Coupling caps test bed

[ivegotmono]

Hi all,
I've got an Eico HF12 which I've been using as a learning and experimenting amp. I would like to start trying out various coupling caps to experience the sound prior to using them in other amps. I realize that different caps will sound different in different amps but I'm trying to get an overall feel for the character of the sound.
I picked up some screw clamp strips in order to make changing them easier and I would like to place these strips outside the amp which would require 12-14 inches of connecting wire. The absolute minimum length would be about 6 inches if I mounted the strips on the back of the amp itself. The wire I've been using is 22awg silver coated.
My question is: is this a feasible proposition and will the wire/extra wire render my test useless.

Steve

[ErikdeBest]

Hi Ivegotmono

Interesting idea and very good question! I think no one is better in answering it than our member tubelab! He build this prototyping board http://www.tubelab.com/The_Tubelab.htm

Here the lenght of wires is considerable. I am also interested in knowing if he experiences differences in sound when the amp si built in a definitive version. Let's hope tubelab jumps in

Erik

[ivegotmono]

I apoligize for not giving credit to tubelab. I did get the idea from his picture on his site.
http://www.tubelab.com/The_Tubelab.htm

In fact the local Radio Shack was closing down and at 90% off I bought all that they had (17cents each) and a bunch of other parts.

Steve

[tubelab.com]

Long (within reason) wire lengths by themselves are not usually a problem. Long wires connected to a high impedance point in proximity to other signals can be a big problem. The grid of a tube is a high impedance point. I don't know the particular placement in question, but I can offer the following. Keep the wires as short as possible, route them away from any signal carrying components (the tubes, other parts, speaker terminals, etc). Add a small resistor (100 to 1K ohms) in series with the wire that connects to the tube grid, mounted right at the tube socket. Above all, since the amp may oscillate, do the initial testing on junk speakers.

The Tubelab is physically large but laid out such that the signal path is no longer than some point to point amplifiers that I have seen. Most of the circuits I design on the Tubelab have sound, distortion and frequency response characteristics similar to the finished design. Tubelab circuits are generally not as quiet as a well designed PC board based amplifier.

What is not obvious in the photos of the Tubelab is the large copper screen beneath the pegboard. It, and all ground leads from the modules come together at a single ground terminal on the rear of the wood base. There is still more hum pick up from all of the wiring than you get in a properly built amplifier (even one that uses a PC board in a plastic box).

When I wanted to test an idea during a time when my lab was being used as a storage shed, I resorted to putting a single stage amplifier together with Radio Shack clip leads. The coupling capacitor is in the foreground connected to the scope probe. I do not recommend this type of construction due to the extreme shock hazard. I ran a frequency response test of this amp and found that even with the long wires the amplifying circuitry (no OPT) was flat to 800 KHz.


http://www.tubelab.com/Active%20load...E%20output.htm