Hello friends!
So, I have an amp that I built and have been using for live gigs. It's fixed bias & tube rectified. Can run KT66/EL34/6L6GC but right now has EL34s installed. The amp had been performing flawlessly and was dead quiet at idle.
The other day between soundcheck & the show the speaker cable became ever-so-slightly unplugged, not enough to be noticeable from a few feet away. I tried playing my guitar for a minute or two until the problem was discovered - there was obviously no sound while it was unplugged - but once plugged back in it worked fine for the rest of the time I used it (about 15 mins). Put it away and the next day upon power up there was no sound. Turns out the HT fuse had blown. It is wired up between the center tap of the HV winding and ground. Replaced the fuse, amp worked fine for soundcheck (prob 20 mins). Powered it up again later and it blew the HT fuse again. Removed the chassis from the box (which I had done to replace the HT fuse each time as it is not accessible from the outside) and this time left it out, installed another fuse, installed a different known-good pair of EL34s, powered it up & watched. The fuse held. Took it off of standby & still held. External bias test points showed the new set of EL34s were drawing current nicely.
Put the amp back in standby, powered it off, waited about 30 seconds, powered it up again and it blew the HT fuse immediately (this is without standby in "ON" position aka the amp was in standby when the fuse blew). It seemed this is what happened the other times I had replaced the fuse and it initially held but then blew on second power up. Throughout entire process the mains fuse never popped.
I'm painfully aware that the cardinal sin of tube amp world is to run signal into the amplifier without a connected load, and that the results can vary from simply a blown fuse to arcing on a tube socket and even a blown output transformer.
I inspected the output tube sockets for signs of arcing and couldn't see anything out of the ordinary, but that doesn't mean it didn't happen.
I'd like to think that if the output transformer did indeed suffer from any flyback voltages it would not be operational at all. Yet, the amp did work & sound normal after being plugged back into a speaker load the night it occurred as well as the next day after the HT fuse was first replaced.
It's not the output tubes since they were replaced and there is no faulty wiring as the amp was working perfectly before the unfortunate incident happened.
Any insight you guys might have would be incredibly appreciated!!
I fear the worst...
Moderators: pompeiisneaks, Colossal
Re: I fear the worst...
I haven't tried this, but I believe it is a proven test.
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Steve
Re: I fear the worst...
I wonder if you may not have any problem with the OT. But rather with the rectifier. Using a standby switch like this is "hot switching" the rectifier tube and is/was considered a bad idea by the folks who made rectifiers. See:
http://www.valvewizard.co.uk/standby.html
When a fuse is on the center tap of the HT winding and blows the HT can still find it's way to ground via the bias circuit if the bias voltage is taken off of another tap on the same PT winding. If your amp has a separate tap for the bias circuit then when the fuse blows the caps in the bias circuit may have been taken out.
It sounds like all of the cases where your fuse blows may be under the above quoted conditions. So, I don't think the output transformer is even involved. I'd try two things:
- Replace the rectifier with a know good one.
- Check the caps in the bias circuit for shorts.
Re: I fear the worst...
Thanks guys!
Rectifier installed in amp at time of initial folly was a 5V3A. After second fuse blew it was swapped for a known good 5AR4 (this was for the test fire with the chassis out of the box). Biased new output tubes appropriately and it held until I switched the standby and power off, then blew when I flipped power switch again. This seemed curious because the HV winding was in standby therefore theoretically there should only have been live voltage on the tube filaments (5V & 6.3V)
PT does not have separate bias winding. Rather, bias supply is taken from pin 6 of rectifier tube a-la old Marshall style.
Rectifier installed in amp at time of initial folly was a 5V3A. After second fuse blew it was swapped for a known good 5AR4 (this was for the test fire with the chassis out of the box). Biased new output tubes appropriately and it held until I switched the standby and power off, then blew when I flipped power switch again. This seemed curious because the HV winding was in standby therefore theoretically there should only have been live voltage on the tube filaments (5V & 6.3V)
PT does not have separate bias winding. Rather, bias supply is taken from pin 6 of rectifier tube a-la old Marshall style.
Re: I fear the worst...
Where exactly is this standby switch located in the circuit? Do you have a schematic? If the HT fuse is blowing when the standby switch is not engaged then there are only a limited number of ways that enough current can be drawn to take out the fuse.
Where is the bias circuit connected to the PT secondary winding? If the bias circuit is connected to the AC side of the rectifier (I suspect it is) then the bias circuit is always on (a good thing) regardless of the state of the standby switch. If the caps in the bias circuit were taken out when the center tap fuse blew then there may be a short to ground in your bias circuit. I would check the caps there and see what their resistance to ground now is.
