Building Soldano SLO 100 Preamp - Need Some Help!

[Frost]

Hello Gentlemen,

I purchased a C3amps pcb board set for an SLO 100 a while back and finally decided to work on it. The main catch however, is that I'm trying to make it into a rack preamp to work with my power amp (no power section, preamp only). I'm pretty good with a soldering iron, and have some experience fixing/modding, but the actual circuit layouts can be over my knowledge much of the time. I'm reading up on tube amp circuitry and picking up on what I can.

I have my preamp board all populated. Looking to see where to go from here. I have quite a few questions.

OUTPUTS
Where would the "preamp out" go on the board? How would I go about wiring it? The 0.02uf coupling cap before V5? A drawing/diagram would really help me out. There's lots of info and layouts on the full amp, but I'm cutting out the entire power amp section so my "outs" are different.

TRANSFORMER
I have an old Antek transformer that can output 280V or 260V, along with a couple 6.3V outputs for tubes and pilot lights. Are these specs okay? I see one layout using an Antek with 200V.

TUBES
Since I'm going preamp-only, I'm assuming I won't need the V5 PI tube? From what I understand the PI flips the signal before it hits the power amp section.

BIAS
Since I'm not using a power section, what do I do about he "bias" section on my board that would use a biasing pot? Do I just leave it empty, bridge the traces, or just add the biasing pot anyways?

Thanks for any and all help, guys. Absolutely appreciated.

[glass54]

Hi Frost
It would be really useful if you got a schematic diagram relevant to your PCB (and share it with us  ).
You would probably get a lot of sensible answers and good guidance.
Regards
Mirek

[tubenit]

Soldano (robrobinette.com)


Here is a schematic from Rob Robinettes site that can be used as a reference for the preamp section


with respect, Tubenit

[thetragichero]

my thoughts: a 280vac winding transformer should work with a full wave bridge i'm getting about 395vdc. tack an extra CR before your first node (choosing the resistor with some math or by experimentation) you should hit that 378v at the B+3 node
i would put *something* in between the tone stack and the preamp output jack to make sure the output of the preamp is at a significantly low impedance

[joesatch]

i'm very intrigued about building a SLO50. I know there are good tones in the amp but i have only seen postings where folks play death metal through it. I know EVH used a SLO to record the for unlawful carnal knowledge album.

[Frost]

Hi Frost
It would be really useful if you got a schematic diagram relevant to your PCB (and share it with us  ).
You would probably get a lot of sensible answers and good guidance.
Regards
Mirek

Sorry about that, man. I post this thread right before bed and had my schematic open but forgot to attach. I'm using the Rob Robinnette schematic that Tubenit posted. Here's the annotated version with the signal paths highlighted

[glass54]

"I post this thread right before bed and had my schematic open but forgot to attach"
  No problems Frost. We often do that one last post before bedtime, then in the morning we realise we goofed!
Obviously there is interest in your project.
+1 for Power Supply ideas by thetragichero. It would also be good to have a line amplifier stage possibly using 12aU7 (Gain around 4 or so) to drive your outputs to amplifiers of choice (basically a line buffer) and if you got into trouble with hum or something, have the opportunity to drive a line isolation transformer. Fitted after junction of LDR3 and LDR4, at 0.2uF coupling cap to PI and as you suggested no PI, etc.
Sounds like a great project.
Regards
Mirek

[tubenit]

This is how I would likely approach this for a line out.  There are other ways of approaching it also that will work quite well.


A 12AU7, a 12AY7 or a 12DW7 would all be options in my thinking for V4 tube.


With respect, Tubenit

[pdf64]

For all but the lowest settings of the master volume, the preamp output impedance there is very high, reaching a max of over 250k; I don't see that changing the valve type in V4 will help with that, as the tone stack and master volume details determine this.
Rather what would be beneficial is a buffer after the master vol, eg a cathode or source follower, as per well designed transparent fx loop send circuits (dumble type fx loops don't seem to be intended to be 'transparent').

[Frost]

my thoughts: a 280vac winding transformer should work with a full wave bridge i'm getting about 395vdc. tack an extra CR before your first node (choosing the resistor with some math or by experimentation) you should hit that 378v at the B+3 node
i would put *something* in between the tone stack and the preamp output jack to make sure the output of the preamp is at a significantly low impedance

Thanks for the help. Seems like my biggest issue here is getting the impedance under control before the output jack.

[Frost]

This is how I would likely approach this for a line out.  There are other ways of approaching it also that will work quite well.


A 12AU7, a 12AY7 or a 12DW7 would all be options in my thinking for V4 tube.


With respect, Tubenit

Thanks for the diagram, Tubenit . Is there another way to approach this without needing a specific tube in V4?

For all but the lowest settings of the master volume, the preamp output impedance there is very high, reaching a max of over 250k; I don't see that changing the valve type in V4 will help with that, as the tone stack and master volume details determine this.
Rather what would be beneficial is a buffer after the master vol, eg a cathode or source follower, as per well designed transparent fx loop send circuits (dumble type fx loops don't seem to be intended to be 'transparent').

