More Amp Construction

paulbottlehead

Active Member
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This is the last pair of 90s Bottlehead amps I'm missing: the Parabees. There wasn't quite a complete schematic available from back in the day, but I did my best and got pretty close electrically to what should be in there, albeit with some different caps and a lot more space.
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I used an Illusion powder from Prismatic Powders for the top plate and African mahogany for the sides. These are unusually hard to photograph because of the sparkle in the green. I'll have to give it another go in the morning.
 

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View attachment 37487
This is the last pair of 90s Bottlehead amps I'm missing: the Parabees. There wasn't quite a complete schematic available from back in the day, but I did my best and got pretty close electrically to what should be in there, albeit with some different caps and a lot more space.
View attachment 37486
I used an Illusion powder from Prismatic Powders for the top plate and African mahogany for the sides. These are unusually hard to photograph because of the sparkle in the green. I'll have to give it another go in the morning.
Paul, I have the manuals for the Parabee and Paraglow. If I can scan them into my computer, I’ll email them to you so you can have them for reference.
 

paulbottlehead

Active Member
Oh, we have scans of the Paraglow and Parabee manuals. If you look in the Parabee manual, the C4S board comes preassembled and there aren't any values stipulated in the schematic ;)

Thankfully with the cathode resistance and the voltage checks I was able to work backwards and figure that out.
 

paulbottlehead

Active Member
Random repair of the week is a pair of MFA-120 monoblocks with "mods". The owner said one works and one blows the 1A fuse.
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The owner presented me with some paperwork for prior servicing, but this amp from the 80s is still rocking its original power supply caps!
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That must be a cap from 1973! It's seen better days, as it's bulging up top and deforming along with a brown hot spot. The main rail caps mounted up top had 1988 date codes on them, and the lights in my house would dim when you turned this thing on, so I didn't have a lot of hope for them either. I let the owner know that it was definitely time for a recap and some thermistors to ease any turn on surge, but that 1A fuse still blew even with fresh caps in.
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It took quite a while to figure out that a metal grounding tab on one of the octal sockets had bent out of position and was just close enough to the board to draw an arc when applying B+, but far enough away that my meter couldn't see it when I checked for shorts.
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I just moved the connections to those pins directly instead of through the board, and now the amp is happy and up and running.
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These are rather controversial amps, as they harvested HK Citation II output transformers , and that means there are a bunch of Citation IIs that went to the junkyard to create these.
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Still, they make an honest 130 watts, even down low, so I at least have mixed feelings about their existence.
 

paulbottlehead

Active Member
This one is all done and ready to go!
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I haven't ever heard a 183 amp, but now I can say it's definitely not the same as a 45, which I would attribute to a filament that's more closely related to the 300B.
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This amp is fully shunt regulated and uses a pair of 6S4A tubes in the regulator circuits. The filaments are DC heated with a CRC supply that has been fine tuned to be within 1% of the nominal 5V with a bogey tube. The iron is the same parallel feed iron set that we use in the SEX amp and Stereomour, but the whole plate choke winding is used to give a 40H plate choke working into a 4K output transformer, so the bass extension is quite nice. The big Miflex caps are in the output stage as parallel feed caps, then there are some Vcaps tucked in by the 12AT7 driver acting as coupling caps.
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This amp is a bit unique because you can also just plug in 300Bs and it works properly, though I prefer the sound of the 183.
 

Wntrmute2

Not So Mediocre Member
Can you describe or show how you wired up those milliamp meters? I received mine and there is just one marking (+) on one of the lugs. What does the illumination require in way of voltage?
TIA. Lovely build as always!
 
