blues deville 'cleans' are fizzy

[acorkos]

i have a blues deville that's producing a slight 'fizz' to the clean guitar sound....the clean is not really clean. it's still there with new tubes, and when playing into a different speaker cab. it's also there when i plug into the 'power amp in' jack (as well as the main input jack). the plate and screen resistors associated with the phase inverter and power tubes all check out OK. the only thing i can find wrong is that the layout diagram shows 413V and 364V on filter caps 35 and 42 (100uF/350V)...i measure 215 and 220 volts on the positive side of those caps. the 220K resistors in parallel with those caps are ok, too. any ideas as to what's going on? thanks.

schematic:
http://www.fender.com/support/amp_sc..._Schematic.pdf

[Rob Mercure]

Ackoros,

Your voltage readings indicate that something is "pulling down" the b+ supply - that is, drawing so much current that the transformer's internal impedance drops the voltage and the most likely culprits are the output tubes. The two less likely culprits are a high resistance in standby switch or an open diode in the rectifier bridge. More likely is that the 6L6GCs are drawing too much current - are the tubes plates glowing red? (you might have to look with the room lights off). Is the power tranny getting a bit hot? Have you measured the outputs control grid voltages? Your schemo calls for 49.6. If the grid voltage is within a few volts try another set of output tubes - while Fender used great tubes when they were made in the USA the "devil" amps use Sovteks and I believe that the assemblers just pulled a couple out of a carton without selecting for match or other desirable characteristics.

If your bias is off after checking the bias supply don't forget that leakage in the coupling capacitors from the PI stage can also offset the bias.

Rob

[stokes]

You cant really depend on the -49.6 grid voltages.With different power tubes the -49.6 bias voltage is likely going to cause various tubes to draw different current,better to check the current draw than the grid voltage.

[WholeToneMusic]

Low voltages

I noticed that the voltages were low on those test points also,
By looking at the schematic C34 is in seies with C35 so shouldn't C35 be half of the voltage of what c34 is because they are in series?

[Enzo]

Ignore the layout, the test points are mislabelled. Lookm at the schematic, 413 and 364 are found on C45, C46. They are the two caps to the left of the ones lebelled with the test points on the layout.

C35, C42 are the bottom half of stacked caps and are supposed to have half the B+ voltage at those nodes. If your B+ was dragged down to half what it should be, the amp would have a lot more problems than "fizzy."

Your B+ is fine.

However, is B+ reaching both plates of V3 the PI tube?

Does the reverb sound OK? A leaky Q2 could put some DC on the input to the power amp - well, where it mixes with the dry signal before entering the PA.

[WholeToneMusic]

Schematic

I had a feeling that the schematic was wrong on those two test points. Can't always go by the schematic when logic tells you different.