Also, I would build/use a lightbulb current limiter until you have things sorted out. It may save you fuses and other parts.
Mike
Where is the bias circuit connected to the PT secondary winding? If the bias circuit is connected to the AC side of the rectifier (I suspect it is) then the bias circuit is always on (a good thing) regardless of the state of the standby switch. If the caps in the bias circuit were taken out when the center tap fuse blew then there may be a short to ground in your bias circuit. I would check the caps there and see what their resistance to ground now is.
Also, I would build/use a lightbulb current limiter until you have things sorted out. It may save you fuses and other parts.
Mike
Re: I fear the worst...
You can shorten this up a whole lot by just telling use about the amp and its history. A whole lot of the questions you have received back and will continue to receive back will be along the lines of "Does it have [X]?" or "Do you have [Y}?", with the intent of trying to understand the situation so we can make good guesses rather than tossing darts at a dart board - over our collective shoulders, while holding our eyes shut.
It will speed things up a lot if you can just tell us:
> What make and model of amp?
> Can you provide a schematic for it, or a link to one?
> How old is the amp?
> Did you buy it new? Was there a previous owner? How long have you had it.
> When was it last recapped and retubed?
> Has this amp ever had any other funny stuff happen?
> How long and how loud do you usually play it?
The reason all this matters is that some amplifiers have what I would call "reckless designs". Some of the Marshalls with the bias supply switched off by standby so every on/off cycle of the standby switch is a brutal no-holds-barred power on with fully hot filaments and no off-bias to do anything about it. Many AC-30s will pop rectifiers and filters after the standby is used; this is so bad that the common if heretical wisdom is to never use standby on an AC-30. There are many other (and stranger) examples. Lord only knows how "clever" the boutique amp builder-for-profit may have been with an amp, or how consistent the build quality is.
Almost forgot - calm down. Although running a tube amp unloaded isn't a good idea. it is not instantly fatal. Some amps simply don't care and suffer no damage from running unloaded. Others have fairly quick problems from it. Depends on the amp, the design, and its condition. And although many people instantly leap to "it's my output transformer, isn't it", it very rarely is the OT. Can be, but that's not the first thing to suspect.
Got some info for us?
It will speed things up a lot if you can just tell us:
> What make and model of amp?
> Can you provide a schematic for it, or a link to one?
> How old is the amp?
> Did you buy it new? Was there a previous owner? How long have you had it.
> When was it last recapped and retubed?
> Has this amp ever had any other funny stuff happen?
> How long and how loud do you usually play it?
The reason all this matters is that some amplifiers have what I would call "reckless designs". Some of the Marshalls with the bias supply switched off by standby so every on/off cycle of the standby switch is a brutal no-holds-barred power on with fully hot filaments and no off-bias to do anything about it. Many AC-30s will pop rectifiers and filters after the standby is used; this is so bad that the common if heretical wisdom is to never use standby on an AC-30. There are many other (and stranger) examples. Lord only knows how "clever" the boutique amp builder-for-profit may have been with an amp, or how consistent the build quality is.
Almost forgot - calm down. Although running a tube amp unloaded isn't a good idea. it is not instantly fatal. Some amps simply don't care and suffer no damage from running unloaded. Others have fairly quick problems from it. Depends on the amp, the design, and its condition. And although many people instantly leap to "it's my output transformer, isn't it", it very rarely is the OT. Can be, but that's not the first thing to suspect.
Got some info for us?
Re: I fear the worst...
R.G. Thank you for your reply and interest!
To your questions:
> I built the amp myself and it has been used for rehearsals for a couple of months as well as a few live gigs. Maybe 1-2 hours at a time @ high volume. Until the "operation w/o load" incident behaved perfectly.
> I don't have a schematic, but I can say the standby switch is between the rectifier and the first filter cap. I know this is a questionable design when using a gradual start rectifier and often times standby switches are used just because. I have more recent build w/me that uses a 5AR4 and has no standby switch. Bias circuit is lifted from Marshall JTM45. If preamp info matters, V1 is a cascaded 6SL7 which feeds into a 6SJ7 and then another 6SL7 set up as long tailed pair phase inverter into the EL34 output section. There is a separate input directly to the 6SJ7 which bypasses the first tube & has it's own volume control. V1 has a volume control as well, and there is a single tone control a-la 18 watt Marshall.
> Build finished this spring and used regularly since then.
> No other funny stuff
> Have run the amp 1-3 hours a number of times without any issue.
