Problem: Weber 5F6A with very low output!

[jibe]

I'm on my way today to try to solve that riddle. I don't work at home cause I have 3 kids and we live in a small apartment. It's just to big a risk that someone get's close to the amp when I turn my head or something. I do my work elsewhere, away from the kids.

I'll take all the pointers and tips with me and try out one thing at a time to see if we can't get a working amp today, hopefully, by this time tomorrow I'll be a rock star instead of an amp-tech n00b. Or maybe I'll have another order in for a new amp by this time tomorrow, you never know, this diseas is contageus. Maybe an AC30 clone, I like that one , goes well with my 2x12 Clestion Blue Cab.



I'll get back with the result of todays experiment.



//Jibe

[MWJB]

"425VDC B+ is a teency weency bit on the low side for that amp, but is still within the ballpark." 470vdc (+/-10v) with a GZ34/WZ34 and reasonable plate current draw is more a ball park voltage for a 5F6A. 425v suggests to me that there is a problem, something is drawing WAY too much current, first step is to bias power tubes to 35mA per plate and see how the B+ settles after that.

[jibe]

UPDATE:

Removed 2 & 4 ohm output, enclosed in electrical tape.
Connected 56 feedback resisitor and 8 ohm output to tip of RCA output connector, and grounded ground of connector. Shouldn't be a problem.

Lowered bias voltage as much as my bias pot would let me.
Remeasured, pin 2 of V3 not a problem, must have made a mistake before.

12AX7B
V1
1: 253
2: 0
3: 1.83
4: 6.54 AC to pin9
5: Same as 4
6: 253
7: 0
8: 1.83
9: 6.54 AC to pin4/5

V2
1: 208
2: 0
3: 1.28
4: 6.50 AC
5: Same as 4
6: 364 D in schema
7: 208
8: 209
9: 6.50 AC

PI
1: 275
2: 30
3: 48
4: 6.56 AC
5: Same as 4
6: 283
7: 32
8: 48
9: 6.56 AC

6L6GC
1: N/A
2: 6.52 AC
3: 468
4: 467
5: -45.7
6: 470
7: 6.52 AC
8: Ground

5UA4
1: N/A
2: 5.20 AC - 472 DC
3: N/A
4: 750 AC to pin6
5: N/A
6: 750 AC
7: N/A
8: 5.20 AC - 472 DC

Some points on the board
A 472
B 468
C 425
D 364
E -46.4

A-E connections can be found on schematic and layout

Have not installed 1ohm resistor from pin 8 to ground on OT for bias measurement but I got them yesterday and will install them for more accurate biasing. After install, will try to bias at 35mA

Have not fixed a signal source to test on the wiper of treble, will do as soon as I find my old CD player. Don't know just yet if were talking preamp problem or output problem.

Still very low ouput, rewiring the output connections fixed nothing.
How does it look? Found an original schematic with voltages on a few places.
Except for my first tube wich differ from the original, everything seems similar.
My measurements seems to be in the vincinity of the original.
What to do?


//Jibe

[MWJB]

Voltages look good.

Something must be miswired.

Look at the schem at Fender.com for the 59 RI, apply a 100-Hz signal at the #1 input jack (as per the schem - see if you can download a sinewave program?) and see what AC voltages you get at the testpoints. Check signal level at the input with your meter, before reading voltages later in the circuit.

In fact, first start off by connecting your meter set to AC <20VAC accross the speaker terminals, play the amp and try an determine an average voltage, amp should be at gig-style settings.

Cross reference this by going over the layout with a highlighter, double checking EVERY connection (especially input jack wiring), post some pics of the circuit board, tube sockets & pots in case any sharp-eyed folks spot a wiring error.

[tubeswell]

"425VDC B+ is a teency weency bit on the low side for that amp, but is still within the ballpark." 470vdc (+/-10v) with a GZ34/WZ34 and reasonable plate current draw is more a ball park voltage for a 5F6A. 425v suggests to me that there is a problem, something is drawing WAY too much current, first step is to bias power tubes to 35mA per plate and see how the B+ settles after that.

Hmmm.. well okay 470 would mean that 425 was out of the ballpark, but ~430 +/-10V for 5881s would mean that 425 is within the ballpark

[MWJB]

It would... IF any 5F6As ran 430v (+/- 10v), but they don't (not the ones built by Fender anyway, nor Kendricks, Clarks, Victorias....etc). Typical B+ AC off 5F6A PT is more like 350VAC+.

It's important to realise that the schem was drawn 5 decades ago, when wall AC was typically lower, plus the schem is a bit of paper not a working amp, voltages may have been noted with the amp plugged into a AC supply that was not typical, or Fender changed things after publishing the schem (often the case). When it comes to troubleshooting it's imortant to deal with real world expected voltages, not a 50yr old ideal, that may or may not, have ever really been the case.

Sorry for the sermon, but 40v off on the B+ could point to a whole host of issues.

[jibe]

Thanks for all the help, for having patience.


Ok, I remember downloading a signal generator long time ago.
For more recent ones: Any suggestions what to download?

The fender test points: Square = DC and round = AC?

I guess I can use this to do the preamp/poweramp test as well, instead of my old CD



//Jibe

[loudthud]

The 56K feedback resistor should be changed to 100K if you are using the 8 ohm tap on the OT.

Just as a sanity check, measure the AC volts at the speaker with no input to the amp. The reading should be very small like 10-20mV at any control setting. If the amp has an ultra-sonic oscillation, it will be several volts.

Resistance checks at some tube pins will confirm things are wired correctly. Readings could vary 5 to 10%.

Resistance checks:

Power off, caps drained, amp unplugged.

V1 pins 2 and 7.
34K with no cables plugged in.
68K with an open ended cable plugged into the #2 input
1meg with an open ended cable plugged into the #1 input

Plug into the #1 input. At the guitar end of the cable you should measure 1meg. Now ohm from the tip of the cable to pin 2 or 7 of V1. You should get 34K on one and about 1 meg on the other. Try the other channel.

V2 pin 2.
135K with both volume controls at minimum
635K with both volume controls at maximum

V2 pin 8.
100K at any control setting

Measure from the Treble control terminal closest to the volume pots to ground. With all controls at max you should measure 1.275meg. Allow 20% tolerance.

V3 pins 2 and 7.
1.01meg at any control setting

V3 pins 3 and 8.
15.06K at any control setting

[jibe]

I can't even begin to understand how one knows such things.
I guess experience.

I'll get right on it.

When you say V2 pin 2/pin 8. From where? Still from the tip of the cable?



//Jibe

[loudthud]

Most readings are taken to ground

[erndawg]

I touch a 100k wired to ground on each of the power filter caps for about one minute

.

Don't forget that caps will partially recharge themselves immediately after a quick discharge. So, you need to repeat that procedure for another 20 seconds or so.

( Then I let it rest for about a week since I'm new to this )

Use a Digital Multimeter to verify no, or very low, voltage at each filter cap. That way you don't have to wait a week...

If I want to install a bleeder resistor, I should put it across C12 on my schematic? 100k 2W? Right?

I prefer using two 220k 2W resistors as bleeders. Put one across C12 or C13, and the other across C14. That way, if one of the resistors fails, or a solder joint fails, you have redundancy. But even if you use bleeders, ALWAYS check the voltage on your filter caps before you start any work.

[MWJB]

"The fender test points: Square = DC and round = AC?" Yes.

"I guess I can use this to do the preamp/poweramp test as well, instead of my old CD" Not sure what you mean here, try the meter across the speakers & play the amp.

Do my last suggestion first, at the end of the day if something is miswired you have to find it to fix it...doesn't matter how you get there.

[jibe]

Ok!

Double up on security sounds like a smart thing.

I've downloaded VA wich turns my laptop into both signal generator and oscilliscope.
I'm ready to try the signal generator and take measurements on speakers and fender
test points.

I'll be back with more info during the weekend.


//Jibe

[jibe]

Another question came to mind:

The RCA connector from the chassi to cabinet gets smoking hot. That is the output from the OT.
How come? Is it really supposed to get so hot you almost can't touch it.
Would that imply that I get a really hot OT, I haven't touched it so I don't know, is that a bad thing if it is?





//Jibe

[loudthud]

It would get hot if there was a short there. Because the OT has a very low resistance, you would need to disconnect it and the speakers and use your ohm meter to check for a short.