Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on January 31, 2013, 10:28:16 am

Title: Blues DeVille Bias Problem
Post by: plexi50 on January 31, 2013, 10:28:16 am

I have a 1994 Blues DeVille. The preamp board had been modified and i had to repair many traces and incomplete connections. This is the kind of job you dread onve you get into it. I am finished replacing many strange part values and the preamp is back to stock. The amp sounds very distorted and has low volume. All cathode voltages and plate voltages look good. Entire preamp verified and all parts good. Im thinking the bias i wrong. The schematic says -46VDC. I am reading -63VDC on the pin 5 grids. I replaced the bias resistors that were in there (12K & 22K with the stock 3.3K & 27K. PI 3&8 measures -38VDC. Whats up Doc?

Edit: I am using the wrong schematic. Going back over values. -49VDC bias is what im looking for

Title: Re: Blues DeVille Bias Problem
Post by: punkykatt on January 31, 2013, 10:36:39 am

Plexi50,  Take your listening device and start at the input jacks, work your way through the amp until the distortion and low volume start. That should pin point the problem.

Title: Re: Blues DeVille Bias Problem
Post by: plexi50 on January 31, 2013, 10:44:21 am

I just found 100K screen resistors on the power board. Not 100 ohm. The bias i think is the main issue i need to tackle at this moment. Bad bias = bad distorted tone. What in the heck was this person thinking?

Edit: 470 5 watt installed. Changing R84 back to 12K

Title: Re: Blues DeVille Bias Problem
Post by: plexi50 on January 31, 2013, 12:02:37 pm

Changed R84 back to 12K. Bias is -46VDC. Not getting -49VDC bias voltage. Loud and sounds pretty good on the Clean channel. OD drive channel volume very low. Rechecking drive channel part values. Man someone really butcherd this board.

Edit:
Found 3 other resistors and one ceramic cap that did not belong in this amp. On Drive relay resistor R16 was 100K instead of 12K. Blah and blah. Amp finished and sounds good. The speaker cab itself has a rattling issue which thankfully for me i do not do or mess with. I earned my money on this one
 

Title: Re: Blues DeVille Bias Problem
Post by: Willabe on January 31, 2013, 03:24:51 pm

Quote from: plexi50 on January 31, 2013, 12:02:37 pm

The speaker cab itself has a rattling issue which thankfully for me i do not do or mess with. I earned my money on this one

Another good job and happy clinent.


               Brad     :icon_biggrin: