Fender deville bias issue

[angelothewolf]

Hi everyone! im working with this Fender Deville reissue. it came with both screen grid resistor open (R61/62). all voltages seemed to be ok so i replace that resistors, set the bias and was working perfectly.
After some hours of testing the volume sudently drops and tube start to overheating. Test bias and the voltage is Running away up to 150mV. So i check all voltages and resistors and everything apears to be fine, also replace the tubes.
So after some troubleshooting i get to the two .1uf coupling caps (C26/27) ...if i lift those the bias becomes stable
any help??

http://support.fender.com/schematics...atic_Rev-A.pdf

[Leo_Gnardo]

Hi everyone! im working with this Fender Deville reissue. it came with both screen grid resistor open (R61/62). all voltages seemed to be ok so i replace that resistors, set the bias and was working perfectly.
After some hours of testing the volume sudently drops and tube start to overheating. Test bias and the voltage is Running away up to 150mV. So i check all voltages and resistors and everything apears to be fine, also replace the tubes.
So after some troubleshooting i get to the two .1uf coupling caps (C26/27) ...if i lift those the bias becomes stable
any help??

One or both of those coupling caps leaky. Unusual to see this in an amp so 'new'.

[angelothewolf]

One or both of those coupling caps leaky. Unusual to see this in an amp so 'new'.

Nop the caps are fine...

[g1]

What do you mean they are fine? Did you replace them and still have the same problem?

[dwmorrin]

Don't those amps have the C- come in through a ribbon cable or something?
Sounds like you're just getting a broken connection from bias power supply to the tube.
I would suspect intermittent wire connection or a partially broken trim pot - something along those lines.
The voltage at the grids probably "rises" to 0V after becoming disconnected from the power supply.

[Justin Thomas]

And after "some hours" of testing, I'd suspect heat might have something to do with it... was the amp in the cab or on the bench during testing?
Could also be something similar around those caps - solder joints opening up, trace lifting, etc. If not the caps themselves, something in their wiring/connections.

Justin

[The Dude]

Nop the caps are fine...

I'm with g-one on this. You said you lifted the caps and bias was stable. That's a relatively certain indicator the caps are bad. Then, "Nop the caps are fine...". How did you check them? Substitution is the only certain method in this case, especially given that the amp has to warm up before the symptoms present themselves.

[angelothewolf]

I'm with g-one on this. You said you lifted the caps and bias was stable. That's a relatively certain indicator the caps are bad. Then, "Nop the caps are fine...". How did you check them? Substitution is the only certain method in this case, especially given that the amp has to warm up before the symptoms present themselves.

the stock caps are rated 630vdc, i tested with a capacitor leakage tester and they masure no leakage at 500vdc. also i replace them with brand new orange drops and i got the same issue. now when i engage the ht supply the bias go straight up to 150mv and start to rise. im not understanding whats going on, i lift the caps and the bias voltage estabilizes... wtf??

im starting to have the feeling that is one of those stupid mistakes...

how do you measure leakage with the caps in circuit? measuring dc at the low side of the cap i get like 20vdc spike in the moment i engage the HT, then it starts to drop and stays around 1,5vdc...

any thoughts?

thanks!

[dwmorrin]

now when i engage the ht supply the bias go straight up to 150mv and start to rise. im not understanding whats going on, i lift the caps and the bias voltage estabilizes... wtf??

The clues I'm getting are "rise" and "stabilize."
The C- should just appear at the grid and stay put.
What about doing voltage checks all along the C- path? Start at the bias pot and go all the way to the grids. You could get a clue if it is dropping further back. Anywhere it drops, check the value of the resistor or check resistance of the trace and any board to board wires.

I'm wondering if your "150mV and rising" reading is due to that node being disconnected completely, thus having no ground reference, and your meter is actually floating there.
Just something to consider...

When you take these caps in and out, are you moving things around alot? Could be masking the real problem.

[Enzo]

Look VERY closely at the solder on the socket pins of the power tubes. Are there small cracks around the pins?

Wwhenever I see one of these amps I just automatically resolder the power tube sockets.

[Zipslack]

Look VERY closely at the solder on the socket pins of the power tubes. Are there small cracks around the pins?

Wwhenever I see one of these amps I just automatically resolder the power tube sockets.

I'm with Enzo on this .... I remember chasing my tail for days when I first worked on a Fender tube amp (oddly enough, a Deville). I would say any recent/current tube amp, regardless of manufacturer, should have this done. Suck off the old ROHS non-lead crap and re-work with good ole' 60/40.

[stokes]

"how do you measure leakage with the caps in circuit? measuring dc at the low side of the cap i get like 20vdc spike in the moment i engage the HT, then it starts to drop and stays around 1,5vdc..."
There should be no positive dc voltage here,only negative bias voltage.The caps seem to be leaking.

[g1]

Agree, testing with a high voltage tester should usually be sufficient, but in this case you need to try replacing them.
Other than that, look for some leakage from a high voltage area into the bias circuit.
That +DC has to be coming from somewhere.