New Guy Warning!!

[ebag4]

Hi Guys,
I just ordered my Stereomour yesterday, I am certain I will be hitting you up for info when the build starts.  This will be my first foray into SET/DHT and I am pretty excited to hear what all the fuss is about.

I am actually not completely new to the forum, I built a Foreplay II about a hundred years ago and did post a few questions on the old forum.

Since I have ordered my Stereomour and have started thinking through the build (no build documentation yet) I do have a few questions if you will bear with me.

I will be building my Stereomour as a straight amp, no volume control or input selector.  I will also be building my own base out of Maple to match my other DIY equipment.  I expect my kit will arrive late next week but there are a few additional parts I may need that I would like to get ordered and have on hand when the kit arrives.

Since I am not using the volume control it is my understanding that I will need to place a 249K ohm resistor across the input jack.  This will be low level audio so I assume a 1/4 watt resistor should work.  Are differences in the quality of this resistor going to be audible?  What type of resistor is recommended here?

I understand that there are a series of components that need to be changed/removed from the circuit(?) when switching from a 2a3 tube to a 45.  I would like to try both tubes at some point and be able to switch between them yet have the operating parameters as close to correct as possible for each tube.  Can this be done with a switch or a switch and a series of small relays?  How many devices need to be switched out or in?

And last but not least, while I wait for my kit I would like to get started on my base.  To do that I need to know the actual size of the mounting plate.  Is it actually 10"x 12", if not can someone give me the actual dimension and how much it overlaps the standard base (how wide is the notch for the lip?)?

I would like to thank Inferno (John) and Jim Rebman for sharing with me their thoughts regarding the Bottlehead amps and their sonic attributes.

Thanks guys and I look forward to the build.

Best,
Ed

 

[jrebman]

Ed,

Funny meeting you here .  As I said, I think you've made a great choice, but since you already know most of what I have to offer on these questions, I'll let some of the other folks chime in.

Welcome again,

Jim

 

[Doc B]

Chassis plate is 10" wide, 12" long and .125" thick.

 

[ebag4]

Hey Jim!  Thanks for the welcome.

Doc, thanks for the info, much appreciated.

Best,
Ed

 

[jrebman]

Ed,

Sorry, I could have answered the top plate question for you, just forgot to type it.  Also, the width of the rabbet should probably be no more than 3/16" as the area around the IEC inlet can get a bit crowded.

I like to have my top plates bolted down in lieu of the gravity mount, so I install some cleats along the bottom edge of the box on the inside that I can drill some holes in for the bolts.  On my stereomour I have the cleats on the front and back inside edges, and then use some 3 inch socket head bolts and threaded brass inserts in the cleats.  A bit of overkill, but it works great.

I have these on my stereomour, Crack and s.e.x amps but the crack and s.e.x. amps have the cleats along the side edges (with notches cut out for the plate chokes on the s.e.x.

I bought the longer length socket head stainless bolts from bolt depot, which is he place I was able to find them the easiest.

-- Jim

 

[InfernoSTi]

Welcome to the club (again), Ed!  I think you will enjoy your Stereomour very much....have fun with the build.

John

 

[Paul Joppa]

There are six places where the wiring is different for the 45 vs. the 2A3. Four of them are simply inserting or removing a resistor which is in parallel with another resistor, and the other chooses which wire is used. If I were to do it, I would use three DPDT switches - one for the power supply voltage, one for the cathode bias resistors, and one for the plate chokes. Because high voltages are being switched, I would install those switches under the chassis to make sure that they are not actuated while the amp is in operation. There is no provision in the design or layout for mounting and wiring the switches.

 

[ebag4]

Hey John, thanks for the welcome!!

Paul, thanks for the info.  Once I dig into it a little further and have a better understanding of the build I will decide whether or not I want to do it via switches it or just make the change when i get the 45s.  I appreciate your response.

Best,
Ed

 

[ebag4]

Since I am not using the volume control it is my understanding that I will need to place a 249K ohm resistor across the input jack.  This will be low level audio so I assume a 1/4 watt resistor should work.  Are differences in the quality of this resistor going to be audible?  What type of resistor is recommended here?

Thanks for the responses so far. much appreciated. 

As I have gone through some of the other threads I noted two different values quoted to be used when the volume pot is eliminated, 100K and 249K.  Can someone let me know which is correct and what type of resistor is best suited for this position?

Thanks,
Ed

 

[jrebman]

Ed,

I think there is a typo on the product page that is a throwback to the paramour II amps in that the input impedance is 100k for the stereomour, not 249k.  Took me a while to figure out what you were talking about, but this must be it.  The normal pot is 100k, so I'd sub 100k resistors for the pot, and it would just go across each inputpair from signal hot to ground, and I'd keep it as close to the grid stopper as is practical -- in other words, not at the input jack.  That would work, but could possibly make the signal cable inside the amp more prone to picking up hum.  I say possibly because it is shielded, but I still prefer to keep loads like that closest to the tube input.

As for type, any good metal film, tantalum, or such should be fine.  I'd use the vishay dale metal films or if I wanted something more exotic, maybe a caddock mk132, and a nude bulk foil if I wanted to get carried away.  I'd also get them matched fairly closely if it were me.

 

[ebag4]

Ed,

I think there is a typo on the product page that is a throwback to the paramour II amps in that the input impedance is 100k for the stereomour, not 249k.  Took me a while to figure out what you were talking about, but this must be it.  The normal pot is 100k, so I'd sub 100k resistors for the pot, and it would just go across each inputpair from signal hot to ground, and I'd keep it as close to the grid stopper as is practical -- in other words, not at the input jack.  That would work, but could possibly make the signal cable inside the amp more prone to picking up hum.  I say possibly because it is shielded, but I still prefer to keep loads like that closest to the tube input.

As for type, any good metal film, tantalum, or such should be fine.  I'd use the vishay dale metal films or if I wanted something more exotic, maybe a caddock mk132, and a nude bulk foil if I wanted to get carried away.  I'd also get them matched fairly closely if it were me.

Thanks Jim, that is exactly the info I was looking for.  Now to get a few items ordered...
Best,
Ed

 

[ebag4]

My order has arrived at the house!  Can't wait to get home to get started!  Thanks for the quick shipment Doc & Eileen!!

Best,
Ed

 

[InfernoSTi]

My order has arrived at the house!  Can't wait to get home to get started!  Thanks for the quick shipment Doc & Eileen!!

Best,
Ed

Yea!!!!!!!

 

[jrebman]

So Ed, is it built yet?

 

[ebag4]

So Ed, is it built yet?

Of course Jim, isn't yours?? ;-D

I hope to get a good start on it this weekend as I have a break in my son's wrestling schedule (YAY!!).  I ordered a quilted Maple board from ebay, it arrived as did my tube sockets and resistors to go across the inputs.  I want to get the chassis sprayed tonight and maybe get started on the circuit boards.

I will update this thread on my progress.

Best,
Ed

 

[ebag4]

I have my chassis components mounted and am currently wiring everything.  I have a question concerning the wiring from the IEC to the switch and to the power transformer.  Is there any benefit to using a larger conductr here?  If so, does it have to be overall shielded or will twisted the conductors eliminate the electrical noise?

Thanks,
Ed

 

[Paul Joppa]

Twisting reduces magnetic fields. Shielding reduces electric fields.

Noise on the power line is going to be high frequency, so it will radiate into nearby circuitry by capacitive coupling, i.e. electric fields, and the shielding is important. A magnetic field is generated by the 60-Hz current in the wires, and will mostly induce hum by magnetic coupling - hence the twisting is also important but for a different noise mechanism.

 

[ebag4]

Twisting reduces magnetic fields. Shielding reduces electric fields.

Noise on the power line is going to be high frequency, so it will radiate into nearby circuitry by capacitive coupling, i.e. electric fields, and the shielding is important. A magnetic field is generated by the 60-Hz current in the wires, and will mostly induce hum by magnetic coupling - hence the twisting is also important but for a different noise mechanism.

Thanks for the reply Paul.  OK, so stay with a shielded cable for the AC input to the amp, do you believe increasing the size of the conductors to the switch and power transformer would be a benefit or are the lengths so short it just isn't an issue?

Other than my humpots which i believe may be on backorder the wiring is complete, now on to the base!!

The kit is basically stock.  I have used Azuma tube sockets instead of the stock units, since I did not use a volume control I have added a 100K resistor across each input, and I used a couple of 2.2 uF Obbligato caps I had instead of the stock 3.3 uF Solens that came with the kit.

Best,
Ed

 

[Paul Joppa]

The wire has more than enough current carrying capacity, and its resistance is a tiny fraction of  the primary winding DC resistance, so there is absolutely no known technical reason a fatter wire would have any audible effect.

Of course, there are always effects unconnected to known technical reasons. Whether they are because of mass hysteria, or unknown technical effects, or something else we've never thought of, is unknown and unknowable. So I hardly ever say "never".  :^)

 

[ebag4]

The wire has more than enough current carrying capacity, and its resistance is a tiny fraction of  the primary winding DC resistance, so there is absolutely no known technical reason a fatter wire would have any audible effect.

Of course, there are always effects unconnected to known technical reasons. Whether they are because of mass hysteria, or unknown technical effects, or something else we've never thought of, is unknown and unknowable. So I hardly ever say "never".  :^)

Thanks Paul, I'll stick with the known factors for now.  Much appreciated.

Ed