Help come up with layout for this great sounding 5W amp?

[SEL49]

You don't need a 100VAC source! I use a low voltage source (less than 5V) and connect to the secondary. Measure this voltage carefully. Then measure the voltage between the primary plate leads carefully. Divide the bigger voltage by the smaller voltage. This gives you the voltage ratio. For example, the voltage ratio may be 25:1. Now square the voltage ratio to get the impedance ratio. For this example, the impedance ratio would be 625:1. Now if you connect an 8Ω speaker to this 625:1 ratio OT, you would reflect 5000Ω to the primary plate leads. Saying it another way, you have a 5K:8 impedance ratio and that's a very common OT.

I like to use a 1VAC source voltage just to make the math easier. How do I get that 1V source? Easy. I connect a 6.3V filament transformer to my variac and adjust the variac to lower the 6.3VAC to 1VAC. If I didn't have a variac, I would use a 6.3VAC with center tap transformer and connect one 6.3V secondary lead to one OT secondary lead and connect the center tap of the filament transformer to the other OT secondary. This would apply 3.15VAC to the OT secondary. Measure for accuracy. Then measure the voltage across the OT primary and do the math as above. For example, let's use the same OT as above. 3.15V on the secondary would give 78.75V on the primary. 78.75:3.15 reduces to 25:1. The ratio is the same.

Smaller voltages are much safer to work with.So,maybeyoudon't have a 6.3V filament transformer laying around. Just use a couple gator clip test leads to connect the amplifier 6.3VAC (use the center tap to get 3.15V) to the OT secondary.

[DeepBias]

...maybeyoudon't have a 6.3V filament transformer laying around. Just use a couple gator clip test leads to connect the amplifier 6.3VAC (use the center tap to get 3.15V) to the OT secondary.

 
 

[waldner]

Ok, so I can use the 6.3V heater connections of the Wangs amp itself to provide what I need.  Or should I use another amp as it might be easier to access given larger size and turret construction with better access to the heater runs?  Then use one side of the heaters and the center tap to get 3.15V.  Am I getting this correctly?

[SEL49]

I don't seem to have any power supplies/adaptors that step AC down from 120V to 12V or similar.  Everything I have is converts to DC.  I do have a variac though.  Could I use that?

You can use a variac by itself, but it might be difficult to set to 1V or 5V. Much easier to accurately adjust if you plug a wall wart into the variac. Don't you know how to open a wart up and remove the diode/cap?
 

[waldner]

I've never taken apart a wall wart and removed a diode/cap.  I'm liking the idea of taking the 6.3V from another amp's heaters though.

[DeepBias]

I've never taken apart a wall wart and removed a diode/cap.  I'm liking the idea of taking the 6.3V from another amp's heaters though.

Look at the image, remove all the tubes before doing that. You could also use 0 - 16 ohm, remember to write the source voltage at 6.3V points.

[kagliostro]

You can use the 6.3V of your amp (assuming it is AC), as you don't have a CT on the heaters winding as to obtain a lower voltage on the primary connect the 6.3V AC between 0 and 16ohm Speaker jack

You can perform this thing as SEL49 say using wires with gators (the green wires on the image), be careful and attach it as they can't release, , see the attached image and measure at TEST 1 & TEST 2 the resulting Voltage

REMEMBER, pull out all tubes



Franco


p.s.: If you want you can also perform a "quality test" on the OT measuring the AC voltage you have between 0 and TEST 1 -- 0 and TEST 2 points, the closer are this voltages the better is

[DeepBias]

You can use the 6.3V of your amp (assuming it is AC)...

As per the schematic it should but who knows, better to verify indeed.
 

[waldner]

Ok, I used another amp heater/ground to get a constant 3.091 VAC.  I connected that to the 16ohm and 0ohm secondary side of the OT with the Wangs amp drained and tubes removed.  Measured at the 3K and 3K points on the primary side and got 98.1 mVAC.  You want to do the math and let me know what you discovered?


Franco, tell me what the HT (assume High Tension) voltage of the power transformer means and where to measure it.  I will also take the other measurements you asked for and report back.

[waldner]

Amp back tubed up and stable, measured DC voltage at point X= 262.5V and point Y= 296.6V

[DeepBias]

Ok, I used another amp heater/ground to get a constant 3.091 VAC.  I connected that to the 16ohm and 0ohm secondary side of the OT with the Wangs amp drained and tubes removed.  Measured at the 3K and 3K points on the primary side and got 98.1 mVAC....

If you got 98,1V AC the turn ratio would be 31,7:1 but mV AC doesn't make sense.
 

[waldner]

I agree that makes no sense.  I tried to set up and measure again and got the same 98mv.  Then I measured at the secondary locations with the 3.091 volts applied and it got 4.5mV.  It seems that this method won't work with the OT not completely disconnected from the amp?

[DeepBias]

With the power tubes removed the voltage is not going anywhere, that method should work. You sure your meter is set to "AC"...
 

[SEL49]

I agree that makes no sense.  I tried to set up and measure again and got the same 98mv.  Then I measured at the secondary locations with the 3.091 volts applied and it got 4.5mV.  It seems that this method won't work with the OT not completely disconnected from the amp?

I suspect your amp may use a shorting jack for the speaker. If so, insert a 1/4" plug to open the shorting contacts. A guitar cable will work if you don't have any unused 1/4" plugs laying around. Just don't plug anything into the other end of the cable!

[kagliostro]

Assuming the grey and black wires comes out from the PT directly, measure the AC HT voltage betwen grey and black and then the other grey and black

Look to the image

Test points are labeled 300VA - 0 - 300VB

Franco

[SEL49]

Franco, I tried to send you a PM but your inbox is full. So, check your email.

[DeepBias]

I agree that makes no sense.  I tried to set up and measure again and got the same 98mv.  Then I measured at the secondary locations with the 3.091 volts applied and it got 4.5mV.  It seems that this method won't work with the OT not completely disconnected from the amp?

I suspect your amp may use a shorting jack for the speaker...

No shorting jack, I checked that before posting, if that were the case he could have burned out the 6.3V secondary of the amp’s transformer he used as the AC source, which would’ve been really sad.

[kagliostro]

Just saying what I’m thinking.

When I read what SEL49 said about the shorting jack, my first thought was that there might be some oxide or other contact issue on that switch — basically acting like a resistor instead of a short — otherwise the voltage would drop to zero.

Now that it’s confirmed there’s no shorting jack, I think the multimeter probes (or the gators) just didn’t make good contact with the test points. Sometimes there’s a thin protective layer on the solder, kind of like invisible paint.

My suggestion: use the meter probes to scratch or punch through the solder a bit, and make sure the alligator clips grab firmly on the tin.

The proper way would be to refresh the solder on the test points by adding a bit of new tin with the iron, or solder short wires to the test/junction points — but that’s more of a last resort and not always needed.


---


When you perform your test control the voltage you feed to the transformer at the moment that it is feeded to the transformer, don't belive in what you read before to connect it, and measure at the test point, not at the alligators

Franco


p.s.: SEL49 you have an e-mail

[waldner]

I tried to make good contact several times with gator clips and probes.  Something else is going on with the and I just don't have the knowledge to test this thoroughly.  I was going to ship the little Wangs amp to DeepBias but he's located in Canada.  I'd send it to one of you,  Steve or Franco, if you are in the USA? 


Or do we have enough info to go on and get a pretty good result as is?


Thanks

[SEL49]

What are you even trying to accomplish with this OT test? All this test does is measure the impedance ratio of an OT. Very useful when you have an OT of unknown specs. But your OT is not an unknown. The impedance ratio is clearly marked on the OT. Seems we've all followed Alice down a rabbit hole for no reason.

Doesn't the amp work very well? If so, just let it be. The more you mess with it, the more likely it will become wounded.

[DeepBias]

What are you even trying to accomplish with this OT test? All this test does is measure the impedance ratio of an OT. Very useful when you have an OT of unknown specs. But your OT is not an unknown. The impedance ratio is clearly marked on the OT. Seems we've all followed Alice down a rabbit hole for no reason.

Doesn't the amp work very well? If so, just let it be. The more you mess with it, the more likely it will become wounded.

That’s basically what I told him in private, you said it in a more concise and elegant way 

By the way, did you notice that in the first photo waldner posted, the markings say 5k–0–5k, while in the last one, which seems to be from his actual amp, it says 3k–0–3k. Probably an earlier version before they changed the transformer.

Anyway, like I mentioned to waldner, if he goes with a Hammond 125C, he’ll have the flexibility to experiment with different impedance settings. The voltages are already known, so he’ll have a good range to work with. I think he wants to build a clone of it.

Edit : And if I’ve followed the thread correctly, his main concern is that the new amp handles overdrive pedals better, which is easily solved with a low/high input. As for the other modifications, they’re pretty much optional, so the layout Slucky proposed is perfect. Personally if I were to build it straightforwardly, that’s exactly what I’d do, and then I’d experiment based on the results. Even if you build two amps with the same components, the sound won’t be completely identical, there’s no rocket science with tube amps, imho.

 

[kagliostro]

I would like to perform the measures for you but there is a problem, betwen us there is the pond, I live in Italy

----

Those values are probably wrong values

A guy who measured the OT talks about of a primary impedance of 25K

One OT is labeled for 10K AA, one other 6K aa and someone say he measured 25K AA

All this different info put on the curiosity to know what is real and what isn't

despite all if someone buid a clone it will be not a pleasant thing to discovery he used the wrong OT

What about the PT ?

