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Hi all.

I'm planning on a 5E3 clone build, salvaging parts from an old Elk amp. The original Elk amp had a pair of 7189 power tubes, an EZ81 rectifier tube, and a bunch of noval pre-amp tubes. For this reason, the PT has a 6.3v tap, but no 5v tap required for the regular 5Y3 rectifier that one would usually find in a 5E3 circuit. Other than the rectifier heater voltage, the PT is okay for the 5E3 circuit (320v - 0 - 320v ample heater current etc)

After an bit of digging, it seems that the EZ81 and 5Y3 have pretty similar ratings, other than heater voltage. Any reason I can't substitute the EZ81 or the 5y3 in this build, all things considered equal, or am I missing something?

Thanks in advance.

Should be totally cool, I did a bassman with 6v6s from a recycled chassis with a 6ca4 before and it was great.

KindaFuzzy wrote:Should be totally cool, I did a bassman with 6v6s from a recycled chassis with a 6ca4 before and it was great.

Hey KindaFuzzy, thanks for the response.

In terms of powering the EZ81, pt says that it puts out 320v - 0 - 320v on the label, but tested with no load, it's actually 350v. I know that this is to be expected, since the PT is rated under load conditions, but my question is : Do I need to use any series resistors (as per the EZ81 datasheet) in order to not over-volt the rect tube? The datasheet states that the max anode voltage for the EZ81 is 350, but should be used with series resistors, to be safe. Would connecting it direct (as it is in most guitar amp circuits) damage it in any way?

I've attached the schematic of the original ELK circuit that this EZ81 was powering. I'm planning on using the same PT btw.

As a matter of interest, I read the same for the 5Y3 tube, but I've never seen them being used (in guitar amps anyway) with any series resistors, and they sem to be just fine. Would the same be true of the EZ81, under similar conditions? It seems that aside from the filament voltage, they should be able to operate within similar tolerances.

Any opinions?

Thanks.

You won't hurt anything by not putting resistors there, but I'd rather do it properly if it's a new build. I'll have to read my valve wizard books when I get home after the holidays, but from what I remember reading, you have to measure the resistance of the B+ winding, then subtract that from the min anode resistance for the 6ca4, and add extra resistors if the PT resistance is lower than the specified minimum. I think the 5y3 minimum resistance is pretty low, so you can get away with just counting on the PT winding giving enough resistance on it's own. As for the max voltage thing, don't worry, if it was working with the same circuit before it'll be fine. I have a feeling your B+ will be up around 400V dc though if it had 7189s, use RG Keen's mosfet follies B+ reducer if it's too high after you build it.