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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Mesa Boogie Mk4 design shennanigans
This is a bit strange, never noticed it before.
The Mesa Boogie Mk4 has a switch for "Tweed", or "Full Power". Basically a PSU voltage drop for reduced power output. I had one in that just would not perform in Tweed mode, but was fine in Full Power mode. I discovered that all 4 of the Electro Harmonix ECC83 pre-amp tubes would not work with the reduced voltage, but substituting 4 new JJTesla ECC83 solved the problem. The EH tubes were actually branded "Watford Valves Cryo" - Watford Valves put these things into liquid nitrogen to make them work better. Apparently. Cough. Anyway, after a bit of investigation I discovered that this Tweed switch is actually on the mains transformer Primary, and switches in some extra windings. The result of which is all the secondary HT voltages are reduced by around 20%, BUT, so is the heater supply voltage. In this case dropping from 6.1v to 5.1v. I'm no expert in valve amp design, but this seems to be rather a strange design choice. But then Mesa Boogie design their amps to work with their own branded valves, so they presumably know the reduced voltages, including heater, will mean their tubes will still carry on working. Interestingly, I had another (non Cryo) Electro Harmonix ECC83 which didn't bug out at the reduced voltage, so perhaps this was a dodgy quad of ECC83, of perhaps something to do with the Cryogenic process? Gareth. |
#2
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Mesa Boogie Mk4 design shennanigans
Gareth Magennis wrote:
The EH tubes were actually branded "Watford Valves Cryo" - Watford Valves put these things into liquid nitrogen to make them work better. Apparently. Cough. Interestingly, I had another (non Cryo) Electro Harmonix ECC83 which didn't bug out at the reduced voltage, so perhaps this was a dodgy quad of ECC83, of perhaps something to do with the Cryogenic process? ** I doubt they put the valves directly in liquid N2 - as it would destroy them instantly by cracking the glass. Even if done some other and safer way, the whole idea is completely nuts. One of the design issues with valves is the vacuum tight seal between the pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps. Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation. ..... Phil |
#3
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Mesa Boogie Mk4 design shennanigans
"Phil Allison" wrote in message ... Gareth Magennis wrote: The EH tubes were actually branded "Watford Valves Cryo" - Watford Valves put these things into liquid nitrogen to make them work better. Apparently. Cough. Interestingly, I had another (non Cryo) Electro Harmonix ECC83 which didn't bug out at the reduced voltage, so perhaps this was a dodgy quad of ECC83, of perhaps something to do with the Cryogenic process? ** I doubt they put the valves directly in liquid N2 - as it would destroy them instantly by cracking the glass. Even if done some other and safer way, the whole idea is completely nuts. One of the design issues with valves is the vacuum tight seal between the pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps. Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation. ..... Phil My bad, their website makes no mention of N2. Maybe they just stick them in the freezer and call it Cryo. Interesting info though. Anyway, any thoughts on 5.1v heater voltage as a viable propostion? Cheers, Gareth. |
#4
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Mesa Boogie Mk4 design shennanigans
"Phil Allison" wrote in message ... Gareth Magennis wrote: The EH tubes were actually branded "Watford Valves Cryo" - Watford Valves put these things into liquid nitrogen to make them work better. Apparently. Cough. Interestingly, I had another (non Cryo) Electro Harmonix ECC83 which didn't bug out at the reduced voltage, so perhaps this was a dodgy quad of ECC83, of perhaps something to do with the Cryogenic process? ** I doubt they put the valves directly in liquid N2 - as it would destroy them instantly by cracking the glass. Even if done some other and safer way, the whole idea is completely nuts. One of the design issues with valves is the vacuum tight seal between the pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps. Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation. ..... Phil Oh, OK, rather late to the party on this one - so there is no bonding at all between the pin and the glass, just an interference fit? Gareth. |
#5
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Mesa Boogie Mk4 design shennanigans
On Tuesday, November 29, 2016 at 9:37:28 PM UTC-5, Phil Allison wrote:
** I doubt they put the valves directly in liquid N2 - as it would destroy them instantly by cracking the glass. Even if done some other and safer way, the whole idea is completely nuts. One of the design issues with valves is the vacuum tight seal between the pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps. Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation. I do not believe they do, either. Here is one "Cryo" vendor describing their process: __________________________________________________ _____________ The BLACK SABLE process is far more than putting tubes in a chamber and lowering the temperature. The BLACK SABLE process starts with tube selection. Only the best preamp, power and rectifier tubes are chosen to undergo this arduous process. All tubes undergo a 24 hour burn-in under load. The tubes are then placed in the Cryo-Processor where a computer-controlled proprietary system lowers the temperature to -300 Fahrenheit / -184 Celsius for 24 hours and then slowly ascends back to room temperature. Preamp tubes are tested and rated for Gain, Microphonics and Noise. Next, all 7/9 pin tubes have their pins cleaned for a better electrical connection. Power tubes are matched to within 3% for both Ip (Plate Current) and Gm (Transconductance). No other cryogenic process comes close. When only the best will do, choose BLACK SABLE. __________________________________________________ ________________________ Which is 21F above the boiling point of liquid nitrogen, a cheap and readily obtainable material in liquid form. That being written, most metals, including coated steel, copper, cupro-nickel and other solderable materials have a vastly greater coefficient of expansion/contraction than even the softest glass (Pyrex). So either the pins are going to shrink out of their surroundings, or all this is done in a *hard* vacuum. There is no real danger of shattering the glass if done slowly, but as the pins will be *smaller* than the opening during the process - that is where all the 'magic' must take place. Color me dubious. Peter Wieck Melrose Park, PA |
#6