To your questions:
> I built the amp myself and it has been used for rehearsals for a couple of months as well as a few live gigs. Maybe 1-2 hours at a time @ high volume. Until the "operation w/o load" incident behaved perfectly.
> I don't have a schematic, but I can say the standby switch is between the rectifier and the first filter cap. I know this is a questionable design when using a gradual start rectifier and often times standby switches are used just because. I have more recent build w/me that uses a 5AR4 and has no standby switch. Bias circuit is lifted from Marshall JTM45. If preamp info matters, V1 is a cascaded 6SL7 which feeds into a 6SJ7 and then another 6SL7 set up as long tailed pair phase inverter into the EL34 output section. There is a separate input directly to the 6SJ7 which bypasses the first tube & has it's own volume control. V1 has a volume control as well, and there is a single tone control a-la 18 watt Marshall.
> Build finished this spring and used regularly since then.
> No other funny stuff
> Have run the amp 1-3 hours a number of times without any issue.
Re: I fear the worst...
But I do have a gut shot!
This amp is actually a rebuild of an amp I built last year. It worked fine but was just two parallel wired 6SL7s into ltp & output. I changed the wiring to cascade the sections of the first 6SL7 and change the second tube to 6SJ7. I added a volume control on the front panel and the input to the 6SJ7 is on the back panel. It might not be the neatest thing, but the wiring is 3D I utilize space when wires will cross and I think that is part of why it ran so quiet.
This amp is actually a rebuild of an amp I built last year. It worked fine but was just two parallel wired 6SL7s into ltp & output. I changed the wiring to cascade the sections of the first 6SL7 and change the second tube to 6SJ7. I added a volume control on the front panel and the input to the 6SJ7 is on the back panel. It might not be the neatest thing, but the wiring is 3D I utilize space when wires will cross and I think that is part of why it ran so quiet.
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Re: I fear the worst...
If it were mine, I'd put a light bulb limiter on it (to keep down on the number of blown fuses to buy) and then start gently poking on wires and stressing solder joints with a piece of wooden doweling or a chopstick.
If the speaker output plug was partly pulled out, start there and see if you can find jack/socket damage from the kick that pulled it. There's also a chance that there's a solder joint in just the wrong place that opens up under stress. Solder creeps over time, so a joint that was accidentally not put together properly and also had some mechanical stress on it will sometimes pull open in a stress fracture. Darned hard to find, though. Since you built it, you might be comfortable just remelting every joint to eliminate possibles. Also pay attention especially to every wire and joint on the power side of things - caps and output tube sockets.
Unless you used NOS or salvaged caps, it's unlikely that you have a cap going shorted, then quasi-healing. Possible, but unlikely.
Does it take a similar amount of time to go to fuse blowing from cold each time? That would indicate a thermal.
If the speaker output plug was partly pulled out, start there and see if you can find jack/socket damage from the kick that pulled it. There's also a chance that there's a solder joint in just the wrong place that opens up under stress. Solder creeps over time, so a joint that was accidentally not put together properly and also had some mechanical stress on it will sometimes pull open in a stress fracture. Darned hard to find, though. Since you built it, you might be comfortable just remelting every joint to eliminate possibles. Also pay attention especially to every wire and joint on the power side of things - caps and output tube sockets.
Unless you used NOS or salvaged caps, it's unlikely that you have a cap going shorted, then quasi-healing. Possible, but unlikely.
Does it take a similar amount of time to go to fuse blowing from cold each time? That would indicate a thermal.
Re: I fear the worst...
If the fuse on the HT secondary center tap is blowing when the standby switch is open (no connection) then there are only two things connected to the HT secondary winding that could be shorting and blowing the fuse. Either the rectifier tube or the bias circuit. Nothing else should be (unless there are more details we don't know) connected to this winding.
One could simply pull the rectifier tube to eliminate that possibility. My guess is that there is a (possibly intermittent) short in the bias circuit. If the rectifier is pulled and the bias circuit is disconnected and the fuse still blows with only the center tap of the HT then connected there is bad news for your PT.
One could simply pull the rectifier tube to eliminate that possibility. My guess is that there is a (possibly intermittent) short in the bias circuit. If the rectifier is pulled and the bias circuit is disconnected and the fuse still blows with only the center tap of the HT then connected there is bad news for your PT.
Re: I fear the worst...
Yep, all true. I was hoping to get the info on what was actually in the amp, details of the biasing circuit, etc. so we could make better educated guesses. Plus, it's easy to miss that the fuse blew exactly as the standby switch was flipped, as that's when any huge surges into now-empty filter caps happens.
