SP 14 Build Frank111

I'm working my way through an SP14 build. I've been in touch with Roy a few times, and also thought I'd discuss some of the issues I've run across here. A lot of it has to do with interpretation of the diagrams and making comparisons with the example builds that are on the website.

Let me start with the Source/Tape switch wiring.  Looking at the switch from the back of the panel, am I correct with this assumption: TOP two lugs - Tape INPUT.  MIDDLE two lugs - one lug to the right gain-balance control, and one lug to the left gain-balance control.  BOTTOM two lugs - the output wires from the INPUT SELECTOR switch connect to these bottom two lugs, AND the TAPE OUTPUT leads connect to these two lugs as well.  

It looks like there are single wires coming from the Input selector switch but in the case of the rest of the connections, it seems to be that they are all a twisted pair with none of the ground wires connected. (Not referring to the ground connections of the three controls to PCB here).
OK. This confuses me. Why do we not connect the grounds from the tape input, tape output, leads from the center switch to gain-balance controls. Is there a technical issue or benefit to doing this? It would be simpler to just go by the diagram and run a single connection to the controls.  But there has to be a reason for doing it this way.  

I've spent a lot of time trying to figure out when ground wires from twisted pairs get connected, and am trying to use the pictures on the web to accomplish that. But sometimes its hard to tell from the pictures because the wires get hidden.

Also, I noticed that the input connections from the master volume control only need a single ground.

I welcome discussion on this. I'd like to build the best pre-amp I can. I know that it's important to get the little things right and that's what I'm trying to do with this posting.

Thanks,

Frank111

in order to eliminate ground loops, (which in itself is quite a complicated subject with many different opinions), only ONE end of the wire is connected from the various RCA input sockets to the selector switch.
So, each connection is a pair of twisted wires. At the RCA socket on the rear panel, connect both the input AND ground.
At the selector switch end, only the 'input' wire gets connected. With the ground wire, makes sure it is a clean cut with no wires poking out, twist it and keep it back about ⅛" from the selector switch. Even though only one end of this hookup wire is connected, it is still shielded by its ground wire connection at the RCA socket.
Same applies to all connections to the tape/source switch as well. At the switch, the ground wire is NOT connected, at the RCA end, BOTH the ground and input wires are connected.
Also make sure that each RCA socket is insulated from the chassis. The ground lugs/tabs of all RCA sockets are connected together with ONE wire connecting them to the PCB ground.
Hope this makes sense.

Yes this information really helps me to understand why it's done this way, and how I need to proceed with the wiring. Glad I asked!!!!

Regarding the RCA jacks. I have all of mine insulated properly from the chassis with one bus wire connecting to each ground lug. What is the proper termination of this long bus wire? I connected mine to the ground terminal of the AC input 'jack'. That's the lug where the purple transformer wire will eventually be connected. This connection seems to go to chassis.

Should the RCA bus terminate at that point or only to the negative of the input point on the PCB?

Frank111

Frank111 wrote:Yes this information really helps me to understand why it's done this way, and how I need to proceed with the wiring. Glad I asked!!!!

Regarding the RCA jacks. I have all of mine insulated properly from the chassis with one bus wire connecting to each ground lug. What is the proper termination of this long bus wire? I connected mine to the ground terminal of the AC input 'jack'. That's the lug where the purple transformer wire will eventually be connected. This connection seems to go to chassis.

Should the RCA bus terminate at that point or only to the negative of the input point on the PCB?

Frank111

connect the wire from the RCA bus to the pcb ground connection. It may well be OK the way you've connected it, but better to keep all this wiring 'standard' for these preamps.

I never would have caught that. I'll make that change asap!

Thanks for your help!

Frank111

Frank111 wrote:I never would have caught that. I'll make that change asap!

Thanks for your help!

Frank111

no worries.
I should mention, that the two wires connecting the selector switch and the Tape/Source switch are not a twisted pair, just a single wire. So are the two wires from the volume pot to the Tape/Source switch. Since this run is usually very short, they do not need to be shielded. You can however, for the sake of tidiness, twist each pair of those wires together.

