Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dennyg on July 01, 2018, 11:41:19 am
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Just finished my first Fender build, (mojotone kit) primarily to learn about tremelo, reverb and traditional Fender layouts/wiring/grounding.
Amp booted up, all voltages within spec, sounds good, quiet, trem and reverb work fine.
However when i cranked MV near full, then turn treble up - as I get around 70%, I get loud scratching sound like DC on the pot.
Sure enough I've got DC where I don't think I should - about 120mv after the 250 treble cap, 80mv after .1 bass cap, and 1.6v after the .02 coupling cap from second stage to third stage (junction of .02 coupling, 500p reverb send and 3.3M grid stopper). My gut is maybe there's something floating relative to ground (e.g. cold ground solder connection) but just a hunch.
A few notes worth mentioning
* I wanted to keep the layout as original as possible so I used the grounding scheme per Doug's general layout scheme - with a bus bar across the pots into the input jack ground - the PS end of circuit grounded at chassis near cap can. This is first build I've done not using pure star ground so maybe i goofed up something
* I ran the b+ wires over top like I've seen in pics of original versions
* Two mods relative to stock: I used the Stokes Mod for a more balanced PI, and added a 1m dwell pot in place of the 1m fixed resistor.
Any ideas/suggestions appreciated.
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You are surrounded by a group who loves to help fix amps, but there has been a common theme recently regarding schematics.
In a nutshell, if you're looking for specific troubleshooting help and give specific component examples you should start by posting an accurate working schematic that includes your mods. This might help you get some help.
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Sounds like some leaky caps. 120mV doesn't sound like much but it may be enough to make the treble pot sound scratchy. I'd replace the 250pF. The 80mV after the bass cap may be caused by the 250pF cap but it could be leaky also. If replacing the 250pF doesn't fix it, then replace the .1 also. May even be the .047 but not likely.
There definitely should not be 1.6V after the .02 so replace that one.
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There definitely should not be 1.6V after the .02 so replace that one.
Is there any chance this is actually -1.6V and you are reading the bias voltage of the following stage?
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The following stage is cathode biased. Grid voltage should be zero.
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I couldn't tell without the schematic :l2:
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I'm wondering if the 250pF is one of those leaky/bad silver mica types? These were discussed about a year and a half ago about some of them being suspect? (although I've yet to personally experience one myself - knock on wood) Also wondering if the leads have been tweaked and abused numerous times being re-soldered or maybe with too hot of an iron? What say you dennyg on the type and condition of that cap???
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Thanks for the replies guys - my responses:
* i replaced the suspect caps (tone stack, .02coupling cap) - no change. I honestly didn't expect a remedy there because i've got suspicious dc on both sides of a stage (i.e. before grid, after plate).
* just for heck of it, i replaced the treble pot. very interesting result which may be diagnostic - i wired up a new pot but didn't mount it (just suspended over the chassis) - worked fine - however when i pressed the pot against the chassis, symptom returned. Gut tells me i've got a grounding issue causing the first couple stages to not be properly referenced. I lean that way also because this grounding scheme is very foreign to me having only done pure star grounding on previous amps.
* attached the stock schematic and layout so Dave will stop giving me a hard time (haha); i've decided to remove the mods and return to stock - get that working first. This amp has many new variables relative to my previous builds, which i think i underestimated (e.g. i've read Aikens' paper on oscillators 3 times and still not sure how a tremelo actually works - maybe will make more sense after a few beers)
I'll report back after I remove the mods and re-flow a few grounds
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Interesting that “ground” means different things to different people. “signal” ground is a return path that can become infected/affected and different than chassis ground or the ground points at a given filter capacitor and is not always a place where voltages and currents go to die. In electronics there are negative voltages and 0/ground is at the higher potential.
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Jojo - I'm encouraged by your note about signal vs chassis ground. I rebuilt the board using all new components (I actually have two kits), and w/o the mods. Also used fresh wiring - gut shot attached. I double traced continuity and resistances both before and after wiring the board in the chassis. I am still measuring a positive voltage potential (not sure if I should even call it 'dc') relative to chassis ground at places i shouldn't, and think I can rule out leaky caps. All other voltages are spot on relative to those printed on schematic. Pulled all preamp tubes, same observations. What could cause the chassis ground to be negative relative to signal ground? Faulty cap can or PT? If I rule out board and wiring, doesn't leave too many components. One new observation - the PT gets hot - not just a bit warm to touch as in my other amps after running a while, but hotter than I'd expect and only after running for five minutes or so. Possibly diagnostic?
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I am still measuring a positive voltage potential
1/10ths of volts?, 1,000's of volts?
what do you measure resistance wise?, anything much more than 1/10ths is probably an indicator of miswired, misread value, 470k instead of 470, failed part
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Is your chassis made of aluminum? If so, you cannot solder to aluminum. You must use a lug and bolt/nut to connect ground wires to the chassis. That's a good idea even if you have a steel chassis.
