Output Transformer Question - Sohpt style amp

[suprleed]

I know tube amps may be a little off topic, but many builders here have ventured into amp building in addition to stompboxes.  I've been following many of the low voltage tube threads here on the forum (like the extensive "valvecaster" thread) and have been researching "sopht amp" threads here as well.  I'm thinking about trying one of the 12V sopht amps as my first foray into tubes.  I've been trying to source some of the parts and have come up with a few questions on output transformers.  Since I'm a total neophyte when it comes to valves and transformers I thought I'd seek a little help from those more experienced and wise on the forum...

1. The 12AL8 power tube datasheet shows a 800 ohm load.  As long as find an output transformer that has an 800 ohm primary impedance (or something close like 1K) and an 8 ohm secondary (to match my speaker cab) I think I should be OK?  I remember reading on some thread here about turn ratio, but I think if I match my primary and secondary impedances I will end up with the correct turn ratio.  Can someone confirm/deny this or point me in the right direction?

2. As I've looked on the net for various output transformers with the correct impedances (above), I found some "line to voice coil" transformers in 25V and 70V versions grouped with some audio transformers.  Would these work as an output transformer or do I need to specifically seek out an "audio transformer"?  The line to voice coil transformers had multiple primary and secondary impedances that would match up to what I'm looking for.  They are obviously used to drive speakers, but would they work in the output section of a simple tube amp?

3. Would running 2 output tubes in parallel yield a higher output wattage just like running them in push-pull yields a higher wattage?  I saw a shemo a while back for a parallel 6v6 output section, but I couldn't find any info on whether the output wattage was greater or not.

[Fender56]

1) You are right. The impedance ratio equal the square of the turn ratio. So for a primary of 800 Ohms and a secondary of 8 Ohm, the turn ratio will be 10.

2) Not sure here about line transformer... Single-ended amp (like this Sopht amp) requires an output transformer specially designed for single-ended applications, since it has an air gap between the core. Otherwise, core will saturate and it can somewhat get damaged. Power transformer usually don't have the air gap. Push-pull output transformer don't need the air gap neither. But at a very low power (12AL8 is about 1/2 watt?), I don't know if it still applicable... Better ask for a second opinion here.

3) Push-pull is another beast, since you will have to design a phase-inverter and use another power transformer... It is easier to put another tube in parallel in a single-end fashion. However, you will have to divide the primary impedance by a factor of 2 (400 Ohms in this case). Or keep the primary at 800 Ohms, and connect your 8 Ohms speaker to the 16 Ohms tap... (turn ratio of 7). Get the picture? 

Doubling the output tubes will double the power, but will not double the volume. An easier solution for more volume is to try more efficient speakers, or use a 4 x 12" cab.

[suprleed]

3) Push-pull is another beast, since you will have to design a phase-inverter and use another power transformer... It is easier to put another tube in parallel in a single-end fashion. However, you will have to divide the primary impedance by a factor of 2 (400 Ohms in this case). Or keep the primary at 800 Ohms, and connect your 8 Ohms speaker to the 16 Ohms tap... (turn ratio of 7). Get the picture? 

Doubling the output tubes will double the power, but will not double the volume. An easier solution for more volume is to try more efficient speakers, or use a 4 x 12" cab.

Just want to make sure I follow you here.  So if I add a parallel power tube, I could connect them both to a 400 ohm primary and connect my 8 ohm cab to the 8 ohm secondary.  Or I could instead connect both power tubes to an 800 ohm primary and use the 16 ohm secondary with my cab.  Am I on the right track here?

Follow up question, are center tapped transformers for push pull use and not single ended use?

Thanks.

[Fender56]

You are right: you could connect both output tubes to a primary of 400 Ohms, and connect the secondary's 8 Ohms tap to your 8 Ohms cab.

The key is to maintain the right turn ratio, whatever the rating:

turn ratio = sqrt(Zprimary / Zsecondary) = sqrt(400/8) = 7

So you can also have the same turn ratio with:  sqrt(800/16) = 7,   sqrt(1600/32) = 7  and so on, using the same 8-Ohm cab.



Push-pull transfo has a center-tap on the primary side. Transfo for single-end has only two wires on the primary side.

Visit AX84.com site and sewatt.com. You will find a lot of information there, as well as schematics. That helps to understand different topologies!

[suprleed]

Thanks Fender!  I've poked around the AX84 site a little bit, but I'll do some more research.  Your input has been very helpful! 

[suprleed]

Anyone have any thoughts on the line transformer question above?

[mydementia]

I've been dinking around with Stephane's amps over at www.sopht.ca for several months now... in fact, gaussmarkov and I have put together a PCB project for the 30V Sopht amp (stay tuned... prototype 1.0 is just about done).  Have you been to sopht.ca?  TONS of information... 
I started with the two 12V versions (one with the 12k5 output tube, the other with the 12AL8) - check out this build log for the parts I used and a little more info: http://www.freewebs.com/dbx-fx/sopht12k5v610.htm.

On the 30V sopht amp, I used a line transformer.  Reading the sopht.ca info carefully, you should be able to figure out which taps to use.  My first attempt at this build is also linked above.  Feel free to PM me if you want more info...