How would I go about adding a cathode/source follower after the master volume? I think I would prefer this method to the tube buffer method

[glass54]

See http://www.valvewizard.co.uk/accf.html Might give you some guidance.
I've used the fixed bias option successfully but raised Rk to 27k 3W Metal Oxide 

[tubenit]

Is there another way to approach this without needing a specific tube in V4?

Sure put in any 12A_7 tube that you like.  I simply listed ones that I prefer in an FX loop that don't add much gain.


You could try this idea using a 250ka and 500ka pot for FX levels?   I think the FX send pot could possibly work well as a  100ka?


With respect, Tubenit

[Frost]

See http://www.valvewizard.co.uk/accf.html Might give you some guidance.
I've used the fixed bias option successfully but raised Rk to 27k 3W Metal Oxide 

Great info in that link  This along with Rob's site have really helped me grasp these tube amp concepts. I think I might go for the fixed bias option as well.  How did you physically go about the cathode follower? I see a few additional components will need to be added, mainly resistors and a diode.

[tubeswell]

How would I go about adding a cathode/source follower after the master volume? I think I would prefer this method to the tube buffer method

A cathode follower is a tube buffer. (A buffer being something that offers a high input impedance to the previous source, but a low output impedance to the following load).


A source follower is similar type of buffer, but using FET instead of a tube. e.g. an IRF820 is an N-channel MOSFET in a TO-220 package with a 1W rating without any heatsink, and so will work effectively if subbed directly in place of a 12AX7 triode in the same circuit, without needing any bias circuit on the gate https://www.vishay.com/docs/91059/91059.pdf. (Only difference is a FET doesn't need a heater supply, and make sure to add a ~12V zener between gate and source)

[Frost]

Ah ok, thanks for clearing that up, tubeswell. As you can see, I'm no whiz with tube amp circuitry outside of a few mods and repairs lol. The source follower concept seems interesting. I'm assuming it wouldn't affect the preamp tone much either since V4 is primary for the loop and cathode follower.

[Frost]

Concerning the power supply board, what changes (if any) need to be made? About to start getting the components for for it, mainly the big radial caps. Going to get a new enclosure for it too eventually but I have a beat up old 1u enclosure from an old failed project I can hook everything up in and test.

[SILVERGUN]

Concerning the power supply board, what changes (if any) need to be made? About to start getting the components for for it, mainly the big radial caps. Going to get a new enclosure for it too eventually but I have a beat up old 1u enclosure from an old failed project I can hook everything up in and test.

Don't order anything yet. You won't need all of the big radial caps since you're just building a preamp.
Post a picture of the power supply board and any documentaion that goes with it.

And, not sure if this was covered above, but you won't need anything to do with the bias circuit either.

[Frost]

Thanks for the heads up. I didn't order anything yet but I was definitely not sure that I'd need all the power supply components since it seems to have a lot of interaction with the power section.

Yeah, the bias pot won't be needed here. What do I do about the spot on my preamp board for the biasing pot? Do I just leave it empty or do I bridge the two lead holes together?

Here's a pic of my power board. It didn't come with any documentation but it's a typical SLO style board with radial caps.

[SILVERGUN]

I, personally, would opt out of that board.
- If you're going to use the Antek PT that does not have a center tap on the HT winding then you will need to change the rectifier arrangement.
- I would use an integrated bridge rectifier package and just mount it right to the chassis.
- The only reason for that many big radial caps would be to provide lots of clean power to a big power section that you are not going to have.
- You "could" just populate what you need on that board, but there will be lots of empty space

You won't have any bias voltage, so the pot (along with other bias components) is not needed - just omit.

[SILVERGUN]

Is that Antek transformer going to fit in your 1U rack?
Do you know you need to insulate/isolate it from the chassis?

[Frost]

I, personally, would opt out of that board.
- If you're going to use the Antek PT that does not have a center tap on the HT winding then you will need to change the rectifier arrangement.
- I would use an integrated bridge rectifier package and just mount it right to the chassis.
- The only reason for that many big radial caps would be to provide lots of clean power to a big power section that you are not going to have.
- You "could" just populate what you need on that board, but there will be lots of empty space

You won't have any bias voltage, so the pot (along with other bias components) is not needed - just omit.

Sounds like a plan. Thanks so much. I was afraid to ask if I'd need the board at all lol but opted to ask if "any changes were needed" haha. I can either get a couple tag strips and use some diodes or mount a 4-pin bridge rectifier on a little turret board. I'll settle on one and go from there.

Is that Antek transformer going to fit in your 1U rack?
Do you know you need to insulate/isolate it from the chassis?

I think it'll either be an extremely tight fit or just barely too tall to put the enclosure top on. The final enclosure can be 2U if that's the case. I think for testing and debugging purposes, the lid can stay off though (right?). I can keep it isolated from the chassis better that way too

[joesatch]

i'm going to try and build this preamp. I have ordered the same PT. Debating whether i should just build it turret style or order the PCB from C3. This PT has dual 6.3v 2 amp taps which i will convert to DC heaters. I have a hammond chassis which i will stuff everything into. Mount the tubes sideways so it will be low profile. Fun. Frost please post progress reports on yours if possible

[thetragichero]

high gain amp like this i would suggest pcb construction and a known-quiet layout to avoid chasing down oscillation gremlins

[joesatch]

Board ordered!

