Trying layout changes in 5f1 home build

[HansVG88]

Hi all.
I am new to this forum and new to building. I stumbled on it searching for information about building a 5f1 Champ-like amp and troubleshooting noises. I am trying to make the amp somewhat smaller with the chassis on the bottom of the cabinet and tubes standing up next to the speaker magnet. But with a Jensen re-issue alnico speaker the 12AX7 tube had to share the side next to the power transformer and the 6V6, 5Y3 and output transformer on the other side. My questions are:

1) I have heard that the 12Ax7 should be as far away from the power transformer as possible. If so, is there any way shield it enough so that it could live right next to it?

2) If you wanted only one input jack how would it be wired? Bridge the 68K resistors or just use 1? Would you still use the 1 Meg resistor to ground?

3) Does it make any difference in the filament voltage if you don't use a pilot lamp?

4) Would the fact that the PT and 5Y3 are on opposite sides of the chassis and require fairly long wires cause problems?

5) Am I an idiot for even trying this?

Thanks for any advice.

[guitician]

Originally Posted by HansVG88

Hi all.
I am new to this forum and new to building. I stumbled on it searching for information about building a 5f1 Champ-like amp and troubleshooting noises. I am trying to make the amp somewhat smaller with the chassis on the bottom of the cabinet and tubes standing up next to the speaker magnet. But with a Jensen re-issue alnico speaker the 12AX7 tube had to share the side next to the power transformer and the 6V6, 5Y3 and output transformer on the other side. My questions are:

1) I have heard that the 12Ax7 should be as far away from the power transformer as possible. If so, is there any way shield it enough so that it could live right next to it?

2) If you wanted only one input jack how would it be wired? Bridge the 68K resistors or just use 1? Would you still use the 1 Meg resistor to ground?

3) Does it make any difference in the filament voltage if you don't use a pilot lamp?

4) Would the fact that the PT and 5Y3 are on opposite sides of the chassis and require fairly long wires cause problems?

5) Am I an idiot for even trying this?

Thanks for any advice.

1. Mu metal....make a box out of it...if you can find any
2. one 68k and 1meg to ground only.
3. Nope
4. The distance wont if you keep the cap on the circuits side
5. Necessity is the mother of invention

[HansVG88]

Originally Posted by guitician

1. Mu metal....make a box out of it...if you can find any
2. one 68k and 1meg to ground only.
3. Nope
4. The distance wont if you keep the cap on the circuits side
5. Necessity is the mother of invention

Thanks for the reply Guitician.

I will check into how expensive MU metal is before making another chassis. Could you explain what you mean by "keep the cap on the circuits side" in regards to the question about the 5Y3 wires running from one end of the chassis to the other? This is my first build and I am learning "on the job".

The noise in the current (12AX7 next to the PT) setup sounds like a hum + a kind of very fast but not loud clicking that gets louder with turning the volume up. Is this oscillation one would expect with this setup?

Thanks again for your kind reply

[guitician]

I meant, don't run the unfiltered DC across the chassis. Also, keep the secondary wires nicely twisted together running along the corner of the chassis for best hum field reduction.

[pentatone]

Hi Hans -

do you already have a cab?

I've built some amps with the chassis on the bottom of the cab with the knobs on the backside like the old Gibsons had and had build the corresponding cabs particularly for use of the planned components.
I said good-bye to the idea of a classical Fendrish cab desing and build them some kind put on the end (the small side on bottom and top). So you have enough space by moving the speaker higher and away from the chassis.

For my current project - a 6V6-PP amp based on the Gibson GA8 (Gibsonette) circuit - I took the dimensions of a 5E3-Deluxe and turned it by 90 degrees.

For me this was the most elegant way to avoid the kind of trouble you fear now.
Over and above that you have some additional space on the surface of the chassis if you want to experiment with different OT positions in the case of "hum-chasing".

Many kind greetings from Germany

Arne

[tubeswell]

Originally Posted by HansVG88

1) I have heard that the 12Ax7 should be as far away from the power transformer as possible. If so, is there any way shield it enough so that it could live right next to it?

Apart from enclosing it with a steel box, you may also probably want to shield the cable from the input to the grid of V1a and from the vol to the grid of V1B and from the coupling cap after V1b to the signal grid of the output tube. Keep the cable shields grounded at one end only

You may also want to put 10k-100k grid stoppers right at the grid pins of the pre-amp tube and 1k5 to 4k7 grid stopper at the signal grid pin of the output tube to kill unwanted HF

Originally Posted by HansVG88

2) If you wanted only one input jack how would it be wired? Bridge the 68K resistors or just use 1? Would you still use the 1 Meg resistor to ground?

You could also wire a 33k in place of the 68k, or wire a switchable 2nd 68k in parallel to the 68k (to emulate the 'hi' input of a typical Fender hi-lo input. - With traditional Fender hi-lo inputs the two 68k are in parallel - making 34k - with the 1M going to ground, when the plug is inserted into the 'hi' input).

Originally Posted by HansVG88

3) Does it make any difference in the filament voltage if you don't use a pilot lamp?

No but the heater winding current draw is marginally reduced. Most 6.3V lamps are between .15A to .3A (or even 0.5A in some cases). I think its worth putting the lamp in - not just because it looks pretty when its on, but its also a good safety feature. Look for 0.15A lamps if you are concerned about the current draw on the heater winding.

Originally Posted by HansVG88

4) Would the fact that the PT and 5Y3 are on opposite sides of the chassis and require fairly long wires cause problems?

Better if they are as close as possible, because of potential EMF coupling from wires. As a rule of thumb wires should preferably be kept as short as possible in order to avoid unwanted signal coupling. Best practice is also to twist the AC pairs together, as this helps to cancel out the EMF surrounding each individual AC wire - thus reducing potential sources of unwanted hum coupling into more sensitive signal wires (because the wires in the AC pair are in opposite phase to each other - while one is negative, the other is positive - hence the EMF in the space surrounding each wire cancels out if these are in close proximity)

[HansVG88]

Thanks for all the great info guys, I am thinking it would be better to make a new chassis and have the input tube and rectifier trade places. I guess I learned there is a good reason that all the amps are laid out that way
We'll see what happens!

[HansVG88]

Arne,

I have a cab built. it is a little smaller so there is less room for the tubes between the speaker magnet and the transformers. I tried putting the preamp tube on the side with the power transformer and found out this is not a good idea because the tube will pick up noise there. Also, the rectifier tube should be there so the wires can be shorter. This is my first build and I am learning many lessons.
So ist das Leben.

[pentatone]

Hans -

a big part of our hobby is finding out how and if things work and trying out this and that - for me this is part of the fun.

If I were you I'd try how it works and if it doesn't you can build a new cab and/or chassis and book it in the chapter "experience".

Der Weg ist das Ziel (the path is the destination)

Arne

[HansVG88]

Thanks to all who responded to my questions I now have a quiet great sounding amp. You guys are the best!