Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Joeandamy on December 10, 2023, 11:27:37 am
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Hi long time "guest" on this forum and can usually read through stuff on here and figure things out but I'm a little rusty and stumped . So my name is Joe about 20ish years ago I built the fender champ so I have some knowledge but that was 20 years ago.
So the other day I got a 74 fender bassman for 300.00 ,not really my kind of amp but I loved the little champ I made years ago so I got it. Brought it home plugged it in and hooked everything up got nothing it turned on tubes heated but no sound. So pulled the thing apart ,pulled the tubes took some voltage readings ,things looked OK, put the tubes back in hpoked it all back up and it worked and I was super happy the I noticed the tube red plated ((6l6gc #2 counting from left to right. )) so I turned the amp off and went to put my guitar away and pulled the amp off the bench broke said power tubes. So ordered new tubes they got here (jj6l6gc) put them in and everything worked good for an amp that sat the last 30 years in a shoe shop. So then the volume on the unused channel started to effect the used side as I was looking at that the new tube red plated so I turned it off and switched tubes to see if it followed the tube or the socket. It stays in the #2 socket it does not follow the tube I have a bias socket comming tomorrow I think to check the bias but any ideas on what else to check. I'm not "in love " with this amp and might look into just getting a kit to make it something else but maybe it won't be a total loss
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Just a pic
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Take the valves out, power up and report the V DC on terminals 3,4&5 of each 6L6 socket.
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V1 ,
Pin3...470.8
Pin4...470.4
Pin5...-41.21
V2 ,
Pin3...470.1
Pin4...469.7
Pin5...-46.01
And thank you
Edit these are all positive volts my mistake
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Last night I was doing the chopstick poke test and the red wire running between the 2 blue caps poking that make loud pops and that is the socket that is doing the red plate also not sure when it all went to negative voltage ..
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V1 ,
Pin3...-470.8
Pin4...-470.4
Pin5...-41.21
V2 ,
Pin3...-470.1
Pin4...-469.7
Pin5...-46.01
And thank you
There’s something completely nuts going on :dontknow:
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OK my mistake i have the leads flipped at the meter I'm so sorry
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OK my mistake i have the leads flipped at the meter they are all positive volts I'm so sorry
Try again. Pins 3 and 4 are positive. Pin 5 is NEGATIVE.
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:BangHead: :BangHead:
V1 ,
Pin3...470.8
Pin4...470.4
Pin5...-41.21
V2 ,
Pin3...470.1
Pin4...469.7
Pin5...-46.01
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Last night I was doing the chopstick poke test and the red wire running between the 2 blue caps poking that make loud pops and that is the socket that is doing the red plate also not sure when it all went to negative voltage ..
Might be an intermittent negative bias voltage connection. With your meter on pin 5 chopstick that red wire some more. See if the -46V goes positive, or to zero-ish.
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I still get anywhere from -46.02 to -46.11 when I poke the caps and red wire .
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So i have been reading all day and came across a guy that said loose sockets can cause some of this so I made sure the caps were all drained and took my dental pick and snugged them up and no red plate for now and everything is working ish . The pre and tube next to the PI is microphonic so I put 3 different 7025 tubes and they all do the same thin on that socket ,the pins are tight so all 3 bad tubes ????
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One thing at a time.
The main cause of redplating is the bias voltage going more positive than it should be given the other parameters of the circuit.
One cause of that could be loss of contact at the grid since 0 is more positive than the -48v or whatever is prescribed by the schematic.
There are other causes that you need to rule out:
The coupling caps at the output of the PI block ~350v plate voltage from ~-48v bias voltage. Sometimes 400v caps are used here. This isn't enough margin and they begin to leak/fail over time. This can happen even with properly rated caps. The symptom here will be redplating some time after starting the amp.
The physical placement of the eyelets on the board where the PI HT node is wired is about 1/8" from the eyelet where the bias voltage is wired. The board itself can become conductive, and if it leaks at this point the bias voltage will tick positive. However this would happen to both tubes. Since you have a bias balance circuit and it's setup for an imbalance at the moment, this could be happening and can't be ruled out yet.
So your task is to suss out which of these is happening.
It sounds like contact with the pin at the grid could have been the only problem you had, but we won't know until you take voltage measurements at the board and monitor bias voltage to each tube after the amp has been powered up for an extended time.
The fix for the first is replacing the caps, and the fix for the second is to decontaminat/dewax and dry the board--failing this the eyelet can be abandoned in favor of a floating connection or tagstrip.
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The pre and tube next to the PI is microphonic so I put 3 different 7025 tubes and they all do the same thin on that socket ,the pins are tight so all 3 bad tubes ????
You have to try those 3 tubes in a known good working amp to see if their good working tubes.
