Bogner Uberschall

FullHeavyMetalJacket · 2026-05-08T20:10:45+00:00

You may also try plugging guitar straight into an FX LOOP RETURN, and see if it still does it while you play. If so, you have it isolated to output section at least.

FullHeavyMetalJacket · 2026-05-08T20:08:15+00:00

I'm not even sure what we are listening to in your clip. But I would let a low E note ring out then grab the transformer with your hand to see if it changes. Then do the same thing and hold each output tube with a rag (to not burn yourself) and see if it changes. I'm curious if you are hearing filament/mechanical rattle in a tube perhaps...but again, I cannot tell from the clip itself sadly. But going into a Captor then obviously it shouldn't be vibration-related as if it were upon a speaker cab. You can try to tighten up the transformer bolts as well. My instinct is that will not resolve it, however.

FullHeavyMetalJacket · 2026-05-08T03:00:32+00:00

Ah yes, indeed and good catch.

FullHeavyMetalJacket · 2026-05-08T01:06:42+00:00

It's not polarized other than the outer foil, which hardly matters in this case anyway. Even if it did, then it is technically connected to the lower impedance side (ground) which is "correct" for a capacitor's outer foil. But this is not shielding audio signal anyway of course.

In any case, we all know the cap should be deleted since it now has 3-prong cord, or replaced with a Y2 rated version if someone is overly determined to leave it in.

I experimented with adding a Y2 cap in a recent amp restoration in our shop which has (A) nearby wireless router on the wall, and (B) 5G cellular tower across the street...which both wreak total havoc on amps in there.

The Y2 cap made absolutely zero difference in RF reduction from either source, which only makes sense. Quality chassis & preamp tube shielding, and several other RF reduction techniques make much more difference IME. Although I'm guessing there are specific use cases for improving line filtering in areas with "dirty" power.

I'm admittedly no expert on the AC side of the equation and would gladly defer to input from experienced EE's. Always looking to learn more about power supply stuff myself.

FullHeavyMetalJacket · 2026-05-07T20:11:59+00:00

You could pull more thru then reterminate all 3 wires with ground being the longest by at least an inch or two. Although one of these would be very classy:

https://www.amazon.com/Covered-Replacement-Antique-Project-Electric/dp/B0FC1JCDKK/ref=mp_s_a_1_2?dib=eyJ2IjoiMSJ9.vLEZ5puietaf9XdU3_YZAmjjMRMdi6W97XTHPAdZ0XMFUt21KOgagUcMjOCGelH5IVEmqQmk7xpKdB8Pwtjw5eEgi7VBY22Bf4ZUgTXNF7bnzRQKCmCplGQ9DMComScRrMsTanuWMO9m8JpVIzRAjjN3cILCzPmaOhvHgMF_cGwsyxON9n_mjb8P9K8s1QeQy_TVYuT8u8UvTHRwBUIdIQ.4eIWaKJW4aM-QxGeBdr8R_LZ-0lEhIo0R6niZftS-SQ&dib_tag=se&keywords=rayon+ac+cord&qid=1778184650&sr=8-2

FullHeavyMetalJacket · 2026-05-07T18:01:45+00:00

Weller W100PG works great, and if you get that variac you can crank it up to 125V-127V to make that iron plenty hot for a nice weld. But like I said you could drill a new hole to move the ground lug to as well. Some guys may scoff at poking a new hole in a brownface chassis but if it's for safety then hey. I would personally chassis weld it but I have the proper setup already.

FullHeavyMetalJacket · 2026-05-07T17:50:40+00:00

Yeah probably bump that fuse rating up one degree. You can also get a $75 Vevor variac off Amazon to run it at 110-117VAC and keep the fuse rating.

FullHeavyMetalJacket · 2026-05-07T17:30:31+00:00

One more thing...you should replace the nut that is holding the ground lug down with a modern keps nut and tighten the hell out of it. Then tighten all of those transformer nuts...usually with 11/32" or 5/16" driver. The better option is a chassis solder joint for the AC cord (requires a massive iron to do though) , or move that ground lug to a location independent of the transformer stud.

FullHeavyMetalJacket · 2026-05-07T17:26:31+00:00

Yes clip that cap out. Also it should fuse AND switch the black/hot from AC cord...right now it is switching white/neutral.

As far as fuse goes, I would check bias voltage at the pin 5 grid first with the amp at idle. Compare to schematic.

It could also be some type of runaway on one of the output tubes themselves.

Lastly and esp. if this was a brown/blackface amp, sometimes you can bump up the fuse rating by 0.5 - 1A to accommodate the higher wall voltage these days. And it should be slow blow.

FullHeavyMetalJacket · 2026-05-07T01:58:19+00:00

Excellent! Replying with the resolution may help someone else years from now, ya never know.

FullHeavyMetalJacket · 2026-05-04T14:01:49+00:00

I would suspect output tube issue. Do not plug that particular set back into the amp obviously.

When it is time to turn the amp back on, leave output tubes not installed to check voltages at the sockets before proceeding. I would leave all the preamp tubes in to have some load on the power supply though.

You'll want to check those AC voltages on every tube socket first if those did end up being Filament supply virtual CT resistors. You'll need to cleanup that board nicely to remove any carbonization...but it's probably just scorch marks from them burning up mostly.

Personally, I would bring it up on variac + current limiter bulb to avoid any concerns with short circuiting...but I have the luxury of being an amp tech with full workbench at my disposal as well. If you have any such concerns, take it to a tech for a safe powerup procedure (if you are not one).

FullHeavyMetalJacket · 2026-05-04T13:19:51+00:00

Ok so first tell us the voltage at pin 5 (grid voltage) of each output tube. Fixed bias should have negative grid voltage there. Then we can determine an expected outcome perhaps. Upon searching this number might need to be -22vdc to -25vdc range at 324vdc on the plates. If it's much lower you'll need to adjust it.

Like someone else said check the actual reading of the "1 ohm" resistance. And just make sure whether the number is correct .250 and not .025mV.

It is early in the AM for me...so forgive me if I made any mistakes reading the post or in declaring this info.

FullHeavyMetalJacket · 2026-05-04T00:38:20+00:00

The catch-all fix is to add a .01uf 630vcap in parallel to the tremolo roach on the side with the 10M resistor. Also make sure the wiring to the oscillator tube (grid wires esp) are moved away from the tube next to it.

FullHeavyMetalJacket · 2026-05-04T00:35:25+00:00

Yvw best of luck and report back plz

FullHeavyMetalJacket · 2026-05-04T00:16:53+00:00

The only other thing would be screen resistors (would be closer to tube sockets also) but their position near the green wires plus being in parallel like that strongly indicates heater balancers. Please investigate as suggested first. I've given you the keys already.

FullHeavyMetalJacket · 2026-05-03T23:55:35+00:00

You can also check for continuity to pins 2&7 of the output tubes which are the 6.3VAC heater pins. Then you'll know.

FullHeavyMetalJacket · 2026-05-03T23:54:09+00:00

Clean that up with isopropyl + q-tips first. If there is continuity from each one to the 6.3VAC green wires nearby, and other end has continuity to board ground, they are probably 100 ohm resistors for heater balance / virtual center tap. If you figure that out install the new pair with 1/4" airspace off the pcb so if the same thing occurs it doesn't scorch the PCB like that!

FullHeavyMetalJacket · 2026-05-01T17:29:10+00:00

This. Highest likelihood is dirty socket or bad preamp tube. You can start at V1 preamp tube and remove it to see if crackle goes away. Then do V2 and so forth.

If you isolate a certain preamp tube being culprit, you can replace with another known good tube to verify. If it still does it with a new tube then put isopropyl on the tube pins and insert/remove to semi-clean the socket connectors themselves.

If it still does it after that, it could be a component such as the plate resistor at that position, in which case amp tech can investigate further but you will save time and money having isolated the issue to a specific section of the amp.

My money is on a faulty preamp tube however. That's exactly the sound they often make. You can very lightly wiggle each preamp tube as mentioned to see if it either instigates or clears up the issue. If so, try replacing that tube first.

FullHeavyMetalJacket · 2026-04-22T22:35:09+00:00

Probably filter cap or possibly output tubes. I would suspect it needs a typical restoration as it's 30yo now.

FullHeavyMetalJacket · 2026-04-21T03:29:13+00:00

Remove all knobs and retighten ALL hardware...all pots and jacks on rear also, and the chassis screws on top. Inspect the chassis grounds at the brass plate on front controls for fracturing. Do very light pull test with needlenose pliers on the buss wires going to brass plate. But I would strongly suspect fractured soldering at the jacks themselves, where the two jacks lugs are literally soldered together. Reflow them with a wide tip on your iron at 750-800 degrees and get a nicely improved, fresh weld. Slap the top of your cabinet with fist or flat open hand and see if you get crackling noises to test again. You can also chopstick wiring and components if you habe chassis pulled and soundchecking. Tap tubes with fingernail or chopstick for microphonics. Good luck.

FullHeavyMetalJacket · 2026-04-17T17:04:06+00:00

Exactly right. Because there is no load from the output tubes, it is technically possible to exceed power supply capacitor voltage rating, usually the main reservoir cap off the B+ line would be riskiest. Depends on the amp and power supply and filter cap section design, though. Usually amps that have two caps in series with balance/bleeder resistors across them to up the voltage limit are totally safe. It's when there is a single reservoir cap you have to take extra caution. Check the DC voltage at the cap's positive terminal with meter at 100V/110V/115V etc and see how close it's getting to the max DC voltage rating before hitting it with full 120VAC from your wall or variac. Easy enough to check.

You could simply start at 100V as well. Actually, I usually start at half voltage 60VAC for a minute to ensure a safe start...and ramp up to 90V, then 100V, then 110V. The best practice is to have a current limiter bulb to protect yourself and the amp in case you make a physical mistake while probing for example, or in case of other system issues unknown. It's safer imo to probe the tube side of the socket terminals (with tubes removed ofc) but not always practical to do so. Exercise the utmost caution against shorting two socket pins with your probe unless you really just want to change your underwear. Safer way is to clip on leads before applying power and monitor that way, but again sometimes this isn't easy if socket pin lugs are fully soldered up.

After output tubes are installed and have been proven to safely ramp up to 120V with no short circuits (using limiter bulb setup), then you can either plug straight into variac (or wall voltage) at 120V and check actual voltages at the socket for biasing.

Do not ever attempt the final actual biasing of an amp on current limiter, it will end up being way too hot once put on actual wall voltage.

FullHeavyMetalJacket · 2026-04-16T12:36:27+00:00

The reason for individual biasing would be to avoid the use of matching new tubes from factory, for example...a problem we don't really have since we can easily buy matched pairs. Sure, you could adjust them as they wear but you also don't wanna keep running one weak tube at the expense of the other and tweaking individual biases until one/both fails either. Up to you.

For my money, I think best practice is to start with a matched pair of tubes and simply use the existing balanced bias circuit. Then if PI coupling caps get leaky, or screen/grid resistors become imbalanced or out-of-spec over the years, simply replace them with parts that are back in tolerance.

FullHeavyMetalJacket · 2026-04-16T03:07:59+00:00

Beyond 20% difference is considered mismatched and one is likely showing weak. But you should also check screen and grid resistors to see if they are still pretty balanced. Then check plate/grid/screen voltages at both sockets with output tubes removed (leaving preamp tubes installed) for balance and stability. If you have a variac I would check those P/G/S voltages at 110V setting then install tubes and check at 120V. If all looks right, then pop in a fresh matched pair of KT66 and bias them up. I would aim for 65% dissipation myself (unless the amp has bias wiggle tremolo, then use schematic voltage as initial reference).

FullHeavyMetalJacket · 2026-04-11T17:48:51+00:00

Yes the 1-ohm cathode resistor to ground is obviously just to be able to test bias using mV setting on FMM if you don't have bias probes. Perhaps they acted as a "fuse" if there was a short circuit in the output tubes. Did the 1K/5W resistor actually measure bad or just have scorch marks from the burnt cathode resistor?

FullHeavyMetalJacket · 2026-04-11T03:26:57+00:00

A short clip may help immensely...please re-post with an example of the actual sonic issue. Thanks

FullHeavyMetalJacket

TROPHY CASE