5F1 Champ Build Diagnosis

[GarlandKelley]

Hey everyone,

     New amp builder here. I apologize if this has already been answered or addressed elsewhere but I have combed the schematics, hookup notes, forums, and combed over my work and cannot find my error. I have included a link to a YouTube video (since I can’t upload video here) that shows the issue I am having. I can provide additional photos/videos if needed.
     I have finished all the wiring and am able to get audio to pass through the first input but something is definitely wrong. It seems that there is a very low output signal coming from the second input but it has the same issue as the first input. The issue seems to be with the volume pot as it worsens when this is being adjusted. The 6v6 and 5y3 were pulled from an old radio but they tested fine on my Heathkit.
     Any help would be greatly appreciated. I am so close to being done with this project and can’t wait to get it functioning properly. Thank you!

Garland

[acheld]

Hi res photos of your wiring will help us help you.  I assume you've used a standard 5F1 schematic, right?

Edit: Also, Doug's got some great tips for new builds here -- https://el34world.com/Forum/index.php?topic=19361.0

[JPK]

Agree with above. Post a schematic, layout, and pics of your amp. Voltage levels. Re-flow any questionable looking solder joints. Use a magnifying glass under a bright light. You might be surprised what you see.


This thread shows a layout with typical voltages. The thread might be useful to troubleshoot yours.


https://el34world.com/Forum/index.php?topic=22672.0
.

[dogburn]

I'd try replacing the volume pot, or maybe first try re-doing all the wiring to that pot. I've had a bad one in one of my projects, which caused frustrating problems (it may have been bad from the start or maybe I killed it from overheating it while soldering).

[Latole]

IMO it is not the pot.

It is mistake on the wiring . Happened often when building a amp, more often if you are a beginner

[GarlandKelley]

Thank you all for the advice, I’ll get some high res pics uploaded ASAP. This is my first amp build so some of the organization in the chassis is embarrassing. I built the 5F1 into the chassis of an old Philco Bullet radio. I can get I have a bad solder joint (or multiple) somewhere but just wanted to check if anyone had a quick answer for the issue in the video. I am going to throughly recheck my work and will post photos. Thank you all for the advice and swift replies.

[Latole]

".....but just wanted to check if anyone had a quick answer for the issue in the video..."

No we can't tell.
Trouble shooting without the amp in front of us is impossible.

Check and check and ....check you wiring

[GarlandKelley]

Okay,

     I have checked and checked and checked and checked….. and checked my wiring. I am having the same issue. I reflowed every solder joint for safe measure.
     As previously stated, this is my first amp build so I am a complete beginner. The organization of the wiring and quality of the overall soldering is not incredible but I am not looking for feedback on those aspects, just a diagnosis of the problem so I can remedy and get the amp functional.
     I have provided an updated video of the issue as well as some high res photos and a slow pan video of the whole board. I reused the 8 pin sockets that were in the recycled chassis so am wondering if that might be an issue. The 5y3 and 6v6 were pulled from an old chassis as well but are testing fine. I ran the lamp to the top to illuminate the front radio panel.
     Thank you all for your help!

Garland

[GarlandKelley]

Photo 1

[GarlandKelley]

Photo 2

[GarlandKelley]

Photo 3

[separateness]

Wild guess in that I'm not exactly sure what is going on but from what I can tell:  Check to make sure you don't have your OT secondary wired backwards which would make it so that you have a positive feedback loop rather than a negative one.

[sluckey]

     As previously stated, this is my first amp build so I am a complete beginner. The organization of the wiring and quality of the overall soldering is not incredible but I am not looking for feedback on those aspects, just a diagnosis of the problem so I can remedy and get the amp functional.

Many times, especially on a first build wiring and quality of soldering ***IS*** the problem.

I see you have a Hoffman board. That's good, but the chassis wiring is a bit helter skelter. Here are a few things to check...

Tubes first since they are easy.

Check resistance from V1 pin 2 to chassis with nothing plugged into either input jack. Should be about 34KΩ. Now insert a plug into the high jack. Resistance should be about 1MΩ.

Check resistance of V1 pin 7 to chassis. Should be about zero ohms with volume pot set to zero and 1MΩ with volume turned up to max. Check both ways.

Check resistance from V2 pin 5 to chassis. Should be 220KΩ.

Disconnect the NFB wire from the speaker jack. Any joy?

Report findings even if none of this helps.
 

[Latole]

Wild guess in that I'm not exactly sure what is going on but from what I can tell:  Check to make sure you don't have your OT secondary wired backwards which would make it so that you have a positive feedback loop rather than a negative one.

OT primary wires too. Try to reverse them.
Look it like motor boating sound.

[GarlandKelley]

Thank you all for your help. I swapped the primary OT and it caused squealing. I then swapped the secondaries which fixed the feedback loop but am still having the same problem. Unless my multimeter is malfunctioning, I am getting resistance values that are way off the previously suggested ones.

