The journey has begun

Wardsweb

Behind The Curtain
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As many of you know, I've started down the path to tubedum. While I'm waiting on the amp, I decided to go ahead and build the tube preamp. To that end , I have ordered the Bottlehead Foreplay. Well, I can't leave anything stock, so here is a VERY crude first pass at what it will look like.
 
the front, top and back will be one piece of metal of which the type is still TBD. That way I will mount the transormer to the back on the inside of the case. There will be one set of inputs, so no need for the selector switch. For now I'll stay with the dual mono pots, but may opt for the single knob dual attenuator. Of course the black knobs gotta go. I will use some slick machined aluminum ones. Just for kicks, I'm going to use the military power switch.
 
Way da go Wards!

Hey Wards,

That should be a good start! I like the power switch. Please keep us posted. Your mods sound sound, or is that sound squared? :) Didn't Harmon Kardon try to sell square sound in the 70's?

Above all have fun.

Rob
 
Question

Because now the signal path is longer from the RCA jacks, I plan on using sheilded twisted pair. Is there a downside to this? I know they supply magnet wire but seems to me there are better ways to help eliminate noise. Anyway, I have loads of magnet wire at work that we use to mod circuit boards, if I decide to change things back.
 
That is so cool!

I like the way you have cleaned up the design - very nice! And I love the military-grade power switch! Gotta love those over-built parts, if for no other reason than ballsy effect. I went with a military Borne pot in place of the Alps volume control in my amp.

MikE
 
I think that magnet wire is Vampire OFC, not typical stuff. Keeping the inputs, pots and tubes as close together as possible makes a lot of sense. Move some, move them all.
 
Wait till you see the tube protectors I'm going to use. My own design that I will have my friend make at his machine shop. Stacked plates like a figure eight. The holes of the eight are where the tubes fit. I don't know the metal yet.
 
Not quite sure what you are getting at but with a minimalist design like the Foreplay the triode, pot and input RCAs are so close together because they should be.
The signal hits the RCA, goes through the pot and into the triode. The less there is to interfere with that the better.
One half the wire has one half the chance of changing the sound.
I understand why you want to change the looks but the trick is to not degrade the sound while you do it.
The reason the RCAs on the Foreplays are where they are have not a thing to do with aesthetics but only with performance.
Since you are buying the kit anyway, build it as is and then think of alterations. Changing layouts for sex appeal will not make it perform better. (Shut up Thor!)
 
If you really think moving them to the back will adversely affect the sound, it's not a problem to keep them beside the tubes or just behind now that there is nothing there. There will only be one set still as I'm not going to be switching sources for this system.
 
I just think if you move your RCAs you should move the whole input package. I don't think the things are that ugly without a good reason.
The cool power (Rocket launcher) switch can go on the front and the tubes in the back with pots in front of them. Maybe a cool power light in between, but I do really think that the reason that the RCAs,tubes and pots were so close is because of sound and sound alone.
You will get much better advice on the Bottlehead forum but my bet is that is the reason.
Good sound at that price will have compromises and I think in this case it is looks.
Just an opinion, and I am not a designer but I do know that less is more in dealing with signal purity and wire.
 
amp layout

Thatch,

I agree with you that the RCA input jacks, the input selector switch and the tubes are clustered close together and away from the power xfmer for low noise performance. Since Wards is wanting to eliminate a whole bunch of input wiring and stray pickup capacitance (these lines are not shielded as I understand it) then with the theoretical noise budget improvement this would bring I figured he could afford to run an input line (shielded) from the back apron from a single unswitched pair of inputs, bringing the noise figure back to where it was in the trade-off for esthetics.

Wards,

Thatch has valid points, however someone with experience in low noise layouts could play with the layout and probably even improve on it. If your design art can tolerate it I'd agree with Thatch and leave the RCA input jack up near the tube.

Be aware that the power transformer and its wires, especially the HV wires to the rectifier and the rectifier components in the AC part of the circuitry all radiate electrical hum and need to be kept physically as far as possible from the amplifying circuits. You could even pick up some reduced hum performance by slipping a grounded shield around the filament supply wires (only an advantage if the filaments are run on AC) up to very close to the pins on the tube socket. The shields for the shielded wires must be insulated so that they cannot contact the chassis anywhere except at the one end where it will go to a selected ground tie point. This is a general true statement that applies to most shield wiring whenever the shield is not required to actually carry signal or power. An RCA patch cord is a good example of an exception.

I think that running a single shielded line to the back apron is OK but it must be dealt with carefully. Cluster these input jacks (2) on the opposite side of the rear apron as the AC power cord entry point. You can put your output jacks between the power cord end and the inputs. Try also to keep the output jacks away from the power cord, fuse etc. You should isolate the ground of the RCA jack from the chassis where it is mounted (to avoid ground loop hum pickup). This can be done by installing a small plastic panel around the jack. You must also shield the wires from the jack. Twisted pair inside a shield will be fine. Don't twist too tight. One turn per 3/4-1 inch is plenty. Float the shield (do not connect) at the jack end and at the tube end connect to the same ground point as the 1/2 of the twisted pair that represents the jack outer shield connection (ground) goes to.

The figure 8 tube shield sounds cool (pun intended). How about forming it from spaced stacked plates of identically punched sheet aluminum with uniform spacers. Like the metal lens on a JBL horn speaker?

Rob
 
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Thanks for the input guys. I've written Fredt300B on his Foreplay that he did move the jacks to the back to get his experience and tips on setup.

Rob you hit it right on with the figure 8 shields. They will be stacked plates separated with standoffs. I'm also working another design that uses one top ring supported by three long standoffs using cap nuts.

It's all flexible. I won't really know how it's going until I can have some hardware to play with. Just getting some thoughts out to learn a little more.
 
The Journey

Wardsweb great information from the guys but l think they may have missed something very important :eek: ;please allow me to help.

What is the most critical thing to remember when begining a journey? - "TRAVEL LITE".;)

Got to rid yourself of all your SS and become one with the "Tubes":p:

Solution pack up the GM truck and move it all to a good home; Northern Ontario comes to mind.:D

Always here for you buddy.
 
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