Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: vampwizzard on June 16, 2021, 02:23:00 pm
-
Hey folks. Doing a repair on a fender champ 12. Only mod is a cannabis rex speaker upgrade. Main issue was "no reverb". Swapped out the original preamp tubes with fresh ones. Started rolling through voltages. Most of the amp checks out fine with fender's test point voltages but I did find a problem with the drive channel. Measuring off R33 the low/clean voltage is fine.. right at 1.68 and then when I hit the footswitch for the OD channel my voltage is still full 24. It then sounds like its not grounded properly, and the OD indicator LED stays off. Also, no reverb but that tank looks JANKY!
Would I be correct in assuming I've got a dead LDR-2 in the switching circuit? And does anyone else agree that shouldn't affect the reverb circuit?
schematic: https://el34world.com/Forum/index.php?action=dlattach;topic=23485.0;attach=71931;image (https://el34world.com/Forum/index.php?action=dlattach;topic=23485.0;attach=71931;image)
-
LDRs are not for reverb
Reverb are solid state driving, ( J-Fet ? ) not tube
Read here;
https://el34world.com/Forum/index.php?topic=23485.0
http://gretschpages.com/forum/other-amps/reverb-tank-repair/54809/page1/
-
Agreed. I believe the issues to be separate.
-
Checkout: https://www.effectrode.com/knowledge-base/how-to-transform-a-fender-champ-12-amp-into-a-champ-deluxe-with-a-few-mods/
-
Thank you. Ive been through that page and the posts in our forum. Asking specifically about my failure and the lack of OD light
-
OD
Are you getting OD? IOW: is the problem with OD; the light; or both?
Reverb
Use a signal generator to run a test tone though the amp with reverb On. Is there signal at the: Reverb tank Output? If not, then at Reverb tank Input?
Is there DC voltage at the test point for the SS reverb driver?
-
OD comes in super hot signal wise. No controls.. cant control tone, gain, volume etc. Touching the chassis makes the typical grounding problem noise plus hot bacon grease splatter noise. I should've clarified that.. my apologies.
the light not coming on and the switching voltage irregularity on the clean/OD toggle seems like one of the LDR's (the second if im tracing this out correctly) isn't giving its internal resistance.
-
...
Reverb are solid state driving, ( J-Fet ? ) not tube...
No. It is drawn backward. The 6L6 and OT drive the speaker (as usual) and (through a resistor) the tank. In this drawing the signal flows in the tank from Right to Left, opposite of convention. The JFET is the reverb recovery preamp; there is another tube gain-stage back to the 6L6.
If the speaker speaks, there is little to go wrong driving the tank. Examine wiring and resistor for rot and rat-chews. The *obvious* next question (actually the first question because it can be done on stage) is: does it BOOINGGG when the reverb knob is up and you kick the amp?
This bizarre bastard has a "debugging port": jack a cassette or iPod into the TAPE jacks to check V2B etc. If OK, pull the tank output cable, put your finger on chassis (to discharge static), then on the center pin of the RCA plug. Does it buzz like a giant bee?
-
Tape input works properly clean. signal mixes and fades out into garbage when i switch it to overdrive.
Turned the reverb trimmer pot off and tapped. Nada. turned it all the way up, very faint tank rattle through the speaker. Ive got an area to recheck. the diagram helps big time
-
one quick and dirty way i check if it's the reverb tank or circuit: rca cable from reverb send to receive. if the reverb knob sounds like and obnoxious treble boost as i turn it up, it's likely the tank (you can take resistance readings at the rca jacks of the tank to see that the transducers measure *something* (you can also google reverb tank dcr and compare your readings to what they are supposed to be). if you're getting zero resistance readings on the tank, you may have a dead transducer or a bum connection inside the pan (these days many reverb tanks don't have the leads from the transducers soldered directly to the jacks. may be a point of failure)
-
resistances look alright on the tank. its hardwired to the pcb so its a bit more complicated to troubleshoot. gonna take a look at it tomorrow.
-
...
Reverb are solid state driving, ( J-Fet ? ) not tube...
No. It is drawn backward. The 6L6 and OT drive the speaker (as usual) and (through a resistor) the tank. In this drawing the signal flows in the tank from Right to Left, opposite of convention. The JFET is the reverb recovery preamp; there is another tube gain-stage back to the 6L6.
If the speaker speaks, there is little to go wrong driving the tank. Examine wiring and resistor for rot and rat-chews. The *obvious* next question (actually the first question because it can be done on stage) is: does it BOOINGGG when the reverb knob is up and you kick the amp?
This bizarre bastard has a "debugging port": jack a cassette or iPod into the TAPE jacks to check V2B etc. If OK, pull the tank output cable, put your finger on chassis (to discharge static), then on the center pin of the RCA plug. Does it buzz like a giant bee?
Thank's, I often learn here.
-
...........no reverb but that tank looks JANKY!....
This guy tells a story you should read.
http://www.chambonino.com/work/fender/fend9.html
-
It looks like ill have to cut open that Tauntaun. :(
ordered a replacement LDR last night, should be here early next week. hoping to have the reverb sorted by then at least.
I appreciate the information!
-
Updates: Reverb tank measured ok and looked fine when I took it out of its little cardboard shell. In the interim the LDR 2 (ordered of ebay, vactrol VTL5C3/2) arrived and I swapped it out. Every once I get something right. That solved the channel 2 problem entirely. The amp now functions perfectly sans reverb, but the tank is disconnected. Time to reattach the springs and see where im at on that front.
Thanks!
-
Testing the amp, I get a little bit of very jagged reverb on just a low A note on the e string.. its distorted and weak. upping the trimmer pot for reverb circuit gain has little effect. otherwise nothing.
-
Voltages around the JFET.
-
16.7 at D, 2.2mf at s and 2.5 at g
-
finally found some spare RCA cables. hooked up the tank from a blues jr. drop test makes reverb, signal in does not. Combined with the overvoltage failure from LDR-2 is it possible i blew that transistor feeding the tank? Just spitballing here.
-
the tank is being "feed" by the OT, the recovery side is the Jfet, look at PRR's signal path
EDIT: check R48 and verify it's 47 not 470K
-
the tank is being "feed" by the OT, the recovery side is the Jfet, look at PRR's signal path
EDIT: check R48 and verify it's 47 not 470K
Value was fine. Then went to trace it in the circuit to R28 and that leg popped off its cold solder joint. Reverb works again. LOL
Thanks for the help folks!
-
Since we're talking about the reverb tank in the Champ 12, that cardboard tank has the worst sounding reverb I ever heard. What would need to be done to be able to use a small tank screwed to the bottom of the cab. I'm talking about a reverb tank like in an Deluxe reverb but the smaller version.
-
This is how I tested it.. not my amp but I’ll bring it up with the owner. I agree though
-
I don't see any changes you have to make to use a proper tank.
-
The existing tank is hardwired in.. I took a mini hdmi to RCA cable out of my dad style junk drawer, clipped off the hdmi end and wired it into the board and plugged it in on the other side. Also have to physically mount the tank.. the cardboard one rides the chassis.
Electrically speaking its a direct upgrade.
-
just got an email from ce dist today that they're carrying small (like 5 1/2") mod reverb tanks that might work if an 8-length tank is too large (i have an old acoustic combo that needs a new tank, mounted inside the chassis with too little room for an 8)