Happy building.
Mike

[suprleed]

I've been dinking around with Stephane's amps over at www.sopht.ca for several months now... in fact, gaussmarkov and I have put together a PCB project for the 30V Sopht amp (stay tuned... prototype 1.0 is just about done).  Have you been to sopht.ca?  TONS of information... 
I started with the two 12V versions (one with the 12k5 output tube, the other with the 12AL8) - check out this build log for the parts I used and a little more info: http://www.freewebs.com/dbx-fx/sopht12k5v610.htm.

On the 30V sopht amp, I used a line transformer.  Reading the sopht.ca info carefully, you should be able to figure out which taps to use.  My first attempt at this build is also linked above.  Feel free to PM me if you want more info...

Happy building.
Mike

Thanks Mike.  I've been pouring over both your site (dbx) and sopht.ca recently.  Both have been very helpful.  I'd also like to get started with one of the 12V amps (start out slowly, I'm still wary of HV)!  Did you like the 12k5 or the 12al8 version better?  I'm leaning a little toward trying something with the 12al8, but am still early in the research stage.  I think it could be interesting to use a 12k5 or 12al8 as a standalone power amp and feed one my existing booster/overdrives in as a preamp.  We'll see how it goes.  I'll PM you after I do some more research and come up with some more specific questions.

[Renegadrian]

Hey, it seems I'll use these information to start some amps too!!! You know I am a Valvy addict (completed #2 yesterday...) and I've seen the sopht site...Always thought that those are GREAT projects, but those trasfo thing got me and put me in the "maybe one day" mood...
Now after reading that dbx page I cleared my ideas about it...So yes, it was that helpful!!! 
suprleed, I'd like to suggest you (if I may...) to build a Valvecaster...Join the club!!!

[mydementia]

Did you like the 12k5 or the 12al8 version better?

I like the 12k5 circuit better... my 12AL8 build just didn't sound as good. 
I always build a circuit for the first time per the schematic.  If it's good - it stays... if there's room for improvement, I tweak until I like it (or until the pads start to lift off and I can't tweak any more!).  I think you'll find that the base 12k5 12V circuit sounds very good - as I said on my build log, it likes boosters and distortion pedals too.
Have fun!
Mike

[Fender56]

I only build the 12K5 version, as a power amp. I did try the preamp, but I was never able to get a good sound out of it (for my taste). I use my POD as a preamp, or the line out of my tube amp (which 50W is really too loud for me, now I'm not gigging anymore). I use it through a Marshall 2 x 12", and it is surprisingly loud (annoying for the neighbor in an apartment, like a loud TV). I'm planning to built a high-voltage tube preamp soon, and use the 12K5. BTW, I use the Hammond 119DA as an output transfo.

I use two different 12K5, and they behave differently. One is warm (dark?) and break up smoothly (Sylvania). The other one has more treble, and has a nasty sound when overdriven (Doron, Caron, did not remember..). It think it is better to buy more than one and swap them to see how they react. They are cheap anyway.

Note that the heater current for the 12K5 is near 0.5 amp @ 12V, so you will need a good power supply or wall wart. Heater for 12U7 or 12AU7 is 0.150 amp @ 12V.

[suprleed]

suprleed, I'd like to suggest you (if I may...) to build a Valvecaster...Join the club!!!

It's actually been on my to do list for a while now!  I think it might help warm up my SS amp.  Just need to get thru some deadlines at work so I can actually have time to work on some projects.

Did you like the 12k5 or the 12al8 version better?

I like the 12k5 circuit better... my 12AL8 build just didn't sound as good. 
I always build a circuit for the first time per the schematic.  If it's good - it stays... if there's room for improvement, I tweak until I like it (or until the pads start to lift off and I can't tweak any more!).  I think you'll find that the base 12k5 12V circuit sounds very good - as I said on my build log, it likes boosters and distortion pedals too.
Have fun!
Mike

Good to know.  Thanks for your opinion.  Maybe I'll focus more on the 12K5 in my research.

I use two different 12K5, and they behave differently. One is warm (dark?) and break up smoothly (Sylvania). The other one has more treble, and has a nasty sound when overdriven (Doron, Caron, did not remember..). It think it is better to buy more than one and swap them to see how they react. They are cheap anyway.

Note that the heater current for the 12K5 is near 0.5 amp @ 12V, so you will need a good power supply or wall wart. Heater for 12U7 or 12AU7 is 0.150 amp @ 12V.

Good tip on trying multiple tubes!  I've been searching the net looking for a good high amp power supply (1-1.2 amps).  I'm trying to find one that's regulated as it seems people have had better luck with those as far as noise in concerned.  I've found a bunch of unregulated 12V 1amp units so the search continues...  Thanks all!

[Fender56]

For the power supply, I use this one, from Digikey:

#T954-P5P-ND

It's a 12VDC @ 2 amp, well regulated. No noise with this one (except class-A single-end hum noise, which is normal, but very very low in this case).


You can built your own using a transfo, rectifier and a regulator. I was too lazy!!!

[suprleed]

For the power supply, I use this one, from Digikey:

#T954-P5P-ND

It's a 12VDC @ 2 amp, well regulated. No noise with this one (except class-A single-end hum noise, which is normal, but very very low in this case).


You can built your own using a transfo, rectifier and a regulator. I was too lazy!!!

Awesome!  You guys have been very helpful!   

I'm in the same boat, much easier to just buy a regulated PSU.