Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RadioComm on May 30, 2020, 07:20:54 am
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Good Morning,
Just finish my 6v6 plexi build. It is a gutted Fender Super Champ XD, for which I am re-using both the power and output transformers. Unfortunately not working. I have low voltage on the output of the rectifier diodes.
134 VAC - input of the 1n5408 connect to the bias circuit
155 VAC - input of the other 1n5408
290 VAC - across both diodes
35 VDC - on the output on the A rail. Not 402 as in Doug's schematic.
Checked direction. Good. Figured bad diode, so replaced both of them, but am getting same reading. I have attached a picture of that section in case it helps.
Thanks for the help,
Paul
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From this picture doesnt look like your capacitor is connected to the rectified side of the diodes on B+1.
Looks like it comes out of rectified diodes and goes straight out of chassis to ??? choke I assume??
If not the remove and check the actual capacitor.
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Doug's 6V6Plexi is designed to work with a power transformer designed for a conventional rectifier and bias circuit. Unfortunately your Fender Super Champ XD transformer will not work without considerable modification. You will need a full wave bridge rectifier and a bias supply that works with a full wave bridge rectifier. I don't think there's enough room on Doug's board for the necessary modifications, but probably a couple extra terminal strips would accommodate the required components. Study the Fender Super Champ XD schematic to understand how the PT is different.
If you don't understand this info you may want to just get a proper PT. Probably much easier.
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Thanks for the replies.
Johntb: Rechecked connections: Seem correct. A Rail -> OT red, choke, 40uf y lead. B point -> Pin 6 power tube, choke, 20 uf x lead.
Sluckey: Didn't realize different PT's needed different rectifier types. Thought out VAC was out VAC. But guess not... What's the difference? Anyway easy is good. Will get new PT and OT while I'm at it. Certainly beats redoing the entire circuit board to accommodate a full bridge rectifier and its bias circuit.
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The difference in PT (power transformer) is that you have 2 HV (high voltage) taps without a CT (center tap). I am no expert but without the CT to ground, as it should be, your cap won't work. If your cap was charging your voltage would be significantly higher.
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Makes sense. New PT with CT it is then. Thanks.
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Sluckey: Didn't realize different PT's needed different rectifier types. Thought out VAC was out VAC. But guess not... What's the difference?
Well, your PT only outputs 290VAC and it has no center tap. Your PT can produce 410VDC unloaded ***IF*** using a Full Wave Bridge rectifier.
Doug's PT puts out 614VAC and it has a center tap. His PT can produce 434VDC unloaded using a conventional 2 diode full wave rectifier.
So you see the B+ operating voltage can be roughly the same from either PT ***IF*** you use the proper type rectifier circuit. The same goes for the bias voltage but you need a different circuit for that too.
Your PT will work fine for the 6V6Plexi but you gotta do the mods. If I had that PT I'd use it.
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Here's some good/easy reading on rectifiers...
http://www.valvewizard.co.uk/bridge.html
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Understood. Good lesson. Also really appreciate the link for the additional learning.
Looking at Heyboer 18 watt PT now. Hoping same dimensions as previous PT. Fingers crossed. Otherwise may have to pull out the nibbler, dremel, and drill. I'll save the Fender PT for another project. I know I could pull out the garolite board and cut out a piece for the full wave rectifier and bias supply. Even though there is an OT underneath the chassis, I think there would be enough real estate in the chassis to fit the new circuit. But I'd rather easy/quick, and the correct PT seems like the most expedient solution.
Thanks for the knowledge.
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FWIW- I built the PLexi 6v6 using a Hammond 270 FX PT. It is 275-0-275 volt and was concerned that the voltage might be a little low. With a diode rectifier I am getting 409 VDC at the first filter cap.
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Good to know. Thanks labb. Options are always good. The 18W PT wasn't an exact physical fit, and MojoTone is temporally out of stock. I will definitely look into the Hammond. Trying to avoid any additional punching, nibbling, and drilling.
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Did some research. Not going to find anything 'plug and play' dimension wise. It was a long shot. Going with the Hammond 290PAZ. It's 290-0-290 140ma / 6.3 2A. It's a replacement for the 18W Combo.
https://www.hammfg.com/part/290PAZ?referer=1063
Thanks to all for help and the knowledge.
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Here's some good/easy reading on rectifiers...
http://www.valvewizard.co.uk/bridge.html
Wow. Thanks again for the link. Really good/easy reading. I now have an understanding of the PT / rectifier type relationship. No wonder the DC voltage was low... :-)
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It works! The correct PT did it. Did have afterwards an OT feedback issue, but the fix was as easy as swapping the OT primary wires. Easy enough. Once again thanks sluckey for picking up on the PT. I wouldn't have never picked up on that. Saved me a lot of heartburn. Plus the valvewizard page will be knowledge that I can reuse.
And it sounds awesome! I will record some samples and post them to the sound clips forum.
Thanks to all.
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:thumbsup: