Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: spunko on March 04, 2021, 10:43:29 pm
-
Hello good people, this is my first post in this forum. I'm from south america, so pardon me for the bad english.
I got this amp like the subject says: V4 tube 6L6 was red plating, it hums, has low and distorted output.
This amp was "repaired" by another person for the exact same problem, and not only the problem persists, but it also returned with V4 socket full of jumpers (picture). I can't seem to find any other modifications besides the jumpers, and a lot reflowed solder joints.
(https://i.imgur.com/BFcaxvb.jpg)
I swaped the power tubes for a short period of time, no more than 1 minute, to see if the tube pass the symptom to the other socket, but it didn't. I asume it wasn't enough time, but I don't want to set a fire.
With the power tubes out (uninstalled) and only the preamp-tubes/PI installed, I took some voltage readings using this diagram:
https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-GB-HotRod-Deluxe-Schematic.pdf
Some of the voltages are ok, but there are some points where the voltage is way to high, or some spots where is not present at all.
Some resistors read a different value compared to the diagram, and R76 and R77 behave weirdly both start reading from 40K and slowly go up to it final value, dunno if reading in-circuit causes it, maybe I have to take them out to test correctly, but these are the only resistors that behave like that.
No voltage present at the "test point" on the PCB for bias adjusment. Don't know if no having power tubes causes to not have voltage there. There is also another test point lacking voltage.
In the following spreadsheet there are all the readings I took, with some notes. I also attached two pictures of the diagram with my readings. Green color for correct values, red color for the ones that are way off. BRW/YLW for R76 and R77.
https://docs.google.com/spreadsheets/d/1xY_tXP3e8xl__2yPQ0U0pK8Tg6M2j99NcM0BDq7P4V0/edit?usp=sharing
I'm planing to take those jumpers out and read voltages again, also some diodes to test out of the circuit.
What would you recomend?
(https://i.imgur.com/KHnM0Gz.png)
(https://i.imgur.com/zfoWFr6.png)
(https://i.imgur.com/aWuvLP2.png)
-
do you have negative voltage on the power tube grids (pin 5)? recently picked up an early 2000s blues junior for dirt cheap with similar symptoms (distorted output, "one of the tubes glow brighter than the other") and there was no connection between one of the power tube grids and the main board. i would say that that caused the symptoms in my case - one power tube was receiving zero signal and no bias voltage so the plates were bright effing red! i suspected the ribbon cable to be the cause so soldered a jumper wire in parallel and everything was good. lent it to the other guitarist at church who is currently ampless with the option to buy
and yes, it appears somebody had been mucking about in mine, the good was fairly recent replacement of filter caps with f&t. the bad was a few lifted traces and jumpers in their place
-
Thank you for the response.
Yes, both PIN5 on the power tubes have -42V
-
First, those test points voltages shown inside an oval are 1KHz sine wave test signal generator voltages and they will only be present when a signal generator are connected to the input jack. They are useful for signal tracing with a scope but are useless for measuring dc voltages with a DMM. So, just ignore any voltages shown in an oval.
Second, those jumpers on V4 socket are likely there because the circuit board traces are broken or burned up. This could have been caused by vibration or heat from the tube, but more likely caused by a careless person with poor soldering skills. If you remove the jumpers you will most likely have to replace them again. I suggest you leave them alone for now.
Now, here's what to do next. Pull the two 6L6s and lay them aside. Measure the voltages on pins 3, 4, 5, and 8 of V4 and V5 sockets, one probe on chassis ground, the other probe directly on the tube socket. Insert the probe into the socket pin from the tube side of socket. Pin 8 will be zero volts because the tube is not plugged in. Pins 3 and 4 will be big positive voltages. Pin 5 should be a negative voltage approximately -50VDC. Both tubes should have the same voltage. Pin 5 is the voltage I'm mainly interested in. Report your findings.
-
I see you have -42V on pins 5. I would expect that to be closer to -50V. Adjust your bias pot to put the maximum negative voltage on pins 5. This is just a preliminary setting and will be readjusted once the amp is working.
-
Red plating must come from;
1- No (good ) negative bias at pins 5 of Output tubes.
2 -Output tubes are short ; put new one
-
Ok, something really bad happened reading the pins on the socket.
The good socket, no problem reading. I got the voltages you mentioned.
The problem happened with the socket with the jumpers. The jumper on PIN3 was loose in its hole (I think broken trace) and it touched with the jumper on PIN2 which has a pointy end.
It caught in flames on the pin3, and I'm worried the OT got burnt.
I immediately turned off the amp, but it smells bad.
Can I test the OT with a multimeter?
EDIT:
I moved the wire on pin3 a bit, so it won't touch with pin2. Put new tubes and powered it up just to test.
The amp sounds clean, not distorted, full power. I hope the OT is good.
