Probably a good idea to add heatsinks to those failure-prone FET's if you get one in that is otherwise repairable. I have installed many hundreds of small bolt-on heat sinks onto transistors and regulators of that size and shape; the only problem being that often times you cannot install the heatsinks without unsoldering the transistors first (and for that you'd have to pull the board), because otherwise when you tighten the bolt with the screwdriver it tends to torque the solder joints and break them or lift the foil pads from the board. We also have spring-loaded heatsinks that can slip over and clip into the transistors, without requiring bolts, but those too require a fair bit of force to push on (even when lubed with heatsink grease) and so once again I usually unsolder transistors first before adding heatsinks.
Yes the other issue is rhat adding the heatsink gives more mass and so it somewhat increases the chances of a dry joint. Not easy to anchor th heatsinks to something solid. Amazing they designed it like this really! I mean, someone actually signed off on it.
@@stuartukguitarampguy5830 , no, we use free-air heatsinks, bolted to the through-hole in the transistors, like metal wings. They don't need to be mounted to the circuit board and they don't interfere with soldering.
Cool video, nice to see the inside of that Black star..😊 Lucky fix there Stuart..Just don't understand why such notorious companies can't carry on supplying those PC boards...!Seems such a waste, and as you say it sends them straight to land fill..Crazy..Thanks for sharing.. Ed..UK.😀
It seems to me, again, that modern engineering is designing to fail. The consumer then has to purchase new. In the long run, it seems better to pay more for a vintage product or some of the newer hand wired amps that can be repaired. It was interesting to see the inside of this amp. Thank you for posting this simple fix with much to learn about the Blackstar.
Hi Michael. I don't think they actually sit down and design something to fail. My belief is that the main driver is cheapness to manufacture (hence lowest price to buy) coupled with a lack of older, very experienced engineers in the design loop. The younger ones keep making the same set of tired old design mistakes.
Great and very valuable information mr stuart, i personally have an issue with a fender frontman 25r, i have a video of the fault, but im thinking could be the J111 JFET in the clean and drive channel selector circuit. The strange thing is that the fault not always appear during operation of the amp. A squeaking noise without connecting the guitar appears (during this issue selector switch of clean and drive channel does not respond and led stays on) and then after a minute the sound damps and dissappear, and the amp is operating normally. Saludos from mexico!
Thanks for the video. My HT5 was accidentally left turned on overnight and now the volume fades away to zero after a couple of minutes of being turned on. Turning off and on again fixes this but only for another couple of minutes. I’ve tested the valves with my Orange tester, both are in good shape. Unless you can suggest anything else I’ll try swapping out the two FETs you mentioned. Thanks for any advice.
PS. I suspect your bad tube (12BH7) had a short between heater and cathode. That might account for the bias resistor being warm but the valve not glowing.
Very nice Video Mr Smith! Always enjoying the in-depth error analysis and all infos. I have exactly the same HT5 (no Reverb) combo and lately the sound fade out after 3-5min. The first 3min is working fine with no issues. Both tubes are ok(tested with valve tester and working fine on another ht5 from a friend). Any help will help!! Thanks in advance!
Thanks a lot! At some point it sounds like the one on your other HT5 video , distorted and fade out sound, so I will check the capacitors also. Thanks again
Does that resistor supply the heaters to both (pre-amp and power) tubes? If so, then it makes sense that the resistor was warm because the pre-amp heater was drawing current, even if the power tubes heater wasn't. Just a thought.
Well done. Perhaps there's enough work (which you could charge a premium on), or soon will be, to justify a decent hot air rework station? Yes, takes a lot of practice, making mistakes etc but looks like you have some donor boards to work with and there's some videos showing proper techniques (northridgefix springs to mind).
Hi Stuart , thanks for these great informative videos,I`m a bg fan. I have one of these HT5`s an early one like this,it just keeps blowing the 500ma internal fuse ,is it likely that the FET`s could cause this kind of short or is it likely to be a power section fault ,such as the diodes do you thin?.Apparently it was left on for weeks! Please keep showing these great vids ,many thanks.
Hi it could be a fair few things if it has been left on all that time. I' do hope you manage to sort it. I cant recall what that fuse does. Is it a mains fuse, i.e. in series with the transformer primary?
@@stuartukguitarampguy5830 No I don`t ,I saw part of a schematic on your video,but I can`t get hold of one anywhere,my current limiter is glowing at about 70volts input on my variac?suspecting either voltage regulator/s or rectifier,never seen so many diodes,circuit if full of them but its a mk1 so they are replacable.
@@electrokatt2246 Hi. At the end of each video there is a brief splash screen. On that screen you will see my email contact details. Send me an email and I'll send you the schematic.
Enjoying your vids! Any advice on why my BlackStar ht5r treble pot may not be working? The treble used to fade/pop in and out, not the treble is non existent and the pot does nothing
I have this early version of the HT5H and i have replaced the tubes after previous owers messed around with it. He ended up turning the biases (the blue ones as in this video) and im not sure which is which and how to adjust. Have u any help/advice. Thank you. And brilliant video 📹 👏
Hi Unfortunately Blackstar provide NO info about how to bias these amps. It;s a complete pain. Here's what I managed to extract from them: Measure voltage across 4,7ohm resistor on cathode. Adjust to mV reading of 46mV. Adjust balance pot for least amount of hum from speaker. 27/1/16 The above seems correct except adjust the balance pot for a symmetrical sine wave output.
Thanks for sharing Stu. I've never seen the guts of the HT5 and found it quite interesting. Did you measure the plate voltages to see how much was on each valve? I didn't see any voltages shown on the schematic and was just curious.
Stuart - love your videos. Do you by any chance know what triggers the bias circuit on an HT 60? I’ve got input and output jacks plugged in, but still getting 95 volts on the output grids.
@@stuartukguitarampguy5830 well its common for the clean channel to bleed dirty. I just purchased one used from Guitar Center used and I am preparing for it to have a small problem. The Haze 40c series seems to have a couple issues that are commonly known and shared online. I will send you another chat once I get it if it has an issue! THANKS!!!!
Hello, I have the same style HT5 board with old, larger components which can be changed. Which points do I measure across for balance adjustment and bias adjustment. I see the pots but no dedicated test points like the newer boards. Thanks
I have the same problem & changed the tubes, worked for a few hours and then it goes mute. I let it cool, turned it on again and sounds good for a few minutes, then sounds deteriorates fastly until mute again. Any advise?
This is what I found on the web.. You can read bias current by measuring voltage drop across the shared cathode resistor of the output stage (R33 at least in plain HT-5) but do note that the cathode resistor is 4.7 ohms (at least in plain HT-5) instead of 1-ohm or 10 ohms so you actually need to employ some very, very basic math to calculate the current from the voltage. Note that since both push-pull halves share the cathode resistor you are reading the current flow through both halves. Current flow through one half is naturally about half of that, given we assume that balance between the tubes isn't way off. Cathode voltage is also hard-limited to maximum value of about 650mV by a parallel diode (though cathode voltage should be well below that under normal operating conditions). So, nothing new here: You measure plate voltage (which should be around 430V - 440V, and which will vary in interaction with the bias setting) and then set bias current according to what's considered safe dissipation for 12BH7. Max plate dissipation for the tube is 3.5W (check datasheet of the EXACT 12BH7 tube type your amp uses to verify the rating) and isn't it common rule of thumb to bias to about 60% - 70% of max plate dissipation for class AB....? I guess the exact setting borderlines subjective issues of preference (hotter or colder bias) and objective issues of safe area of operation (which in the amp is already quite close to limits to begin with). Given that, there is no exact setting and even everything you measure will fall to certain margin of tolerances. Anyway, this is just as ordinary fixed bias tube amp as any other one. If you have a scope you can simply try biasing that removes the crossover notch, assuming the idle dissipation falls to ballpark of safe figures that way. No, Blackstar won't give any guidelines or setup procedures. According to them the amplifier has an automatic bias system and therefore you don't have to worry about biasing replacement tubes. ...And NO: There is no proper "bias voltage" setting. If there was, there would be no point to make that voltage user adjustable to begin with. Where you set bias voltage depends on individual tube characteristics and plate voltage, which you probably should know already. With certain grid bias voltage on a given tube you end up to certain current draw and dissipation figure at given plate voltage. I don't see the point of the threads quoting "stock" / measured grid or bias voltage figures as they are completely meaningless variables. You rebias, they change. That's the point of biasing. Given that, there's much more sense in this thread: www.ax84.com/bbs/dm.php?thread=473836 Reading between the lines, the stock bias setting seems to be insanely hot 7.5mA * 430V = 3.2W (!!!), which is well above the 70% figure of class-AB, in fact, close to cooking the tube at maximum plate dissipation even when idling. Add to that the plate voltage is also very, very close to maximum ratings of the 12BH7 (450V). If there ever was deeper rocket science in proper and safe biasing then rest assured that Blackstar at least didn't seem to give a damn about it. As said, this is a plain fixed bias amp. The only difference is that there's also a bias balance adjustment and that the plate dissipation of a medium power triode such as 12BH7 is about decade lower than that of generic power pentodes or beam tetrodes. Biasing routine, however, is just the same.