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Mesa Boogie Mk4 design shennanigans
On Wed, 30 Nov 2016 13:44:18 -0800 (PST), "
wrote: On Tuesday, November 29, 2016 at 9:37:28 PM UTC-5, Phil Allison wrote: ** I doubt they put the valves directly in liquid N2 - as it would destroy them instantly by cracking the glass. Even if done some other and safer way, the whole idea is completely nuts. One of the design issues with valves is the vacuum tight seal between the pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps. Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation. I do not believe they do, either. Here is one "Cryo" vendor describing their process: _________________________________________________ ______________ The BLACK SABLE process is far more than putting tubes in a chamber and lowering the temperature. The BLACK SABLE process starts with tube selection. Only the best preamp, power and rectifier tubes are chosen to undergo this arduous process. All tubes undergo a 24 hour burn-in under load. The tubes are then placed in the Cryo-Processor where a computer-controlled proprietary system lowers the temperature to -300 Fahrenheit / -184 Celsius for 24 hours and then slowly ascends back to room temperature. Preamp tubes are tested and rated for Gain, Microphonics and Noise. Next, all 7/9 pin tubes have their pins cleaned for a better electrical connection. Power tubes are matched to within 3% for both Ip (Plate Current) and Gm (Transconductance). No other cryogenic process comes close. When only the best will do, choose BLACK SABLE. _________________________________________________ _________________________ Which is 21F above the boiling point of liquid nitrogen, a cheap and readily obtainable material in liquid form. That being written, most metals, including coated steel, copper, cupro-nickel and other solderable materials have a vastly greater coefficient of expansion/contraction than even the softest glass (Pyrex). So either the pins are going to shrink out of their surroundings, or all this is done in a *hard* vacuum. There is no real danger of shattering the glass if done slowly, but as the pins will be *smaller* than the opening during the process - that is where all the 'magic' must take place. Color me dubious. Peter Wieck Melrose Park, PA Especially dubious because if the pins really did shrink away from the glass the glass/metal seal would be broken and the tube would leak air in once exposed to the atmosphere again. When tubes are made the glass really does seal to the pins, just like glue. But glass is elastic (more elastic than the metal pins) and the movement would be small so maybe the glass does move with the pins. Steel pins on a tube would shrink about .0001" going from room temp to 300 below. Glass would move about .000067" so about 33 millionths of an inch difference. So the glass/steel seal would probably hold. Eric |
#7
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Mesa Boogie Mk4 design shennanigans
wrote:
Especially dubious because if the pins really did shrink away from the glass the glass/metal seal would be broken and the tube would leak air in once exposed to the atmosphere again. When tubes are made the glass really does seal to the pins, just like glue. ** True when the steel pins are as hot as the molten glass that is formed around them. But this happens at a very high temp ( about 1500C ) so during cooling thermal contraction will put considerable stress on this seal even at room temp. ..... Phil |
#8
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Mesa Boogie Mk4 design shennanigans
Gareth Magennis wrote:
The result of which is all the secondary HT voltages are reduced by around 20%, BUT, so is the heater supply voltage. In this case dropping from 6.1v to 5.1v. I'm no expert in valve amp design, but this seems to be rather a strange design choice. ** The same idea was used by MusicMan in several models to create a half power switch. I did a quick test on a few 'X7 and 'U7 types and found good operation was maintained to under half voltage on the heaters. At about 2 volts instead of 6.3V, voltage gain is low or sub unity. ..... Phil |
#9
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Mesa Boogie Mk4 design shennanigans
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#10
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Mesa Boogie Mk4 design shennanigans
Jeff Layman wrote:
And why would they do it to a rectifier tube? What would they expect to gain? ** You are kidding - right ?? .... Phil |
#11
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Mesa Boogie Mk4 design shennanigans
Jeff Layman wrote:
How would they do this in practice? If it had to be done in a hard vacuum, how would you transfer the heat away from the valve to cool it down to -184? Maybe they hire a rocket to fire the valve into space and let it cool down by radiation, obviously keeping it out of sunlight while it's up there. ;-) And why would they do it to a rectifier tube? What would they expect to gain? Your money. if you are gullible enough -- Never **** off an Engineer! They don't get mad. They don't get even. They go for over unity! ;-) |
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