BFDRRI low current

[AxeAmpTN]

Dude just brought in his 65 Deluxe Reverb RI complaining that V7 is redplating.  Haven't replicated the problem, but looking at the circuit board, R62 looks like it's gotten a little toasted (still measures 470 ohms).  Checked the current and V7 is measuring 8mA and V8 is at 18mA.  Bias voltage is at the prescribed -37VDC.

Anyone have any insight into what might be going on here?

[shooter]

i'm using this schematic;


https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_reissue_sm.pdf


roll in your bench spares n see if the current becomes closer


i'd suggest 1 ohm cathode R's, simply to eliminate "Inducing" issues


did you measure the -37 in the tube sockets?

[tubeswell]

V7 is measuring 8mA and V8 is at 18mA.  Bias voltage is at the prescribed -37VDC.

Anyone have any insight into what might be going on here?

If you swap the tubes around, does the problem move with the tubes, or remain at the sockets?

[AxeAmpTN]

V7 is measuring 8mA and V8 is at 18mA.  Bias voltage is at the prescribed -37VDC.

Anyone have any insight into what might be going on here?

If you swap the tubes around, does the problem move with the tubes, or remain at the sockets?

Haven't tried that yet. (It was right before closing when this came in.)  I'll do that first thing in the morning.

[AxeAmpTN]

i'm using this schematic;


https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_reissue_sm.pdf


roll in your bench spares n see if the current becomes closer


i'd suggest 1 ohm cathode R's, simply to eliminate "Inducing" issues


did you measure the -37 in the tube sockets?

-37 was at TP36.  Hadn't considered the 1 ohm cathode resistors.  I may try that after swapping the tubes to see if the problem follows.

[AxeAmpTN]

V7 is measuring 8mA and V8 is at 18mA.  Bias voltage is at the prescribed -37VDC.

Anyone have any insight into what might be going on here?

If you swap the tubes around, does the problem move with the tubes, or remain at the sockets?

Tubeswell, the problem definitely followed the tubes.

[tubeswell]

V7 is measuring 8mA and V8 is at 18mA.  Bias voltage is at the prescribed -37VDC.

Anyone have any insight into what might be going on here?

If you swap the tubes around, does the problem move with the tubes, or remain at the sockets?

Tubeswell, the problem definitely followed the tubes.

Time for some new tubes then

[AxeAmpTN]

UPDATE:

So I replaced the power tubes with a brand new pair of Tung-Sol 6V6GT tubes.  Current measured the same in each and I adjusted the bias to get 20mA (still cold, I know).  Tested it and everything sounded good again.  Customer played it and was happy with the results.

Then tonight, he was using it at his rehearsal.  I called after an hour and he said it was making a popping sound during quiet passages and between songs.  He said it kind of went away when he pushed the tubes more firmly into their sockets.  Some time later, he said there was a sudden drop in power and noise started drowning out his playing.  He looked in the back and saw V7 glowing blue before it started redplating.  He saw it flash and before he shut off the amp and called me again.

Haven't had a chance to open it, but I'm guessing something is on the verge of catastrophic failure if only one out of a brand new pair of tubes is acting up like that.  This takes me into somewhat unfamiliar territory.  So I guess now I'm curious if anyone has witnessed this kind of problem.  Do I need to go back to R62 like my original post?  Or is there something worse to look at like the OT?

I can't stress enough how much I appreciate y'all experience and insight with these things.

[tubeswell]

You were definite about the mismatch in tube current following the tubes when you previously swapped the tubes over. However, we didn't get the chance to see if there was anything else awry before you put new tubes in and sent it back to the customer.


Red plating output tube is lack of bias voltage. So suspect anything related to getting a negative VDC on the output tube grid pins:
1) bias supply diode and bias filter cap
2) loose tube socket pin clamp, cold/cracked solder joint, or dodgy PCB trace* in that part of the circuit.
3) leaky coupling caps from the PI (probably less likely than 1 or 2 above, but still a possibility)

*really common problem with the Fender RI PCB amps as they age.

[AmberB]

When I was still repairing electronics for a living, before I retired, if I ran into a problem with a copper trace on a circuit board, especially if it had a crack or break on it, I would scrape off a bit of the varnish on each end of the trace and solder a wire onto it to make sure it would have a good connection.  Perhaps a bit messy, but it solved the problem. 
It's "fun" when you have a cracked circuit board with multiple traces to fix...

[shooter]

 
fun indeed, I pretty much quit working on PCB amps, because I've had a life-time of THAT fun 

[AxeAmpTN]

Okay, so this is definitely my bad for not doing due diligence with the rest of the circuit.  Got it open and the first thing I notice is the bitter smell of burn.  If R62 looked toasty before, it now looks like Georg Ohm's bitch...and it's only half the man it used to be (216Ω).