1. When you say 'micro-phonic' do you mean they act up when you tap on them?
2. Or are they acting up when your just playing through the amp?
Tubes are not made to be tapped on.
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Willabe...yes that was tapping on them with a chop stick ,something I seen on YouTube
Startomaster ...I will replace the caps the power side is all original so got 50 years out of them lol and will start testing thank you guys so very much for the help
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I'm not talking about the electrolytic caps.
Read again.
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Stratomaster...I should have been more specific I found a site that has a complete cap replacement "kit" if you will just so I have all the right parts and was going to replace the power side just for peace of mind and then will do the coupling caps sorry I was thinking of my to do list . Is the amp worth the work ? I was buying a tube amp more for the platform , I wish it had a gain knob
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Only you can answer if it's worth the work.
Don't replace anything (other than old electrolytics) without first establishing a fault. The coupling caps I mentioned could be fine. The board could not be conductive.
It's entirely possible tightening the pins fixed the redplating. Power up the amp and monitor voltage. Then you'll know for sure.
If the amp isn't worth it, or isn't what you want, get it working well enough, sell it and get something more to your liking. Someone somewhere will appreciate the amp for what it is.
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Staromaster , understood and will take your advise on the caps .
So 20 years ago before the internet is what it is today (FaceTime and stuff) I met a guy on one of these forums and i SO wanted a tube amp and anyhow he helped me over the telephone build this champ clone. And what was so great about it it had a 6l6 ,1 preamp tube a full tone stack amd some kind of rectifier tube a 5y?? Anyhow when you picked soft it was so clean and when you dug in it would break up and distort , I can't find that thing any place and this gentleman that helped me might have been like you guys and just knew how to put a tone stack in ,he would help me and as a tool maker I would send him stuff he needed .It was the coolest project I have ever worked on and want to do something like it again hence the bassman . But i do really appreciate all your time in this.
Joe
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Your Bassman can be made to do what you describe. If you're wanting a project tube amp, then you've got a good one. This amp will be much louder than the Champ and the way you go about getting the overdrive will likely shift to preamp style to keep the volumes reasonable and behavior more consistent across the volume range.
Once you get your amp working reliably, start a new thread with what you want your amp to do that it isn't currently doing. Or do a search on the forum for others that have done similar projects. You'll have plenty of help.
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So waiting in the bias tube tester to show up (stupid Amazon) but been playing it all night and no more red plates ,been checking the voltage and it seems stable but was wondering if this thing soldano did would be possible with some pointers in the right direction...just curious
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Since it’s an old amp, did you clean the tube sockets? Just rub some WD40 with a Q tip on the tube pins and plug the tubes several times in and out of the sockets. You wouldn’t believe what a bad connection can do when the sockets are corroded or dirty
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Just rub some WD40 with a Q tip on the tube pins and plug the tubes several times in and out of the sockets. You wouldn’t believe what a bad connection can do when the sockets are corroded or dirty
Sigh. That's two believers this month. I know what I want Santa to put in my stocking. :icon_biggrin:
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but was wondering if this thing soldano did would be possible with some pointers in the right direction...just curious
Very doable, but it's a major mod. Are you up to the task?
BTW, if you use input #2 the second triode must rely on the guitar for it's grid ground reference. Not good and certainly not something I'd expect to see on a Soldano drawing.
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Just rub some WD40 with a Q tip on the tube pins and plug the tubes several times in and out of the sockets. You wouldn’t believe what a bad connection can do when the sockets are corroded or dirty
Sigh. That's two believers this month. I know what I want Santa to put in my stocking. :icon_biggrin:
LOL. Anyway, it’s a harmless effective way to ensure contact. Unless the pin grabbers are loose too.
I use the more expensive stuff too😎
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WD40 is much more than just a water displacer these days. There's a whole family of WD40 brand products, including electrical contact cleaner. I even have three different WD40 products in my shop, but no electrical cleaner.
https://www.amazon.com/WD-40-Specialist-Electrical-Contact-Cleaner/dp/B00AF0OFVU/
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Well got my tube bias socket tester thing today , everything seems to be fine with the amp played for about 4 hours total yesterday and today and no more red plate but as I understood when I bought this it had life as a bass amp and while it is clean I mean super clean sounding it's not what I want in a tube amp SO....here we are and the what to do next stage . I wish I had more knowledge on this stuff but think I will just see if I can sell it and get something with some bite to it. I would like to mod this but by the time I rewire it almost would be easier to just get a kit ...I don't know I'm confused 😐 😕
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you can use pedals for distortion, reverb, tremolo, etc.
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Good on ya if you can JnA -- I get attached and amass fun toys but would have more of a variety if I could let go.
The herd is ... 7 pre-1980 amps now =) -- in my defense they all get played monthly if not weekly or daily!
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Winterturtle: the 80s were such a good time I'm still stuck there :icon_biggrin: