amps, resistors, and wattage

[Spivonious]

Yep, the biggest resistors on the board are the box shaped ones at 3W and 5W.

I'll try to do a trace, but this board was hooked into other pieces by 11 connectors, so it might be a bit difficult. If it helps at all, the amp has a pentode/triode switch, an active EQ, and a spring reverb. While the power tubes do get mounted to this board (the sockets are the only component on the other side), I'm sure it's not just the power amp section.

I'll try to get at least a partial trace tonight.

Do you think it's worth going to a tech? The closest authorized Genz Benz tech charges $60 just to look at it, and the amp is out of warranty.

I'd love to learn more about amps in general. This is no longer my main amp so it's okay if it's out of commission for a while. I'm educated when it comes to physics but have zero experience building circuits, or even understanding why a tube amplifies a signal. Am I in over my head?

[DonP]

In pentode mode, screen grids are connected to a regulated supply (through the resistors). In triode mode, they are simply connected to the anode. So you'll see some logic like that with the jumpers going to a panel switch.

Amps in general are easy, but when they start putting in all sorts of bells and whistles they get messy real fast. It might be worth it to take it to a tech, as long as he can guarrenty it will function correctly when he's done. Some factors in the decision is how much is the amp worth, how much are the repairs, and how much will you get for it fixed vs. not fixed.

[Spivonious]

Well, all fixed I could probably get about $400-$500 for it. Broken I'd be lucky if I could sell it, but if so, I'd guess around $150.

I do like the sound of it, and it would be nice to have it back in commission to provide a different sound from my B-52.

Cyg - I searched for Weber and AES and couldn't find any stores selling electronics. Do you have links?

On a sidenote, are there any books/websites you recommend for learning the theory of audio amplifiers? Like why does a tube amplify a signal.

[Cygnus X1]

http://www.tedweber.com/

http://www.tubesandmore.com/

[DonP]

I've shopped at Antique quite often.

I've also shopped at Angela. Cyg, I've still not found out what was so bad about them?

How a tube or transisitor amplifies:

You have a low voltage (or current for certain transistors) signal that controls a grid / valve / gate. This gate is like the spicket of a water hose. As you open and close the spicket, you control a large flow of water (the amplified signal). All tubes and transistors are "biased" because you don't want them to shut off, or max out. You want to be able to change the speed of the water from low to high but not bottom or top out (distortion).

[Cygnus X1]

Don, I've only shopped Angela once, and they did not fill my order or
bother to notify me before they shipped it.
The very item I needed the most is the one they failed to
ship.
I quickly ordered it "next day" from Weber and ate the shipping charge.

At least if Weber or AES can't fill an order they will email me
and ask if I can choose a replacement or backorder.

[Cygnus X1]

On a sidenote, are there any books/websites you recommend for learning the theory of audio amplifiers? Like why does a tube amplify a signal.

Here's a good start:
http://www.diyguitarist.com/Misc/J_Darr.htm

Old, but timeless info.

[Spivonious]

Thanks, Cyg. That seems like just what I'm looking for.

[Spivonious]

Sorry to bump this thread again, but I'm finally getting around to ordering these parts (and getting motivated to fix the amp). Does the voltage rating on the caps matter as long as it's >= the existing ones? It's the capacitance that matters, right?

Would I be okay ordering 100uF, 450V, electrolytic caps from Weber?

[Infernal Death]

Generally speaking you are fine replacing a cap with one with a higher voltage rating. However caps with higher voltage rating tend to be bigger and / or have a wider lead spacing so they might not fit on your board. That's something you should pay attention to when ordering replacement caps.
Also be on the lookout for 105 °C caps (if possible) as those have a longer lifespan, especially in hot tube stuff. Something i always try to do (if possible).

Flo

[Cygnus X1]

Good to see you Flo!

Spiv-I have had no problems with Weber caps.
Yes, same voltage or higher as Flo said, and watch the sizes.

[Twitch]

I'm almost positive it's the screen resistor. It's in the power section and when it went before I got crazy bias reading and burned up a tube. I started getting these symptoms again so it hasn't burned up a tube yet.

Is there a way to test transistors? There are some on the board near it. I don't have a schematic, so it's hard for me to say what depends on what.

As I alluded to above, this happened before and both screen resistors were replaced (one for each EL34 tube).

Here's a photo of it on the board. It's the blue one.



Here's a pic of the whole board.

That looks to be a 1 watt. The one next to it for comparison is a 1\2 watt. So actually the big blue one may be as much as a 2 watt. What you can do if in doubt is replace with 2 - 4 watt resistor and youll be fine.