great channel and enjoyable vids. can i please ask where you sorce your deoxit d5. i just checked on amazon and it was £40,00 im certainly no tech and its just for cleaning my guitar and amp pots at home. i have just run out of my 1 can of servisol super10 that i have had for years. i see thet there is a rapide branded switch cleaner on amazon ebay etc that is fairly cheap. any experience of this.
Hi JAmes. Woah, that's a crazy price! Keepo using the Servisol, it;s perfectly good. Tbh I usually just go on eBay and buy half a dozen cans. I think I paid about £15 a can last time?
I have one of these amps, and it's a shame they don't make the boards, but on the upside, I don't really think it's a great amp. Clean channel is dull as they come. OD channel doesn't seem particularly good at anything. To me it's milk toast.
That's why Blackstar are frowned upon in the US. They have no service, nor few to no parts. They are quite literally considered a disposable amp. Also, they use cheap inferior parts for the most part. The thin small traces and pads don't help either. Not roadworthy amps.
@@stuartukguitarampguy5830 The amp may have been dropped, causing the heater filament to get shorted or broken. I guess you could check the old tube with a multimeter for continuity. Or the tube socket may have had some corrosion and the act of removing the tube and installing a new one temporarily "cleaned" the corrosion.
Hi Mark, yes it's SO annoying that they can design and release an amp with what I'd call 'schoolboy' errors in the design. Same with Fender though, really hot ppwer resistors mounted onto the board so they burn the board and come loose. Ditto zener diodes. It's just totally basic stuff.
Probably a good idea to add heatsinks to those failure-prone FET's if you get one in that is otherwise repairable. I have installed many hundreds of small bolt-on heat sinks onto transistors and regulators of that size and shape; the only problem being that often times you cannot install the heatsinks without unsoldering the transistors first (and for that you'd have to pull the board), because otherwise when you tighten the bolt with the screwdriver it tends to torque the solder joints and break them or lift the foil pads from the board. We also have spring-loaded heatsinks that can slip over and clip into the transistors, without requiring bolts, but those too require a fair bit of force to push on (even when lubed with heatsink grease) and so once again I usually unsolder transistors first before adding heatsinks.
Yes the other issue is rhat adding the heatsink gives more mass and so it somewhat increases the chances of a dry joint. Not easy to anchor th heatsinks to something solid. Amazing they designed it like this really! I mean, someone actually signed off on it.
@@stuartukguitarampguy5830 , no, we use free-air heatsinks, bolted to the through-hole in the transistors, like metal wings. They don't need to be mounted to the circuit board and they don't interfere with soldering.
Always well detailed videos on troubleshooting tube amps a wealth of knowledge. Thank you for taking the time in uploading these videos.
Thanks Wayne
I've been away recording a new album, but I'm here again to see your videos, Regards, Stuart.
Excellent, glad to have you back!
Boring Saturday evening and then Young Stuart turns up!! Great!!
I only do it to be called 'young Stuart'. Had my 70th last week, Aaaaggghhhhh......
@@stuartukguitarampguy5830 A mere whippersnapper!! x
@@ferraridinoman 70 is the new 69!
Stuart, always look forward to your videos even if it turns out to be just a valve replacement.
Cheers Jerry!
I like the tone of this little amp.
Yes they are great little amps but getting harder to repair now that Blackstar have stopped supplying replacement boards for them.