Obviously this isn't normal for a 1W resistor, especially with R63 still so clean.  I'm looking for bad connections, cold solder joints, loose traces, etc.  Everything else looks good so far.  Solder joints on the sockets seem solid.  Not sure how to retension pin clamps in these white ceramic sockets.  And the output on the bias pot isn't looking so hot...

[tubeswell]

R62 is the V7 screen grid resistor - either cooked when the tube red-plated, or this is what was causing the uneven tube current in the 1st place (and it wasn’t just following the tube)

If bias voltage is there for the other tube, then the problem must be around the solder joints, traces, Pin-5 clamp, arcing on the socket for V7, etc so check everything to do with the bias on v7, including whether C27 is leaking DC.

Check the traces for DC continuity by R-metering between the related component leads

If the bias isn't there for the other tube, then the problem will be further upstream in the bias supply (diode, bias filter caps, traces, solder joints etc)

[AxeAmpTN]

Thanks y'all!

So this time I carefully checked out the circuit, traces, solder connections, pin clamps, etc....
-no DC leakage on C27
-attempted to retention the pin clamps (not too thrilled with the design of these sockets)
-checked solder connections (noted that the lead connected to pin 4 was was in the top hole and solder was questionable, so I moved it to the bottom hole like everything else and cleaned up the connection)
-removed burnt R62 and found the pads on each end to be loose, so I hardwired it to the socket and the other end of R63
-V7 tube was shot, so it got replaced.
-Adjusted bias and load tested with good current and voltages on relevant test points
-Hooked up the speaker and did 5 hour operational test (at the request of the customer)

Everything is finally working stably.  Thank you all for your insights with this!

PS- I did give the customer a quote to replace the printed boards with a point-to-point wired board.  Out of curiosity--for those of you who do this kind of thing--what do you usually charge for this kind of service?

[tubeswell]

PS- I did give the customer a quote to replace the printed boards with a point-to-point wired board.  Out of curiosity--for those of you who do this kind of thing--what do you usually charge for this kind of service?

Giving away sensitive commercial information LoL
25hrs plus parts cost (discounted hourly rate)
Somebody else is gonna undercut this now

[PRR]

A full P2P conversion makes the amp MUCH more valuable (reliable, fixable, and clean/toneful) than the store-sold PCB amp. You should get much of that difference for your labor. Sadly most players do not understand the difference, or how much work it can be.

[AxeAmpTN]

PS- I did give the customer a quote to replace the printed boards with a point-to-point wired board.  Out of curiosity--for those of you who do this kind of thing--what do you usually charge for this kind of service?

Giving away sensitive commercial information LoL
25hrs plus parts cost (discounted hourly rate)
Somebody else is gonna undercut this now

A full P2P conversion makes the amp MUCH more valuable (reliable, fixable, and clean/toneful) than the store-sold PCB amp. You should get much of that difference for your labor. Sadly most players do not understand the difference, or how much work it can be.

Huh.  I have a feeling I way underquoted for this one.  (Might be for the best since I'm still building my business.)

[HotBluePlates]

... PS- I did give the customer a quote to replace the printed boards with a point-to-point wired board.  Out of curiosity--for those of you who do this kind of thing--what do you usually charge for this kind of service?

After you do the first one you'll figure out how much work it is.  Unless you've built a bunch of amps from scratch, you probably under-quoted the job.

Allessandro apparently quotes his at $600, before up-charges for custom options.  That's 8 hours @ $75/hour, and I'd bet he's making that work by having a production process worked out to greatly reduce labor-time.

[tubeswell]

Allessandro apparently quotes his at $600, before up-charges for custom options. 

Interesting, that works out about the same overall cost as me  - when you take into account the currency exchange rate and shipping
I just can't pretend that it takes 8 hours. That's just BS

[HotBluePlates]

Allessandro apparently quotes his at $600

Interesting, that works out about the same overall cost as me  ... I just can't pretend that it takes 8 hours. That's just BS

I think I got down to under  6-8 hours for installation when I was still making & installing Hoffman boards in DRRIs (but that was nearly 20 years ago).  But what made it work for me was having an assembly line production process.  I never made 1 board at a time, it was always 3-4 boards at once to have them on-the-shelf for future installs.  Of course, that was on turret boards, and eyelet boards are much easier & faster to make.

If Allessandro is billing less than $75/hr, he's probably under-charging for tech work.  Check out some videos on Youtube of builders doing "production carpentry."  If you put in some unpaid hours to think about more efficient ways to get work done, you can speed the job up and effectively earn more per hour while accomplishing the same work.  Completing the job slower given the flat fee just means you're settling for making less per hour.