Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Hiwatt25 on August 18, 2025, 04:50:52 pm
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I've built a couple amps from kits but for my next project, I would like to scratch build a 6G6-B Bassman. My plan was to source the appropriate transformers but I'm clearly out of my depth because the replacement transformers I've found online puzzle me.
The 6G6-B replacement transformers available at Mercury and Pacific Audio Magnetics have 360V secondaries but the 6G6-B schematics I've seen show 428V on the plates of the output tubes. It's my understanding that the secondaries of the power transformer is the source of plate voltage, right?
So, am I looking at the wrong replacement transformers or is there something about this circuit I'm ignorant about. I have a feeling I'm just missing something and would be grateful for any help.
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The PT secondaries are AC. These need to be rectified, filtered and loaded to get to the final B+ DC voltage
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... 360V secondaries but the 6G6-B schematics I've seen show 428V on the plates of the output tubes. It's my understanding that the secondaries of the power transformer is the source of plate voltage, right?
So, am I looking at the wrong replacement transformers or is there something about this circuit I'm ignorant about. ...
Review 1.7.4 through 1.8.4 of NEETS Module 2 (http://compatt.com/Tutorials/NEETS/New_PDF/14174A.pdf) (about AC), and 4.2.1 to 4.2.2.5, then 4.2.3, 4.2.3.1, 4.2.3.3, and 4.2.3.4 of NEETS Module 7 (http://compatt.com/Tutorials/NEETS/New_PDF/14179A.pdf) (Solid-State Power Supplies).
They will tell you the rectifier/filter-cap charges to a DC Voltage output close to the Peak Voltage Output of the transformer. This post (https://www.thegearpage.net/board/index.php?threads/mercury-magnetic-transformers-for-blonde-bassman-6g6b-6g6c-build.2563652/post-38990096) says you want 323v AC output from a power transformer to match the 125P19A power transformer, and this post (https://www.thegearpage.net/board/index.php?threads/transformers-for-bassman-build-help.2513700/post-37965365) says Mercury Magnetics cloned his Blonde Bassman's 125P19A and offers it for sale as model F-BMAN-P (https://www.mercurymagnetics.com/product/f-bman-p-2/).
323v AC x √2 = 456.8v DC, and after some losses due to the cap not-being ∞µF a DC Voltage output of about 430v DC is believable.
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Holy smokes, thank you for filling one of the gaps in my understanding. I'm recently (mostly) retired so I've been dedicating time to an interest I've put off for a looong time.
I really fell in love with tube amps when I was 19, working at a little guitar shop in Saratoga, Ca called Torrez guitars. The owner always happily answered my questions and even gave me his old Hickok tube tester for Christmas one year...but I digress. I've been reading Dave Funk's book and frequenting Uncle Doug's YouTube channel and learned a lot but clearly have much more to learn. Which is all just to say, I'm grateful for the help.
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... filling one of the gaps in my understanding. ...
NEETS Modules (http://compatt.com/Tutorials/NEETS/NEETS.html) cover basic electrical/electronic topics. Modules 1-4 and 6-8 are good background knowledge to have for understanding amp-electronics.
Pete Millett's site (http://tubebooks.org/technical_books_online.htm) is another great resource for old tube books.