tube substitution and amp modification guidelines

[sad mafioso]

I recently picked up an old Bogen MX60A PA head, with the intention of using it for vocals during my band's rehearsals. I didn't do too much reasearch before buying this amp, but it was dirt cheap, so I figured 'why not?'

It wasn't until I brought the thing home that I found out it uses 8417s in the power stage. I'd like to be able to use more readily available (and affordable!) tubes in this thing, like KT88s or 6550s. I scoured the net for info on how to do this, but I managed to dig up was the Quicksilver article at Triode Electronics.

I think the QS article has some great info, but I wouldn't be surprised if those mods won't translate over to the Bogen. So here's where I'm hoping you guys can help me out: First of all, what is it about an amp that makes substituting tubes more than a just case of swapping in new tubes and rebiasing? (Assuming they have the same pinout, of course.) Do these types of mods only serve the purpose of supplying the right amount of bias current, or is there more to it than that? What other things should I consider when planning a modification of this type?

-Nick

[Sch3mat1c]

Why? Everything. Gm, Rp, plate, screen and grid voltages and currents, dynamic characteristics such as peak and cutoff currents, etc.

For instance, IIRC 8417 was made with extreme Gm so it took very little drive capability = cheaper circuitry. So bias voltage may only be maybe -20V, whereas for a 6L6 or KT88 type you'll need -30 to -50V. That peak current is different means either a different screen voltage or load resistance for optimum conditions.

Lastly, the driver may not be able to handle the extra drive it has to provide. Chances are it uses a cathodyne or something, which can't provide much drive on its own.

To sum, to change output tubes may require completely modding the amp... at which point it obviously won't be a Bogen MX60A anymore.

Tim

[SHiFTY]

Well assuming the tubes in it are OK, you may get many more years of useful life out of it. Power tubes can last for many years, even decades, if run within their ratings. Old tubes have a cooler sound too.

One thing you will want to do is replace any wax paper capacitors in it (very important!) and any selenium rectifiers. You might want to replace all the capacitors in the thing, actually: but if the big can caps are ok (not leaking goo) then just leave em.

If you have a schematic, post it, I'm sure people here can tell you how to modify it for 6L6s or whatever. Tubes are EASY, man!

[SHiFTY]

Hmm, after further investigation, it seems to have a voltage doubler power supply, from a SS rectifier, and the output tubes are well known for their frequent meltdowns. You might have to do some more drastic mods to get this working right.

I trust you know basic electronics, and how to solder?

[sad mafioso]

Originally posted by ShiFtY
Well assuming the tubes in it are OK, you may get many more years of useful life out of it. Power tubes can last for many years, even decades, if run within their ratings. Old tubes have a cooler sound too.

That's encouraging. I have no idea how old the tubes are, but I figured it would be best to replace them. I think the amp was made sometime in the 60s.

Originally posted by ShiFtY
One thing you will want to do is replace any wax paper capacitors in it (very important!) and any selenium rectifiers. You might want to replace all the capacitors in the thing, actually: but if the big can caps are ok (not leaking goo) then just leave em.

Done and done. Well, not actually done, but I have new caps ready and waiting to go in.

Originally posted by ShiFtY
If you have a schematic, post it, I'm sure people here can tell you how to modify it for 6L6s or whatever.

Unfortunately, I don't have a schematic. The only one I know of is a Sams Photofact, and I'm not sure if posting that on the web will fly.

Originally posted by ShiFtY
I trust you know basic electronics, and how to solder?

Yeah, I recently built a tube guitar amp, so I'm not totally in the dark here.

[SY]

Here's what I'd do: pull the output tubes, then using a scope and generator, see how much drive voltage the drivers can provide. if it's more than 50V peak (100V p-p), you're in gravy. Now, see what voltage the bias supply can provide; measure the voltage at the output of the supply at the top of the voltage dividers which are used to adjust bias. If it's 60-70V or more, you're in good shape.

If you pass both of these tests, your amp can be modded pretty easily. It will be a matter of resizing a resistor or two in the bias string, some minor resoldering at the tube socket bases and (depending on what the output stage looks like) making a tweak or two to the operating points. Is this configured as a pentode, triode, or ultralinear?

[sad mafioso]

I think it's ultralinear. However, I don't have access to a scope or signal generator, so I think this may be out of my reach at this point. I guess I'll just stick with 8417s for now. Thanks guys!

[Original Burnedfingers]

Already put 6550's in a Bogan with no ill effects. Bias set ok and front end has plenty of drive capability.


J