By volume pot, do you mean each gain-balance control? I don't see any connection from master volume directly to the source-tape switch. Just the middle tabs on the source-tape switch going to the gain-balance pots.

Frank111 wrote:By volume pot, do you mean each gain-balance control?  I don't see any connection from master volume directly to the source-tape switch. Just the middle tabs on the source-tape switch going to the gain-balance pots.

yes, forgot yours is an SP14

Thanks for checking!

Is there a preferred way to run the wires from the RCA jacks to the front panel? On the web I saw where it looked like three groups of wires were run underneath the PCB.  Some preamps on the site show them running in one group along the side of the chassis. I'm not sure if this would be possible with the SP-14, as the PCB is close in proximity to the chassis side. What would be best?

Thanks,
Frank111

all the wires going from the RCA sockets to the rotary selector, keep those together, neatly bundled together and running along one side of the chassis, preferable the side closest to the rotary switch. If the pcb's are close to the chassis wall, keep that input wiring elevated above the pcb running along the side of the chassis, maybe use self adhesive zip tie tie down pads.
All power supply wiring, can be either run under the pcb or somewhat elevated above the pcb. Try and keep DC and AC wiring separate, and as short as possible. Again, use zip ties to keep all the DC and AC wiring in their own 'bundles' and neat.

OK got it. Thanks again MontanaWay!

This is my last work week day, and I'm getting ready for some serious bench time over the weekend to wire this SP-14 up. I did some checking last night regarding the wire runs from the front panel to the back. From what I can determine, the only way to run these wires is underneath the PCB. There is not enough room to route them along the bottom side due to the PCB placement. Running them over the top and along the side of the PCB results in the wires coming in contact with the tubes. It looks like the only way to do this is to run them in three groups from front to back underneath the PCB. Combine the Right channel into one group of five pairs, the Left channel into another group of five pairs, and the Tape In-Out into the third group. Each group would clear the standoff mounts. I see no other way to get this done.  Please advise on this; does this sound ok? I will keep the runs short and neat as possible.

The large transformer has two high voltage windings. From the documentation, it appears as though only one set is used.  The documentation mentions to not use a Gray wire. My transformer does not have a gray wire on it. So, from what I can determine, am I correct in my assumption that I use only ONE high voltage winding?  I think that goes for the 6 volt section also. But as I write this, I'm drawing a blank about that. This is enough for my brain to handle at this point anyway!

I think once I get this resolved and implement the advice given earlier, I should be ok. The PCB really looks nice. It really is a nice pre-amp. Just need to get past the little details.

Thanks,
Frank111.

Go for it Frank you got plenty of room this is how I routed my wiring with no issues that I'm aware of
Also Montana way did I understand correctly that you recommend grounding RCA directly to circuit board?
I ask because mine is grounded right to the power ground lug and to my knowledge I have had no issues....
Love the SP-14 outstanding Preamp

Frank111 wrote:This is my last work week day, and I'm getting ready for some serious bench time over the weekend to wire this SP-14 up. I did some checking last night regarding the wire runs from the front panel to the back. From what I can determine, the only way to run these wires is underneath the PCB. There is not enough room to route them along the bottom side due to the PCB placement. Running them over the top and along the side of the PCB results in the wires coming in contact with the tubes. It looks like the only way to do this is to run them in three groups from front to back underneath the PCB. Combine the Right channel into one group of five pairs, the Left channel into another group of five pairs, and the Tape In-Out into the third group. Each group would clear the standoff mounts. I see no other way to get this done.  Please advise on this; does this sound ok? I will keep the runs short and neat as possible.

The large transformer has two high voltage windings. From the documentation, it appears as though only one set is used.  The documentation mentions to not use a Gray wire. My transformer does not have a gray wire on it. So, from what I can determine, am I correct in my assumption that I use only ONE high voltage winding?  I think that goes for the 6 volt section also. But as I write this, I'm drawing a blank about that. This is enough for my brain to handle at this point anyway!

