El34 / 12AX7 SE Build (My 1st build)

[John Berard]

This is my first attemp at an amplifier build. I plan on using the Skunkie Design schematic with some minor tweaks. I am finishing up the chassis but am concerned I did not leave enough room between the output tubes and the transformers. There is only one inch. If I ever went with KT77, it would be a no go.

So should I just stop and and start a new chassis? If so, what would be a more comfortable distance to use EL34 or KT77 valves. Also, please voice and other concerns such as the rectifier valve/choke layout.

PS: I am waiting on the PT to arrive thus not on the chassis.

Thanks in advance.

As suggested by an esteemed member, herewith are the schematics:




Output transformers are ISO Tango Model FC-12S:

 

[Lavane]

The 6CA7's I'm using in my ST-70 are pretty close. Even closer when I used KT-88's. So are my 300B's in my SET amp. A little more space would be nice to have though.
Why would KT-77's be any closer than a EL34? I have some KT-77's and they have a pretty straight bottle like an EL34.

 

[wparks]

Congratulations on starting a build thread for your first build - what a wonderful journey you are starting out on and have invited us to follow.
I can't help but feel like we are beginning mid-show however, so lots of questions-

What inspired you, and how did you decide to build this amp?
Can you post the schematic that you are beginning with? Is this based on Skunkie's Ultra Linear Single Ended?
How did you punch your holes (with the notch?) and fancy ventilation holes?
What output transformers are you using?
Is the meter for VU or bias? (I'm guessing bias due to the switch next to it?)

This is your dedicated build thread, so feel free to capture as many thoughts put as much information as you like here- the more the better, and the more help we can offer (if you need any). Great start, please keep it up!

I have learned the hard way that it is beneficial to lay out the components in paper cut-outs on the blank chassis so I can take a lot of time to fully consider all of the possible interactions and complications.

 

[opnly bafld]

KT77 and EL34 are @ 1.3" diameter.

FWIW my Magnavox 6BQ5 single ended console pull has one tube right against the OPT, of course it's a smaller tube (less heat?).

 

[John Berard]

The 6CA7's I'm using in my ST-70 are pretty close. Even closer when I used KT-88's. So are my 300B's in my SET amp. A little more space would be nice to have though.
Why would KT-77's be any closer than a EL34? I have some KT-77's and they have a pretty straight bottle like an EL34.

Yes, most Kt-77s are straight but some newer ones are Coke bottle shaped.

 

[BmWr75]

Yes, most Kt-77s are straight but some newer ones are Coke bottle shaped.

I've never seen a coke bottle shaped KT77 and could not find one in a search just now. What brand?

 

[Lavane]

I've never seen a coke bottle shaped KT77 and could not find one in a search just now. What brand?

Same here. Are you thinking if the KT-88 maybe?

 

[opnly bafld]

KT66?

 

[John Berard]

I've never seen a coke bottle shaped KT77 and could not find one in a search just now. What brand?

Sorry guys. I got it confused with this 6CA7:

 

[gadget73]

Purely for visual reasons, I might consider subbing the 12ax7 for a 6SL7. Not quite as much gain, but I think the small octal tube looks less dwarfed by an EL34 than a 12ax7 does.

Not sure I'd be overly worried about an inch from the transformer. It'll get hot but I have seen commercial designs that were more tightly packed. If you want to help cool the transformer you could stick a shiny plate of something just in front of it as a reflector. A small air gap between that and the transformer itself will make a pretty significant difference in trafo temperature.

 

[Lavane]

Sorry guys. I got it confused with this 6CA7: View attachment 3560126

Ya, that's one of those fancy shmancy 6CA7's. Most I've seen including the EH 6CA7 fat boys I'm using are either straight glass, or just a little fatter like the EH tubes.
I like Gadget's idea of using an octal 6SL7. I use the lower gain octal 6SN7's in one of my preamps. I recently built a single ended EL34 amp with a 6SL7 driver tube. I tried a Tung Sol 6SL7, and a Soviet era version of the 6SL7.

 

[wparks]

Thank you for posting the schematic and other information.

I was looking into those ISO Tango transformers, and the datasheet that you posted (which is the same from the website), and I am surprised that nowhere does it mention at what percentage of the primary the Ultra-Linear tap is placed. It lists a range of tubes they can be used with, but each tube will require a different percentage tap to be optimal, so again I'm surprised they do not seem to mention it.

 

[John Berard]

Congratulations on starting a build thread for your first build - what a wonderful journey you are starting out on and have invited us to follow.
I can't help but feel like we are beginning mid-show however, so lots of questions-

What inspired you, and how did you decide to build this amp?
Can you post the schematic that you are beginning with? Is this based on Skunkie's Ultra Linear Single Ended?
How did you punch your holes (with the notch?) and fancy ventilation holes?
What output transformers are you using?
Is the meter for VU or bias? (I'm guessing bias due to the switch next to it?)

This is your dedicated build thread, so feel free to capture as many thoughts put as much information as you like here- the more the better, and the more help we can offer (if you need any). Great start, please keep it up!

I have learned the hard way that it is beneficial to lay out the components in paper cut-outs on the blank chassis so I can take a lot of time to fully consider all of the possible interactions and complications.

View attachment 3559950

Thanks for your suggestions. I added the schematics and output transformer specification to my starting post so anyone new to the thread would see them up front. What inspired me? Well, I have always been a solidstate guy but last year while recovering from bypass surgery, I decided to try a tube amp. I got a Will Vincent ST-70 and hooked it up to my McIntosh MHA-150 headphone unit and used the preouts. Wow, I was hooked. Now I am going down that rabbit hole and want to do some tube/valve rolling. I figured a SE amp would give me the most from it. I also constantly need to be doing something with my hand.

