Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => PC Express and JSchem - Schematics and Layout diagrams => Topic started by: tubenit on February 20, 2009, 08:54:03 pm
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The schematic is a version of a 6L6 Champ that Big Daddy drew up and built. I added a layout to it.
Thought I'd post this for others who might be building a 6V6 or 6L6 Champ and wanted a schematic to edit.
With respect, Tubenit
You can find most of these schematics in the "Broken Links" file. Write down the name of what you are looking for and then click on to this Broken Links file list. Some of the files on this thread may open and some may not. Use the list on the "Broken Links file" to hopefully find what you are looking for:
http://el34world.com/Forum/index.php?topic=17420.0 (http://el34world.com/Forum/index.php?topic=17420.0)
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My 6L6 VibroChamp conversion is in this older thread: http://www.el34world.com/Forum/index.php?topic=610.0
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Here is my updated schematic with NFB & RAW control.
How I got here from the last set of mods: One of the input jacks went bad; the shorting leaf spring would not hold tension. So I built a new input jack assembly. While inside I replaced the old cap can. I had a 4X 20uF can on hand, so I used that and put 2X 20uF caps in parallel for the 1st filter stage. 40uF is the stock value for the AB764 VibroChamp.
Then 2 old problems kept recurring: a) intermittent squealing; b) nasty static like distortion on the attack of an overdriven note or chord. A voltage check showed oscillating DC voltage exceeding 300mV on the far side of the coupling cap to the power tube. This was a blue vintage cap shaped like a Sprague radial cap. To confirm, this cap tested bad on my cap tester. It was replaced with an orange drop, which I had on hand. Otherwise I may have been more creative with cap-type selection. The distortion was tamed with a heavy dose of NFB; also I re-inserted the local feedback suppression cap on the power tube. That smoothed-out the overdrive tone. (I tested NFB values with a 50K pot in series with the stock NFB circuit now tapped-off the 16 ohm OT secondary.)
Preamp gain was fine-tuned to 3.3K cathode resistors to match the 220K plate resistors on the gain stages. A/B tests were done with a Strat. That seemed to produce the best tone, but it's subtle differences.
The RAW control sounds very good on this amp; much better than on my Princeton which gets very ratty sounding with its new RAW control. I think I need to go back into the Princeton and play with NFB values in the hope of taming this rattiness. Will this ever end? :D
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Photo's of 6L6 VibroChamp. On the rear chassis is a standby switch & RAW Control. On the chassis top is a choke where the stock OT used to be (see next panel). The choke is exactly the same dimensions as the stock OT! On the sidewall of the cab is a supplemental chassis, recycled from an old '80's stereo vol control unit. The new Hammond 125DSE is mounted on the far aside of the supplemental chassis, which houses an impedance switch, 2X phono speaker-out jacks; and an RCA jack for NFB back to the main chassis. The OT primary is fed by 2 banana plugs from the main chassis. The Hammond OT is way too large to fit on the main chassis.
Speaker is a 10" Emi 105 @ 16 ohms (I thought I was buying an 8ohm speraker on eBay but what the heck.)
As you can see lead dress is nothing special. Somehow, the amp is dead quiet.
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More gut shots & chassis top.