Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pjd3 on March 06, 2019, 06:51:29 pm
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Feedback very welcome now!
So, I have my Bogner Alchemist on the kitchen table. After 10 years of reliable service over hundreds of gigs the channel began to rapid switch back and forth by themselves. After a fair amount of internet searching I noted in a few cases (including Line 6's claims) that one of the reasons this can happen is that a tube can begin to go bad drawing excessive current and bringing down necessary power to other parts of the circuit. (being an electronic technician by day I've seen this occur frequently in products I have serviced, in particular the Talon 4 military robot). So I pulled each of the 5 12ax7's one by one, not much change (except for a slight deviation in #5).
BUT!! Upon removing 2 6L6GC power tubes, the rapid toggling of the channels ceased, leading me to entertain the possibility that one of the power tubes was going, and it was infact depleting current, voltage or power to some component(s) of the channel switching circuit. If there was a 5V logic chip, a flip flop of some sort, it may not take much to make it dither, or become indeterminate in its states. I was hoping to hear from anyone what they think about this situation. Considering the situation, its tempting to order up a couple of nice ANOS power tubes (and one or two 12ax7) and hope that this was in fact the source of the issue. I have heard about tubes going bad and drawing excessive current. I did start a Fender amp build to replace this amp, so, buying good tubes is by no means a waste. They will be put to good use one way or another!
Thanks for chiming in, geez, my amp tech couldn't even this Bogner Alchemist but then again, he's an expert in more vintage amps, not new arrivals with digital FX and logic style switching circuits! And he didn't believe that a tube could be an instigator in this issue. But, even Line 6 outlined this symptom as 1 of 3 reasons why chaotic and automatic channel switching can occur.
Would love to hear your thoughts!
Thank you,
Phil
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easy enough to "prove" with what I call the 90% factor
now that is doesn't clicky-clack, switch your channels 10 times in a row, 0 fail = theory 90% to proof.
and if your bored, put any standard TTL chip on the BB and brown down the 5vdc to 4.4vdc, 1 1 0 11 1 00 1 :icon_biggrin:
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Thanks for coming by Shooter.
I ended up a tad disappointed when I returned the 5 12ax7's to the preamp, and the problem returned. With amp on, (no power tubes installed), one by one I pulled each 12ax7 out observing any change. With each preamp tube pulled the intensity of click-clack decreased, still switching back and forth until the last (V1) preamp tube was pulled, upon which the click clack faded out until it stopped. So, it looks like the issue is in the actual switching circuitry. And it doesn't seem to take much to push it into "click-clack" mode. Damn, I was hoping on a bad power tube! Ahh, Time to pull the chassis out, get the schematic and find the components most directly associated with the channel switching action. I assume they are just Relay, transistor, logic IC components that won't break the bank to purchase and replace myself. Line six spoke of a relay that is a critical component in the switch circuit.
Wish me luck, Sure going to need it!
Thank you,
Phil
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Could be nearly anything. I happened to have acquired a copy of the Alchemist assembly and troubleshooting manual, factory issue, some years ago. 10 Meg, so I can't post here; and 240 pages English.
Let me know if you need it and how I could get it to you. If you can isolate it to an area I may be able to send a couple pages relevant.
I had a really bad experience with Line6 and would never consider their products again. Reinhold is a different story but if you've seen the factory manual- Yeah, lot of tech in there.
silverfox.
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Any "logic" *should* be on its own supply. Ha-ha.
There is an occasional trick to pull small (even not-so-small) DC power from large power-tube cathode resistor.
I would not think the 12AX7 would be involved, they are small tubes, and don't suck enough to clack a relay. (In fact other beefier tubes were used to clack relays.) I sure can believe a 6L6 team could spare some power. Or not, if a 6L6 went dead.
But whatever it is, it is strange, and I would not try to guess what it was doing without a schematic or a lot of tracing. If the Fox has a manual, get together.
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Hello, pjd3/all:
Were you able to correct this problem? I logged into this forum after searching the 'net, as I am having an identical problem.
On my Alchemist, the channel switching issue only pops up when all of the footswitch functions are engaged ie boost, delay, reverb on, switching from clean to high gain. I did change the power tubes, and this helps but the problem is still there.
Any help with how your problem was solved? Thanks in advance for your reply.
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I have one of these on my bench as well, I also have access to the schematics and have found the AC heaters for the power tubes get rectified to 3.5vDC for the switching relays. there is another part of the amp that powers the FX board which rectifies from 14vAC to 18vDC. I am only getting 12vDC on the (+) of THAT rectifier. and only getting 2.5vDC on the rectifier side that should be giving me 3.5vDC+/- which is supposed to feed the relays AND I believe the preamp heaters..........................SO would a good guess be failed or failing rectifier diodes at these to low voltage points?
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without looking at a schematic, a drag on the DC "indicates" loading, could be caps, R's, .......
what's happening on the AC of the equation?
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Try banging a little on the foot switch assembly. There is a ground tab of copper grounding the foot switch pcb to the foot switch chassis, that may have become corroded.
As you may know, the circuit ground appears to be through the connector shielding; the metal case of the XLR connector. A bad connection there, or the other end could cause the problem. Broken solder connection in foot switch connector?
Check the molex type connectors on the preamp control board.
If I'm reading things right, the relay power is 3.5 volts and taken from the heater supply. There is a hum balance pot in the circuit. Not sure if that could cause the problem.
silverfox.
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It’s likely to be either a defective part in the PS for the channel relay; (one or more) bad filter cap(s) or bad voltage clamping diode, or a defective switch/transistor in the channel relay. Look for signs of heat damage (charring) on the board or on nearby resistors. Measure the supply nodes in the channel switching power rail - look for voltages that are way out of spec. Look for burned emitter resistors on the switching transistors, etc
http://schems.com/bmampscom/bogner/Alchemist%20Schematic.pdf (http://schems.com/bmampscom/bogner/Alchemist%20Schematic.pdf)
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Thank you EVERYONE for chiming in!!!! I did in fact order two new caps for this part of the circuit! the 4700uF and 1000uF caps in this part of the PSU. C6 and C11. I tested them with my fluke and one of the them registered as "disc". I installed the two knew caps............................TA-DAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA like magic the channel switching is restored and awesome!. I also re-scoped the 3.5v+/- and the 18v+/- and "saw" MUCH better voltage wave forms for three of the four voltages!! so the wacky looking 3.5v's are now very smooth looking since waves, and BOTH sides of the 18v look like pretty properly filter DC!!!!