I found Stephe of Skunkie Designs on Youtube and started watching her builds. She is what I call an advanced hobbyist and admits that she is not an electrical engineer and comes across as being very honest. That led me to watching DLab and Blueglow. So here I am hooked.

I am using the keyed punch because asthetics are important to me and I didn't want any screws on the top plate. Stephe (Skunkie) mentioned she used a Greenlee chassis punch for some of her builds. I found one on Ebay for about $20 and then use the older style sockets. It gives a really clean look. The gold plated sockets were a bit harder to find but I did manage. Here is a photo of the punch along with a snapshot of the Greenlee mini-catalog that arrived with it:





The cooling holes around the sockets were individually drilled using Skunkie Designs template: https://www.skunkiedesigns.com/2a3-6sl7-diy-info?pgid=mcp1uc3r3-61a1f9a7-1aff-410d-8dab-aa3fcb0c5c4d

 

[wparks]

Bypass- ouch! Glad you made it through buddy! This is the first build of your new life!

Once you get hooked on 'tubes, it's hard to go back. Not all distortion is a bad thing!
Single ended is indeed a very good direction to go- I started small with a clone of the Dave Gillespie modified Magnavox 8600 using two 6BQ5 I call the Fat Daddio SE (that picture was from my build thread). I use it frequently with my headphones and headphone adapter (what better for flea power!) and once you achieve that breathtaking level of clarity, dynamics, and tonality, it's hard to go back to solid state.

Very cool tool- I did not know Greenlee had one that did a keyed hole. Very nice. I'll have to look for that- Thank you for the photos.

I assume from the meter and switch that one of the tweaks you are adding is adjustable bias? (please correct me if I am wrong) If so, do you have bias pots in mind for this? (I really need to watch Skunkie's 'tube videos on this build.) Wondering what bias currents it will use, and also curious about the 360K feedback resistor between the two plates.

 

[John Berard]

Purely for visual reasons, I might consider subbing the 12ax7 for a 6SL7. Not quite as much gain, but I think the small octal tube looks less dwarfed by an EL34 than a 12ax7 does.

Not sure I'd be overly worried about an inch from the transformer. It'll get hot but I have seen commercial designs that were more tightly packed. If you want to help cool the transformer you could stick a shiny plate of something just in front of it as a reflector. A small air gap between that and the transformer itself will make a pretty significant difference in trafo temperature.

I would prefer an octal for looks also and avoiding screws to mount the socket. Alas, I do not have the knowledge of how to properly integrate one into the schematic I am following. Think of me as a Dynakit / Heathkit kind of guy.

 

[wparks]

I would prefer an octal for looks also and avoiding screws to mount the socket. Alas, I do not have the knowledge of how to properly integrate one into the schematic I am following. Think of me as a Dynakit / Heathkit kind of guy.

I'm pretty sure we can help with that. I'm guessing two resistors will have slightly different values, and different pin wiring. That's about it.

 

[John Berard]

Thank you for posting the schematic and other information.

I was looking into those ISO Tango transformers, and the datasheet that you posted (which is the same from the website), and I am surprised that nowhere does it mention at what percentage of the primary the Ultra-Linear tap is placed. It lists a range of tubes they can be used with, but each tube will require a different percentage tap to be optimal, so again I'm surprised they do not seem to mention it.

Not sure if this helps but I got these readings from my multimeter:

Pins 8/10: 187 ohms
Pins 9/10: 116 ohms
Pins 8/9: 71 ohms

 

[John Berard]

I assume from the meter and switch that one of the tweaks you are adding is adjustable bias? (please correct me if I am wrong) If so, do you have bias pots in mind for this? (I really need to watch Skunkie's 'tube videos on this build.) Wondering what bias currents it will use, and also curious about the 360K feedback resistor between the two plates.

Sorry, I forgot to respond to this. On some of her builds, Stephe has a volt meter purely to monitor how each power tube is performing. I wanted a meter for asthetics but would also like the ability to adjust current so each tube is perfectly matched. I noticed Decware uses milliampere meters. Not even sure how to incorporate my wants into the schematic along with another members' idea of using an 6SL7 with an octal socket. Oh heck, let's add a 12v trigger and a Dennis Had fuse circuit to protect the amp from a tube short while we are at it. lol

 

[wparks]

I believe the most common tap point is at 40% of the primary winding, which we can only loosely infer from a resistive measurement. 71 Ohms is 38% of the 186, so this is probably a 40% tap. To know this correctly we would have to apply an AC signal and do a voltage measurement to calculate the turns ratio, but I don't think that is necessary. It is curious that the datasheet indicates this OPT can be used for both the 6L6 and the 6V6, because while 6L6/KT88/KT66 family of tubes like 43%, the 6V6GT requires more like 20%. You're probably good. I'm wondering if Skunkie talks about this at all in the build videos.

 

[John Berard]

I believe the most common tap point is at 40% of the primary winding, which we can only loosely infer from a resistive measurement. 71 Ohms is 38% of the 186, so this is probably a 40% tap. To know this correctly we would have to apply an AC signal and do a voltage measurement to calculate the turns ratio, but I don't think that is necessary. It is curious that the datasheet indicates this OPT can be used for both the 6L6 and the 6V6, because while 6L6/KT88/KT66 family of tubes like 43%, the 6V6GT requires more like 20%. You're probably good. I'm wondering if Skunkie talks about this at all in the build videos.

I believe the recommended tubes on TubesUSA,.com are not from ISO Tango but rather added by the USA distributor to help guide buyers. Note that the KT-77 isn't mentioned at all.