Bias Pots

[Wayne Alexander]

I attended one of Gerald Weber's amp seminars two weeks ago. Gerald said that it was dangerous to use a standard (carbon film) pot like the ones we normally use for volume and tone controls on an amp for bias, because the thin carbon film can wear/arc out and then there's unlimited current flow through the pot. He suggested using a cermet pot. Those are often very small, and I wanted a big pot just because, and the only one I found was this Bourns:


http://www.surplussales.com/Potentiometers/Pots-4.html

part number (RWV) 3500S-20K Panel Bushing Mount Multi-Turn $ 15.00
Potentiometer, 20k Ohm

I had used a standard Alpha 25K carbon film pot before, but Weber convinced me to upgrade, so there it is. There's a picture of it in the "How about we post some pics and layouts?" thread, just to the left of my power tube sockets. It's blue.

[Omar]

Wayne,
Thanks for the info. So how was the Weber seminar? Can you tell us some details about it? Was it worth the $$?

Omar

[Wayne Alexander]

Omar, I think the Weber seminar was worth the $500. I don't have an engineering background, and it was a useful tutorial on how and why power supplies work in guitar amps, how voltage division works, how tone controls work and what can be done to change the frequency response, how capacitors work in general and their functions in amps. If you already know the engineering stuff it may be less useful, if you don't its a good thing. Some of it is a sales pitch for stuff Weber sells, but he's a good guy and knows his stuff. I dont agree with him on everything (his taste in tubes is good, but I think mine's better, for instance) but he's a good resource for a semi-private amp engineering tutorial if you need that. He also runs "amp camp" down at his facility in Texas, which I gather involves bringing some amps down there and going through them in a tutorial setting with him and his ampbuilding/ampfixing staff helping out over the course of a several day session. That might be useful as well.

[Allynmey]

How do I find out when/where for a Weber seminar? I think I'dlike to go.

Allynmey

[Wayne Alexander]

Weber describes his seminars on his Kendrick Amps website (also in Vintage Guitar Magazine ad). scroll down this page http://kendrick-amplifiers.com/Merchant ... ry_Code=05 to see the description, then click the "tube amp seminar" purchase link to see the schedule (that doesn't commit you to buy it.

[Dai H.]

maybe he's a nice guy and all, but I'm not sure if I'd rely on him for technical information. For example, re-forming alu electrolytics by putting them under a lamp(!). Another time, (IIRC) a transformer engineer (a guy who is a childhood friend of the guy who runs EMC--which is the company Kendrick uses to reverse engineer and build their trannys) took him to task at some gtr. show for advising people to use Rustoleum on transformer laminations (again, IIRC--not good because of the carbon content in the paint). So he's not exactly an engineer. Carbon (composition--I *think* he means) pots for bias circuits, I would think it depends where/how a bias pot is placed in the circuit whether it matters if it goes open. For example, common to a lot of Marshalls, the bias pot and a fixed R is between C- and ground--if the pot goes open you just get more negative bias voltage which runs the tubes cooler, so it seems a bit alarmist to say carbon pots are dangerous to use in bias circuits.

[Flames1950]

Aren't the bias pots in the vast majority of blackface era Fenders carbon comp, or are they a different composition? Look like any other CTS/etc. brand pot that Fender used, except for the screwdriver-friendly shaft.

[Omar]

Flames - first off welcome to the board. Glad to have you here!

You make a good point. Every blackface Fender that I have seen has what appears to be a carbon comp bias pot. I've never heard anyone, including Weber, say that this is dangerous.

Omar

[Wayne Alexander]

I'm not trying to sell Gerald's viewpoint, but note that Vox and Marshall use the cermet pots. They don't necessarily cost more if you use the little ones. Cermet is sturdier than a thin carbon coating and a voltage spike can't burn a hole in it.

[Bear]

As a non-technical amateur at this stuff, I've been giving more slack to Gerald - for someone who picked this stuff up out of love and effort without real background, he's done pretty well. The problem, though, is that he has become one of the leaders in education on tube guitar amps. I respect that he wants to give back what he has learned. Then again, it looks like he's making pretty good money on it. Yeah, color me ambivalent.

Bear

[ampdoc1]

I tip my hat to Gerald for a lot of things, but a lot of what he says involves personal bias. I know I've worked on better than 1000 tube amps (Fender, Marshall, Crate, Peavey, Mesa, Gibson, Vox, etc) over the years, and I have NEVER had to replace a bad bias pot. I've also read some of his hokum about point to point wiring vs. PCB boards and his theory of "parallel" capacitance on PCB boards. He's a good guy and knowledgeable in a lot of ways, but don't take all he has to say as the holy grail. This site as well as the AMPAGE pages and others have a lot of people that are very good and discuss most every aspect of tube amps. $500? Well, I hope you had a good time in Texas!

DEH

[Wayne Alexander]

I went to Gerald's seminar when he came to Santa Monica, CA, near where I live. I have to say I learned a LOT about voltage division, how and why capacitors work and what they do in a circuit, power supplies and rectification, how tone controls work and voicing for more/less bass response, and a bunch of other things that I've simply been unable to grasp from reading amp books and from these boards. I'm not trying to convince anyone else to take his seminars but I got a lot out of it. (I've had no personal engineering education, you might be less interested in this if you have). Overall the seminar vastly improved my comprehension of circuit design and tweaking.