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Discussion in 'Amplifiers' started by donemad, Aug 23, 2011.
Are there any changes to be made to use the 6LB6?
Indeed they will but good luck finding a 2057 at a reasonable price. They are probably the only sweep tube to cost more than the 8950.
Better to do the 6lb6 conversion.
The 8950's are going for around $100 or so when you can find them. I don't do cheap tube amps at all myself but all the guys around here are only going with the common cheaper tubes either by design or the above conversions.
The heaters ( filament ) on the 8950 are 12 volt.
the heaters on the 6LB6 are 6 volt.
If it has an even number of tubes, you can feed two of the 6LB6 heaters in series and do it, or else supply a 6 volt filament transformer.
How many tubes does it have?
Just tie the filaments in series with the 6LB6's, and you should be good to go. Most of the inter-electrode capacitances are fairly close, so you shouldn't have too much trouble tuning up the amp. Just keep your input wattage down so you don't over-drive the tubes. Good luck!
Question was answered by previous posts.
I have done the 6lb6 conversion in siltronix gear and worked fantastic.
Just like Jeff said, wire the heater voltage in series to run a pair of 6lb6's.
Power out maybe a little less than what the 8950 gives but not enough for the receiving station to notice the difference.
Data sheet for 6lb6
IT IS THE 750 Maco, looks like it has 1 8950 x 2 8950 x 6 8950......thanks for any help
I dont really know what you mean-by odd number!...I did check them on a tube tester and 5 were checked at 80%+ 2 were at 35% and 1 was dead, not even light-up. what would you do? all I need is 4, and I had these other 6lf6 tubes,that checked at 85% just a few laying around..........thanks for any help
..OH and 1 at 22%
Two 6lb6 will use 12v for filament
One 8950 will use 12 volt for filament.
Since there is an odd number of tubes, not even. You will need to keep one 8950 at 12v filament.
then the other eight 6lb6 can be wired in series. Using two 6lb6 wired together for 12v filament from transformer.
Hope that did not confuse you as I re read it may be confusing.
I would sell the 8950's and then you would have enough $$ to buy 6lb6 to last a lifetime for that amp.
Pin 1 and pin 12 are heaters.
8950 pin 1 will have 12v and pin 12 will be at ground.
to make it work for 6lb6 two tubes are required.
Pin one will have 12 volts on tube A.
then on tube A pin 12 jumper it to pin 1 of tube B.
Pin 12 of tube B will go to ground.
I would put a cap at tube A pin one to ground for RF bypass.
hope that clarifies it better.
If you are unsure of how to do this modification then please take it to your technician.
Very good advice, if you are not sure how to proceed, you can wind up doing damage to the amp, or your new tubes, or you, these old amps have lethal voltages in them.