Amp tinkerers and people who aren't afraid of 700 volts or so in here....

Moderators: Slowy, Capt. Black

User avatar
BY druz15
#764361
Hey guys.

So i've recently started playing bass in a band, and am using the Hiwatt DR103 clone I built.

It had a few issues way back when I first built it that had been intermittent.
When I first fired it up the bias circuit was not working properly and one of the tubes nearly started to red plate. I switched it off in time but it sort of fried the tube a little and at the time I couldn't afford a new set, so I had it biased as best as possible, but it was a little on the low side as the one that was sharing the bias supply with the weak tube was obviously drawing more current.

Fast forward a year or two and I've finally put some new tubes in as the way it was operating wasn't getting anywhere near enough max power.
Swapped a new matched quad in, got it biased in beautifully, and all good. HOWEVER it has brought back ANOTHER old issue that I thought had gone away.

When you play a note quite hard (especially bassier notes, and obviously the low e on a bass) it occasionally does a loud crackle or popping sound, and it also sometimes triggers a spurt of harsh noise.
The crack is super loud and sometimes almost sounds like a gunshot. The noise won't go away unless you either turn the presence knob down or tap or bump the top of the amp in the right spot. Tapping or bumping the amp can also trigger the noise and/or a pop.

I took the amp in to a tech to diagnose this issue a while back and he put it down to the amp going into cutoff when playing bassy notes as it was biased wrong and managed to bias it right with the half cooked set in there.

Somehow in the process of getting the chassis out of the headshell and AFTER replacing the tubes / biasing it up this popping issue has come back.
(after I biased the new tubes I played both guitar and bass for 30-40min happily with no issues, it was only after I started inspecting some stuff in the amp that it started happening again and now it won't go away)

Some things that I notice trigger it
-playing bassy notes (in this band I will be playing a drop C bass, so this is a biggie)
-bumping the amp headshell.
-tapping certain capacitor bodies in the preamp circuit (it tends to be on the brilliant side I think as the hissing noise gets way quieter when you turn down the brilliant channel too) tapping these same capacitors can also stop the noise.
-tapping one or two of the mains capacitors' bodies with a screwdriver.

Suspected culprits.
-bass overloading circuit -> bigger capacitors in audio circuit? (doubtful as it seems to be triggered physically and acoustically, not electrically)
-microphonic tube (fairly certain I checked all of these the last time this happened, as well as swapping them out with known good ones, and it didn't go away, but maybe worth checking again?)
-dodgy solder joint where one or more capacitors meets the board. It all looks fairly good and I touched out some of the less nice looking ones, but you never know.
-Loose wire somewhere (have done a fairly thorough *tug wire with chopsticks* run through so doubtful, although worth trying again
-Bad mains capacitor, this seems possible, not really 100% sure how to test for this or determine which one, and it also doesn't explain why other capacitors set this off.
-something up in the brilliant channel circuit that is reacting to physical vibrations.

Any leads chaps? Anyone had this issue before and found the fix? Seems like something that must happen fairly often as it's sort of a physical vibration/acoustic issue, and I'm sure techs have found and diagnosed this before, just can't seem to find anything substantial on the google.


TL;DR
my amp makes weird popping/crackling sounds and hissing noises when you play a really bassy note or thump the top of the headshell/tap various capacitors in the amp
Last edited by druz15 on Thu Nov 16, 2017 11:12 pm, edited 1 time in total.
BY harveysj
#764362
I had an issue with a Hotrod deville once, getting that loud gun like crack follwee by some time ther crackle. It turned put to be a faulty PI tube. I’m not familiar with the amp your talking about but if it uses a PI tube it might be worth a try.
User avatar
BY jeremyb
#764363
My only experience with crackling was the ryan built amp I had which used to crackle on some notes when I got it, I re-seated the tubes and the problem went away, must have moved in shipping! You've obviously changed all the tubes so perhaps it's a faulty tube socket, or maybe dry joint on a socket that needs re-flowing? changing the sockets might be a cheapish next step?
User avatar
BY sizzlingbadger
#764368
Seems like vibration is the cause of the issue so it could be a poor joint, connection, tube socket or a tube's internals moving about (ie microphonic issue). Refining your approach with the chopstick is probably the way to go.
BY Delayman
#764373
Does this amp have an effects loop? Total left field, but I had an amp that would react badly to some vibration, but putting a patch cable across the loop sorted, so I figured it was a connection in the loop somewhere.
User avatar
BY druz15
#764375
yep will crack it open and check the sockets and tubes again for microphonics. May a bit of contact cleaner in all the sockets as well.
Still seems super weird that tapping certain capacitors will trigger it though. Can a capacitor be microphonic?
User avatar
BY jeremyb
#764376
druz15 wrote:yep will crack it open and check the sockets and tubes again for microphonics. May a bit of contact cleaner in all the sockets as well.
Still seems super weird that tapping certain capacitors will trigger it though. Can a capacitor be microphonic?


It's possibly the tap being transferred to something else, capacitors are just two surfaces with a bit of goop between them :mental:
User avatar
BY sizzlingbadger
#764379
druz15 wrote:yep will crack it open and check the sockets and tubes again for microphonics. May a bit of contact cleaner in all the sockets as well.
Still seems super weird that tapping certain capacitors will trigger it though. Can a capacitor be microphonic?


Caps can go bad, possible they are shorting out internally when you tap them. I would think its a fairly unusual issue though. Is it signal caps or smoothing caps ? I'd still look at a loose connection at this stage.

Replace or swap a cap, does it still have the issue, if so its likely to be something else.

Is it only on the brilliant channel that you get the issue ? If so it would narrow it down a fair bit.
User avatar
BY RectifiedAmps
#764396
It really does sound like a bad connection somewhere. If it was a faulty mains cap, you'd likely have other symptoms - hum, sucking down the B+ voltage, weak output. Likewise, I doubt it's overloading - this is a DR103 after all and the design should handle anything you throw at it. Probably not a microphonic tube either - that'd give you more ringing sort of issues.

Start with the easiest things to fix and work your way down the rabbit hole:

Play a low E and gently wiggle each tube (preamp + poweramp) in its socket to see if anything happens. Try flicking them with your finger too, to see if they have an intermittent short. After that, patch cable across the fx loop, contact cleaner in the pots & jacks and then back to chopsticking the guts. If tapping the mains caps induces the issue, it's probably a bad connection somewhere in that vicinity of the board (tubes included).

The most annoying thing it could be is a parasitic oscillation, but I'd think that wouldn't just be crackling bass notes. Moving the wiring around with a chopstick while the amp is on is the best way to tackle this one, but it can be elusive.

Oh, and caps can definitely be microphonic - especially coupling/signal caps in high-gain amps. But again, that doesn't sound like the problem you have.
User avatar
BY druz15
#764458
sizzlingbadger wrote:
druz15 wrote:yep will crack it open and check the sockets and tubes again for microphonics. May a bit of contact cleaner in all the sockets as well.
Still seems super weird that tapping certain capacitors will trigger it though. Can a capacitor be microphonic?


Caps can go bad, possible they are shorting out internally when you tap them. I would think its a fairly unusual issue though. Is it signal caps or smoothing caps ? I'd still look at a loose connection at this stage.

Replace or swap a cap, does it still have the issue, if so its likely to be something else.

Is it only on the brilliant channel that you get the issue ? If so it would narrow it down a fair bit.

Signal caps mostly, one or two power supply caps do it also though.
Seems to be worst on the brilliant channel, although there still is also a bit of noise on the normal channel just not as extreme
User avatar
BY druz15
#764459
RectifiedAmps wrote:It really does sound like a bad connection somewhere. If it was a faulty mains cap, you'd likely have other symptoms - hum, sucking down the B+ voltage, weak output. Likewise, I doubt it's overloading - this is a DR103 after all and the design should handle anything you throw at it. Probably not a microphonic tube either - that'd give you more ringing sort of issues.

there is a bit of hum and the output does get weaker when this happens. Basically tapping a cap will create a pop and send it into this weird state where it's thin/weak and noisy, and tapping the chassis/ the same or sometimes a different cap will get it out of this state. Bass notes also trigger it

Start with the easiest things to fix and work your way down the rabbit hole:

Play a low E and gently wiggle each tube (preamp + poweramp) in its socket to see if anything happens. Try flicking them with your finger too, to see if they have an intermittent short. After that, patch cable across the fx loop, contact cleaner in the pots & jacks and then back to chopsticking the guts. If tapping the mains caps induces the issue, it's probably a bad connection somewhere in that vicinity of the board (tubes included).

Thanks will check all of these out


The most annoying thing it could be is a parasitic oscillation, but I'd think that wouldn't just be crackling bass notes. Moving the wiring around with a chopstick while the amp is on is the best way to tackle this one, but it can be elusive.

Oh, and caps can definitely be microphonic - especially coupling/signal caps in high-gain amps. But again, that doesn't sound like the problem you have.

yeah thought as much, considering there are a bunch of different caps all creating the same problem it seems unlikely, unless something in the power supply cooked a bunch of caps when it was first being dialled in. This is a home scratch build using the transformers and chassis from a fender dual showman, no effects loop, was considering adding one but will iron out these issues first hahaha
User avatar
BY druz15
#764472
update:

cleaned all tube sockets, pots & jacks and still occuring.
Discovered that merely tapping the turret board material causes the noise/issue as well, so I suspect something underneath the board is either occasionally shorting out, or one of the underboard wires has a dry solder joint
User avatar
BY druz15
#764480
UPDATE:

Solved it. Thanks for the tips dudes.
The wires under the board on the tube side all seemed ok, until I got to one and went to wiggle it and SNAP (that'll teach me to use solid core wire)
it was super hanging on by a thread and I reckon every little bump or bass vibration was jolting it loose and what not.
turned out it was the wire connecting the cathode of V2 to it's cathode resistor & bypass cap. Replaced the wire, touched up a few solder joints on the underside of the board while I was there, and fixed a little mistake I'd made ("past participle of get" the two presence pot's capacitor values in the wrong place).

Works perfectly now. Still a little bit of background hum, which I put down to my less than Hiwatt quality wiring layout (dealing with a shallower chassis meant a few compromises unfortunately) but the output volume compared to the hum is pretty decent, and the louder the amp goes, the less obvious the hum is compared to the signal which is good.

Popped it back in the chassis, cranked it halfway up and banged out a few big bass notes with no issues, sounding glorious!!!
Cheers!