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EL34s need to have pin 1 grounded, is that right? And the 6550 will be OK with that if I swap them back in? I see on the 6550 data sheet it says pin 1 is NC or base shell. What is base shell?
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6550's have a metal base shell that connects to the cathode. This poses no problem unless the amp is cathode-bias, then you need to use the Sovtek 6550WD, which has a phenolic base.
Beam power tubes (6550, 6L6, 6V6, etc.) have Pins 1 & 8 connected internally (cathode to beam plates). EL34's, which are power pentodes, need to have their suppressor grid connected to the cathode externally, hence the reason for the jumper. -
Great! Thanks for the info, John.Comment
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Say, how are the EL34s gonna feel about a 1.5k grid stopper? If I have to up that to 5.1k, will the 6550s squawk about it?Comment
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6550's have a metal base shell that connects to the cathode.
The metal base shell connects to pin 1. The Cathode is on pin 8. There are 6550 amps out there that have B+ *(450-600VDC) connected to pin 1, using the n/c? pin as a terminal point, probably before the metal strip ones came about. I have found this out the hard way, by getting myself between the shell and chassis. Only if you jumper pin 1 and 8 together will the metal base shell will be connected to Cathode. An EL34 has the Suppressor Grid (G3) connected to pin 1. The common convention is to connect this grid to the Cathode by jumpering pins 1 & 8. There are EL34 amps out there that connect pin 1 to the negative bias supply. Some Traynor amps were designed this way. I remember trying to convert one to a grounded Suppressor once and it didn't respond well at all. Red Plates galore. I might have missed something along the way, but putting it back stock made the amp run stabily, so I figured - why fight it.
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P.S. - I don't think you will have any trouble with 5.6K Grid resistors. Seems to me that Marshall has used either those or 1.5K for years. If in doubt - try them both and record the results for future reference.Comment
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OK, well I don't have cathode bias in this amp nor do I have B+ connected to pin 1, so I should be OK to try a swap.
Right now it's got 470R screen resistors and 1.5K grid resistors. Just for a quick test, that shouldn't bother an EL34 shoud it?Comment
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