I think once I get this resolved and implement the advice given earlier, I should be ok. The PCB really looks nice. It really is a nice pre-amp. Just need to get past the little details.

Thanks,
Frank111.

yes, routing the wire bundles neatly under the pcb is OK, as long as you keep them away from the power supply section and any AC wiring.
What is the part number on your toroid transformer?, I can look it up and let you know about which wire to use.
I am assuming you have two toroid transformers. A larger unit for the high voltage AC and filament for the rectifier tube and a smaller unit for the preamp tubes filament supplies.
Like I said, give us the part numbers printed on the toroids.

Maintarget wrote:Go for it Frank you got plenty of room this is how I routed my wiring with no issues that I'm aware of
Also Montana way did I understand correctly that you recommend grounding RCA directly to circuit board?
I ask because mine is grounded right to the power ground lug and to my knowledge I have had no issues....
Love the SP-14 outstanding Preamp

standard practise is to run the ground from the RCA sockets to the main pcb ground. But if your system is working for you, great.
The really strange thing about ground loops is, that if I build a preamp like yours and run the RCA ground to the power inlet, I might just end up with a ground loop issue. As far as I can determine, although there are certain rules on audio gear ground connections, its not really set in stone!

@MW,
I terminated the RCA ground as shown on the schematic provided by Roy I am an amateur at this stuff although I have successfully built a ST-120 & SP-14 which is probably more of a testimonial to Bob & Roy's customer service, patients and instruction provided with the kits.
I may move the ground to the PCB just for Sh*tz N Giggles to see what changes if anything.
OBTW Beautiful work on your amp designs.

Maintarget wrote:@MW,
I terminated the RCA ground as shown on the schematic provided by Roy I am an amateur at this stuff although I have successfully built a ST-120 & SP-14 which is probably more of a testimonial to Bob & Roy's customer service, patients and instruction provided with the kits.  
I may move the ground to the PCB just for Sh*tz N Giggles to see what changes if anything.
OBTW Beautiful work on your amp designs.

sounds like a good plan!
Thank you for your positive comment.

Took a few minutes to move the RCA ground termination to the PCB as suggested by MontanaWay and do believe there has been a positive change although it maybe a placebo effect that is being discussed in another thread on this forum
Just another reason to love this forum everyone is ready to help regardless of skill level
thanks MontanaWay

Maintarget wrote:Took a few minutes to move the RCA ground termination to the PCB as suggested by MontanaWay and do believe there has been a positive change although it maybe a placebo effect that is being discussed in another thread on this forum
Just another reason to love this forum everyone is ready to help regardless of skill level
thanks MontanaWay

you're welcome.
When I first joined up here, the assistance I got from members was, and still is, incredible.
I'd have to say that this is one of THE BEST forums I've ever been a member of, regardless of the genre. So thanks to all members here and especially Bob, who keeps it all in check!

Congrats... very clean work...
Let me recommend some tubes.

I have tried the following tubes...

Sylvania Vt231
Sylvania 6SN7 GTB Chrome top
RCA 6SN7GTA Black base 60's
Raytheon 6SN7 GT
Russian Novosibirsk 6N8S  
Russian Foton 6N8S

after months of rolling finally I keep Sylvania VT231 at input voltage amplifier and Foton 6N8S at output
I assure you will be suprised with the performance for the money both with Novosibirsk and Foton factories 6N8S.

There are tubes maybe even better then vt231s, such as real 1578 and Raytheon 6SN7WGT (brown base) but for one 1578 or Raytheon wgt you can buy 50 foton 6N8S...

I've been away from the forum over the past few days, but did make progress on wiring the preamp.
My wiring is similar to Maintarget's with the exception that I ran the left channel in a separate bundle between the bottom side of the chassis and the first two standoffs. So I have one run like Maintarget's that comprises of the Right channel wires and tape in/ tape out wires in one bundle, and another run bundled with just the left channel runs. Looks nice and neat, and hopefully will be correct.  I took a continuity meter and tested for proper connections by connecting it to the Main attenuator out and then inserting it in each RCA jack; checking for proper continuity per the selector switch. Did this for Left and Right sides. I also checked operation of the attenuators. Both channels operated the same, so that's good.

