5F1 First Build Issues, need guidance

mcasemo · 2016-05-25 5:44 pm

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.

mcasemo · 2016-05-26 12:22 am

Forgot link to the schematic

http://www.tedweber.com/media/kits/5f1_schem.jpg

mozz · 2016-05-26 3:47 am

Voltages all look good. Best advice i can give you with that chassis is.............print out schematic, look at your wiring and then highlight each wire on the schematic, either somethings missing or somethings extra.

mcasemo · 2016-05-26 10:51 pm

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

ArcticBreaze · 2016-05-27 3:00 am

what is the resistor across the first filter cap on your turret boards

ArcticBreaze · 2016-05-27 3:11 am

just a thought

these resistors arn't on the weber schematic😉

mcasemo · 2016-05-27 4:11 am

the turret at the top had some parts point to point still, the one on the bottom had all on turret. the resistor you circled is the bleeder resistor i added to drain voltage after power down.

mcasemo · 2016-05-27 4:17 am

i've posted on madbean as well, link below. i may be cramming stuff together too much

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ArcticBreaze · 2016-05-27 6:56 am

I think that would make hum and his but not the problem you have try it with out the bleed off resistors might not help but worth a try.

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5F1 First Build Issues, need guidance

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