- Thread starter
- #41
I am so happy about this thing!
Scott vintage radio
- Thread starter xtiaan2000
- Start date
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The custom can capacitors arrived a few days ago. My landmate has been gone so i just yesterday found a few minutes between chores to drop the first 2 cans in.
They came out really nice as always.
Its time to start thinking about buying some tubes for this bad machine and ill duscuss that privately with the owner.
Basically it has 4x 12ax7s.
1 is missing and another is weak but 2x are looking good.
I think the owner should consider buying 2 telefunken ribbed plate 12ax7s on ebay.
The power tubes are also shot and power tubes get replaced in matched sets.
Ill probably have him order a set of whichever flavor of 6bq5 is within the budget. Power tubes ard what switches the power to drive the speakers so they take a beating after 60+ years of use and neglect.
Furthermore, getting a set that all pulls the ssme current will go a long way to cancelung the inherent noise in things.
When a source of noise is amplified and applied equally to both sides of the transformer out of phase it cancels out and you hear nothing.
Once the tubes begin to drift and conduct differently, you dont have perfect cancelation and the leftover causes distortion. We try to eliminate distortion down to under 1% at full power of 15 watts per channel which should be no problem on this unit.
They came out really nice as always.
Its time to start thinking about buying some tubes for this bad machine and ill duscuss that privately with the owner.
Basically it has 4x 12ax7s.
1 is missing and another is weak but 2x are looking good.
I think the owner should consider buying 2 telefunken ribbed plate 12ax7s on ebay.
The power tubes are also shot and power tubes get replaced in matched sets.
Ill probably have him order a set of whichever flavor of 6bq5 is within the budget. Power tubes ard what switches the power to drive the speakers so they take a beating after 60+ years of use and neglect.
Furthermore, getting a set that all pulls the ssme current will go a long way to cancelung the inherent noise in things.
When a source of noise is amplified and applied equally to both sides of the transformer out of phase it cancels out and you hear nothing.
Once the tubes begin to drift and conduct differently, you dont have perfect cancelation and the leftover causes distortion. We try to eliminate distortion down to under 1% at full power of 15 watts per channel which should be no problem on this unit.
I got some time to dig back in this morning.
Today's work was to overhaul the filament and fixed bias supply.
One brilliant aspect of the HH Scott design is the stout DC bias supply which runs at ~50 volts dc.
Its very well filtered and a very robust supply because theyve used it not only as a bias supply but also to light up the filaments of the preamp tubes which can run on 6.3 or 12.6v ac or dc.
Corresponding to the voltages of a fully charged lead acid battery which was the technology of the time.
Dc provides the quietest mode of operation.
So in their brilliance, the engineers at HH SCOTT company wired all 4 in serries and fed them 50v dc which, conveniently enough the bias supply was already supplying.
Elegant in its simplicity.
Much like an FJ55.
Anyways....
In order to chenge the caps i had to lift one terminal strip.
In order to lift it i had to drill 2 rivets and replace them with screws.
The terminal strip broke and i had to replace the terminal strip too.
65 year old plastic often doesn't like to be moved. No surprise there.
Put it all back together and im one step closer to firing this thing up.
Next step is to try and find some tubes.
Today's work was to overhaul the filament and fixed bias supply.
One brilliant aspect of the HH Scott design is the stout DC bias supply which runs at ~50 volts dc.
Its very well filtered and a very robust supply because theyve used it not only as a bias supply but also to light up the filaments of the preamp tubes which can run on 6.3 or 12.6v ac or dc.
Corresponding to the voltages of a fully charged lead acid battery which was the technology of the time.
Dc provides the quietest mode of operation.
So in their brilliance, the engineers at HH SCOTT company wired all 4 in serries and fed them 50v dc which, conveniently enough the bias supply was already supplying.
Elegant in its simplicity.
Much like an FJ55.
Anyways....
In order to chenge the caps i had to lift one terminal strip.
In order to lift it i had to drill 2 rivets and replace them with screws.
The terminal strip broke and i had to replace the terminal strip too.
65 year old plastic often doesn't like to be moved. No surprise there.
Put it all back together and im one step closer to firing this thing up.
Next step is to try and find some tubes.
- Thread starter
- #44
Great update!
I got tubes ordered yesterday.
A pair of good testing 12ax7, and a quad set of new Sov-Tec power tubes.
When they arrive ill fire it up and see whats left to do!
A pair of good testing 12ax7, and a quad set of new Sov-Tec power tubes.
When they arrive ill fire it up and see whats left to do!
Tubes arrived yesterday and i got them in and fired up the amplifier.
With a load, on it the filament supply reaches 40v which should be about 50v
Due to the same supply being responsible for biasing the tubes the output tube bias adjustment is all the way one direction to get the tubes to bias correctly and they aren't quite where i want them.
I need to replace the old rectifier which i believe is a selenium stack with a modern silicon bridge rectifier and once thats done i may need to shift a few resistor values around to get to my desired result of 50v at the filament string and a bias supply where the pots operate near center.
Progress is being made.
I sure love to see an old set like this fire up and all of the bottles are glowing and things are looking reasonably correct voltage wise.
Its a good feeling.
I end up doing a lot of stuff prior to even thinking of putting music through it.
With a load, on it the filament supply reaches 40v which should be about 50v
Due to the same supply being responsible for biasing the tubes the output tube bias adjustment is all the way one direction to get the tubes to bias correctly and they aren't quite where i want them.
I need to replace the old rectifier which i believe is a selenium stack with a modern silicon bridge rectifier and once thats done i may need to shift a few resistor values around to get to my desired result of 50v at the filament string and a bias supply where the pots operate near center.
Progress is being made.
I sure love to see an old set like this fire up and all of the bottles are glowing and things are looking reasonably correct voltage wise.
Its a good feeling.
I end up doing a lot of stuff prior to even thinking of putting music through it.
- Thread starter
- #47
Psyched!