Cool video, nice to see the inside of that Black star..😊 Lucky fix there Stuart..Just don't understand why such notorious companies can't carry on supplying those PC boards...!Seems such a waste, and as you say it sends them straight to land fill..Crazy..Thanks for sharing.. Ed..UK.😀
Glad you enjoyed it Ed.
Thanks Berkshire Amp Repair. Appreciate the videos.
Glad you enjoyed it!
U got a schematic ! Good for u . I understand that doesn't happen very often .
Yes, gold dust. We wouldn;t want anyone COPYING our crap design now would we???
It seems to me, again, that modern engineering is designing to fail. The consumer then has to purchase new. In the long run, it seems better to pay more for a vintage product or some of the newer hand wired amps that can be repaired.
It was interesting to see the inside of this amp. Thank you for posting this simple fix with much to learn about the Blackstar.
Hi Michael. I don't think they actually sit down and design something to fail. My belief is that the main driver is cheapness to manufacture (hence lowest price to buy) coupled with a lack of older, very experienced engineers in the design loop. The younger ones keep making the same set of tired old design mistakes.
Great and very valuable information mr stuart, i personally have an issue with a fender frontman 25r, i have a video of the fault, but im thinking could be the J111 JFET in the clean and drive channel selector circuit. The strange thing is that the fault not always appear during operation of the amp. A squeaking noise without connecting the guitar appears (during this issue selector switch of clean and drive channel does not respond and led stays on) and then after a minute the sound damps and dissappear, and the amp is operating normally. Saludos from mexico!
Hi I hope you manage to get it fixed!
Thanks for the video. My HT5 was accidentally left turned on overnight and now the volume fades away to zero after a couple of minutes of being turned on. Turning off and on again fixes this but only for another couple of minutes. I’ve tested the valves with my Orange tester, both are in good shape. Unless you can suggest anything else I’ll try swapping out the two FETs you mentioned. Thanks for any advice.
PS. I suspect your bad tube (12BH7) had a short between heater and cathode. That might account for the bias resistor being warm but the valve not glowing.
Very nice Video Mr Smith! Always enjoying the in-depth error analysis and all infos. I have exactly the same HT5 (no Reverb) combo and lately the sound fade out after 3-5min. The first 3min is working fine with no issues. Both tubes are ok(tested with valve tester and working fine on another ht5 from a friend). Any help will help!! Thanks in advance!
The only other thing I look at on these amps is the two power FETs. They get very hot. Replacing these MAY sort the problem.
Thanks a lot! At some point it sounds like the one on your other HT5 video , distorted and fade out sound, so I will check the capacitors also.
Thanks again
Does that resistor supply the heaters to both (pre-amp and power) tubes? If so, then it makes sense that the resistor was warm because the pre-amp heater was drawing current, even if the power tubes heater wasn't. Just a thought.
Pretty certain it is exactly this
Well done. Perhaps there's enough work (which you could charge a premium on), or soon will be, to justify a decent hot air rework station? Yes, takes a lot of practice, making mistakes etc but looks like you have some donor boards to work with and there's some videos showing proper techniques (northridgefix springs to mind).
Hi Ralph. Nah, I don;t want to do that. Trying to do fewer amps not more! 40 years ago, maybe!
Hi Stuart , thanks for these great informative videos,I`m a bg fan. I have one of these HT5`s an early one like this,it just keeps blowing the 500ma internal fuse ,is it likely that the FET`s could cause this kind of short or is it likely to be a power section fault ,such as the diodes do you thin?.Apparently it was left on for weeks! Please keep showing these great vids ,many thanks.
Hi it could be a fair few things if it has been left on all that time. I' do hope you manage to sort it. I cant recall what that fuse does. Is it a mains fuse, i.e. in series with the transformer primary?
@@stuartukguitarampguy5830 Yes it is ,primary and secondaries test fine,the short I`m thinking may be in the power supply though?
@@electrokatt2246 Yes it's just a question of working through it now. Do you have a schematic?
@@stuartukguitarampguy5830 No I don`t ,I saw part of a schematic on your video,but I can`t get hold of one anywhere,my current limiter is glowing at about 70volts input on my variac?suspecting either voltage regulator/s or rectifier,never seen so many diodes,circuit if full of them but its a mk1 so they are replacable.
@@electrokatt2246 Hi. At the end of each video there is a brief splash screen. On that screen you will see my email contact details. Send me an email and I'll send you the schematic.
