Thanks Chris,
Since I have dealt with this elsewhere (although a long time ago), I will summarise.
It is difficult to note the plate gauge (unless you are wealthy enough to smash one for the purpose of investigating!), but I look at (1) the G1 cooling fins (on top) and (2) the general mounting below. For the classic GE 6L6GC (7581A etc.) the mounting pins are relatively short and thick. I found them thinner for the Ruskies, and sometimes the general construction seemed just a little, er, well, less rugged.
We have an importer of tubes here in Johannesburg RSA, not far from me. (He is an insurance appraiser and does this for a hobby.)He had Sovtek WXT, EH, Svetlana etc., and I found the Svetlana best looking, but relatively expensive. They had rugged grooved anodes, the classic oval shape. Then came the rest with box shaped anodes (no grooves or ridges), etc. Of those the EHs seemed to look somewhat more rugged, but I am now getting to subjective impressions. This guy sells 95% of his power valves to music bands of one kind or another. Imports are not matched, but he will match for a nominal fee.
Mentioning cost would not mean a lot; the EHs go for the equivalent of about US $18, which might appear cheap - but keep in mind that our whole economy here is a little different to yours.
Regards
Johan
Since I have dealt with this elsewhere (although a long time ago), I will summarise.
It is difficult to note the plate gauge (unless you are wealthy enough to smash one for the purpose of investigating!), but I look at (1) the G1 cooling fins (on top) and (2) the general mounting below. For the classic GE 6L6GC (7581A etc.) the mounting pins are relatively short and thick. I found them thinner for the Ruskies, and sometimes the general construction seemed just a little, er, well, less rugged.
We have an importer of tubes here in Johannesburg RSA, not far from me. (He is an insurance appraiser and does this for a hobby.)He had Sovtek WXT, EH, Svetlana etc., and I found the Svetlana best looking, but relatively expensive. They had rugged grooved anodes, the classic oval shape. Then came the rest with box shaped anodes (no grooves or ridges), etc. Of those the EHs seemed to look somewhat more rugged, but I am now getting to subjective impressions. This guy sells 95% of his power valves to music bands of one kind or another. Imports are not matched, but he will match for a nominal fee.
Mentioning cost would not mean a lot; the EHs go for the equivalent of about US $18, which might appear cheap - but keep in mind that our whole economy here is a little different to yours.
Regards
Johan
Stuart,
I have not checked all the posts back: I presume you talk of 100 ohm screen-to-anode resistors - those could be 1/2 W or less; they only dissipate about 50 mW or so.
Btw, I asked some time before: You do not have screen taps on the output transformer? (Would be advantagous if you could use UL.)
Hard-wiring to triode would simply mean mounting the 100 ohm resistors between screen and anode posts on the socket, but a circuit might be useful just to check whether other things like capacitors lurk around the output transformer. There is also the question of altering feedback component values if you have.
Regards.
I have not checked all the posts back: I presume you talk of 100 ohm screen-to-anode resistors - those could be 1/2 W or less; they only dissipate about 50 mW or so.
Btw, I asked some time before: You do not have screen taps on the output transformer? (Would be advantagous if you could use UL.)
Hard-wiring to triode would simply mean mounting the 100 ohm resistors between screen and anode posts on the socket, but a circuit might be useful just to check whether other things like capacitors lurk around the output transformer. There is also the question of altering feedback component values if you have.
Regards.
Hi guys,
Heres a coupe of pics of what I have done...
http://www.fossilfarm.co.uk/stuebay/board1.jpg
http://www.fossilfarm.co.uk/stuebay/board2.jpg
The scorch mark went right the way through the board (literally, its almost a hole), hence the black patch still there. Ive checked the caps and resistors etc , all appear ok.
I have a new relay coming, but the existing one works fine, and the whole amp SEEMS to be fine.
Please let me know if I have got my wires crossed with some of the advice given, and whether my solution is more dangerous than an unattended chip pan.
The resistor is 1w, but I have smaller ones if I should make a change. I chose a 3 amp quick blow fuse, looking at the 5v on the relay.
Another thing, i finally got the darned manual translated from chinese and it turns out the bias was set too high from the factory. The manual suggests drawing 40-45mA, before the explosion, they were running at over 55mA (without me tinkering with the settings). Could this have contributed to the fireworks?
