5F1 First Build Issues, need guidance

Hello everyone – first time posting here. Getting into amp building after a bunch of pedal builds. But I’m having trouble with my first one and was hoping for some guidance, it’s a 5F1 (simple right?). I initially bought a Chinese kit off ebay that came with chassis, tubes, transformers, turret, wires, etc. But I decided to put it in a small octagon shaped box (I make guitar pedals in the octagon shaped Hammond enclosures, so got it in my head I needed an octagon shaped amp). A guy who makes octagon dice trays made me a cabinet, looks pretty cool.

So with the size of the box I could not fit the original chassis, so I did point to point in a small 4.5 x 3.5 x 1” Hammond chassis, following the weber schematic with all the parts I got in the kit. After a few issues (transformer primaries labeled wrong in the provided schematic, thought I fried it) I got everything looking ok. The point to point was pretty messy in the small chassis, but voltages looked ok, and I got sound, but it was sputtery and jumpy. Meaning as I turn the volume up it is erratic on the level. Also at a constant setting, hitting the strings harder will jump to a louder level (video/audio below). Troubleshooting it I replace each tube, hooked up a different PT and OT, but still had the same issue. Speaker works fine with another amp driving it, and when I play this through another cabinet, it does the same thing. It would act differently with a new 12ax7, where I think it would be ok, but then back to original issue. Could I be frying the 12ax7’s?

I was posting a build log on madbean and was getting some help but never figured it out (post lost in a crash on the forum). Many thought the point to point could be dangerous (I admitted to a few shocks) and I should start from a kit and turret for my first build. So I took it apart got some turret board and turrets and laid out a version that still fit in the small chassis. Got it all wired up with the original components and sockets, but it did the same thing.

Now, thinking it was a bad component, I got all new caps, resistors, laid out another turret board, new sockets, and it’s still doing the same thing!!!!! Any help, suggestions, guidance, criticisms appreciated. Some notes, video, pics below. I did figure out the switch on the pot for power was wacky and contributing to some of popping, but still left with what is in the video.


  • Single input with 34K input resistor to mimic the two 68K in parallel, is this the correct thing to do?

  • B+, B+2, B+3, bias voltages change with volume position, is this normal? I just noticed this during last troubleshooting run. I show them in the video.

  • Initially used the 330 V windings on the PT, but got kinda high voltages, it had 300 taps, so used that and got the voltages more in check. The issue was there using both voltages.

  • Tried probing with audio probe, but not sure I was smart enough to figure anything out. Output of first stage sounded fine. Had trouble hearing the audio probe over the actual amp speaker on preamp pin 7. I have an L-pad, I suppose I could set it for 4 ohms and hook the output transformer to it so I could just get the probe volume. Or is there a problem doing that because of AC impedance vs DC resistance?

  • I was checking continuity on some spots and got a continuity beep on my DMM when testing the speaker jack tip to ground, but maybe that was because the low resistance of the speaker, so I pulled the plug to disconnect the speaker, but still was continuous. Should this be continuous across the transformer secondary?

  • Is my grounding scheme OK (see pic). All go together to one wire to the chassis. All internal go to same point. Both are u terminals bolted to chassis.


Voltages (first check with volume at zero, they vary with volume position)


B+: 349.2
B+2 (6V6 pin 4): 296.9
B+3: 256.3

12AX7 pin 1: 162.9
12AX7 pin 3: 1.38
12AX7 pin 4-5: 3.4 vac
12AX7 pin 6: 163.9
12AX7 pin 8: 1.21
12AX7 pin 9: 3.4 vac

6V6 pin 2: 3.4 vac
6V6 pin 3 (OT blue): 318.7
6V6 pin 4 (B+2): 288.8
6V6 pin 7: 3.4 vac
6V6 pin 8: 20.15

OT Disconnected, resistance
Red-Blue: 560 ohms
Black-Green: 1.2 ohms


Video
https://www.youtube.com/watch?v=O8C_HOXXDXw


Point to point, abandoned
https://lh3.googleusercontent.com/-...jjbPj2-whPsVQXufGoFhpJv2K7u_YQCCo/s512/15+-+1


https://lh3.googleusercontent.com/-...S2Rk1eXLT9p2yEJGxqlangCCo/s512/IMG_4139-2.jpg

a tale of two turrets
https://lh3.googleusercontent.com/-...0Q4AyeKwZeE8iLi5EYXavgCCo/s576/IMG_4530-2.jpg

Tubes
https://lh3.googleusercontent.com/-...XS33onSZMRTU3TlX2RNStQCCo/s640/IMG_4541-2.jpg


Sockets and grommets
https://lh3.googleusercontent.com/-...CX35HiODMphRq1Wny3SmsACCo/s640/IMG_4545-2.jpg

Chassis ground
https://lh3.googleusercontent.com/-...qHh0Xo_1CCfj8oqB95RYmgCCo/s640/IMG_4546-2.jpg


Turret #2
https://lh3.googleusercontent.com/-...dE39g_hqdLn0RESsLtZ_rgCCo/s640/IMG_4547-2.jpg

Power transformer
https://lh3.googleusercontent.com/-...MpznzrPH05ZKSgerN_KTJwCCo/s640/IMG_4549-2.jpg

way to many screws
https://lh3.googleusercontent.com/-...KiIM5E-X0xTXa0wX-PqJrgCCo/s640/IMG_4552-2.jpg
 
thanks mozz, i saw that tip somewhere, going to do it tonight. of course in my head i'm certain it's all correct, haha. I've done some stupid things with pedals.

More info

B+ seems to vary from 334v to 350v, think B+2 and B+3 vary more.

Measuring the pot i get the following:
Pot pin 3 (after 22n coupling cap)
starts at zero
-0.323v @ 12 o'clock
-0.665v @ 1 o'clock
-0.825v @ 3 o'clock
-1.2v @ 4 o'clock
then falls back to zero from 4 o'clock to all the way open

Wiper (pin 7) follows same pattern but peaks at -0.500v at 4 o'clock

Pot Pin 1: grounded and stays grounded

the 6V6 pin 5 (post coupling cap from 12ax7) does a similar thing peaking at -10v
 
just a thought

these resistors arn't on the weber schematic;)
 

Attachments

  • turret board.jpg
    turret board.jpg
    74.1 KB · Views: 43