- Thread starter
- #41
I am so happy about this thing!
Scott vintage radio (1 Viewer)
- 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!
I got some time again today to look at the amp.
I had a great vacation, put ~1100 miles on the fj55. Came back to a farm load of chores and my land mate took his vacation (so I adopted all of his chores).
Got everything under control.
He returned yesterday with his 300k 1997 tacoma belching white clouds. After a pep talk about head gaskets we went swimming while the truck cooled off.
In the AM we ruled out a head gasket with my block checker. Coolant is coming through the idle air control valve . Now he's back and I have some free time (because he doesn't need a head gasket afterall) and I got the Scott back on the bench.
Put a signal through it and measured roughly 18 watts at the speaker terminals.
Despite what the vintage Scott ads say, 18 watts is about all you're going to get from a new pair of 6bq5s properly biased, so we are at the level we want to be at in terms of power output.
Next step is to drop it into the system and see if there's any additional problems that show up. My guess is that there will be few ...
That's the update for today.
I had a great vacation, put ~1100 miles on the fj55. Came back to a farm load of chores and my land mate took his vacation (so I adopted all of his chores).
Got everything under control.
He returned yesterday with his 300k 1997 tacoma belching white clouds. After a pep talk about head gaskets we went swimming while the truck cooled off.
In the AM we ruled out a head gasket with my block checker. Coolant is coming through the idle air control valve . Now he's back and I have some free time (because he doesn't need a head gasket afterall) and I got the Scott back on the bench.
Put a signal through it and measured roughly 18 watts at the speaker terminals.
Despite what the vintage Scott ads say, 18 watts is about all you're going to get from a new pair of 6bq5s properly biased, so we are at the level we want to be at in terms of power output.
Next step is to drop it into the system and see if there's any additional problems that show up. My guess is that there will be few ...
That's the update for today.
This might be the only tube amplifier-related post I've ever seen on Mud - a hobby I dabble in as well. Cool stuff so far! Love the Cinemag xformers back on page one and the meticulous process so far on the Scott 299.
I would expect your bias voltage to be high since AC wall voltage was 110V when this thing was made, and most everyone sees 120-125VAC nowadays. Strange that it's 20% low. Is there a dropping resistor in that part of the circuit that's out of spec perhaps? Or perhaps a bad cap with undue current flowing through it causing a voltage drop? I mainly deal in guitar amplifiers and the increase modern wall voltage yields less headroom. There's a little trick circuit you can install with high wattage zeners to drop the B+ back to original spec - I've done that on a few amps and it might be good for a hi-fi application so you can preserve clean headroom and reduce THD.
Have you ever used CE Distribution for parts? They have the original machines for making Mallory can caps and started reproducing them a few years back. Good quality. I've poked around Hayseed's website but never ordered anything from them. That also looks like quality stuff.
Right now I'm avoiding replacing filter caps on my Ampeg SVT. 300w with 6x 6550 output tubes. B+ rail is somewhere around 600V and there isn't a great access point for discharging the caps without juggling multiple PC boards. Not a fun job. After that is converting an old Roberts tube real to real unit into a dual mono microphone preamplifier. That one involves a lot of chopping to get rid of unnecessary circuitry that will only cause hum. All of that of course is after working on my 60 & 40, and looking out for a 55 which is what my wife really wants more than any of the others.
Bias voltage was low due to the old solid state rectifier going high resistance.
It went in the trash, replaced it with a new silicon unit and bias voltage is on the money at 50v.
4x 12.6v filament tubes with the filaments in serries should in fact make a 50.2v string in operation right.
Its at 50v even, im not going to sweat that. The power tubes bias up nicely with even current and no red plates. Functionally things look good but right now I've got a pair of home brew, mono 6l6 ultralinear amps in my system right now, and after some more playing on the bench, the HH Scott will go in the system in a week or 2.
I have used CE can caps and I stopped. I can stuff my own cans and often do, but also the cans that certain offers are rated at 55° Celsius.
