Fender Blues Deluxe Reissue

[Enzo]

The bias winding appears to be a 39v winding on the 36958. On the 41752, the winding appears to be 54v CT, so 27v per side. Due to this lower voltage, that cap is part of the voltage doubler with the diodes.


ANy way to tell if this power transformer is a replacement? Someone may have replaced a bad transformer, looked up the "correct" part for the Deluxe, not realizing this amp needed the DeVille transformer. It's possible...

Check the voltages on those two power resistors for the 16v supplies. On the hot ends, do you see closer to 33v or 48v?

[Enzo]

Having 70-100v bias voltage on the grids of those power tubes is MORE than enough problem to solve.

[bsco]

The resistors have plus and minus 47 volts on them. The ac winding of the transformer has 39 volts across...it is not center tapped......I ermoved the output tubes and plugged a cable from the pre-amp out to another amp and plugged a strat into the input of the deluxe...great sound..so the problem is definately in the bias circuit and ot output circuit somewhere.

[Enzo]

OK, I'll bite. Remove D10, install W2 jumper, and see if it falls into place.


MAy I assume W1 is already installed?

[bsco]

I though about doing this but the amp had previously worked with this particular configuration...The bias pot measures 39k it is supposed to be 25k??? Could this cause the bias voltage to be off by that much??? If I remove D10 and install J2 (have to remove the cap of course) what are the possibilities of something else getting damaged or fried?? ANd you are right W1 is already there.

[Enzo]

J2 shorts across the capacitor, so it is irrelevamt whether we remove the cap or not.

The problem is way too much bias voltage to start with, the problem is not the adjustment range. The stock circuit has -53v to play with. That is as high as it will go.


Yes, you could have any number of other things wrong, but without proper bias, the amp was not going to work right, so we needed to fix it. If you can get the bias down to -50v instead of twice that, then there is a good chance you will be able to find to 60mv on the test point.

Then we can move on from there.

[bsco]

OK. I will go downstairs and try it right now. I'll let you know what happens shortly....Thank you very much....

[Jazz P Bass]

That is a good idea.
You could also "scope" the output tube grids themselves (without the 6L6's in).
You should have a nice healthy signal there.
If you do, install the 6L6 tubes and a dummy load on the output.
Measure the voltage of the 6L6 plates to make sure they are getting voltage through the output transformer primary.
If the tubes are good, without a signal, you will see a voltage (measured in millivolts) across the 1 ohm resistor.

[bsco]

I tried what you suggested Enzo.....guess what..........it worked. I have the bias set for 60mV and pluged the strat in and tried it out. Great sound and tons of volume. The drive channel works great as well. Here is what I do not understand...this amp had to come from the factory like this and it had actually worked for some time. There were no signs of any work being done to it before. When you mentioned that the transformer could have been replaced, I checked the board for the metal stakes that the transformer harness connects to...the center-tap for that particular winding had no stake to connect to...and the solder pads were never touched. And there were no other signs of any work ever being done on this unit. According to the schematic it looks like somebody at the factory started to wire it up as a Deville maybe?? What for I would not be able to guess. But without the help of the people from this form I doubt if I would have figured it out as like I said, it came from the factory like this so you would think it should still work like this...I think before I even assemble the board back into the chassis I am going to run this beast for a while..Is there anything else I should check in this general area?? I am going to monitor the plate, screen and bias voltages for a bit. And make sure that the tubes don't red-plate...I thank everybody who tried to help me in any way they could and if you guys were closer I'd buy you a few beers. As a matter of fact I am going to have one right now to celebrate...Once again everybody..thanks very much. I'll let you know how the amp makes out over the next few days on test.

[g1]

this amp had to come from the factory like this and it had actually worked for some time. There were no signs of any work being done to it before

I'm guessing this is a transition model like Enzo suggested, probably with the upper bias circuit shown on schematic but with the 036958 transformer.
You should check R83 and R77 in case either has opened or changed value. This could have caused the increase in bias voltage.
What is the value of R76? If it is 10K, change it to the 1.5K shown in the lower bias drawing. Also remove R83.

[bsco]

Thanks Enzo. I replied back to your post but it appears furhter up for some reason. Thank you very much for your help..It worked. To g-gone.....the resistor R76 measures 1.5k and R83 is not there to begin with. This was a very interesting learning experience for me for sure and I can assume that this not the only amp out there that has this sort of thing done to it.......Once again, thanks to everybody who had attemped to help me in any way. You guys are fantastic...

[g1]

Just make sure that the bias pot and resistor are ok (R77 & R82). You did mention the pot reading 39K rather than 25K which seems out of tolerance to me.
Otherwise you should be good to go.

[bsco]

Thanks. I have been running the amp all day...And I keep checking the volatges and so far...so good. I was thinking of replacing the bias pot anyway just to be safe....just because of the measurement difference...I am going to recheck that measurement again later to see what it reads but I just might go ahead and replace it anyway for what the pot costs...Once again...Thanks for all the advice and help.

[Enzo]

The value of the pot doesn;t matter much, as long as the sweep of its range results in voltages that go high and low enough to cover your needs.

[bsco]

Hi Enzo. The pot has lots of range going from way below the 60mV setting to way above it. And when the range is varied it is not erratic or jumping..it looks to be very smooth. I was just concerned about the overall value..I am after runing the amp for quite a bit and have been monotoring the plate,screen and bias voltages and everything is ok. So I guess the next step is for me to reassemble and install in the cabinet and then run it again for a few hours just to be sure. After that, it should be good to go. Thanks for the tip.
Cheers.....