My response to mystery situations is to get the subject into captivity and set up so debugging can actually find a specific problem.
My response to mystery situations is to get the subject into captivity and set up so debugging can actually find a specific problem.
Re: I fear the worst...
Thanks again guys!
I'm on tour right now so am without the luxury of my bench at home which has current limiter, variac, etc.
But I want this thing up & running if at all possible so I have no problem buying a crate full of fuses and tearing through them in the debugging process.
Will test all filter & bias caps today. More soon...
I'm on tour right now so am without the luxury of my bench at home which has current limiter, variac, etc.
But I want this thing up & running if at all possible so I have no problem buying a crate full of fuses and tearing through them in the debugging process.
Will test all filter & bias caps today. More soon...
Re: I fear the worst...
If you pull all the tubes ....besides the rectifier tube... and turn it on.. does it still blow the fuse??...and what voltage do you have on the plates of the power tubes?? Start simple first... then work your way up to a bigger problem. It may be simply bad power tubes... even tho you put in a different set ...there is always a chance ...the new ones are bad also...
Last edited by Cameron on Mon Jul 31, 2017 7:49 pm, edited 1 time in total.
Re: I fear the worst...
Well well well...
It would APPEAR that the issue was unrelated to the speaker cable being mistakenly unplugged during use. Opened up the amp again today and took a closer look. Filter caps all show resistance/seem ok. Bias electrolytic caps seem ok too. Hmm...
The speaker cable was unplugged at the cab, not the amp, still I started checking for stress points, etc around speaker jacks. If you look at the gut shot posted earlier you will see that I used a trick I read about (maybe on Randall Aiken's site?) of grounding the speaker jacks at the same point as the first filter cap. Turns out the red wire running from the speaker jacks to ground had come unsoldered somehow. It was still in the hole on the terminal strip, but tugging on it came free very easily.
So - re-soldered the ground wire, replaced the HT fuse, fired it up and BOOM! Working properly & sounding great like before. Used it for soundcheck, perfect. Powered it down & back on. Still fine. And again, still fine, and again etc. Seems it's good! (For now at least, ha ha)
Thanks to everyone for all your help as usual, especially R.G. Much respect!
It would APPEAR that the issue was unrelated to the speaker cable being mistakenly unplugged during use. Opened up the amp again today and took a closer look. Filter caps all show resistance/seem ok. Bias electrolytic caps seem ok too. Hmm...
The speaker cable was unplugged at the cab, not the amp, still I started checking for stress points, etc around speaker jacks. If you look at the gut shot posted earlier you will see that I used a trick I read about (maybe on Randall Aiken's site?) of grounding the speaker jacks at the same point as the first filter cap. Turns out the red wire running from the speaker jacks to ground had come unsoldered somehow. It was still in the hole on the terminal strip, but tugging on it came free very easily.
So - re-soldered the ground wire, replaced the HT fuse, fired it up and BOOM! Working properly & sounding great like before. Used it for soundcheck, perfect. Powered it down & back on. Still fine. And again, still fine, and again etc. Seems it's good! (For now at least, ha ha)
Thanks to everyone for all your help as usual, especially R.G. Much respect!
Re: I fear the worst...
Just call it a lucky guess based on the thousands of bum solder joints I've made on my own equipment. And notice that I guessed other stuff that wasn't the problem, too. 
An amp tech friend of mine told me he was brought a Fender amp that had started cutting out at random intervals. Upon investigation, the output tube sockets had the leads and wires that went to the sockets stuck through the eyelets on the terminals and properly wound around the terminal. It's just that they were never soldered. It worked for over 20 years like that.
The very worst ones to find are hairline cracks in components ( I've found one resistor and one cap like that over the years) and broken wires >> inside the insulation << . That one took a while to find. I think it happened when the wire was stripped with a blade-type stripper and the insulation was pushed over the nick the blades left while it was being constructed. Some years later, fatigue cracking broke the copper, but left the insulation holding the ends together.
An amp tech friend of mine told me he was brought a Fender amp that had started cutting out at random intervals. Upon investigation, the output tube sockets had the leads and wires that went to the sockets stuck through the eyelets on the terminals and properly wound around the terminal. It's just that they were never soldered. It worked for over 20 years like that.
The very worst ones to find are hairline cracks in components ( I've found one resistor and one cap like that over the years) and broken wires >> inside the insulation << . That one took a while to find. I think it happened when the wire was stripped with a blade-type stripper and the insulation was pushed over the nick the blades left while it was being constructed. Some years later, fatigue cracking broke the copper, but left the insulation holding the ends together.