Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jordane on January 30, 2023, 03:07:59 pm
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EDIT - I did not actually find my problem, but it ended up resolving itself, so who knows what it was...
I have spent some time trying to track down a problem I have on my Deluxe Reverb (Hoffman board, using Weber 6A20 parts). Layout and schematic here - 2 channel short board:
https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf (https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf)
I had it working at one point, so it is something that happened after I had everything soldered together.
Symptoms:
Can play through channel one like normal. Channel 2 (reverb) has some problems. The volume pot doesn't seem to add or reduce gain. The channel sounds as though it is at 100% gain the whole time, regardless of whether a guitar is plugged in to the jack, or whether the volume pot is at "0" or all the way up.
At first I thought it was a bad volume pot, but I unsoldered all connections and tested with multimeter - tests normal. Resoldered all connections, same problem.
Then I checked the grounding at the jack, seems correctly done.
With the amp on, I touched the mid lug on the volume pot with a chopstick, and got an audible noise (this doesn't happen on channel one). I then touched each lug on the V1 and V2 preamp tube sockets - V1 is all silent, V2 makes noise on the output lugs - 6, 7, and 8.
I swapped to a different tube in V2 and ran the same tests - same results.
I unsoldered the wire from the mid volume pot lug and #7 V2 lug, so no signal at all should be getting from the volume pot to V2. I still have noise at idle. Tapped lugs 6, 7, & 8 on V2 again and got a "tap" noise again through the amp. Again, all other lugs and pots are silent when touched with a chopstick.
I hope I have provided enough information (and clear descriptions). Any help would be greatly appreciated.
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what's your skill set?did you take any VDC measurements?
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High resolution photographs of the amp would be helpful and allow other eyes to look everything over to aid in trouble shooting.
How does the vibrato channel work when you pull V3? (reverb obviously will not function)
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Lot of components in your build and very easy to wire something wrong. Picture might help others if they’re clear enough but try this: Might take some time but will likely fix the issue. Switches, pots sockets, bad "new"components, etc., might be the culprit but very doubtful. Use the layout you posted and a magic marker, start from each tube putting a dot next to a wire on the layout then check your bd. to make sure it's correct put a dot on the bd or component (you can use alcohol to wipe the mark off, not whiskey). :icon_biggrin:
I think you get my point, a complicated layout like the Deluxe Reverb has to be exactly right. All the wires and connections you know are right will have dots on the layout and bd., any one without a dot hasn't been checked. Go slow, take your time even if it takes a day or two, just make sure every connection matches the layout, very easy to miss a dot, so go slow. (1 out of 100), a new build will be an error you made. I usually do this after about ten connections, then ten more as I build.If a wiring error you'll find it.
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I re-read your post, if you had the amp playing ok, then most likely your wiring is correct. What was the last thing you did that cause the issue..? Could be a non-soldered wire or a bad solder joint, if amp played well at first.
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Thanks all, I appreciate the responses. I was hoping everyone would say, “yeah that’s the culprit!”
I will continue to troubleshoot, work my way backwards. I am not sure what I did wrong but I am motivated to find it.
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... The volume pot doesn't seem to add or reduce gain. The channel sounds as though it is at 100% gain the whole time, regardless of whether a guitar is plugged in to the jack, or whether the volume pot is at "0" or all the way up.
... Resoldered all connections, same problem. ...
If you unsoldered the ground of a previously working Volume pot, it would exhibit the symptoms you describe.
Check the ground lug of the Volume pot, and make sure it has 0Ω (or mighty close) to the filter cap negative terminals in the amp.
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Check the ground lug of the Volume pot, and make sure it has 0Ω (or mighty close) to the filter cap negative terminals in the amp.
I will double check this, thank you.
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Thanks for all the ideas, I finally had time to try some of these methods. Put the amp back together and turned it on so I could start with DC voltage measurements and work my way down the responses... no noise. Amp back to working as normal, as it has since last September.
:worthy1:
Go figure. I do not know what was wrong, or what I changed... may well have been a cold solder joint at the volume ground, or a bad solder joint at the tube socket. I literally spent several hours checking my work over the past few weeks.
You guys are brilliant, carry on and sleep well.