The person that gave me the amp said that the initial problem was hum, low output and distorted, so he took it to a tech who installed all 5 tubes new and the amp worked fine for 2 days, and then it started again the same problem, took it to tech again, did the jumpers and the amp ca,e the same but with a red plating tube.
Could be something in the amp damaging one of the power tubes?
I have a set of new 6L6, but I don't want to burn them too.
-
Pin 3 shorted to pin 2. That's a pretty common failure. Can be costly. I guess you will have to redo those jumpers after all. Hope you do a better job than the last guy. Inspect the ribbon cable/connectors for signs of burning. Check each wire in the ribbon cable for continuity.
Measure the resistance of the OT primary. One meter probe on the red, the other probe on the blue. Repeat for the red to brown leads. Resistance readings should be approx. equal. Then measure resistance between the blue and brown leads. Should be about double your other readings. Hopefully the OT is OK.
R80 and/or R81 are probably burned up.
-
Sounds like you got lucky. Check R80 and R81. Then adjust the bias pot so you have 60mV at TP30. Inspect those jumpers and make them have safe/reliable connections.
-
Measure the resistance of the OT primary. One meter probe on the red, the other probe on the blue. Repeat for the red to brown leads. Resistance readings should be approx. equal. Then measure resistance between the blue and brown leads. Should be about double your other readings. Hopefully the OT is OK.
Red - blue: 108ohm (this is were the jumpers are in the socket)
Red - brw: 82.5ohm (good socket)
R80 and/or R81 are probably burned up.
Yes, they are bruned.
-
Sounds like you got lucky. Check R80 and R81. Then adjust the bias pot so you have 60mV at TP30. Inspect those jumpers and make them have safe/reliable connections.
So, it seems it was a bad tube which causes it to red plate and distort/low output. But also those jumpers are trash.
What I am wondering is, if there is a problem in the amp that burns that tube. Or did the prevoius owner have bad luck and got a new bad tube.
Im going to replace R80/R81, re solder the jumpers and put new tubes.
I will return with the voltages on PIN3,4,5 and 8.
Q: do I need to replace R80/81 before reading the voltages?
-
It would be very beneficial to build and use a light bulb limiter to spply power to the amp, until these big problems are resolved and it’s operating happily.
-
It would be very beneficial to build and use a light bulb limiter to spply power to the amp, until these big problems are resolved and it’s operating happily.
Thank you very much. I have all the parts in hand, I will have it ready in like 20 minutes :icon_biggrin:
-
Q: do I need to replace R80/81 before reading the voltages?
No, but you should adjust the bias as I mentioned earlier.
-
Q: do I need to replace R80/81 before reading the voltages?
No, but you should adjust the bias as I mentioned earlier.
Great, thanx again. I will come back when I finish the jumpers and change those resistors.
I forgot to mention that all ribbon cables test good for continuity and no burn sings.
-
Ok, I redid some of the jumpers with shorter cables, not all the jumpers were needed so I took them out. Replaced R80/81, but for some reason the new ones doesn't look like 1/2 watt to me, the leads are thinner. Maybe 1/4 watt? or leads does not matter that much? The seller said they were 1/2 watt, but as you read I'm not sure.
I noticed broken traces from R78 and R79 that were still attached somehow, so I took care of that too. Is there any way to hold them in place, like a heat resistant glue?
After all the work done, and using a light bulb limiter, I powered on the amp and no smoke this time. Yay!
I read some voltages and proceded to power on the amp without the bulb limiter. I also used two tube probes to measure the plate current on both power tubes.
The amp was on for 15 minutes before I took the readings (no bulb limiter.).
BIAS test point: 60mV
(https://i.imgur.com/Snwh5f2.png)
-
Replaced R80/81, but for some reason the new ones doesn't look like 1/2 watt to me, the leads are thinner. Maybe 1/4 watt? or leads does not matter that much? The seller said they were 1/2 watt, but as you read I'm not sure.
I ordered some 1/2 watts from Mouser and only paid attention to the ohm and watt ratings. This is what they sent me (that's the smallest US coin, about 18 mm):
-
I ordered some 1/2 watts from Mouser and only paid attention to the ohm and watt ratings. This is what they sent me (that's the smallest US coin, about 18 mm):
Mine are a bit bigger, but the leads are the same.
So I was playing the amp and I think something is wrong. The clean channel stays clean until 3, pass that and it gradually starts to distort. I only manage to get to 6 on the volumen pot and it wans't clean at all.
The drive channel sounds kinda good I think, and also got it at 6 in the master volumen.
Talking with a friend that owns the same amp, he told me he can't pass 2 on both channels because is too damn loud, otherwise he will be kicked out of the house. He can't figure out how I manage to get to 6.
I forgot to mention that all 5 tubes are new.
What would be the next step?