[T Wilcox]

Is that Antek transformer going to fit in your 1U rack?
Do you know you need to insulate/isolate it from the chassis?

The transformer is 1.6", with the supplied mounting hardware it exceeds a 1U rack @1.75". Would like to know if anyone is able to make one work in a 1U and how. I have not
Not sure what is meant by isolating from chassis? They come with 2 rubber donuts for shock absorption but winding is wrapped which would isolate it from the chassis regardless. The only part that actually touches the chassis is the mounting bolt. I've built about 15 preamps in 2U racks with these torroidals, they are great

[SILVERGUN]

Is that Antek transformer going to fit in your 1U rack?
Do you know you need to insulate/isolate it from the chassis?

The transformer is 1.6", with the supplied mounting hardware it exceeds a 1U rack @1.75". Would like to know if anyone is able to make one work in a 1U and how. I have not
Not sure what is meant by isolating from chassis? They come with 2 rubber donuts for shock absorption but winding is wrapped which would isolate it from the chassis regardless. The only part that actually touches the chassis is the mounting bolt. I've built about 15 preamps in 2U racks with these torroidals, they are great

Just making sure you had the rubber donuts and knew what to do with them.

[Frost]

i'm going to try and build this preamp. I have ordered the same PT. Debating whether i should just build it turret style or order the PCB from C3. This PT has dual 6.3v 2 amp taps which i will convert to DC heaters. I have a hammond chassis which i will stuff everything into. Mount the tubes sideways so it will be low profile. Fun. Frost please post progress reports on yours if possible

Absolutely. I'll post my progress later today. Didn't get a whole lot done but I have my old enclosure with tube sockets mounted on an L-bracket from my old project. There's also a pilot light, IEC socket, power switch, and holes for jacks and pots. Currently removing the old wiring and components. This enclosure will serve as a debugging/testing area and then I'll move everything into a nice 2u rack.

Questions for anyone who can help out:

What should I do about the power supply? I know the included power board for the full amp is overkill, but Soldano preamps still use a power supply with big caps like the x88R and SP77. Should I lift a power supply off of their designs and integrate it into my SLO?

How do you guys get perfectly straight holes for mounting pots? A long time ago, I tried printing a paper layout and carefully punching holes for my drill press but they still came out slightly off. Naturally, there was human error on my part, plus my drill's slight oscillation so there's always going to be some degree of error. Is there some kind of drilling template I can get? Something with evenly-spaced holes that my drill can follow. I was thinking about a piece of pegboard, like what you can hang tools on in your garage.

[joesatch]

i use a Greenlee radio tube punch. They are expensive IF you buy new. I bought a used one off ebay for $20 been using it very clean sockets. As for the PS section i would assume you could put a Full Wave Bridge rectifier on a terminal strip (or buy the ready made one as mentioned above and bolt to the chassis). From there you're just gonna go to your main filter cap then filter cap stages separated by dropping resistors. I haven't delved into yet but i wouldnt think the PS section would be hard to put together.

[SILVERGUN]

If it's easier for you, you can use the ps board you have.
Use the attached pic as a jumping off point.
You'll wind up using smaller value caps. Maybe someone else will want to chime in with exactlty how to do that. I'm just too busy right now, but it is nice that you can see the traces through the board.

[Frost]

If it's easier for you, you can use the ps board you have.
Use the attached pic as a jumping off point.
You'll wind up using smaller value caps. Maybe someone else will want to chime in with exactlty how to do that. I'm just too busy right now, but it is nice that you can see the traces through the board.

Awesome idea, Silvergun. Why didn't I think to flip the diodes? lol . Yeah, I wouldn't mind making a little separate turret/tagboard for the bridge rectifier and caps but the power board is pretty convenient. Especially if it only requires flipping some diodes and changing some cap values. Thanks for the help

Perhaps we could put together some kind of guide for converting these SLO kits to preamps for future reference. The pcbs are readily available, and many people might not want or need the big power section. Or want to shell out for a big output transformer. I personally just want something soldano/mesa-flavored to record direct with or use with my carvin power amp.

[Frost]

Cleaned out the old enclosure. Boy, that thing was dirty lol. Dust, sawdust, and possum droppings everywhere . I removed most of the old components from the previous project and left the very basic stuff like the fuse and power jack. I think for the final enclosure, I might go with straight-mounted tubes instead of mounted at a 90-degree angle. I'll see.

Going to to try wiring everything up either tonight or this weekend, or at least most of it. Still need help with the proper filter cap values. What should I use: 10uf? 16uf? 22uf? How much does it even matter? IIRC the cap values have really wide tolerances anyways

[shooter]

How much does it even matter?

in just a preamp, probably not much, although you want the ripple riding on the DC to be real small, in the 1st n 2nd gain stages, so bigger caps for those/that/them taps