Enjoying your vids!
Any advice on why my BlackStar ht5r treble pot may not be working? The treble used to fade/pop in and out, not the treble is non existent and the pot does nothing
No, that's odd. Can you get on the back of the pot with a meter to see if there is any resistance there?
I have this early version of the HT5H and i have replaced the tubes after previous owers messed around with it. He ended up turning the biases (the blue ones as in this video) and im not sure which is which and how to adjust. Have u any help/advice. Thank you. And brilliant video 📹 👏
Hi Unfortunately Blackstar provide NO info about how to bias these amps. It;s a complete pain. Here's what I managed to extract from them:
Measure voltage across 4,7ohm resistor on cathode. Adjust to mV reading of 46mV. Adjust balance pot for least amount of hum from speaker.
27/1/16 The above seems correct except adjust the balance pot for a symmetrical sine wave output.
@@stuartukguitarampguy5830 thank you
Thanks for sharing Stu. I've never seen the guts of the HT5 and found it quite interesting. Did you measure the plate voltages to see how much was on each valve? I didn't see any voltages shown on the schematic and was just curious.
Ah, actually I didn't, that was a mistake. My guess is lowish, say about 250V for preamp and 300V for 12BH7.
Stuart - love your videos. Do you by any chance know what triggers the bias circuit on an HT 60? I’ve got input and output jacks plugged in, but still getting 95 volts on the output grids.
Hi Tim. Unfortunately not and I don't have the schematic for that amp either. I hope you get it sorted.
do you have a Marshall Haze repair video? Thanks for all the information in your videos :)
Hi, no that's npt an amp I have ever come across. What is the issue?
@@stuartukguitarampguy5830 well its common for the clean channel to bleed dirty. I just purchased one used from Guitar Center used and I am preparing for it to have a small problem. The Haze 40c series seems to have a couple issues that are commonly known and shared online. I will send you another chat once I get it if it has an issue! THANKS!!!!
Hi I have this amp and suddenly began to low and high the volume what can be ?
Hello, I have the same style HT5 board with old, larger components which can be changed. Which points do I measure across for balance adjustment and bias adjustment. I see the pots but no dedicated test points like the newer boards. Thanks
Hi SSCott Alas I don't have this info.
@@stuartukguitarampguy5830 No problem, thanks.
I have the same problem & changed the tubes, worked for a few hours and then it goes mute. I let it cool, turned it on again and sounds good for a few minutes, then sounds deteriorates fastly until mute again. Any advise?
It may be the FETs as detailed in this video.
hi, can you tell what are the spots for biasing the tubes ?
This is what I found on the web..
You can read bias current by measuring voltage drop across the shared cathode resistor of the output stage (R33 at least in plain HT-5) but do note that the cathode resistor is 4.7 ohms (at least in plain HT-5) instead of 1-ohm or 10 ohms so you actually need to employ some very, very basic math to calculate the current from the voltage. Note that since both push-pull halves share the cathode resistor you are reading the current flow through both halves. Current flow through one half is naturally about half of that, given we assume that balance between the tubes isn't way off. Cathode voltage is also hard-limited to maximum value of about 650mV by a parallel diode (though cathode voltage should be well below that under normal operating conditions).
So, nothing new here: You measure plate voltage (which should be around 430V - 440V, and which will vary in interaction with the bias setting) and then set bias current according to what's considered safe dissipation for 12BH7. Max plate dissipation for the tube is 3.5W (check datasheet of the EXACT 12BH7 tube type your amp uses to verify the rating) and isn't it common rule of thumb to bias to about 60% - 70% of max plate dissipation for class AB....? I guess the exact setting borderlines subjective issues of preference (hotter or colder bias) and objective issues of safe area of operation (which in the amp is already quite close to limits to begin with). Given that, there is no exact setting and even everything you measure will fall to certain margin of tolerances. Anyway, this is just as ordinary fixed bias tube amp as any other one. If you have a scope you can simply try biasing that removes the crossover notch, assuming the idle dissipation falls to ballpark of safe figures that way.
No, Blackstar won't give any guidelines or setup procedures. According to them the amplifier has an automatic bias system and therefore you don't have to worry about biasing replacement tubes.
