So I'm wondering if anyone might have some insight as to why my plates are glowing on my tubes. The fuse blew on the amp some time ago when a friend of mine accidentally dropped an empty pint glass on the amp. After replacing the fuse, the amp worked for a little while, only to blow another fuse after emitting an orangish flare from the back of the amp that I saw reflected off the wall. Well, after checking out the tubes, I noticed that the plate on one had completely burned through and then burnt a hole in the tube. I bought some cheap chinese tubes on eBay to start figuring this out, and their plates are glowing too. When standby's switched off, there is no glowing, but as soon as you switch it on the creases (for lack of a better term) of the plates are glowing. Any ideas?
If you put a known good tubes in an amp and it starts red-plating right away, it's conducting full blast like WildOne says.
I've seen more than my fair share of tube circuitry gone bad due to problems like cracked solder joint or opened resistors in bias circuitry. Fixing is easy once you know what it is, but you will have to troubleshoot and identify the source of the problem somehow. Big part of what you are paying the repair techs for is the time to identify these problem.
If you don't have the expertise to do this, it's really best to have a tech with those skills look over it.
I've seen more than my fair share of tube circuitry gone bad due to problems like cracked solder joint or opened resistors in bias circuitry. Fixing is easy once you know what it is, but you will have to troubleshoot and identify the source of the problem somehow. Big part of what you are paying the repair techs for is the time to identify these problem.
If you don't have the expertise to do this, it's really best to have a tech with those skills look over it.
There you go, I was just going to get it from schematic heaven and upload it, but the file size was to much. Anyway, are you up to making some voltage measurments? Let's start with the B+ voltage from ground to the top of the 20 ufd main filter caps- you can leave the standby switch off to prevent the nasties. I have to inject here that there is a considerable safety hazzzard with this measuement!
I agree with the previous recommendation to take this to an experienced repair person.
If you have to ask then maybe yo need to hire a pro. The voltages inside are lethal. If it can burn through a steel plate it will do even worse on your nervous system. You don't get a second chance. Also this is a very complex amp and you will not be able to fix it unless you can pass this simple test: Look at the schematic, point to the bias supply and tell us specs on it's filter cap. What kind of rectifier does it use? what is the expected voltage?" Those are dead easy questions, right up there with "how many beers in a six pack". If the answers are not obvious hire some help.
OK still with us?
(0) Remove the tubes. For this level of debugging it's best to not have them in.
(1) Do NOT place both hands in the amp. There is high voltage even with the amp unplugged from the wall. Filter caps can store power for a good long time. So with one hand in back pocket, connect a meter to the B+ and ground, then apply power and wait for volts on meter to stabilize. Leave this meter in place. It wil be the "safety meter" when it reads over about 30 volts keep clear. Meters cost only $5 at harbor Freight and there way-cheap meters are good enough. Buy four of them.
(2) you can make a "discharge stick" for about $0.50. Get a (say) 100K 1W (or 3W or 5W) resistor and clip one lead kind of short. Tape the resistor to the end of a wood or plastic chop stick so that the short lead points out from the end. put a short length of wire on the other lead and solder a clip to the wire. clip the wire to the chassis. Poke anything with the pointed tip before you touch it. notice the "safety meter" quickly goes to zero if you connect the stick tip to the B+. This stick will save you a lot of time, otherwise you'd have to wait a while for the amp to be safe after powering down. The safety meter will verify the stick is doing the job.
I suspect a broken bias supply. What you need to do is look at each part. is the diode shorted, filter cap OK. The common problem is the adjustment pot fails but I don't see one. Seems to be non adjustable. So maybe a fix resisor has failed.
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Thanks for the advice guys. Do I just need to drain the filter caps? Look, I understand the safety implications. I'm wondering what I need to do to get the amp working. Thanks for your concern, but I am poor and need to get my amp up and going again.
filter cap: 8ufd, 150 volts, solid state rectifier (the diodes), and -55 volts, right? I'm a bit of a newbie reading schematics, but took a course in electronic circuits as a physics major. If the problem could be solved by opening up the amp, dissipating the filter caps, replacing the two 220 k resistors I think I could handle it.
Hi,
OK the usual safety warning.
Always test with a meter before you touch anything!
Notice that your B+ is isolated by the stand by switch. This is on the (Bottom right) after the diodes. This will remain live after power off so don't get thinking it's dead when you measure B+ on the tubes you need to use a resistor across them to discharge. The switch will be OFF so its not connected to the tube circuit!
