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Looking for some advice before I move too far along on this project...I am piecing together an amp from some parts I've accumulated over the years, and it looks like it will shape up to be close to a "Foxy 18" layout from Brown Note. The problem is that my chassis is rather small (a "stout" chassis from Hoffman, 12x6.5x2.5"), and it would require the turret board (about 8.5" long) being on 1" standoffs and one end projecting out over the power transformer (laydown type). The chassis has a depth of 2.5" so its not an issue as far has having the board elevated. Are there any inherent problems with having the end of the board directly over the PT?...e.g. hum/noise problems that might surface as a result? Other factors?

Should be fine

I would try to optimize the layout to just make the board smaller though

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well I picked up a pre-populated board on ebay for the Foxy 18 about 5 years ago ($30...pretty good deal!), so I was hoping to stick with that vs dismantling and re-doing the board. However there is a lot of "space" on the board..it certainly could be more compact probably by about 1-2 inches.

Output tube cathode resistors and bypass caps are easy to pull off and mount near the tubes with terminal strips (more airflow too)

Grid stoppers shouldn't be on the board, those should be as close to the tube socket as possible. So pull those off if there are any.

And so on...

I would try to cut it down and optimize

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Thanks for the advice..prompted me to consider to try the whole amp wired on terminal strips. Will have a little bit of planning to work out but shouldn't be too complicated....hopefully.

Looking at the board and considering your advice, I should be able to literally "cut off" the end of the board containing the cathode resistor and bypass cap as well as the 3rd filter stage. I will mount those on terminal strips. this will leave me room to easily fit the shortened board in the chassis.

edit...need to figure out how to post pics correctly.

So here's what I had to start with...a Foxy 18 board I picked up off ebay a few years ago real cheap:


And here's how it looks in the Hoffman Stout chassis (I already removed the components on the right side). Obviously would result in being directly over the PT unless some surgery is performed.


Some work with a fine tooth saw resulted in this:


There's a spot on the chassis wall behind the mess of PT wires where I can mount a terminal strip...that's where the filter cap and resistor will go. As for the cathode bypass cap/resistor, I'll mount those next to the EL84's.

I think that will work nicely for you.

You could even mount that little piece of turret board somewhere if you really wanted to mount the resistor and cap.

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Thanks Kurtlives...if I can find a spot for that board, I will do that. The chassis is 2.5" deep so I may use the side of the chassis near the cap can and mount the board.

Here's everything in place...I still haven't drilled mounting holes for the turret board..its just sitting on top of the standoffs. note that the tubes and controls are on the same side of the chassis. I have a Hall VVR that I will likely install (shown on far left). Also will have an indicator lamp to go next to it.

So should I pull the 8.2K resistors, run a wire from the lugs on the board to pin 1 on each EL84, and connect each resistor across pins 1 and 2 for each tube?
below is the Foxy 18 layout

Try to keep grid wires as short as possible and grid stoppers as close to the tube as you can.

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An update...I've removed the input grid resistors (2x68K) that were on the center of the board. This board was originally designed for a hi and low input. A single 68K resistor is now attached directly to pin 9 of the EF86 socket using a tag strip and is where I will tie in the input from the jack. The filter cap and resistor that were cut off the end of the board are now placed where the input grid resistors used to reside and will be fed under the board from the cap can. The cathode bypass cap and resistor are now on a "ledge" directly over the EL84 pair. I've also moved the EL84 grid stoppers directly to the EL84 sockets. All this is coming together but I have a couple of questions/concerns:
1. The power/standby switch is looming right over the end of the board...will this be problematic?
2. The Hoffman stout chassis has the tube sockets on the same side of the chassis as the controls...will the heater wiring cause problems with the input/preamp section?

Below is a pic showing what I have at this point....still some more wiring to do and some cleaning up a little. Thanks for your advice and comments!

1. No, the B+ is low impedance filtered DC. You should be fine but as always try to keep your lead dress neat.

2. Yes the preamp (signal) wires can pick up the 60Hz signals emanating from heaters. Normally you want the signal wires to lie flat along the chassis. In your case you probably want to take the wires straight from the turret to the connection points, keep the wires as short as possible. It won't look as pretty as standard lead dress but it will keep the signal wires away from the heaters.

This pic shows the direct connections. http://lh6.ggpht.com/_EJpKfY8EgW8/S4ShF ... G_0224.JPG Notice how the wires fly straight to the tube.

If you do get 60Hz hum injected into your signal wires it will be easy to diagnose. The hum will be constand "low" hum that doesnt change regardless of where the controls are set.

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Thanks for the info. I'll wire it up as you suggested and keep my fingers crossed that their will be minimal to no hum.

Ok...Got this finished this afternoon. Here's how it turned out:


I've got a NOS NEC EF86 in V1, EH 12AX7 as the PI and 2 JJ EL84's and a Tesla EZ81. Classic Tone Transformers. Parts that i scrounged together over the past few years.

I hooked up a 1x12 closed back cab with a green beret, got the tele and it sounds really good. The contour control really shapes the sound, as well as the cut and the single tone knob. Side note...I think I should get some sort of promotion in my soldering skills for wiring up that contour control...that was a real challenge!

Just a couple of issues that I'll have to work out..
1) I either have a bad soldering joint from the input jack to the EF86, or part of the shield is shorting out on the signal wire...no biggie..I can handle that.
2) When the volume is turned up past "9", I pick up a lot of interference. The frequency of the interference changes with the position of the gain control, as well as the cut control. Any trouble shooting tips would be greatly appreciated.

Overall I'm very happy with the amp...sounds great and has that nice Voxy chime.

Looks good, nice job! No hum issues?

In regards to #2, which is a common problem in high gain EF86 amps. Is it interference or oscillation? Regardless, things to try...

shield some of those signal wires, use tube shields if you can, dampening O-rings on tubes, make sure grid stoppers are as close to the tubes as possible. Reduce the gain of the EF86 stage slightly, this will go a long way in making the amp more stable and less picky towards tubes.

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Thanks! No hum issues at all. In fact its pretty quiet.

I'm embarrassed to say that I'm not sure about the difference between oscillation and interference. I would imagine that oscillation would be a certain pattern, perhaps "pulsing"? If so, that's not the case here. I can have the gain control maxed out and the volume on 9, no problems at all. When I take the volume up any higher, I get a screech/hiss sound. If I leave the volume control in that position and adjust the cut control, the pitch of the hiss changes. This pitch will also change when I make slight decreases in the gain setting. At gain positions less than 5-6 or so, the screech goes away. This leads me to suspect that the problem is more with the power section rather than the pre-amp...is this correct?

The grid stoppers on the EL84s and EF86 are directly on the tube sockets.

As far as shielded signal wires, should I just change out all the ones going from the master volume and tone controls across the board (the thicker cloth covered wires in the photo) to shielded cable? Would the best place for the shield grounds be to tie them in with the input jack ground?

Edit: Controls left-to-right in photo...Gain, Tone, Contour, Cut, Volume

Thanks for all your suggestions..this has been a great learning experience for me.

What you describe is oscillation.

Shield the Gain and Volume controls to start and see how the amp is. I would guess that shielding those two control’s wires (lugs 3 and 2) will cure your problem though.

Ground one end of the co-ax. Take the ground to the preamp ground point (so yes wherever your input jack ground is most likely).

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Swapped out 5 wires with RG174 shielded cable. Definitely an improvement. Got rid of the symptoms described above, but have some fizzies at maximum volume/gain settings when playing through the amp. They were likely there before but masked by the oscillation problem. Not concerned *too* much, as the sweet spot for the amp seems to be when the volume is just under maximum with the gain around 8-9.

I may try my hand at making a head cabinet for it as the next part of this project. Also working on getting the front and rear control plates set up in Corel and sending off to BNP Lasers.

Thanks again Kurtlives for all your help. The amp is definitely a keeper!