External Bias Pot and Test Points

[gearhead]

Has anyone completed an Express with an external bias pot (thru top of chasis) with test points too? How did you implement it wrt method (x ohm resistors at y socket points etc,) and locations?

The real estate around the Francesca bias pot is pretty crowded, and don't know if running everything off into the "rural" part of the chasis would be problematic; lossy, crosstalk, etc. I did do a search, found a few "I'm going try it xyz way", but couldn't find an implemented solution with feedback report or pics.

Thanks all. A great, great forum with a great bunch of ampheads.

edited because I couldn't spell things right the frist tiem.

[UR12]

Gearhead

Here is a thread by Jeph where he did exactly what you're talking about.

https://tubeamparchive.com/viewtopic.php?t=620

[Jeph]

I'm actually thinking of pulling them out. Not that I think it makes much difference, or is causing any issues with the amp, but I'm finding the amp is pretty much set and leave alone. I put -30 on pin 5 and I haven't really had to touch it with either 6v6s or 34s.

[gearhead]

Jeph's post is the one that prompted me to ask about running off into "rural" chasis real estate. Did it work? How were the runs lead dressed to minimize probs?

On the other hand, Elcabong put his test points (and bias pot?) right next to the power tubes on the chasis. How did that work out?

Just looking some post-build info.

My more basic question is not necessarily unique to express's, but while I'm here :

Given the EL-34 pinout:

Pin 1 - g3 (suppressor)
Pin 2 - Heater
Pin 3 - Anode
Pin 4 - g2 (screen)
Pin 5 - g1 (control)
Pin 6 - N/C
Pin 7 - Heater
Pin 8 - Cathode

Your biasing by changing the bias pot value (20-25k) to ground as the last in the chain of voltage drops across a 47k(?), 220k, and a 1.5k resistor connected to pin 5.

You can measure the effects of the grid bias by measuring at the cathode? By putting a 1 ohm resistor between the cathode and ground, you can tell the voltage (and current) by measuring the voltage across this resistor, and hence the current (V=IR).

My q's are: why should there be any voltage across this resistor if pin 8 is connected to ground already? Even if you disconnect the pin 8 to ground and run it only thru the 1 ohm resistor, isn't this artificially creating a voltage not really there in an amp without this setup?

Thanks,
Dave

[Jeph]

I just kept the wires close to the chassis, and tried to stay away from everything else. I do have some noise in the amp, but after poking around the wires added for the bias pot/probe jacks I don't think they're the cause. Here's a pic of how I ran 'em:

[img768]http://www.chassi-swing.com/amps/express/guts.jpg[/img]

[gearhead]

Jeph,

Thanks so much for sharing the pic and feedback! It's not a "classic" Trainwreck, but if it doesn't affect the tone or responsiveness of the original design, I'm all for sensible mods too.

I could post my practical app bias q's elsewhere if there are no takers.

Thanks all,

Dave