Fender Blues Deluxe

[rodrigosalvati]

Hi,

I am helping a friend out with his Fender Blues Deluxe from 1993. Ive replaced all the capacitors in the power supply, and all the voltages going to the different sections seem right. But I do have a question about the filtering section. Capacitor C35 only has around 50vDC when powered up. Is that how it should be? Also, I measured the resistance over R83, and it was well over 220k when in the circuit, but when I lifted it I did measure the correct resistance.

These are just two things I noticed, that hopefully understanding why will help me learn a bit more.

Thanks

 

[rayma]

The voltages across C34 and C35 should be close to equal (each about half the B+ voltage).
Otherwise one of the capacitors will fail sooner than usual.

If the voltages are not close, then check both R82 and R83 out of circuit.
If either is off, replace both with 220k 5%, with a rating of at least 1W or more.
They are only rated 1/2W at present, and will degrade quickly.

Also check R88 and R89 for the same reasons.

 

[rodrigosalvati]

Thanks very much. Of the 4 resistors, two were bad.

 

[rayma]

Not surprising, they were operated at their maximum power rating.
Parts must always be derated below the maximum rating for decent life.

 

[rodrigosalvati]

I've replaced the resistors and the voltages are equal now. One of the power tubes was defective so I replaced both with new matched set, and trying to bias.
I replaced the biasing resistor(r87) with a 10k trim pot and a 22k resistor in series, set it to the same 27K ohms and took measurements.

72ohms and 80.5 ohms on output transformer
1.66vdc and 1.87vdc voltage drop
439vdc and 437vdc Plate voltage
0.0231 Amps and 0.0232Amps Plate Current
10.14Watts and 10.15

Anyone know if this looks correct for this amp? Some values seem a bit low, so I am wondering if I am doing something wrong before trying to adjust the new bias pot. I dont think this version has a test point like the reissue.

thanks

 

[gijser]

I don't know anything about this particular amplifier, but looking at a datasheet for the 6l6gc tube, operating in class AB push pull the plate current should be around 80 mA, power dissipation at idle around 30 watts. https://frank.pocnet.net/sheets/093/6/6L6GC.pdf

 

[Rikaro]

Bias should be adjusted to get 60mV at test point T26. This corresponds to an average idle current of 30mA per tube.
Means a plate dissipation of 12.9W or 43%.

See reissue schematic here: https://www.thetubestore.com/lib/th...Blues-Deluxe-Reissue-2004-Schematic-Rev-A.pdf

 

[Rikaro]

I don't know anything about this particular amplifier, but looking at a datasheet for the 6l6gc tube, operating in class AB push pull the plate current should be around 80 mA, power dissipation at idle around 30 watts. https://frank.pocnet.net/sheets/093/6/6L6GC.pdf

You don't bias to 100% PD with fixed bias class AB.

 

[rodrigosalvati]

thanks, this was the original from the 90s, not the reissue, so I didn't find any test points. I left the plate dissipation at 10.15W, which seems low, but he says it sounds great.