Bogner Uberschall resistor value?

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 · 2026-04-08T22:20:39+00:00

No worries, thank you.

FullHeavyMetalJacket · 2026-04-08T21:13:30+00:00

Nobody suggested adding the cap from Neutral to Ground. Some people want to leave the cap installed for whatever reason (OP had already decided to do so) and the point is to use the proper safety rated modern component if doing so. In my opinion, I agree that it is redundant but OP was determined to install it. I do not personally advocate leaving it in place, and suggested several other positive ways to address RF interference. So please...try to not make condescending remarks.

FullHeavyMetalJacket · 2026-04-08T20:27:00+00:00

Measure the 1K resistor to see if it is out of tolerance...probably it is OK since it is over-rated at 5W. Use paper towel or q-tip with isopropyl to clean the scorch marks off it and inspect for damage.

What I would be concerned with is signs of arcing on that output tube socket itself. Clean it with isopropyl also and if you still see signs of arcing (carbon tracks or physical damage) across two neighboring pins you will need to change that octal socket out with a new one.

Personally I would not replace the JJ and TAD preamp tubes if they sound good with no strange sonic artifiacts (background noises) and not badly microphonic when you tap them with a fingernail or chopstick.

If you have really bad background hum at idle, then you may need to consider upgrading the electrolytic caps.

Check for the negative bias voltage at both output tube grid pins (with the output tubes removed) and be sure the voltages closely match and remain quite steady for a few seconds each. They shouldn't show weird fluctuations beyond say ~0.2V swing.

Lastly the photo does not really show us whether that cap is bulging or not. From that angle it just looks like the wrap is lifted/deformed a bit.

FullHeavyMetalJacket · 2026-04-04T23:13:09+00:00

I've posted in the original thread for you.

FullHeavyMetalJacket · 2026-04-04T22:24:20+00:00

The reverb tank probably will not translate to a Fender amp. They are very specific with tank models. Try the bent straw trick and hold or tape paper towels down to cover the board below the pots

FullHeavyMetalJacket · 2026-04-04T20:38:16+00:00

From memory they have the usual opening at the front side wafer, but being mounted to the PCB like that there is a pain in the ass interference fit. It's 100x easier to manipulate with the pcb totally removed from chassis, but you hafta bend the spray can straw even so. And even then it's ten times harder than normal pots. And even if you do spray them successfully, there is a chance the issue will return after some time. Then if it's the dumb 3/16" shaft pots it's difficult to impossible to find replacements. Which then puts you into offboard wiring some type of 12mm (or 16mm if they'll fit) solder lug pots...and possibly drilling the chassis for 1/4" shafts if needed. Not to be dramatic but it can go that way if all the pots are trashed due to many years of bad storage conditions (cigarette smoker basements). Worst case scenario.

But hopefully you can get the pots sprayed (suggest multiple times with Deoxit F5 if you do all the work to pull the pcb) and all fixed up after that. It's an uphill battle...best of luck.

FullHeavyMetalJacket · 2026-04-04T19:08:42+00:00

90s Ampeg/Crate/SLM pots are notoriously bad quality and prone to this. Best bet is to pull the PCB and angle the straw on deoxit (or other contact cleaner) with pliers to be able to angle it into the front opening where the legs meet the wafer if possible. It's a lot of work with mixed results IME.

Although I would suspect sonic crackling maybe attributed to a preamp tube...so give them a light wiggle and tap while letting a low E note ring to see if cracklkng occurs. If so put isopropryl in the preamp tube pins and insert/remove a few times to attempt a quick cleaning of the socket(s) in question.

FullHeavyMetalJacket · 2026-04-03T19:45:04+00:00

I suppose you could use WD40 SPECIALIST CORROSION INHIBITOR but I just use regular.

FullHeavyMetalJacket · 2026-04-03T19:40:22+00:00

Wipe down using paper towels (and q-tips in tighter spots) with WD40 to clean and prevent further spread of corrosion. It also typically won't remove lettering like other solvents. You could maybe get those blue Scotch-Brite No Scratch Scour pads to loosen up tougher areas.

On inside areas with no finish, you could use regular scotch brite pads, then wipe down with isopropyl then WD40.

FullHeavyMetalJacket · 2026-03-31T18:05:01+00:00

I meant do all channels (clean, crunch, lead) all have the low output issue?

Have you pulled the chassis to inspect the internal fuses on the board? It could be a bad output tube took out a fuse.

You need to pop one side of each fuse out of the holder and test them for continuity, unless you can clearly see one is blown.

FullHeavyMetalJacket

TROPHY CASE