Im remaining vigilant. In a couple of days I'll check the transformers. But theres only so much PCB green I can take in one day...
Many thanks,
Stu.
Heres a coupe of pics of what I have done...
http://www.fossilfarm.co.uk/stuebay/board1.jpg
http://www.fossilfarm.co.uk/stuebay/board2.jpg
The scorch mark went right the way through the board (literally, its almost a hole), hence the black patch still there. Ive checked the caps and resistors etc , all appear ok.
I have a new relay coming, but the existing one works fine, and the whole amp SEEMS to be fine.
Please let me know if I have got my wires crossed with some of the advice given, and whether my solution is more dangerous than an unattended chip pan.
The resistor is 1w, but I have smaller ones if I should make a change. I chose a 3 amp quick blow fuse, looking at the 5v on the relay.
Another thing, i finally got the darned manual translated from chinese and it turns out the bias was set too high from the factory. The manual suggests drawing 40-45mA, before the explosion, they were running at over 55mA (without me tinkering with the settings). Could this have contributed to the fireworks?
Im remaining vigilant. In a couple of days I'll check the transformers. But theres only so much PCB green I can take in one day...
Many thanks,
Stu.
Hi Stu,
Remove all the charred board with an exact-o (hobby knife) knife, it's conductive or can be in higher humidity. You can fill it in with epoxy if you wish.
The higher bias could have certainly helped the tube to fail. Just think, the plates weren't glowing red yet but I'll bet the screen grid was. The control grid may have got hot enough to get the tube into thermal runaway too.
R18 looks like it may have gotten warm there. The 3A fuse is useless where it is, the resistor will burn out first. Speaking of resistors, I see you have a carbon film as a replacement. You should be using a metal oxide type as the original was. Since you have to dig out all the burned stuff, replace the resistor with the proper type. Sorry.
-Chris
Remove all the charred board with an exact-o (hobby knife) knife, it's conductive or can be in higher humidity. You can fill it in with epoxy if you wish.
The higher bias could have certainly helped the tube to fail. Just think, the plates weren't glowing red yet but I'll bet the screen grid was. The control grid may have got hot enough to get the tube into thermal runaway too.
R18 looks like it may have gotten warm there. The 3A fuse is useless where it is, the resistor will burn out first. Speaking of resistors, I see you have a carbon film as a replacement. You should be using a metal oxide type as the original was. Since you have to dig out all the burned stuff, replace the resistor with the proper type. Sorry.
-Chris
stueyp said:
A well designed fixed bias stage one should be able to set the tube quiescent I range from 25mA up to 80mA without any fuss. One needs a good range to take up variations between unmatched tubes.
I have KT88's when swapped into other sockets, the quies I can vary easily by a third. Your "burn out " problem probably wasn't caused by the bias variation, unless the bias pot was faulty and o/c as to cause excess anode current.
It pays to double check bias circuits, that approx neg volts appears on pin 5 on octal sockets.
richj
Fuse should be about 100mA, this will blow quick enough to prevent serious damage, but will not nuisance blow. This should be in series with the cathode circuit. NOTE: Standard 5 x 20mm cartridge fuses do not always behave well at 500V (ARC!) and above, which is why it should be in the cathode circuit.
The fuse should not be located in series with the screen grid resistor.
Incidentally while the screen grid resistor should be the same on all channels I doubt you will hear the difference. HOWEVER I do think these should all be replaced with 1W 5% none flaming fusible resistors if you can get them - otherwise I would replace them with 1/2W 5% carbon films which will self destruct quickly and do a lot less damage to the pc board, particularly if mounted somewhat clear of the board.
I don't recommend digging out all of the pcb material, first off the voltage between those points will be virtually the same under non fault conditions and secondly you can measure the resistance between the two points and verify that it is at least a couple of hundred K to several M ohms minimum - I would then seal it with a non conductive high temp epoxy if it passes the above test. (Both sides) Otherwise remove material until it does pass, and then seal as before.
The fuse should not be located in series with the screen grid resistor.
