bad in the span of a few years. Often when you hear a fade out its because the heater has a bad connection to a tube and the sound fades as it cools off or whatever. While it's silent, turn off the lights and check that all the preamp heaters are glowing.

[tubenoob]

Cool, I'm getting back on this. I had 64 Vox to antagonize me while I was taking a break from the Fender, with it properly slapped down and fixed, all attention back to the BD.
It's encouraging to hear there may be hope yet.
I've always heard that people mod these amps into something decent.
Without getting too far ahead of myself, what are the changes that people do to these?

[Leo_Gnardo]

I've always heard that people mod these amps into something decent.
Without getting too far ahead of myself, what are the changes that people do to these?

Some de-brightening, pad pots, fatten mids & lows. We'll get into that, let's make it work first.

I'll give you one E-Z mod for now. Fender if I'm not mistook, uses a 130K slope resistor, that makes the tone stack shattering bright. It's jammed right behind or maybe between the tone controls on the circus board. You can hang a parallel resistor on it without having to unass the board. About 470k in parallel should give you 100K the stock blackie/silver value, and you can even go less to bring out the lows & mids, dial back the highs. You could select all the way down to 47K which gives you 34k, about as low as I'd go. If you have some good mini clips try some different values & pick the one that makes your ears happy.

[matt h]

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[tubenoob]

What is the negative voltage on pin 5 of either output tube, above -52?

Ok I checked this and it's -44v
I finally have some time to work on this and I plugged it in after sitting for a few days and it seemed to work fine at first then I turned it up and the cracking started again, then the Drive LED flickered as it cut out, then it slowly faded out to silent.
Voltages all still present on the plates, etc.
It seems like it's either loading up somehow or heating up and the same series of symptoms occurs and it's unplayable for a while.
What is happening here??
I thoroughly cleaned the FX send and return jacks, the speaker jack, replaced the input jack, replaced the power supply caps, all those connections are good, the work is solid I just don't have a clue what's causing this, help... Anybody? Please help me get a grip on this confounding thing

[Stevem]

That -44 is a little low, I would set it at -48 to -50 just to be on the safe side for now, Fender calls for -46.8.

When the amp cuts out can you still pull signal off of the effects send jack?

Also how are all the solder connections on the boards from the tube sockets, any that look even somewhat flaky should be reflowed with the tube out of the socket.

All the secondary PT wires connect to the board with spade connectors, are they all tight, the same with the OT primary side .

[tubenoob]

Does anybody know hot to adjust the bias on these amps? There doesn't appear to be a bias pot, I see a 27k resistor, R87 on the schematic, do I need to change this value? How would I replace that with a trimmer?
Anybody? Help...

[tubenoob]

Weird thing, I tried another amp's preamp out to the Fender's power amp in and it worked! But then it slowly got more distorted, with hard picking it was distorted then after a minute or so it faded out and no signal.
I measure all the plate voltages of the power amp and preamp and the all seemed normal.

Another strange thing is no sound when I probed the plates in the power amp and the preamp except the PI which you could hear a click.
The reverb was mute as well, before I could rattle the tank and hear the springs but now it's dead UNTIL I switched off the standby switch and turned it back on, then I could hear rever again but still no signal otherwise.

I have no idea what's going on here.

[ampdoc1]

I've found that that the 91k (or maybe the 9.1k) resistor in the PI was always the trouble in these amps. I don't know if they were from a hugely bad batch, or they were under-rated.

Also a lot of times Q1 is bad.

I don't remember fixing anything else on these amps except bad traces on that shitty PCB.

[Sazafraz]

Here's the link for getting rid of the unused triode http://www.duncanamps.com/technical/bluesdeluxe.html have not tried the mod

There is a re verb mod that brings it to life think its clipping c50 (not sure)

Thanks for the hint on the 750p coupler never thought about that one !

Should all ready have the 100 k slope resistor if its the original made in usa one

After blowing the orginal speaker a V30 is a nice upgrade in this amp

You should be able to replace the bias resistor with a pot or a pot and resistor like 10k resistor and a 25k pot (not an expert on this)

[Stevem]

Yea, that 91k in the PI section always goes as both of those plate resistors need to be 1 watt, not 1/2 watt like they are, this resistor failing will not make the amp drop off and fade out however!
Are the output tube heaters going out when the amp fades off?
It would also I seem at this point test it's time to pull all the boards out
( yup that main board is a pain) and check all the solder connections!
Also just for kicks jump the standby switch and see if that cures the fade out.


In regards to the bias adjustment go to radio shack and get a 50k pot and replace that 27k for now, let it just dangle by some wire for now and when you install it set it for max resistance and back it down to like -50 volts on pin 5.
When you are done with the rest of the amp repair measure the resistance you have the pot set for and make up the needed resistors to get that value.

Note that fender ships these amps biased on the cold side so that the amps / tubes stand a better chance of making it through the warranty period and the resulting cross over distortion from the output stage sounds like crap when you push the amp hard enough!

[tubenoob]

...
Are the output tube heaters going out when the amp fades off?
It would also I seem at this point test it's time to pull all the boards out
( yup that main board is a pain) and check all the solder connections!
Also just for kicks jump the standby switch and see if that cures the fade out.


In regards to the bias adjustment go to radio shack and get a 50k pot and replace that 27k for now, let it just dangle by some wire for now and when you install it set it for max resistance and back it down to like -50 volts on pin 5.

The heater voltages do not drop when it fades out, neither do the plate voltages.

To replace that resistor, can I tie the wiper of a trim pot to one of the outer legs and put that pot in place of the resistor? Or do I need to have a tail on the trim pot like a 10k resistor and a 25k trim pot?

[eddie25]

It could in theory be a bad connection from the socket to the tube which might still show proper heater voltages on the socket. You would have to check that each tube has heater glow after it fades out. I hate to harp on that, but I've seen it happen more than once.