...And NO: There is no proper "bias voltage" setting. If there was, there would be no point to make that voltage user adjustable to begin with. Where you set bias voltage depends on individual tube characteristics and plate voltage, which you probably should know already. With certain grid bias voltage on a given tube you end up to certain current draw and dissipation figure at given plate voltage. I don't see the point of the threads quoting "stock" / measured grid or bias voltage figures as they are completely meaningless variables. You rebias, they change. That's the point of biasing.
Given that, there's much more sense in this thread:
www.ax84.com/bbs/dm.php?thread=473836
Reading between the lines, the stock bias setting seems to be insanely hot 7.5mA * 430V = 3.2W (!!!), which is well above the 70% figure of class-AB, in fact, close to cooking the tube at maximum plate dissipation even when idling. Add to that the plate voltage is also very, very close to maximum ratings of the 12BH7 (450V). If there ever was deeper rocket science in proper and safe biasing then rest assured that Blackstar at least didn't seem to give a damn about it.
As said, this is a plain fixed bias amp. The only difference is that there's also a bias balance adjustment and that the plate dissipation of a medium power triode such as 12BH7 is about decade lower than that of generic power pentodes or beam tetrodes. Biasing routine, however, is just the same.
great channel and enjoyable vids.
can i please ask where you sorce your deoxit d5. i just checked on amazon and it was £40,00
im certainly no tech and its just for cleaning my guitar and amp pots at home. i have just run out of my 1 can of servisol super10 that i have had for years.
i see thet there is a rapide branded switch cleaner on amazon ebay etc that is fairly cheap. any experience of this.
Hi JAmes. Woah, that's a crazy price! Keepo using the Servisol, it;s perfectly good. Tbh I usually just go on eBay and buy half a dozen cans. I think I paid about £15 a can last time?
@@stuartukguitarampguy5830 thanks for the reply Stuart.
i have actually just run out of servisol. any opinion on the rapide brand switch cleaner.
@@jameshunt4339 I HAven;t heard of that one. My unscientific gut opinion is that pretty much any solvent cleaning spray will do!
👍
Hello I try to solve problem on HT5RH: I change TR3 but when I turn on the amp , R39 (22k) melts and fall !! Have you got an idea? thanks a lot
Franck
That would only happen if TR3 was a short I think. Have you measured it? Not easy to fix these amps.
@@stuartukguitarampguy5830 thanks for your answer: what's short for you? What can I measure: the gain? best regards, Franck
Hi Franck I meant short circuit you shoud be able to check it with a meter. Tbh I think this might be one for an amp tech now.
Stuart if i send you a full marshall stack sized letter are you ok to recieve it?
YEs that should be fine. Do you have my address?
I have one of these amps, and it's a shame they don't make the boards, but on the upside, I don't really think it's a great amp. Clean channel is dull as they come. OD channel doesn't seem particularly good at anything. To me it's milk toast.
Yes I've heard that from a few people.
That's why Blackstar are frowned upon in the US. They have no service, nor few to no parts. They are quite literally considered a disposable amp. Also, they use cheap inferior parts for the most part. The thin small traces and pads don't help either. Not roadworthy amps.
You're summed it up perfectly!
Shorted filament?
Yes could have been. That's the only thing that makes asense. How on earth does a filament get shorted though???
@@stuartukguitarampguy5830 Heater cathode short I would guess?
@@stuartukguitarampguy5830 The amp may have been dropped, causing the heater filament to get shorted or broken. I guess you could check the old tube with a multimeter for continuity. Or the tube socket may have had some corrosion and the act of removing the tube and installing a new one temporarily "cleaned" the corrosion.
I have a blackstar amp now and what i can tell is not buying any more of them.
Hi Mark, yes it's SO annoying that they can design and release an amp with what I'd call 'schoolboy' errors in the design. Same with Fender though, really hot ppwer resistors mounted onto the board so they burn the board and come loose. Ditto zener diodes. It's just totally basic stuff.
One good reason NOT to buy Blackstar!! Sod 'em!
They make good hand wired amps, toss the boards and make a champ out of it
@@robertstrickland2121 That's actually quite a good idea!!!
@@stuartukguitarampguy5830 if anyone in west Alabama has a dead one,let me know, I’ll take it rather than trashing it.
@@robertstrickland2121 Another viewer suggested chucking the board and converting it to a champ. Great idea!