My next area is hooking up the transformers to AC power, and then move to the PCB connections.

MontanaWay, both of my transformers were installed to the bottom plate when the kit arrived. The big torroidal was installed towards the front which is opposite of what the pictures on the net have shown. The smaller one was installed towards the back, and is a square blue one.  Looks to be potted with some sort of black material.

The larger transformer has two windings for primary, two windings for high voltage, and two windings for 6.3 volts. The directions only show connections using one HV and 6.3 winding. The primaries get connected in parallel then go to the AC connection. Very confusing to see two unused windings. I'll see if I can find a part number for the large transformer. The small blue one does not show a part number on it. But I can see the numbers 120V and 7V written in dark marker on the black potting on top of the transformer. Any help to clarify this will be appreciated. We'll get to the PCB after this gets done.

Thanks for all of your input!

Frank111

Frank111 wrote:I've been away from the forum over the past few days, but did make progress on wiring the preamp.
My wiring is similar to Maintarget's with the exception that I ran the left channel in a separate bundle between the bottom side of the chassis and the first two standoffs. So I have one run like Maintarget's that comprises of the Right channel wires and tape in/ tape out wires in one bundle, and another run bundled with just the left channel runs. Looks nice and neat, and hopefully will be correct.  I took a continuity meter and tested for proper connections by connecting it to the Main attenuator out and then inserting it in each RCA jack; checking for proper continuity per the selector switch. Did this for Left and Right sides. I also checked operation of the attenuators. Both channels operated the same, so that's good.

My next area is hooking up the transformers to AC power, and then move to the PCB connections.

MontanaWay, both of my transformers were installed to the bottom plate when the kit arrived. The big torroidal was installed towards the front which is opposite of what the pictures on the net have shown. The smaller one was installed towards the back, and is a square blue one.  Looks to be potted with some sort of black material.

The larger transformer has two windings for primary, two windings for high voltage, and two windings for 6.3 volts. The directions only show connections using one HV and 6.3 winding. The primaries get connected in parallel then go to the AC connection. Very confusing to see two unused windings. I'll see if I can find a part number for the large transformer. The small blue one does not show a part number on it. But I can see the numbers 120V and 7V written in dark marker on the black potting on top of the transformer. Any help to clarify this will be appreciated. We'll get to the PCB after this gets done.

Thanks for all of your input!

Frank111

OK, thats easy to explain.
The way the circuit is designed, the filament supply for all the tubes are split up.
The rectifier tube gets its filament supply from the main, large, transformer, which then leaves one spare 6.3V winding. This is usually not used. So just insulate the ends and tuck it way nice and neat.
The preamp is designed so that each pair of tubes for each channel have their own dedicated filament supply regulator, each of which requires its own 6.3VAC supply, so, in comes the smaller toroid. In your case, it is a potted unit. This smaller toroid should only have a primary and two 6.3VAC secondaries, each secondary 6.3VAC is used to supply EACH channel's filament regulator.
If either of your toroids have the choice of 120VAC or 220VAC primary, thats is, two 120VAC windings, for us here in the US, just parallel each 120VAC winding up. If you were to have 220VAC mains, then you would series the primaries up.

What about the two high voltage windings on the big transformer? Is it correct that you use only one of them? Would it be possible, and is there any advantage to parallel each of the unused transformer windings to utilize them for more current rather than just let them go unused?
(Two high voltage and two 6.3 on the big transformer).

I'm clear on the small transformer now. Thanks!

same applies to the larger toroid. Each primary, I am assuming, is 120V, so parallel them up.
As for the spare 6.3V on the larger toroid, sure you can parallel it up with the used 6.3V, but not at all necessary.
Sometimes it is easier, when you don't need higher current, to just clip those wires short on the unused 6.3V, insulate the ends, I use heat shrink, then tuck them away neatly.