Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: zephyrblau on August 26, 2023, 01:43:55 pm
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I'm back dating a TV front 1 X 15 tweed amp for a harp player buddy. this is the progress so far. I'm hoping I haven't done anything to let the smoke out.
comments ?
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2 more (crappy) pix. I'll take more. if it's important, the PT was harvested from a 50s era Wurlitzer driving 2 X 6L6 and a handful of octals. chassis is a BUD.
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Looks like a very good amp waiting to happen but i wouldn't be too happy about how some of the solder joints turned out to be.
Fuse and mains switch not in line may not matter when you can plug in both ways but i am guessing you cannot choose, so better haver fuse and switch on the hot side and directly connected. Where i live you can change mains polarity by reversing the mains plug so good practice would be to switch hot and neutral on and off.
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@Williamblake
thanks for chiming in. I may have done part of this working from the schem and part from the layout. I'm not sure they agree or I'm just confused.
I'll have another look.
thanks again
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following the schematic and layout is a good idea - except for the power mains. They were drawn before modern 3 wire circuits were common. Best practice is typically considered - green ground directly to chassis, neutral directly to one of the PT leads, hot to fuse, then to switch, then to the other PT lead. But you have some other weirdness going on here. I suggest you spend some time reviewing some builds here and elsewhere to learn some tried and true techniques. Sluckey's projects are very well documented on his site - http://sluckeyamps.com/ (http://sluckeyamps.com/) Better to get it right the first time.
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Just to give a visual of what bmccowan is stating. :icon_biggrin: I think it's also good practice to connect the Line or "L" to the center connector of the fuse holder. I also think the Ground wire should have its own chassis bolt, not a transformer bolt, etc., since that could work loose over time (but I have been guilty of doing that). If you are using a power cord with a strain relief - not an IEC inlet as in my drawing - then make the Ground wire about twice as long as the "N" and "L" wires: the idea is that if the cord ever gets yanked out of the chassis, the Ground will not be the first wire to break loose.
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my thanks to you both. the visual layout is most helpful. I've corrected the hot side to hot-fuse-switch with neutral going to the other PT primary.
I thought the power supply would now be ready to go but I can't remember why I grounded the center tap of the 5V secondary.
the PT is from either a 7020 or 6144 Wurlitzer. IIRC the trannies are the same (18453)
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I grounded the center tap of the 5V secondary.
NO NO NO! You will burn up a lot of stuff. Just tape the end of that 5V centertap and don't use it.
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I knew there was a reason I came here for help :worthy1:
done.
...and thanks again.
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...I can't remember why I grounded the center tap of the 5V secondary. ....
You only do that once.
In the 1930s, the 5V CT went to the B+ filter, main capacitor or, in those days, maybe a choke. It can be shown that this minimizes 60Hz hum on top of the expected 120Hz ripple. However someone here was provoked into trying it, and with modern-size filter caps the difference was less than a trace-width. And since nobody has seen that connection in a lifetime, it would just be confusing. Cut the CT wire short and hide it.
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this should do the trick. a terminal strip would have been tidy.
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I grounded the center tap of the 5V secondary.
NO NO NO! You will burn up a lot of stuff. Just tape the end of that 5V centertap and don't use it.
After doctoring your tiny pics I'm not convinced that wire is a center tap for the 5V secondary. I think it's the center tap for the B+ winding. If that's true it must be connected to chassis. You really need to sort this out. Easy to do with an ohm meter. Turn the power OFF! Connect one probe to the wire in question and leave it connected. Then connect the other probe to pin 4 of the rectifier socket. I bet you have a resistance reading. Now move the probe to pin 4. I bet you have the same resistance (or close). This would prove the wire is the center tap for the B+ winding.
Now move the probe to pin 8 or 2. I bet there is no reading. This would prove the wire in not a center tap for the 5V winding.
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probe from mystery wire to pin #4 = 43.8 ohms
#6 = 40.6 ohms (assuming that was a typo on your part)
#2 = 0 ohms
I thought that when I originally wired the PT I was referring to a schem dated 1955 but, now I can't find that.
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reviewing the pinouts in the tube manual first may have eliminated the most recent confusion.
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probe from mystery wire to pin #4 = 43.8 ohms
#6 = 40.6 ohms (assuming that was a typo on your part)
#2 = 0 ohms
The meter is showing OL for pin 2. That's infinity, not 0 (ZERO). So that mystery wire is the center tap for the B+ winding and should be connected to ground.
Nice rabbit hunt!
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again my thanks.
as I get ready to install the board I have a question about the orientation of the O/T. apparently it's not conventional to mount as shown but, the chassis is 17" wide. is this going to be an issue ?