Issue With Bassman 5F6A Build

[martin manning]

Looks like there is a bug in the TSC.

It's been stated her before by Gary and Scott L IIRC that the TSC is not accurate.

Not accurate? How about out to lunch!

[Guitarnut]

Looks like there is a bug in the TSC. Here is a SPICE comparison. You can measure from the top of the treble pot to ground with the bass pot all the way up and look for 1275k, then turn the bass pot all the way down and look for 275k.

Does the settings of the treble and mid pots matter?

[martin manning]

Does the settings of the treble and mid pots matter?

No, the wipers are isolated by caps.

[Guitarnut]

I show 1238K all the way up and 299K all the way down.

[martin manning]

Sounds ok... the bass pot must be 939k.

[Guitarnut]

Sounds ok... the bass pot must be 939k.

Maybe they're interactive enough that I'm just not hearing what I think I should. But I would think at some point I could make it sound thin and it just never loses any bass response...could be the cab and speakers I guess.

I'll try the cap and resistor change and see what happens.

Thanks again,
Mark

[Guitarnut]

In thinking about tone controls in general, the higher the number setting, the less of that particular band is being sent to ground. With NFB on the presence control would that be the opposite? Since the wiper is going to ground, the lower the number the more NFB applied to the circuit and as the control is turned up, more NFB is being sent to ground?

Here's why I ask. I just set my presence to 12 and the tone stack is far more responsive. I think I like it much better this way. To my ears, it's not really adding presence as much as it is dynamics once I get the TMB dialed in.

Just an observation.

Peace,
Mark

[martin manning]

That's correct. The presence cap allows higher frequencies to bypass to ground, so there is less HF FB, which increases the loop gain for those frequencies.

[Structo]

I always wondered why the TSC showed the bass roll off like that.

Too bad, could have been a great tool....

[Firestorm]

I always wondered why the TSC showed the bass roll off like that.

Too bad, could have been a great tool....

I wondered, too. But since the rolloff it depicts happens mostly at frequencies that aren't present to begin with, I ignored it. Thinking about it though, there has to be a rolloff somewhere on the bass side since passing lower and lower frequencies requires the caps to become exponentially larger. I still find TSC to be directionally useful, but since it can't know how frequencies are shaped before and after, you never get quite what you expect.

[Reeltarded]

I never sweep it. I use it as a median baseline.

It is useful like that.

[Guitarnut]

Thanks again for all the help and support. I am really glad I stopped in at The Amp Garage. You guys are amazing!!!

[IMG:800:600]http://www.crenshawweb.com/bassman/fin01.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/bassman/fin02.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/bassman/fin03.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/bassman/fin04.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/bassman/fin05.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/bassman/fin06.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/bassman/fin07.jpg[/img]

Peace,
Mark

[statorvane]

Great looking cab. Nice finger joints and finish.

Is that a functional ground switch left of the fuse holder?

[Guitarnut]

Great looking cab. Nice finger joints and finish.

Is that a functional ground switch left of the fuse holder?

Thanks! The switch is just sitting in there to fill the hole. I wouldn't fool around with a ground lift.

Peace,
Mark

[JazzGuitarGimp]

That's one helluva cabinet. Did you make that, Mark?