-40 V is probably not enough the tubes will run hot! 50-55 maybe OK.
OK that said, for your plates to "Glow" you may have low bias or you may have a leaking coupling cap. Remember do not power up with no speaker or load attached you may stuff the Output Tx.
You could measure or change the 56K if its value has changed
So the .05 uF between the 7025 and the 6L6's may be faulty this would effect your bias supply.
Is it both 6L6's or more on one than the other? Which one failed?
Because if the 220K is open (Between the Bias supply and the Grid) it will red plate!
Just for interest it is a bit difficult to make out on the schematic, however it seems that the bias supply may be on with the B+ off in standby. So you coud measure your bias in stand by and with the B+ on and see it it changes due to leakage from the coupling caps! If there is no change then the caps should be OK.
A safety mod you could do, is put a 500K-1Meg resistor 2watt or more across the B+ caps to give a slow discharge after power off.
Regards
M. Gregg
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You do not say if this is at each tube or at the supply?
If its at each tube then the 220K's should be OK. Remember that they may have changed value! With no tube in the socket you may not see the "actual" voltage on the bias!
What is the actual value of bias you have between the 10K-56K before the 220K's?
Also remember that the working voltage of the resistors needs to be correct for replacement!
Regards
M. Gregg
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If you, Nooel, do not understand the guts of the amp, it's bits, pieces, voltages, ect...I also say take it to someone who does.
From experience, plates that have had a red glow (even once) are damaged and they will glow again even if the problem is corrected.
Are you running bad speakers? Is the total Ohm load higher than 8 ohms? Tube amps are not to fond of loads lessthan 6 ohms and 4 ohms is like have a fire in the hole for most....ESPECIALLY if you are a musician and like to crank up and hit the bass.
Good chance the bias is way off and running the tubes to hot.
Whoever looks at the amp also needs to check the screen resistors, if they are damaged or shorted, the glow will be on every tube you put in.
Stay away from ebay sellers of tubes ( especially that one with JJ and has Premium plastered on every item) unless you are absolutely sure of their credentials...ebay is full of rip off con men looking for the inexperienced. Very few power tubes or any tubes sold there are matched (and same gm or same emission IS NOT a matched set, only plate current matches are what your trannie want). And, most buyers leave feedback before they really know if they have a crap tube.
From experience, plates that have had a red glow (even once) are damaged and they will glow again even if the problem is corrected.
Are you running bad speakers? Is the total Ohm load higher than 8 ohms? Tube amps are not to fond of loads lessthan 6 ohms and 4 ohms is like have a fire in the hole for most....ESPECIALLY if you are a musician and like to crank up and hit the bass.
Good chance the bias is way off and running the tubes to hot.
Whoever looks at the amp also needs to check the screen resistors, if they are damaged or shorted, the glow will be on every tube you put in.
Stay away from ebay sellers of tubes ( especially that one with JJ and has Premium plastered on every item) unless you are absolutely sure of their credentials...ebay is full of rip off con men looking for the inexperienced. Very few power tubes or any tubes sold there are matched (and same gm or same emission IS NOT a matched set, only plate current matches are what your trannie want). And, most buyers leave feedback before they really know if they have a crap tube.
Wait, what? We dissipated the filter caps, is this not what you are saying? We had the B+ (standby) off. We did test with a meter and the filter caps had about 3 volts which we dissipated anyway.
The two 220k are fine, we replaced the 10k, and the 56k is fine. We measured -35v with the power tubes out and it was reading -35v for both. After changing the 10k resistor it went to -40V for both. With the B+ on it went to -39V. That being said we have not measured the bias supply with the tubes in. Again, its supposed to be -55V.
Just tried the grid screen resistors (the 470k) with the amp off and just through the tube pin holes and am getting like .5k (between pins 4 and 6) for each power amp. Is this serious or am I missing something here?
The tubes that we are testing with have red plated before, so you're saying they will always red plate?
Since we have tested all the resistors in the bias supply, doesn't it have to be either the coupling caps or the bias supply filter caps? I'm almost positive the speaker load is 8 ohms. Had the 4 speaks plugged in every time when testing, and the amp was fine pre-pint glass drop.
Also, finally, if there are any safety precautions other than dissipating the filter caps, let me know. Feel free to add the "if you didn't know this you should take it to someone who does" after.
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