Wow! that looks great! I had never heard of a 183 tube, much less a 183 amp. I would love to hear it. Wondering if this Wednesday, Aug 4th, I could stop by with my son and his working Moreplay for a visit? We would be available around 4:00 pm.
Don Palmer
 

paulbottlehead

Active Member
Can you describe or show how you wired up those milliamp meters? I received mine and there is just one marking (+) on one of the lugs. What does the illumination require in way of voltage?
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+ goes to the tubes, the unlabeled post goes to ground. To illuminate, I had to remove the white circular rings you see in this photo, then line the inside of the meter bodies with foil, then I drilled a small hole in the back of the meter and popped an amber LED through. I use a 200 ohm resistor as a current limiter and lit the LEDs off the 6.3V AC winding. I would recommend ordering a few different amber LEDs, as some "amber" LEDs look really yellow.
 

paulbottlehead

Active Member
Wow! that looks great! I had never heard of a 183 tube, much less a 183 amp. I would love to hear it. Wondering if this Wednesday, Aug 4th, I could stop by with my son and his working Moreplay for a visit? We would be available around 4:00 pm.
Don Palmer
Sure thing!
 

Wntrmute2

Not So Mediocre Member
View attachment 38546
+ goes to the tubes, the unlabeled post goes to ground. To illuminate, I had to remove the white circular rings you see in this photo, then line the inside of the meter bodies with foil, then I drilled a small hole in the back of the meter and popped an amber LED through. I use a 200 ohm resistor as a current limiter and lit the LEDs off the 6.3V AC winding. I would recommend ordering a few different amber LEDs, as some "amber" LEDs look really yellow.
Well mine are a bit different.
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The ad claimed they could be backlit but no details.
 

marantzfan

Administrator
Staff member
This one is all done and ready to go!
View attachment 38539
I haven't ever heard a 183 amp, but now I can say it's definitely not the same as a 45, which I would attribute to a filament that's more closely related to the 300B.
View attachment 38540
This amp is fully shunt regulated and uses a pair of 6S4A tubes in the regulator circuits. The filaments are DC heated with a CRC supply that has been fine tuned to be within 1% of the nominal 5V with a bogey tube. The iron is the same parallel feed iron set that we use in the SEX amp and Stereomour, but the whole plate choke winding is used to give a 40H plate choke working into a 4K output transformer, so the bass extension is quite nice. The big Miflex caps are in the output stage as parallel feed caps, then there are some Vcaps tucked in by the 12AT7 driver acting as coupling caps.
View attachment 38541
This amp is a bit unique because you can also just plug in 300Bs and it works properly, though I prefer the sound of the 183.
I have to say. I really like this style you’ve developed with the meters, windows and cutout sections, all in a utilitarian black powder coat. Looks rugged, almost like a military piece of equipment but with some pizzazz.

Your one off builds never cease to impress. If I was smart, I would commission you to build me an amplifier and be done….
 

paulbottlehead

Active Member
I decided to go after some low hanging fruit yesterday and finish up a pair of SR-45s for my amp archive. A long time friend/customer had me rebuild his SR-45 amps a while ago and left me the carcasses, which included a pair of custom power supply PC boards that sat above the original Paramour power transformer. He also left me a pair of filter chokes on there and the power transformers themselves. Unfortunately the chassis plates were bent in shipping so I couldn't just use those. A second local customer bought some really weird and ugly Bottlehead Paraglows on eBay, then harvested the Magnequest output iron and some of the hardware and left me with those.
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These were stuffed with what I would call fashionable mods for about 1998, but just got too messy to work on for the long term. and some of the solder joints were cold and noisy. I stripped the plates down and ran them on my belt sander for ages to get the oxidation off, then lacquered over the top. I also drilled out the back of the wood bases and added IEC power entry modules.
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I'm glad both the custom PC boards are still working well, and I had the rest of the necessary parts just sitting around here to get these up and running.
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I'll have to give these and the 183 some break in hours and compare the two, since they are almost the same circuit and using the same iron. I'm grateful to have the space back that these four amp chassis were occupying!
 

marantzfan

Administrator
Staff member
The 240 I felt was a pleasure to work on. The layout is so accommodating compared to some other vintage tube pieces I’ve been in….
 
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