Any ideas why this mid pot is not working?

[Bluefinger]

Hi,

https://taweber.powweb.com/store/5e7m_schem.jpg
https://taweber.powweb.com/store/5e7m_layout.jpg

I am using this tone stack in an amp build but the added mid pot is absolutely useless. Nothing happens when I adjust it, even with a 250K pot. I have double and tripple checked everything twice and it's wired correctly ... any ideas? Since it's posted on the weber site, I assume it should be working ... any ideas? Has anybody tried it yet?

thanks!

[tubeswell]

Unsolder the pot and clip your Rmeter cross each of the lugs to make sure nothing is open. Could be one (or more) of the terminal lugs has lost contact with the resistive trace inside the pot - in which case you'll have to ditch that pot.

[Bluefinger]

naw, that's not it. I have already tried that and used two different pot values (100k, 250k) without any success. Resistance lug to ground is fine ... it's like that schematic isn't working at all. But many seem to be happy with their 5E7M so I guess that tone stack should be working propperly. I don't see how .1 coupling caps or a long tailed PI would make that pot completely useless either so I'm a bit confused now ...

[R ski]

The configuration doesn't look right, surf the web, look at various tone stacks used after a cathode follower, similarities will be recognizable after some redrawing and you would be surprised how one error in a tone stack configuration will render one tone control useless.

After many years of building guitar tube amps from scratch, I learned that if a drawing is wrong a minor correction will resolve the issue.

[Bruce / Mission Amps]

Off hand, I don't really see how that mod circuit would greatly effect the mids anyhow...Anyone else???

[loudthud]

Off hand, I don't really see how that mod circuit would greatly effect the mids anyhow...Anyone else???

Playing with Duncan's Tone stack calculator, the E-Series circuit is really an interactive mess. I don't have an amp with that type circuit to play with but I might try temporarily installing one in a 5F6A type amp as an experiment.

In the typical 5F6A circuit you have a low pass and a high pass in parallel. The high pass is pretty straight forward, except the lows are fed into the bottom side of the Treble control. This side has to be a relatively low impedance to highs for the control to attenuate the highs.

The Mid control is in the ground side of the low pass capacitor so increasing the resistance from ground brings up the mids (and highs) instead of just completely rolling them off. The Bass control is more or less in parallel with the low pass capacitor so it can attenuate the lows by shorting them to ground.

If you look at the Bass control in the 5E7 circuit before the Weber Mid mod, one side of the pot has the .005uF cap to ground and the other end is just grounded. When the pot is at the end with the cap, a low pass network is formed and the lows can proceed to the output. When the Bass pot is at the ground end, nothing gets pass the control to the output. The Treble control looks like a high pass network but the cap in the ground leg of the pot allows highs (that can bleed thru from the Bass control) to be shunted to ground. The Mid pot is positioned where the Bass and Treble networks normally would go to ground. As the resistance to ground at that point is increased, Mid frequencies are allowed past the Bass control as long as it's not at the end with the cap. At the same time, Mid frequencies won't be attenuated as much by the Treble control if it is in the lower half of it's rotation where it just shunts the highs to ground. So the Mid control is going the interact with the Bass and Treble controls quite a bit.

One change that might help is to connect the Bass control's .005uF cap to the Mid pot instead of to ground.

[Bluefinger]

Thanks, that makes a lot of sense ... so the mid pot is least effective at the bass and treble settings where they would normally be when used with harp.

For guitar I have come to like the 5F4/5E7 bass treble configuration but it seems like the Weber mod is a bad choice for harp. I think I will make another board with a 5F6A style tone stack and see how this one will work. I hope it won't change the overall sound of the amp since like it very much, except for the useless mid pot.