Old Selmer PA amp - some help reqd.

[Reeltarded]

If the NFB connects to the speaker jack instead of a particular tap you will notice a difference, otherwise no.

4 ohm tap less voltage, less headroom, less NFB. 8 ohm tap more voltage, better headroom, more NFB. 16 ohm tap most voltage, most headroom, most NFB. Early Marshalls have lots of NFB. 27k on the 16R typically. That is where I would set and forget that part for now.

You are trying to cheat physics though. When you get clean headroom you lose volume, when you get volume you lose clean headroom. For a bass the best you might hope for is a lower volume than youd like but it might sound very good. Expectations for 100w and bass might be a little high.

No reason to stop tinkering.

[old tube]

well I've tried a couple more values and at 5.6k parallel with the 15k the distortion is sufficiently tamed and it's still loud enough to shake the house.

I'll rewire the speakers tomorrow and see how it is at 16ohms. But tonight it looks like a result!

Really appreciate your help.

Martin said that really the permanent change should be made to re-proportion the 2k2 and 100R. I'd really appreciate some help with that, if possible.

[martin manning]

The schematic shows the FB signal coming off the 16-ohm tap, so it's getting the full monty. If you have a good result with the 5k6, I don't see any reason you couldn't keep it that way. Othewise you'd have to figure out a new combination of the 2k2 and 100 ohm to get you the FB you're looking for. With that combination you have 15k//5k6=4.1k, much bigger than the 600 ohm there going to ground, making it 520 for the combination.

Using a different speaker load, connected to the correct tap, shouldn't make any difference as long as the FB stays on the 16 ohm tap.

[old tube]

Indeed the FB comes off the 16ohm tap. Changed the speaker config. to 16ohms. Compared to running out to 4 ohms, I'd say the amp now has a "bigger" sound, a little more clean headroom and the bottom end is definitely "deeper" and more satisfying. It is remarkable the difference this simple mod has made! I'm back with 12AX7 in V1 but I've kept a 12AT7 in the PI.

Given me exactly what I was looking for. This rig will have no problem being heard and felt at small / medium venues and has the deep tone I wanted with just a hint of growl when you hit the strings hard.

Can't thank you gentlemen enough.

[Reeltarded]

You can thank Martin enough. He accepts cash and rubber chickens.