Incidentally while the screen grid resistor should be the same on all channels I doubt you will hear the difference. HOWEVER I do think these should all be replaced with 1W 5% none flaming fusible resistors if you can get them - otherwise I would replace them with 1/2W 5% carbon films which will self destruct quickly and do a lot less damage to the pc board, particularly if mounted somewhat clear of the board.
I don't recommend digging out all of the pcb material, first off the voltage between those points will be virtually the same under non fault conditions and secondly you can measure the resistance between the two points and verify that it is at least a couple of hundred K to several M ohms minimum - I would then seal it with a non conductive high temp epoxy if it passes the above test. (Both sides) Otherwise remove material until it does pass, and then seal as before.
Stuart,
Regarding your question about screen taps (post #28), it was I who was unclear. I only now studied all your previous photographs and the description given elsewhere carefully, and see that the output stages are indeed switched between tapped screens (ultra-linear) and triodes. I thought switching took place between triode/pentode operation. So the mentioned taps are used.
Two things from my side at this late stage. Looking at the size of the switching relays (and having had trouble with similar ones before, if not the same), I feel very sceptical about their ability to handle the some 800Vp that could be present there. The structural spaces in the ones that I opened in the past were small. I do not want to cast undeserved doubt, not having seen the actual ones on your amplifier, but the ones I am talking about had very similar markings and sizes.
Then, having often used UL and measured characteristics fairly broadly, I for one am not sure why one would like to switch to triode operation in the first place. I know there exists some controversy here and respect relevant opinions, but UL operation affords the best of both worlds (triode and pentode), and I honestly could never find enough reason to go "back" to triodes, let alone for a switching option.
With that I do not wish to open an off-thread controversy here (the characteristics of UL have been explained elsewhere), but without confusing you: If it was my amplifier, I would get rid of the relays and use permanent UL operation, especially with the danger of that type of relay. You intend (if I am correct) permanent connection in triode mode with your previous question on how to mount the resistors?
Hope this does not confuse you - that is not intended.
Regards.
Regarding your question about screen taps (post #28), it was I who was unclear. I only now studied all your previous photographs and the description given elsewhere carefully, and see that the output stages are indeed switched between tapped screens (ultra-linear) and triodes. I thought switching took place between triode/pentode operation. So the mentioned taps are used.
Two things from my side at this late stage. Looking at the size of the switching relays (and having had trouble with similar ones before, if not the same), I feel very sceptical about their ability to handle the some 800Vp that could be present there. The structural spaces in the ones that I opened in the past were small. I do not want to cast undeserved doubt, not having seen the actual ones on your amplifier, but the ones I am talking about had very similar markings and sizes.
Then, having often used UL and measured characteristics fairly broadly, I for one am not sure why one would like to switch to triode operation in the first place. I know there exists some controversy here and respect relevant opinions, but UL operation affords the best of both worlds (triode and pentode), and I honestly could never find enough reason to go "back" to triodes, let alone for a switching option.
With that I do not wish to open an off-thread controversy here (the characteristics of UL have been explained elsewhere), but without confusing you: If it was my amplifier, I would get rid of the relays and use permanent UL operation, especially with the danger of that type of relay. You intend (if I am correct) permanent connection in triode mode with your previous question on how to mount the resistors?
Hope this does not confuse you - that is not intended.
Regards.
Good quality relays are ideal for switching sources, I have used them in several of my designs and are generally regarded in audio as a sign of quality.
Implemented correctly it greatly reduces the amount of internal wiring running around the inside of the amplifier and picking up noise.
Hermetically sealed relays are usually far more reliable than the standard mechanical rotary switches often used.
Capacitive cross-talk in a good relay may be lower as well. (Definitely true of any inexpensive rotary switch.) This becomes an issue if the source impedance of the driving component is not low, and manifests itself as a reduction in separation as the frequency increases. Hard to beat a Shallcross rotary though..
HOWEVER cheap relays can be much worse, and low current contacts must be used, (Usually gold or rhodium plated) otherwise an oxide barrier can form on the contacts that the low voltages and currents in used line level audio can't punch through. (Ask me how I know... LOL)
Looking at the pictures I was pretty impressed with the build quality of this amplifier - it would be considered very good to excellent in most circles at this price point. My HK OEM came nowhere close to this level of engineering competence. Parts choices are another issue..
Implemented correctly it greatly reduces the amount of internal wiring running around the inside of the amplifier and picking up noise.
