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mix and match tubes in amp?
- Thread starter NEONNIGHT34609
- Start date
Which tubes?
What amplifier?
The trouble with general rules like this is that not every amplifier is the same.
Not every tube substitute is the same.
Remember that when you have two or more tubes in a parallel circuit, you want them to pull pretty much the same current each. They need to balance the load equally, or you risk one of them overheating and failing
If one tube in a parallel set of four is stronger than the other three, it will overheat before the others even reach normal operating temperature. Naturally you can throttle down the drive-power level so that the hottest tube is below the danger level.
Nobody every uses that method, though.
Some substitutes work better than others.
Some amplifiers are more tolerant of substitutes than other amplifiers.
But making sure that a parallel set of two, three or six tubes are the same simply improves your chances of a decent service life.
Installing one strong tube in a circuit with five weak ones will nearly always cause the strong tube to overheat and fail very soon.
There are other considerations as well, but any time you want to substitute one tube type for another type, the first thing to check would be the specifications for each type number.
Here's one handy online source: http://frank.pocnet.net/
If the specs look similar, and the base-wiring diagrams are the same, the substitute may work okay.
But if the pins are wired differently, that's likely to cause trouble unless you change the socket wiring to match your substitute.
And if the other specs are too different, you may need to tweak your amplifier circuit to accommodate your new substitute.
73
What amplifier?
The trouble with general rules like this is that not every amplifier is the same.
Not every tube substitute is the same.
Remember that when you have two or more tubes in a parallel circuit, you want them to pull pretty much the same current each. They need to balance the load equally, or you risk one of them overheating and failing
If one tube in a parallel set of four is stronger than the other three, it will overheat before the others even reach normal operating temperature. Naturally you can throttle down the drive-power level so that the hottest tube is below the danger level.
Nobody every uses that method, though.
Some substitutes work better than others.
Some amplifiers are more tolerant of substitutes than other amplifiers.
But making sure that a parallel set of two, three or six tubes are the same simply improves your chances of a decent service life.
Installing one strong tube in a circuit with five weak ones will nearly always cause the strong tube to overheat and fail very soon.
There are other considerations as well, but any time you want to substitute one tube type for another type, the first thing to check would be the specifications for each type number.
Here's one handy online source: http://frank.pocnet.net/
If the specs look similar, and the base-wiring diagrams are the same, the substitute may work okay.
But if the pins are wired differently, that's likely to cause trouble unless you change the socket wiring to match your substitute.
And if the other specs are too different, you may need to tweak your amplifier circuit to accommodate your new substitute.
73
Friend of mine has played whack-a-mole swapping tubes, just can't figure out why the new one blows. I tried to explain the above explanation, but unfortunately.......there goes another one.
Do your best to keep tubes used in the same stage (wired together on top plate caps) as similar as possible. If you're lucky, none will turn the plates cheery red after 60 seconds of AM carrier or full PEP output on SSB. If any tube turns red, failure is likely.
If you're looking to find tubes that match or work well with existing used tubes, the answer is NEVER to buy a new one. It will always hog the most drive power and work harder than the rest. Get on eBay (or other used parts source) and look for another used tube by the same manufacturer. While it's no guarantee you'll find a match, I've had the best luck doing it like this when a full set of new tubes was not an option.
Sometimes a driver tube works well when placed in the final stage. Then a single driver tube can be replaced with one that doesn't need to match the others. It's also a good idea to test the other tubes before proceeding. Others in that stage were probably subjected to the exact same use or abuse and if several are weak, no sense trying to match anything to them.
If you're looking to find tubes that match or work well with existing used tubes, the answer is NEVER to buy a new one. It will always hog the most drive power and work harder than the rest. Get on eBay (or other used parts source) and look for another used tube by the same manufacturer. While it's no guarantee you'll find a match, I've had the best luck doing it like this when a full set of new tubes was not an option.
Sometimes a driver tube works well when placed in the final stage. Then a single driver tube can be replaced with one that doesn't need to match the others. It's also a good idea to test the other tubes before proceeding. Others in that stage were probably subjected to the exact same use or abuse and if several are weak, no sense trying to match anything to them.
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Another thing to consider is not all sweep tubes were created equal to begin with. Even tubes with the same number and manufacturer can have very noticeable differences in their construction. For example there are six variations that I know of with RCA's 6LF6. They have short plate and long plate like GE. Side getters and top getters that provide improved shielding. Long glass envelope and short that fits Palomar. The shape and thickness of the two additional radiators on each side of the plate also varies. The best were made in Europe by companies like Mullard and Amperex. Just holding one in your hand, you'll notice they weigh about twice as much as western versions and last twice as long too. All things to look at when trying to find another tube similar to what you have.
PS: Stay away from any tubes that have glass that looks tinted darker around the area that lines up with the plate inside. This is an indication that the tube has been worked hard. I believe the glass becomes discolored from electron bombardment or migration of materials from the filament. I forgot which one it was. This is not to be confused with the silver mirror colored flash marks left from the getters underneath. You want to see those inside sweep tubes. If the edges of the silver look white, that tube was either severely over heated or has lost some of its vacuum. If all the silver area has turned white, the tube probably has a crack in the glass somewhere.
PS: Stay away from any tubes that have glass that looks tinted darker around the area that lines up with the plate inside. This is an indication that the tube has been worked hard. I believe the glass becomes discolored from electron bombardment or migration of materials from the filament. I forgot which one it was. This is not to be confused with the silver mirror colored flash marks left from the getters underneath. You want to see those inside sweep tubes. If the edges of the silver look white, that tube was either severely over heated or has lost some of its vacuum. If all the silver area has turned white, the tube probably has a crack in the glass somewhere.
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