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Chassis is steel but I'll replace soldered grounds with bolted.
As to Shooter's question:
The spots with positive voltage relative to chassis ground that shouldn't be there:
* 80mv after .1 bass cap
* 200mv after treble cap
* 200mv after 500pf cap into grid of reverb driver (V2)
* 2-3V after .02 coupling cap into 3.3M grid stopper of V3B
* 600mv on grid of V3B
I think I can rule out the board, it's components and wiring since I rebuilt board using all new components/materials from the second kit, double checked component values and connectivity/continuity before and after board installation, both to the layout and schematic. I've also grounded every point following a cap in the signal areas of the circuit to try to isolate a single source with no change to the other points when measured. I think ground to chassis would be the only thing common to all stages?
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2-3V after .02 coupling cap
un-solder the "right" side (no longer connected to the 3.3m or rev) so it's floating, now measure the cap to pre-amp ground, Power ground, and chassis, BUT before you do that, does the TS/Vol make the DC go up & down?
measuring directly at grids can and usually does create false assumptions.
200mv
if it's constant, not fluctuating with signal or pots, it's probably a test equipment thing
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* I replaced the soldered-to-chassis main grounds (PT, cap can, bias supply) with terminals/bolted to chassis - no change
* The dc reading does vary with varying impedance via MV - from 0 to 200mv (i.e. same as reading across the treble cap); increasing impedance of .1 bass cap via Bass pot adds another 80mv.
* I floated the .02 coupling cap - voltages to all three grounds (signal, PS, chassis) are same, around 11V (vs 2-3V when connected), but of course they are all connected via chassis. Should I remove those ground connections from chassis and measure again?
This is the most bizarre trouble-shoot I've done yet.
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Try a different meter.
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This is the most bizarre trouble-shoot I've done yet.
-What is the board material you used?....is it a re-used board?
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I think Dave wins the prize :bravo1: So the board from the kit is like card board. After your post, I looked at the board closely and see a resin-like substance along a quite a bit of the edges, and in other spots away from eyelets which a few do have solder flux around them - but the resin is like in streaks, not typical flux spatter. Turned the amp on and measure 10-12V across several spots around the edges. There is very low current - 2V across the 3.3M yields less than a micro-amp, but still enough to make the treble and MV pots a bit scratchy, but not bad enough to cause loud scratching like the first board. I inspected the first board and sure enough, resin around the edges and in spots where no soldering flux would be.
I inspected the bottom piece that insulates the board from chassis and no resin around edges, or anywhere else, so that material must've been from different batch.
Below a pic of one of the edges with the resin. Interested if anyone else has suffered from a conductive board, particularly in vintage builds using the cardboard-like material.
I believe this case can be closed. And I'll be reporting it to Mojo - I doubt I would be only customer suffering from the issue.
Thanks Dave!
BTW for Sluckey - I confirmed meter functioning properly by measuring voltages on one of my other amps that I'd documented voltages for reference - 0mv after all coupling caps, all other voltages measured correctly.
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Glad I could help! I had a similar experience a couple years back with a black board I got from AES.
It was a big jump to go from the dumbest thing I've ever said in a thread to getting this right, but I feel better now.
After some research I found out that there is carbon content in the black coloring of some boards out there.
Doug recently commented in a different thread about how painful a conductive board can be, so you're in good company.
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The old original fender boards were notorious about becoming conductive. I'm kinda surprised that you still find that in new boards such as in that mojo kit. I'll try to remember that!
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Not just one but TWO bad boards! Bought the kits at the same time so likely just had a bad batch. I'm ordering two new boards from Doug tomorrow - those boards I KNOW I can trust.
I'm just glad it wasn't something obvious/stupid. Only my 10th build but I'm trying to earn some respect on here as only posting for help when I've exhausted my limited (but growing) set of experiences. You guys take your valuable time to help and much appreciated. Maybe I'll actually be able to help others at some point.
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G10 FR4 only for me. My shop is full of fish paper circuit cards that came with chassis. I would never use that material in a high voltage amplifier today. There are some things my idol Leo did because it was standard at the time. Some of those things made for happy tonal accidents and some just accidents. I have had to pull out the hair dryer on more than one Fender fish paper circuit card and learned early on that they are trouble. Fish paper without wax treatment is hygroscopic.
https://en.wikipedia.org/wiki/Fishpaper
Barry
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Confirmed the conductive board was the issue. Rebuilt using one of Doug's FR4 boards and the amp fired up first time, no DC on pots or in any of the places it formerly had. Now I get to pull apart the second board and rebuild it but will do so with pleasure knowing that the issue was in the board.
My luck to get not one but two conductive boards -grrrrr. At least I now know the symptoms but will never use those old carbon fiber boards again.
Pic of new build attached.