Hermetically sealed relays are usually far more reliable than the standard mechanical rotary switches often used.
Capacitive cross-talk in a good relay may be lower as well. (Definitely true of any inexpensive rotary switch.) This becomes an issue if the source impedance of the driving component is not low, and manifests itself as a reduction in separation as the frequency increases. Hard to beat a Shallcross rotary though..
HOWEVER cheap relays can be much worse, and low current contacts must be used, (Usually gold or rhodium plated) otherwise an oxide barrier can form on the contacts that the low voltages and currents in used line level audio can't punch through. (Ask me how I know... LOL)
Looking at the pictures I was pretty impressed with the build quality of this amplifier - it would be considered very good to excellent in most circles at this price point. My HK OEM came nowhere close to this level of engineering competence. Parts choices are another issue..
Kevinkr,
I think in paragraph 4 you meant: "Definitely not true of any inexpensive rotary switch."?
Otherwise agree with you.
Stuart,
As Kevin said, the main advantage of switching inputs with relays is that you can mount them nearest to the specific input. Reed relays are even more convenient, although I have not had experience of them. The carrying around of a few volts dc to every relay is far less problematic than carting the (low) signal all over the show, especially in complete amplifiers or complex pre-amps. Push-button switches are somewhat more convenient than rotaries but you must still bring the signal there. (But I have seen better quality rotaries than push-button.)
Regards.
I think in paragraph 4 you meant: "Definitely not true of any inexpensive rotary switch."?
Otherwise agree with you.
Stuart,
As Kevin said, the main advantage of switching inputs with relays is that you can mount them nearest to the specific input. Reed relays are even more convenient, although I have not had experience of them. The carrying around of a few volts dc to every relay is far less problematic than carting the (low) signal all over the show, especially in complete amplifiers or complex pre-amps. Push-button switches are somewhat more convenient than rotaries but you must still bring the signal there. (But I have seen better quality rotaries than push-button.)
Regards.
tube failures
The key to PREVENTING this type of catastrophic tube failure is to have a series-connected cathode resistor that has a power rating based on 60-80 % of the normal power dissipated in normal operation. Abnormal current due to a tube short, or due to failure of the bias voltage (allowing the plate current to shoot out of sight) will then result in the cathode resistor acting as a fuse and
protecting the tube (power = current squared times resistance value).
I had a Harmon-Kardon Citation amplifier that had been built from a kit by a friend whose soldering skills were dubious, and it kept frying cathode resistors because his cold solder joints led to loss of negative grid bias at times. Fortunately the designer had used the correct wattage for the cathode resistors, so all i had to do was replace the fried resistors and correct the soldering job.
Tubes CAN short out, but many failures are due to bad solder joints or the shorting of grid signal coupling caps, causing grid bias to go out of control.
The key to PREVENTING this type of catastrophic tube failure is to have a series-connected cathode resistor that has a power rating based on 60-80 % of the normal power dissipated in normal operation. Abnormal current due to a tube short, or due to failure of the bias voltage (allowing the plate current to shoot out of sight) will then result in the cathode resistor acting as a fuse and
protecting the tube (power = current squared times resistance value).
I had a Harmon-Kardon Citation amplifier that had been built from a kit by a friend whose soldering skills were dubious, and it kept frying cathode resistors because his cold solder joints led to loss of negative grid bias at times. Fortunately the designer had used the correct wattage for the cathode resistors, so all i had to do was replace the fried resistors and correct the soldering job.
Tubes CAN short out, but many failures are due to bad solder joints or the shorting of grid signal coupling caps, causing grid bias to go out of control.
Aha, I think I get it now. Now I see why amplifiers that dont use this sort of relay method have long rods connecting the switches. Kevinkr, do you mean the contacts inside the relay? Go on, tell us how...and is this still a problem for sealed relays?
All the loud switching noises have me almost diving for cover every time I do anything with this amplifier. Theres also a warm up sequence when you switch it on...which culminates in another relay orchestra....
Thanks again guys.
Stu.
All the loud switching noises have me almost diving for cover every time I do anything with this amplifier. Theres also a warm up sequence when you switch it on...which culminates in another relay orchestra....
Thanks again guys.
Stu.
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