-
lol, I was wondering the same thing....I had to keep my HRD on 1 or 2 on the volume too .......
-
Hello, back again with my amp.
When testing in my last post, I noticed one cap (C36) leaking some material, so I changed only that one.
Then I got another hot rod(2) from 2009, same amps basically, and the other one (2) sounds a litte bit louder and maybe more bassy than mine :dontknow:
Also a friend of mine has another one from 2002, and it sounds exactly as the other one I recently got (2), a bit louder and more bassy than mine.
The testing was made with the amps side by side and a A/B switch with a les paul.
- First tested the guitar in both inputs.
- Then tested the guitar pluged directly into the "power amp in" fx-loop.
*Also swaped power tubes between amps to check if the lower output followed the tubes, but it didn't. The "problem" (if I can call it that way) stayed in my amp, no matter what tubes were in it.
All test with those amps gave the same result, both of them sound the same, but mine didn't. It has a little less output and is less bassy.
Is weird because the amp sounds good, it has good output and is suuuper clean, is cleaner than the other two amps. But those break up the same, sound the same, and the testing lead me to think that maybe there is something to check in my amp.
PD: I also noticed that R78/79 were replaced with cheaper ones that are sold locally. These ones don't have those tiny bumps on the corners, so they sit flat on the pcb.
-
Speaker?
-
Speaker?
It could be, but the 3 amps have the same speaker.
-
I have been working a lot in this amp, and now I have another problem.
Fixing the solder joints on the power tube sockets, the ribbon cables broke (fisrt preamp tube and the next one).
I didn't noticed at first because I only tested the clean channel which it was super loud, when switched to drive channel it sounded like a 1 watt amp, super low output. Also the reverb didn't work.
I checked those 2 ribbon cables and saw them broken, so I changed them all with single wires. After two hours of work, all the wires are soldered, hooked the amp to the bulb limiter and powered ON.
Now the amp makes some crackling noises, I'm thinking maybe there is a bad solder joint, but dunno where else to start looking, cuz I resoldered all my work, so it must be somewhere else, or maybe a bad component? What else could be causing this noises?
Also the drive channel got fixed but is still weak, there is a notable difference in output between both channels, clean sounds louder.
-
The problem for the crakling noises was some flux I forgot to clean on the ribbon cables :BangHead:
Once I cleaned that, the amp is dead quiet.
There is only one problem now, the reverb pot makes a lot of noise when I move it. Is not like a dirty pot, besides I tried to clean it with deoxit but it didn't work.
The only way to describe it, is like the ZVEX - Box of Rock when you move the drive pot, it makes a lot of noise, but that pedal works that way.
All hot rods I have played, the reverb pot is dead quiet, this one makes a looot of noise when dialing the pot.
-
suspect a leaky coupling cap in line with the pot
-
thanx for the answer.
So looking at the schematic, could it be C52 or C17? Could I test those caps with this tester I have in hand (picture).
(https://i.imgur.com/5Ertcsw.jpg)
(https://www.valuetronics.com/media/catalog/product/cache/image/beff4985b56e3afdbeabfc89641a4582/b/k/bkprec_820_1.jpg)
-
Well done for perseverance with those failing PC boards. Seeing as how you seem to be continually coming across issues, you may want to consider gutting it and rebuilding with new eyelet or turret boards
-
those works be the first two I'd suspect. I'd just clip my meter negative lead to ground and test for voltage (dc) from TP22 on. could be something after the pot but that's where I'd start poking around
-
Hello friends. I have solved all the problems listed, but a new has appeared (or maybe an old one and I didn't notice with the other issues).
The reverb problem was caused "by me". I messed up soldering the replace for the ribbon cables, two wires were crossed like an X, put them in the correct spot and the reverb pot is dead quiet now.
The new problem: The drive channel is lower in volume than the clean channel which sounds super loud. I have to set master pot for the drive channel up to 5 to match the volumen on the clean channel at 2, and it also sounds a bit thin :dontknow:. the clean channel has bit more body or bass on it, just a bit.
I said in a post above that I have another hot rod 2009. The "good amp" sounds with the same power on both channels, the drive is super loud.
Both amps have the same power or volume on the clean channel, tested with an A/B pedal.
- I changed pre-amp tubes with the same negative result.
- I inserted a signal with the drive on, and poked with a plastic stick all over the PCB, components, solder joints and wires, without noticing any difference.
- Sprayed conctact cleanner on the drive/more drive switches.
I have been looking all over google about this issue and I only manage to find just one result with the same problem "lower volume on drive channel" but is from 2016, and it has only 2 replies with no solution.
Is so close to be a good working amp and I have learned a lot with it.
Is it ok if I open another thread with just that issue? or better stay in this one?
I ask because there is no info about this exact issue on google.
Again, thank you all for the help, and sorry for my bad english.
I'll be waiting for your replies :occasion14: