Sluckey AC15 build. Another one.

[EricR]

Well im in the process of ordering all my parts. Read the entire thread on Legos spin. So much info. I want to make sure I am getting everything right. For Norm. Vib. channel mods I was unsure if these 2 variations achieve the same result. With the 3 position switch in middle and one jack does this cascade norm into vibe? It seems from my reading that most liked doing it this way.

[sluckey]

Those two drawings represent two completely different mods. The top drawing will cascade the normal channel into the vib channel. The switch is a common 2-position DPDT. This drawing shows only the changes you would make to a completed amp to cascade the channels. It makes no sense unless you also look at the complete schematic/layout. I DO NOT RECOMMEND THIS MOD!

The bottom drawing shows how to wire a special 3-position DPDT switch to be able to use one input jack to connect to either preamp, or use the center position to connect to both preamps (running in parallel, not cascade) simultaneously. I do recommend this mod.

Lego's lengthy thread is nice to read because it shows the progression of this amp. It also contains a lot of derailment info that will most likely just confuse you. Also, Lego's board layout is quite different from the final version. Now that you've read it, let it be. All the info you need to successfully build this amp is on my AC-15 webpage...

     http://sluckeyamps.com/VAC15/ac15.htm

[EricR]

Those two drawings represent two completely different mods. The top drawing will cascade the normal channel into the vib channel. The switch is a common 2-position DPDT. This drawing shows only the changes you would make to a completed amp to cascade the channels. It makes no sense unless you also look at the complete schematic/layout. I DO NOT RECOMMEND THIS MOD!

The bottom drawing shows how to wire a special 3-position DPDT switch to be able to use one input jack to connect to either preamp, or use the center position to connect to both preamps (running in parallel, not cascade) simultaneously. I do recommend this mod.

Lego's lengthy thread is nice to read because it shows the progression of this amp. It also contains a lot of derailment info that will most likely just confuse you. Also, Lego's board layout is quite different from the final version. Now that you've read it, let it be. All the info you need to successfully build this amp is on my AC-15 webpage...

     http://sluckeyamps.com/VAC15/ac15.htm

Thank you. Ya it was a lot of info. I have your schematics downloaded from your website plus the mods you recommend. I have the special 3 position dpdt switch so I will stay with the recommend mods. I'll post updates once I get all my parts and get some progress. Thanks again fornyour time. I got a pre-made board already from EL34s site. Real nice. Just going to try and emulate your work. Take it slow and make the work clean.

[EricR]

Waiting on chassis and transformers but in the mean time got all my jumpers done and the board loaded. Triple checked all. Hope I didn't miss anything. Had a good time doing this on a couple snowy days.

[EricR]

Well still waiting for the chassis. Should be soon. But in the mean time working on a cab for speaker and the head. Never done dovetails before. Having fun.

[Lectroid]

That is a nice looking cabinet, the dovetails look professional.

[EricR]

That is a nice looking cabinet, the dovetails look professional.

Thanks! Appreciate it.

[tubeswell]

^agree^ sure looks pretty. I learned how to make dovetails in year 7 (‘junior high’) woodworking class using a tenon saw and a hammer and chisel. But man it was fiddly. Never bothered with them since but nowadays I think I’d use a fancy table router jig if I had access to one. I wonder how it’d look with a 1/2” round over along the joins?

[acheld]

. I wonder how it’d look with a 1/2” round over along the joins?

It'd look great!   I normally use 3/8" roundover bit myself.

What is the wood used?  Very pretty.

[sluckey]

Looks like oak to me.

[EricR]

^agree^ sure looks pretty. I learned how to make dovetails in year 7 (‘junior high’) woodworking class using a tenon saw and a hammer and chisel. But man it was fiddly. Never bothered with them since but nowadays I think I’d use a fancy table router jig if I had access to one. I wonder how it’d look with a 1/2” round over along the joins?

I've been going back and forth if I should route the edges. I'd have to buy a router so I might leave it. Not too late to do it. Still on the fence if I should buy one.

[EricR]

Looks like oak to me.

Yup. Red Oak. Making a cab for the head too. Should be a nice looking amp!

[EricR]

Making some good progress. The chassis should be here in a week or so. Excited to put this one together!

[Jalmeida]

Looks great!!

[tubenit]

Tiger striped oak is beautiful!!!  Nice job!  I personally like rounded over edges.  Either way it's a beautiful cab!!


With respect, Tubenit

[Ronquest]

Do you have faceplates yet? 

[EricR]

Do you have faceplates yet?

Chassis gets here Monday. I was going to paint the Chassis, do letters by hand and clear coat. We will see if I can pull it off.

[EricR]

Well a lot going on in life. Felt good doing the dovetails. Slows down the mind. Got the cab for the head fitting together for the dry fit.

[sluckey]

I've always had a great appreciation for handmade dovetails.

[EricR]

I've always had a great appreciation for handmade dovetails.

I definitely found appreciation doing this. The top joints came out really tight. It was a good feeling. Got in a nice lil rhythm. Watched a few vids and looked at other people's work prior. Man some peoples work is next level. Amazing to see.

[EricR]

Happy days. Got the chassis. Gonna fit real nice. Lots of family stuff at the moment. See where I get today the  it'll be a bit before I can work on it.

[EricR]

Well looks like I'm not done making mistakes in life. I ordered the chassis and had the iec mount placed on the wrong side. In the end I decided I can mount the board upside-down and all will line up again. Not the end of the world.

Today I went to put a enamel clear coat on the chassis and the finish crinkled. It rustoleum, I followed all wait times and made sure all where enamel sprays. Sand, primer, base and clear. It did it pretty uniformly. Not the worst look but definitely not intended. So much is on my plate right now I'm not sure I have time to sand it back and spary again. Might slap it together just to have it together and after the move if it bugs me to much refinish it.

[sluckey]

I've got a bad feeling about this! You will regret not being able to get to the component side of this board. I'd cut my losses and get a proper chassis.

[mresistor]

agree with Sluckey,,  if you are dead set on this, then please install underneath (bottom) mount sockets so you can pull all of them if you have to get to the component side of board.

[EricR]

I've got a bad feeling about this! You will regret not being able to get to the component side of this board. I'd cut my losses and get a proper chassis.

I agree. With a bit of a family emergency going on we are having to move in the next couple months. My head has been distracted. I would love to have this buttoned up before the move but maybe I should just hold off, get a correct chassis arrangement and finish when life lets up. Ill think a bit. I do have bottom mount tube sockets so I could flip it when needed but that would be a real hassle.

[EricR]

Well after thinking it over you guys are right. I had this other chassis. The IEC is on the wrong side also but with no tube sockets predrilled I can make everything the right way. I pushed hard and got everything drilled. Figure I can make a plug of some sort for the unused IEC and put the footswitch there. Hopefully everything goes smoothly from this point. I'll have a couple days to really focus when I get back from work.

[EricR]

Allright got the power supply and heaters wired. Making good progress. If anyone sees anything wrong please say. Always appreciated. I'm getting 288VAC at pins 1 and 7 at the rectifier

[sluckey]

What vdc do you have at the two filter caps? Now would be a good time to plug in all tubes to verify that all filaments light up.

[EricR]

What vdc do you have at the two filter caps? Now would be a good time to plug in all tubes to verify that all filaments light up.

Okay so all tubes light up with heaters. I'm getting 404VDC on the stay side and 402VDC on A side. The line voltage is 121VAC. This transformer had 115 and 125 options. I went with the 115v. Seems like I'm a little low coming out the transformer and a little high coming off the cap.

[sluckey]

I suggest using the 125vac primary tap.

[EricR]

I suggest using the 125vac primary tap.

With the 125vac tap puts me at 313vac into the rectifier and 440vdc out.

[sluckey]

Sounds backwards to me. The 115v tap has a higher voltage/turns ratio as referenced to the secondary. The 125v tap has a lower voltage/turns ratio as referenced to the secondary. Therefore, if you put 121v on the 115v higher voltage/turns ratio tap you will get a higher secondary voltage which will result in a higher B+.

For example, let's say your secondary puts out 300v.

300:115 = 2.6 voltage ratio which is also the turns ratio.
300:125 = 2.4 voltage ratio which is also the turns ratio.
 So, if you feed 121v into the 115v tap you will get 121 x 2.6 = 315v on the secondary.
And if you feed 121v into the 125v tap you will get 121 x 2.4 = 290v on the secondary.

Bottom line, if you want to lower the B+ then use the 125v primary tap.

[dude]

Can I ask where you got the black grill metal material in previous pics? Nice build.

[EricR]

Sounds backwards to me. The 115v tap has a higher voltage/turns ratio as referenced to the secondary. The 125v tap has a lower voltage/turns ratio as referenced to the secondary. Therefore, if you put 121v on the 115v higher voltage/turns ratio tap you will get a higher secondary voltage which will result in a higher B+.

For example, let's say your secondary puts out 300v.

300:115 = 2.6 voltage ratio which is also the turns ratio.
300:125 = 2.4 voltage ratio which is also the turns ratio.
 So, if you feed 121v into the 115v tap you will get 121 x 2.6 = 315v on the secondary.
And if you feed 121v into the 125v tap you will get 121 x 2.4 = 290v on the secondary.

Bottom line, if you want to lower the B+ then use the 125v primary tap.

Well that makes sense. I think the lable on the box was wrong it is showing grey 125 and black 115. Hammonds website matches what you are saying and I do have grey hooked up now.

So I should switch it back to lower the voltage at the cap? 440vdc is to high it would seem.

[EricR]

Can I ask where you got the black grill metal material in previous pics? Nice build.

Thanks! I got it at customcargrills.com This one was made from aluminum and powder coated black. They sell scrap pieces at discounted prices. It worked well

[sluckey]

Yes, that box label is wrong! Shame on Hammond. Use this pdf instead...

     https://www.hammfg.com/files/parts/pdf/270FX.pdf

And use the black wire for lower B+.

[EricR]

Allright good progress today. Most of the out put transformer is done as well as the control panel ground buss.

Thank you Sluckey for repeating  the answer in different ways untill I understand. Just want to make sure I am understanding.

[EricR]

Getting real close. Hoping to get it finished Wednesday and start her up. I was wondering since my B+ is a bit high is there anything I need to watch out for?

[Esquirefreak]

Damn, that's pretty. Nice work!

[sluckey]

I was wondering since my B+ is a bit high is there anything I need to watch out for?

Your B+ voltage will drop considerably when you finish the amp so the tubes can draw some load current. Total B+ current should be about 100mA. Easy to check if you are curious. Turn the amp off and put the STBY switch to standby mode (switch open). Set your meter to current mode to measure about 100mA and connect the meter leads to each side of the STBY switch. Turn the amp back on and your meter should tell you the total B+ current. I thought it was useful to know even if only to be sure my choke was rated for enough current.

[EricR]

Damn, that's pretty. Nice work!

Thanks. Has really been fun building it.

[EricR]

Allright. Got it done. Turned it on. No smoke.
100ma across stay.
A 330VDC
B 300VDC
C 301VDC
D 220VDC
voltages are looking good.
Made a little getup for the footswitch to fix the mistake of iec cutout.
One question. I remember reading about the 1ohm resistors on the power tubes making it easy to check bias. I can't find that thread now. Can someone explain how to calculate again. Thank you.

[sluckey]

I remember reading about the 1ohm resistors on the power tubes making it easy to check bias. I can't find that thread now. Can someone explain how to calculate again. Thank you.

1. Measure the plate voltage (pin 7) for each tube. Write it down.
2. Measure the cathode voltage (pin 3) for each tube. Should be the same. Write it down.
3. Measure the millivolts ***ACROSS*** each 1Ω resistor. Write it down but call it milliamps.
4. Subtract cathode voltage from plate voltage. This is the voltage across the tube.
5. Multiply the voltage across the tube times the milliamps through the 1Ω resistor. This is the static dissipation. Repeat for the other tube.

[EricR]

Thanks Sluckey. Everything works! Love the tone but have to play with it and get to know it a bit. I did the vib volume mod. I used a 67k resistor and if the norm channel is at 5 the vib in 6 is equal in volume. Real close. Not sure how far off the original circuit was but from what I read it sounds like this mod fixed it.

[sluckey]

Thanks Sluckey. Everything works! Love the tone but have to play with it and get to know it a bit. I did the vib volume mod. I used a 67k resistor and if the norm channel is at 5 the vib in 6 is equal in volume. Real close. Not sure how far off the original circuit was but from what I read it sounds like this mod fixed it.

Good to hear. You may be the first to try the mod.

[EricR]

Well those 1 ohm resistors sure made it easy checking for bias. I had 8 tubes to try n find a matched pair. I had a 125ohm for a cathode resistor which I know is slightly low making it run a bit hot. All my tubes were in the 14 to 15 watt range, well over 100%. I changed the cathode resistor to 250ohm and and chose a amprex tube and a Hammond tube cause their original read outs were real close. 15.3 watt a d 15.14 watt. Whit the cathode change I'm at 9.53 and 9.43. So 79% and 78%. Sounds real good with the changes. Here is a couple final pics too.