|pontiacpete||11/13/2017 7:36 AM|
|Fender tweed bassman 5F6 no sound|
This amp is not passing signal. It has signal up to the grid of the phase inverter, pin 2. The voltages on the phase inverter seem really high. They are:
pin 1 430vdc
pin 2 330v
pin 3 385v
pin 6 430v
pin 7 starts at 300v and goes down to 200v
pin 8 385v
I've don't quite a lot of work to find out why the phase inverter has these voltages: changed socket, lifted board to observe whats going on underneath, lifted/suspended all the components 1M , 470R, 1M and 10K resistors off the board keeping them in circuit and still have high voltages.
Am I reading these voltages wrong?
does anyone have any ideas what's happening here.
Thanks in advance.
|alexradium||11/13/2017 7:58 AM|
|PI is grounded via the presence pot,check out all ground connections first.|
you could have a wrong wiring,if its really 330V on grids you can toss the tube,take it out and measure what you have pin by pin.
is this a kit you built?
post pics of the amp
|J M Fahey||11/13/2017 8:39 AM|
|Those cathode voltages are incredibly high, measure resistance from each cathode to ground.|
You might have lost connection from preamp ground to chassis ground (the metallic chassis itself) and real power supply ground (negative of filter caps).
Place multimeter black probe at first cap ground at the PSU and remeasure voltages, also what voltage you have available across the first filter cap, also what voltage appears at chassis and preamp grounds.
No, do not assume itīs a waste of time because "everybody knows it should be 0V" , just measure and report.
Also am amazed at the very high plate voltages, pins 1 and 6, what is rail voltage at the other end of corresponding plate resistors?
Of course, also check multimeter, use another one and compare results, it might be out of whack or have a dead battery.
|pontiacpete||11/13/2017 9:44 AM|
|Thanks for responding so quickly. |
I started with hooking the feedback resistor in place. It had been taken out by another tech when I got it and that was it to getting sound.
Pin 1 280v
pin 2 69v
pin 3 110
pin 6 280
pin 7 70
These voltages are looking a lot better, still a bit high compared to the schematic but it's passing a pretty signal.
|Chuck H||11/13/2017 9:52 AM|
|Most clones and all vintage amps of that model will have higher voltages than the originals because most clones aren't corrected for modern, higher mains voltage and, of course, none of the originals were either. Just make sure the bias is appropriate for the existing voltage.|
|pontiacpete||11/13/2017 10:55 AM|
|Thanks Chuck, bias is good at about 27mA. This is an original 1960 bassman 5F6. It needs a cap job, that will probably change the bias a bit, but we'll find out after that happens.|
|g1||11/13/2017 11:25 AM|
Or maybe it was just coincidence and there is a bad connection or presence pot fault like alexradium suggested.
Disconnect that resistor again and recheck. If you lose sound and the PI voltages go funny, something is miswired.
|alexradium||11/13/2017 11:48 AM|
|pontiacpete||11/13/2017 12:02 PM|
|The amp came to me with the transformer wire for the output dangling and I was clipping the black wire to the chassis and to a load when working on it. But wiring in the 27K resistor brought the sound back. Just tried it again, lifted the 27k feed back resistor and no sound and very high voltages on PI|
|Chuck H||11/13/2017 12:22 PM|
|pontiacpete||11/13/2017 1:23 PM|
|Here's a picture. I went through the layout/schematic many many times and there aren't a heck of a lot of connections in a 5f6. When I've built these before I clearly remember connecting it up without the feedback resistor so I agree something's up. I hope someone can see what's wrong.|
edit: here's another picture after I touched up horrible solder joints
|g1||11/13/2017 1:42 PM|
|Maybe the presence pot is bad? It looks like it may not be wired up correctly either.|
|alexradium||11/13/2017 1:42 PM|
|being an original,i'd take in consideration also leaking caps and conductive board,pretty strange anyway.|
|Enzo||11/13/2017 1:46 PM|
|So lift the 27k NFB resistor, and measure resistance to ground from the point it was connected to.|
|pontiacpete||11/13/2017 2:04 PM|
|Enzo, not knowing which ended I should lift of the 27K I lifted both. The one that is closest to the speaker jack has .3 ohms to ground the other is open.|
Across the output transformer from black to green it measures .3 ohms.
|eschertron||11/13/2017 2:11 PM|
|pontiacpete||11/13/2017 2:15 PM|
|I'm not sure if this is a 5f6 or a 5f6a, I don't have the cabinet here. I have been using the 5f6a schematic. Where would the 2.2K resistor be?|
|Chuck H||11/13/2017 2:29 PM|
|pontiacpete||11/13/2017 2:42 PM|
|Thanks chuck, I was just looking at the 5f6 and 5f6a. This amp is definitely like the 5f6a in that the 10K resistor was not grounded like it is in the 5f6 schem. That connection was under the board and there's no chassis solder point above the 10K. I did pull that you can see it is above ground now.|
|eschertron||11/13/2017 3:20 PM|
Still odd is that if you have a 5f6 and not a 5f6a, how the NFB is related to the PI at all. On the 5f6a the NFB resistor and the presence pot connect to that 10k tail resistor. Did some one start to mod from one version to the other and then get confused?
|Enzo||11/13/2017 3:49 PM|
|When you lift a resistor or other part, you are disconnecting an end. It really doesn't matter which end, it is disconnected. There would then be two end points where the resistor had been. In your case, the one spot would be the one connected to the speaker winding of the OT. That would have a VERY low resistance, as you found. The other end point would be the top of the presence pot and as well the bottom end of the middle pot. On the drawing, there ought to be 5k to ground from that point - the presence pot. You got open. That means either your presence pot is open, or it is miswired. Or the ground connection is missing. That is the 5F6.|
In the 5F6A, that point is still the presence pot, though now it is the ground path for the phase inverter too.
|J M Fahey||11/13/2017 7:01 PM|
Circuit is closed through the feedback resistor, when and if you connect it, but you still have a wiring error.
PLEASE answer the questions posted in #3.
G1, Chuck H and Escherton are also worried about that but you donīt yield
|pontiacpete||11/14/2017 6:23 AM|
|Thanks guys, Got to the bottom of this issue for sure. |
The presence pot was most certainly wired wrong. The pot was going to ground through the .1 cap, or should I say, not going to ground because of the cap. Also the pot was the wrong value, 50K instead of 5K. So now when measuring at the top of the presence pot it's 5K.
So with or without the feedback resistor lifted the amp makes good sound.
Voltages are way better now. The plates of the PI are at 260 and 285vdc. The cathode 53v and about 35v on the grids.
Also, clipped out the 56K at the top of the middle pot that went to ground.
Many Thanks to you all for seeing me through this repair.