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Vintage threads from the first ten years

Marshall 6550/EL34 tube swap

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10/9/1997 12:39 PM

Aaron

Marshall 6550/EL34 tube swap Last weekend I converted my friend's 6550-equipped JMP MV to EL34s. Here's the problem - I used an EL34 schematic, and my friend's amp matched the schematic bias circuit, except his amp had 5.6k's on all the grids.     Thinking these resistors were what made the bias work for 6550s, I removed them, resoldered the leads, cranked the bias pot full-up, checked the bias voltage (-39V), installed the tubes (Sovtek EL34WXT), let it warm up, then played it. The plates didn't glow, and the amp sounded great! I know, I know, I really should check the idle current. I will this weekend when I get my hands on it again...     But now, I've learned from "other web sources" that the resistors I removed were grid stoppers, and should have stayed in the amp. They weren't on my schem, and the rest of the circuit matched my EL34 schem, so what gives?? If I put the grid stoppers back in will it bias up right for the EL34s? Did Marshall just plug 6550s into an EL34 amp?? What's going on here??     As I recall (fuzzily), the amp has a 22k bias pot, 56k upstream of that, and a 15k on the opposite node of the 56k (the node where the cap is)...What do I need to do to do a "correct" EL34 switch?

 

10/9/1997 12:54 PM
Steve Morrison	The first thing you should do is to measure the idle current of the EL34's. If you can get them in range with the existing bias supply circuitry, then no further modification is needed.

 

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