Original equipment can caps were rated at 85 or 105c. The CE cans also have a sloppy crimp on the base which causes them to bulge out asymmetrically it doesn't effect their function but for the price, they could be putting out a better product. Folks look at photos of tube amplifiers with lust in their eyes so why not give them something beautiful to look at. I go with Hayseed Hamfest because they look great and offer excellent customer service.
Ive had some oddball requests including male octal bases and they have come through with the goods.
Cinemag is another great American company.
I definitely love vacuum tube equipment and high quality hand built audio equipment in general.
Its beautiful functional industrial art and i hoard it because it represents spent man-hours. Its incredibly good stuff, works better than the modern stuff and is totally worth rebuilding when it does break.
It went in the trash, replaced it with a new silicon unit and bias voltage is on the money at 50v.
4x 12.6v filament tubes with the filaments in serries should in fact make a 50.2v string in operation right.
Its at 50v even, im not going to sweat that. The power tubes bias up nicely with even current and no red plates. Functionally things look good but right now I've got a pair of home brew, mono 6l6 ultralinear amps in my system right now, and after some more playing on the bench, the HH Scott will go in the system in a week or 2.
I have used CE can caps and I stopped. I can stuff my own cans and often do, but also the cans that certain offers are rated at 55° Celsius.
Original equipment can caps were rated at 85 or 105c. The CE cans also have a sloppy crimp on the base which causes them to bulge out asymmetrically it doesn't effect their function but for the price, they could be putting out a better product. Folks look at photos of tube amplifiers with lust in their eyes so why not give them something beautiful to look at. I go with Hayseed Hamfest because they look great and offer excellent customer service.
Ive had some oddball requests including male octal bases and they have come through with the goods.
Cinemag is another great American company.
I definitely love vacuum tube equipment and high quality hand built audio equipment in general.
Its beautiful functional industrial art and i hoard it because it represents spent man-hours. Its incredibly good stuff, works better than the modern stuff and is totally worth rebuilding when it does break.
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So far so good on the Mallory cans I’ve gotten from CE Dist, although it’s been maybe a year or more since I’ve bought any. I have a couple in a guitar amp I use several times per week - a 1963 Gretsch (made by Valco in Chicago). No heat issues after about 7 years.
Glad you’ve got the 50V nailed. seems like this 299 is pretty close!
I too love tube equipment. Relatively easy to repair for most basic circuits you run into, which is important to me - a big part of why I drive old trucks too. The beauty in it is undeniable too.
Yesterday my landmate came home to me sitting in my chair staring at the wall catatonic, water pipe in hand...
After 2 or 3 hours in the scott i decided to swap out the old board in my Mcintosh c8 for a new one id made.
5 hours later: this is your brain on drugs.
Sizzle....
Seriously working through something as complicated as a Mcintosh c8 or c8s will be like putting your brain through a chicken plucker after a few hours of it.
Its never insurmountable.
I always get it done
Once they're done and Ill have a completely rebuilt set of Mcintosh 30s with the correct preamps for stereo reproduction which actually predates the agreed upon 1958 "invention/addoption" of stereophonic sound.
Not only that, these were about as good as you could get in terms of hi fidelity equipment in the mid 1950s, and of course its all made in the USA.
Not many folks can say that about their gear
Mcintosh equipment is still made in America to this day and is still on the top of the heap in terms of overall build quality.
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I’ve dreamed of some MacIntosh tube monoblocks … but I could buy a couple cruisers for the price haha!
Alan Blumlein, a genius in his own right, was working on stereophonic sound during WWII. He also worked on, I believe, radar systems and there’s a stereo microphone technique named after him that I use on occasion when I’m working in a studio. Interesting cat.
I can't really afford Mcintosh, initially it sort of fell into my lap because it didn't work and I had cash and ive sort of hunted it ever since, but at the end of the day I could never afford one that runs.
Just like landcruisers.
Nice ones that run are way out of the budget but I've got one anyhow.
We are the same