V1 Pin 2 is reading at 2.8 ohms with no input inserted

Pin 2 with jack inserted is reading 5.5 ohms

V1 Pin 7 with volume all the way down is 0 (good there)

V1 Pin 7 Volume all the way up is reading 67.5 ohms

V2 Pin 5 is reading 18 ohms
The resistance value

[GarlandKelley]

Disconnecting NFB did not work either

[dwinstonwood]

Hello GarlandKelley, it's difficult for me to tell from the photos where your filter caps are grounded. I'm assuming they're connected to ground under the board?

[sluckey]

Hello GarlandKelley, it's difficult for me to tell from the photos where your filter caps are grounded. I'm assuming they're connected to ground under the board?

Here's the board layout. The cap ground connection is under the board (no reason for it to not be on top in plain view.). There would be a completely different set of symptoms if that underboard jumper were missing. A bad solder connection might be the issue. Worth putting a jumper on top of the board.

     https://el34world.com/Hoffman/files/Hoffman_5F1.pdf

All of those resistance readings are terribly wrong (except the zero ohm reading). I suspect faulty meter or operator error. Check the resistance of some spare resistors to gain confidence in your meter or your operator skills. However, none of those low readings would account for your symptoms.

Also, that old wafer style socket for the 6V6 has a reputation for having unreliable connections to the tube pins.

[separateness]

Thank you all for your help. I swapped the primary OT and it caused squealing. I then swapped the secondaries which fixed the feedback loop but am still having the same problem. Unless my multimeter is malfunctioning, I am getting resistance values that are way off the previously suggested ones.

V1 Pin 2 is reading at 2.8 ohms with no input inserted

Pin 2 with jack inserted is reading 5.5 ohms

V1 Pin 7 with volume all the way down is 0 (good there)

V1 Pin 7 Volume all the way up is reading 67.5 ohms

V2 Pin 5 is reading 18 ohms
The resistance value

Well, sorry to waste your time with the NFB suggestion. Taking wild stabs like that is not good practice (I should know better as I am a troubleshooter by profession) but it seemed similar to something I had seen before.  These are most peculiar readings.  You don't happen to have DC voltage on either of the V1 grid pins do you?
V1a grid to ground should be around 34k
V1b grid to ground with the volume all the way up should be 1M

[GarlandKelley]

Thank you all for the suggestions and advice. I will as the jumper to the top to see if that helps. I think my multimeter is malfunctioning as I am not getting proper readings in anything. I will try a new battery or get a new one if necessary. I suspect that the wafer style 8 pin socket might be the culprit. It came from a chassis from the 1940s and the chassis was far from pristine. I may try a new 8 pin socket for the rectifier as well.

[GarlandKelley]

Board

[GarlandKelley]

Am I supposed to have 100k resistors on the heater lines or do those go straight to the lamp for the 5F1? I see them mentioned in the hookup notes but not the board diagrams. Could that be a contributing issue?

[separateness]

Am I supposed to have 100k resistors on the heater lines

I should say not. You want to be getting the full 6.3V to the heaters and lamp which are all in parallel.

[PRR]

> issue seems to be with the volume pot as it worsens when this is being adjusted.

My 2c bet is oscillation. The Fender layout IS critical. Your build does almost everything different. At some level of gain the thing goes into supersonic oscillation, all the biases go way-out, audio gain goes small.

Measure the DC voltage at 6V6 cathode. Does it sit near +20V with VOL at zero, then go way-small as VOL hits a magic spot?

[GarlandKelley]

PRR, this sounds like what is happening. I’ll have to check voltage when my new multimeter arrives. I am also getting early breakup/distortion before it hits the oscillation point. It’s not the natural distortion expected from the amp when pushed harder. The oscillation occurs at a specific point and when I turn the potentiometer beyond that point, the audio essentially cuts out. What would be the solution if the issue you are describing is the culprit?

[Latole]

Hello GarlandKelley, it's difficult for me to tell from the photos where your filter caps are grounded. I'm assuming they're connected to ground under the board?

I don't like these solders.
Photo on answer #16

[Esquirefreak]

All the wires bundled together crossing sensitive areas and the OT right under the input jacks might also be a culprit for oscillation and all sorts of weird behavior. Maybe try to give the leads some space and run them away from sensitive areas. I'd try that. The stuff around the VOL pot also looks a bit crowded.

It takes some time to understand why lead dress and placement of components in an amp is critical. Look around on other builds and amps to get some inspiration. I'm sure you'll figure this one out soon

/Max

[GarlandKelley]

Thank you all for the advice. Max, I was concerned about the OT being so close to the input jacks. Even though the chassis is large, a big portion of the underside was unusable due to a large radio component that controls the station scroll wheel that I wanted to remain intact. The current configuration was the only feasible option I saw besides the OT placement. I was using the original OT slot to minimize chassis modifications but may try moving the OT to see if that helps.

[PRR]

...concerned about the OT being so close to the input jacks....

6V6 Plate lead to all the 12AX7 and tone stuff is more likely. (Signal voltage on 6V6 Plate is 25 times the voltage on speaker leads.)