BIAS - Blues Deville

[Valvehead]

I searched on here , didnt find the answer... Blues deville Reissue

So, I have bias issues... -70v at pin5..both tubes , should be -60v........Bias pot does NOT affect anything--thats what is odd....

The Bias Test point on the edge of the pcb (tp26) should show +60mvdc..........its at 3mV

Heres are the parts ive tested and they are all GOOD =
R66 / D3 (tp26)

R64/65

R59/60

R57/58

R77

R76

R78/ 79 (THE 5W CEMENTS)

D15

Test Point 41 has -74vdc = OK

Ive pulled both pcb's and went over all the solder joints . grounds are verified .

Any ideas ?

why would the bias pot have no affect ??

cant seem to attach a PDF schematic here....

so = https://www.thetubestore.com/lib/the...atic-Rev-A.pdf

[Enzo]

"Grounds are verified" doesn't mean much to me. What did it mean to you? The reason the trim pot would have no effect on voltage at bias would be either it or the R77 circuit was open.

Remember, it is about the circuit, not the parts. R77 is "good"? OK, but does the circuit see it? Power off and drained, what resistance to ground do you measure from eich end of R77? One end should measure close to zero ohms to ground. WHat should the other end read? tempting to think 100k, but no. If you do measure 100k there, then the part is not connecting to the trim pot. There is a parallel circuit to ground up through the trimmer and back down through R83. Those add up to 76k, which is parallel the 100k in R77. I get about 43k, do you?

ANother possible test: measure resistance to ground from pin 5 of either power tube. Turn the bias trimmer up and down. Does the resistance change?

Or just troubleshoot. The whole bias supply is one big voltage divider. There is -74 at TP41? Great. That fights towards ground through R76 and then parallel/series group R77/82/83. It is late, and I am enjoying a screwdriver, but let me fire up the calculator. R77 plus R82 is 125k. That is parallel R83 51k. WHich I get 36k for a total. SO the overall divider is 10k over 36k. And that gives me about -58v at the right end of R76.

But what if you get about -62 there? That would mean only the 51k resistor was in the divider, and the trimmer side was open to ground.

If only the 51k side were open, then I get about -69v at the right end of R76.

But all that assumes the circuit is as wired on the schematic. WHat if the ground connection of R83 and R77 doesn't actually get there?

[bsco]

Excellent breakdown Enzo...

[vintagekiki]

The Bias Test point on the edge of the pcb (tp26) should show +60mvdc..........its at 3mV

Heres are the parts ive tested and they are all GOOD ... ...

Test Point 41 has -74vdc = OK

Ive pulled both pcb's and went over all the solder joints . grounds are verified .

Any ideas ?

should show +60mvdc..........its at 3mV

Everything is fine but not work.

When was the last time surprised your Blues deville for birthday?

Everything has its age, so and 6L6. Tube is not perpetual motion so it lasts forever.

[Valvehead]

should show +60mvdc..........its at 3mV

Everything is fine but not work.

When was the last time surprised your Blues deville for birthday?

Everything has its age, so and 6L6. Tube is not perpetual motion so it lasts forever.

I tried different 6L6 in it.

[Valvehead]

"Grounds are verified" doesn't mean much to me. What did it mean to you? The reason the trim pot would have no effect on voltage at bias would be either it or the R77 circuit was open.

Remember, it is about the circuit, not the parts. R77 is "good"? OK, but does the circuit see it? Power off and drained, what resistance to ground do you measure from eich end of R77? One end should measure close to zero ohms to ground. WHat should the other end read? tempting to think 100k, but no. If you do measure 100k there, then the part is not connecting to the trim pot. There is a parallel circuit to ground up through the trimmer and back down through R83. Those add up to 76k, which is parallel the 100k in R77. I get about 43k, do you?

ANother possible test: measure resistance to ground from pin 5 of either power tube. Turn the bias trimmer up and down. Does the resistance change?

Or just troubleshoot. The whole bias supply is one big voltage divider. There is -74 at TP41? Great. That fights towards ground through R76 and then parallel/series group R77/82/83. It is late, and I am enjoying a screwdriver, but let me fire up the calculator. R77 plus R82 is 125k. That is parallel R83 51k. WHich I get 36k for a total. SO the overall divider is 10k over 36k. And that gives me about -58v at the right end of R76.

But what if you get about -62 there? That would mean only the 51k resistor was in the divider, and the trimmer side was open to ground.

If only the 51k side were open, then I get about -69v at the right end of R76.

But all that assumes the circuit is as wired on the schematic. WHat if the ground connection of R83 and R77 doesn't actually get there?

Cheers Enzo --
Did some testing tonight ---

R77 is at 0 ohms 1 end to gnd and 100k at the other to gnd ........not 43k as you mentioned

R77 and R83 are getting to ground and are connected to bias pot .

Pin 5 to ground , either 6L6 , i get no change turning bias pot

tested R83 out of circuit - i get 51k

does R76 have -58v ? Nope , it is -73v one end and -75v other end

testing ohms across the bias pot pins i get something like 135k i think , forgot to write that down , , turning the pot nothing changes on the meter

could the bias trim pot be bad ??

[Enzo]

Yes the trim pot can be bad. Probably open at the lower end. OR OR OR the copper trace that connects that end of the trimmer over to the resistor could be cracked or damaged. SOMETHING between the wiper and the resistor is open.

R77 measuring 100k in circuit is the clue, that means the other parallel circuit is not connected to it. The fact you get bias voltage at all to the tubes means the path through the trimmer is intact to the wiper. That is what I mean by it is the circuit, not the parts, that we should consider.

[vintagekiki]

I tried different 6L6 in it.

How much bias voltage changes (point PW3B-6) with changing bias adj R82 (25k) from min to max.

[SoulFetish]

The bias Pot sits right next to those 5W resistors in the low voltage supply(R78 & R79).
those generate a lot of heat and are installed flush with the board. They usually will cause a lot of heat stress and damage to the traces and solder pads underneath the board.
I would lift the board and check that out, Particularly if you can see any discoloration from the top. Then you can be sure that there need to be some repair work done To those traces and solder pads in that area.

[Valvehead]

fixed 1 trace. Checked continuity to ground , etc.. all good everywhere .. Bias pot measures ok with ohmeter now --it works. Same problem-- around 2 mv at bias test point and -65v at pins 5 . Amp works but clean channel definately is not clean--background distortion . caps in bias supply ???

[Helmholtz]

-- around 2 mv at bias test point

Check if D3 is shorted. Requires lifting one end.

[g1]

Are you able to vary the -65V with the pot? What are the lowest and highest voltages available at power tubes pin5 ?

[vintagekiki]

... ...

How much bias voltage changes (point PW3B-6) with changing bias adj R82 (25k) from min to max.

[Valvehead]

Are you able to vary the -65V with the pot? What are the lowest and highest voltages available at power tubes pin5 ?

D3 is ok

its at -70v -- the bias pot works according to my ohmeter , but has no affect on the bias V when turned . I do have a new 25k here , i can try and swap it . also checked c38 = good . I guess ill pull the main pcb again and go thru all the traces . The tube pcb looks good .

[g1]

Check solder connections at R77 and R83. When you do the ohmeter check on the pot, put one probe at R82/R83 junction, and one probe to ground.