Actually, now that I look more closely at yours, it's not "correct" either. The diodes should be BETWEEN the red wires and the plates of the 5AR4s. This looks like someone has created a bridge rectifier, which doesn't correspond with the schematic. The correct way to do this mod in this amp is in the photo below. I really can't say what the purpose of your mod is. I could be a factory revision, but there's no way to tell. Sorry to add more confusion, but when folks get into these amps, they can make a mess.
Do you have a friend who is experienced with tube amps and could check it out and take some voltage measurements for you?
Do you have a friend who is experienced with tube amps and could check it out and take some voltage measurements for you?
They are. The diode is going from pin #7 (red wire) to pin #6 (plate) and there's a jumper to pin #4 ( the other plate).
jeff
Hmmmm. Look again. Red wires to pin 6 + 4, diodes from pin 4 to what looks like a ground wire. Looks like a bridge to me. Am I wrong?
The SS diodes partially protects tube rectifiers, but they don't help anything to prevent overload the 5AR4s.
Must to decrease first capacitor value to maximum 120uF.
If it even hybrid bridge (BTW the PT has CT 350V or not?) helps nothing to rectifier tube's overload (due large first capacitor).
Simulated:
Simulated:
Agreed. I don't see a center tap wire but the secondary wiring doesn't look any different from the other examples I've seen. If the original power supply used a 700VCT transformer, you couldn't just lift the center tap and slap a bridge in there, right? You'd get, like 900VDC, right? So maybe it's a factory revision using a 350 volt secondary?
Sorry, I thought you were referring to the pic in your post #41. Yeah, now that I look at it, it's totally different. Don't know how I missed that. 🙁
jeff
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So here's the full pic of the mod I posted at #41:
Note the HV secondary center tap (red/yellow) and the filament center tap (green/yellow) going to circuit ground at the test jack between the output tubes. So logically a standard full-wave rectification scheme.
Now compare to the OP's version:
No red/yellow center tap. So it looks like he has a factory revision using a full-wave bridge rectification scheme.
The bias voltage supply has change from red/black tap off the HV winding in the first amp, to brown and brown in the OP's amp, indicating a separate bias winding. At least that's my guess.
Also, the unused secondaries in the OP's amp lead me to think that maybe the original transformer was replaced with a newer version that required a modification to bridge configuration, hence the little tag strip and diodes added.
What remains is euro21's concern about the rectifiers, since there is no modification to protect against the effects of the huge input cap. So that's something the OP will have to consider if he decides to keep the amp.
Note the HV secondary center tap (red/yellow) and the filament center tap (green/yellow) going to circuit ground at the test jack between the output tubes. So logically a standard full-wave rectification scheme.
Now compare to the OP's version:
No red/yellow center tap. So it looks like he has a factory revision using a full-wave bridge rectification scheme.
The bias voltage supply has change from red/black tap off the HV winding in the first amp, to brown and brown in the OP's amp, indicating a separate bias winding. At least that's my guess.
Also, the unused secondaries in the OP's amp lead me to think that maybe the original transformer was replaced with a newer version that required a modification to bridge configuration, hence the little tag strip and diodes added.
What remains is euro21's concern about the rectifiers, since there is no modification to protect against the effects of the huge input cap. So that's something the OP will have to consider if he decides to keep the amp.
Wow, you guys really know your stuff, but I'm totally lost now. I'm definitely a newbie, but I have recently completed a recap of a vintage Scott LK-72-B. I have a volt meter and can follow instructions if you want me to test anything. It would be nice to know I wouldn't be burning through tubes if I decide to keep these amps.
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Don't be lost. ;-) I was very concerned about the way your 5AR4s were configured, but comparing your helpful photo to others on the internet, I can see what was changed and why. Euro21's concerns about the large input capacitor are probably justified. It's a question of how long the 5AR4s will last. Watch the video he linked to. If you want to keep the amps, you might want to see if there's someone close to you who could do some modifications.
I finally watched the video. He mentions the 56mF 500V cap you circled and the green 63ohm resistor. It would be easy enough to install those, but that 56mF cap is between pin 8 on tube socket and left terminal on large cap. However, the red wire on mine goes between pin 8 and the right terminal on large cap.
I'm also seeing a couple other things that aren't on my amp. Like the diode I circled below and the blue wires on the tube sockets. Mine also has the two diodes on the tubes sockets that aren't on his amp, as well as the black wire that runs from the diodes to the left side of the large cap. I've attached a new image below that shows the wiring in my amp a bit better.
So there are several differences between my amp and the amp in the video, and I'm unsure about how to modify my amps to protect the rectifier tubes, or if it's even necessary. I'm also seeing several other differences that I haven't mentioned since they may not be relevant to the rectifier tubes. Unfortunately, there is no one in my area that works on tube amps like this.
AMP FROM VIDEO:
MY AMP:
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In your amp, the fix would look like this. Remove short red wire, solder resistor between pin 8 of the left 5AR4 to positive terminal of large cap. The new capacitor would go positive to pin 8, negative to ground terminal of large cap. Forget the rest of the differences between the two amps, he's made a lot of modifications that aren't relevant to your amp.
That's looks easy enough. In the video he says the green resistor is 63ohm, but doesn't mention wattage. Do you know what would be recommended here?
Do I only need to add the one resistor? The amp in the video has a second green resistor across the #2 pins where the short yellow wire is on mine.
Thanks very much for all your help with this.
Do I only need to add the one resistor? The amp in the video has a second green resistor across the #2 pins where the short yellow wire is on mine.
Thanks very much for all your help with this.
I would use a 50 ohm minimum 7 watt resistor. I don't know what the other green resistor is for, again it's not relevant as far as I can see. The guy may have a point about rectifier life span, but otherwise he's kind of trashed the amp...IMHO.
https://partsconnexion.com/mills-resistor-51r-ohm-12w-mra-12-series-1-tolerance/
https://partsconnexion.com/f-t-capacitor-47uf-500vdc-series-a/
The axial cap will be easier to install. Be sure you sleeve the leads on both the cap and the resistor.
https://partsconnexion.com/heatshrink-3-64-yellow-polyolefin-2-1-shrink-ratio/?searchid=517619
(Boy, their web site is a PITA to navigate now...)
https://partsconnexion.com/mills-resistor-51r-ohm-12w-mra-12-series-1-tolerance/
https://partsconnexion.com/f-t-capacitor-47uf-500vdc-series-a/
The axial cap will be easier to install. Be sure you sleeve the leads on both the cap and the resistor.
https://partsconnexion.com/heatshrink-3-64-yellow-polyolefin-2-1-shrink-ratio/?searchid=517619
(Boy, their web site is a PITA to navigate now...)
The cap could also go like this, where the two diodes connect to the tag strip and the black ground wire:
Thanks again! That's incredibly helpful. The second option you described seems like it would be easier. So I may go with that.
Please use great caution around tube circuitry. There are extremely dangerous high voltages present, both AC and DC.
I want to live. 
In the interest of safety, I would like to collect whatever safety tips and practices this forum kindly and experienced members will share, so that all may benefit.
First, some warnings and disclaimers:
In the interest of safety, I would like to collect whatever safety tips and practices this forum kindly and experienced members will share, so that all may benefit.
First, some warnings and disclaimers:
- Any voltage can be deadly.
- Amplifiers are dangerous--each person (me included) is responsible for deciding if they are willing to risk their life, and for ensuring that whatever information they receive is correct
- This forum and its members accept no responsibility for any death, injury or property damage that result from any...