Franco

[DeepBias]

I tried to make good contact several times with gator clips and probes.  Something else is going on with the and I just don't have the knowledge to test this thoroughly.  I was going to ship the little Wangs amp to DeepBias but he's located in Canada.  I'd send it to one of you,  Steve or Franco, if you are in the USA? 


Or do we have enough info to go on and get a pretty good result as is?


Thanks

Those values are probably wrong values


A guy who measured the OT talks about of a primary impedance of 25K


One OT Is labeled for 10K AA, one other 6K aa and someone Say 25K AA


All this different info put on the curiosity to know what Is real and what Isn't


What about the PT ?


Franco

Have that little baby sent to you and we'll know 
 

[kagliostro]

I'm  posting from the phone and have some problems on posting, re read my message, I revised It

Franco

[DeepBias]

I would like to perform the measures for you but there is a problem, betwen us there is the pond, I live in Italy

----

Those values are probably wrong values

A guy who measured the OT talks about of a primary impedance of 25K

One OT is labeled for 10K AA, one other 6K aa and someone say he measured 25K AA

All this different info put on the curiosity to know what is real and what isn't

despite all if someone buid a clone it will be not a pleasant thing to discovery he used the wrong OT

What about the PT ?

Franco

Hey Franco, lots of Italian people in my family, cheers from Canada!

I’m kind of like you I’m curious. It would’ve been interesting to know the actual impedance value, same for the power transformer. But really, that method of checking impedance using voltage is usually pretty straightforward. I can’t quite see what the issue would be… oh, maybe he forgot to turn on the source amp for the 6.3V, and that 98mV reading is just the static from beginner’s nerves.

Just kidding, guys 
 

[kagliostro]

Hey Franco, lots of Italian people in my family, cheers from Canada!

the world is small 

When I was in high school, a classmate of mine was from Canada. His father was Italian, and after several years in Canada, he decided to return to Italy, but he stayed there only a short time. He couldn't adjust to a country different from the one he remembered. His father returned to Canada, while my classmate stayed there for five years to finish high school, and then he too returned to Canada. Many years have passed, but I believe his last name was Dalla Giustina.

Excuse me for the OT (Out of Topic)

Franco


p.s.: May be a malfunction of the multimeter or a wrong set on it ?

[SEL49]

Mojo's Studio One amp uses this 11.5K primary OT with push/pull 12BH7s. One of the major reasons for proper impedance matching is the transfer of maximum power when properly matched. Seems to me that "proper" impedance matching is not a big consideration in a 1 watt amp?

[kagliostro]

I agree with you SEL49 power isn't a problem, what I think is a problem for the autor of the thread is the "tone" of the amp

he say it is a great sounding amp and he is interested to know if is possible to clone it, so I imagin he wants the clone has the closer sound to the original

all the investigation is around to discover the more about the amp to perform a very close copy

all is going around this

---

Nice score the Mojo OT

Franco

[DeepBias]

... p.s.: May be a malfunction of the multimeter or a wrong set on it ?

That's what I asked a couple of post above but no answer yet, sometimes I forget myself to switch on AC since my DMM is powering on DC by default.

You have a really good point SEL49, and 10K seems like a good compromise without overthinking. I’ve got a little one watter but it uses a 12AU7, I was looking at it and thought I could throw it on the bench and try different load impedances, but honestly it’s well-built and sounds great at 22K. For the amount of work it’d take and the small difference it would make, my curiosity kind of fades away, I'm focussing right now on adding a new preamp stage on an old 18W, I'm kinda almost there.

For some still-unknown reason, we’ll just have to put our curiosity on hold for now.

But if I wanted to twist the knife a little, I’d ask waldner to give it one last try by plugging a speaker cable into the 16-ohm jack, and then applying the 6.3V from the other end of the cable. That way we can be sure the voltage is actually going into the secondary, it gives a nice solid spot for the alligator clips to grab onto.
 

[kagliostro]

I agree with you, two male jack (or two power or guitar cables), one on the 8ohm intake and the other on the 16ohm intake will solve the problem

and result in good connection using gators and we will be sure that if present any shorting switch will be open (despite all we are not sure 100% and may be there is one one)

--

Going on with research I've found something that can help us

MOJOTONE has that OT for the 12BH7 and for that use the reflected impedance is 5.75K-0+-5.75K or 11.5K aa if you prefer)



the same brand has a Kit (The MoJotene Studio ONE) with 12BH7 as Power Tube (single tube in self split PP configuration) and the PT they use is this



I've find online the schematic of the Studio ONE (rev. 3)



As we can see on the documentation the PT HT AC is 250V (@120mA) (near the voltage of 240V that was calculated starting from the measured B+) and on the schematic B+ is 325V (hope you can read it on the posted schematic)

So the Voltages measured by Waldner of 329V using the 6Z4 and 342V using SS diodes are very close

As SEL49 told the Mojo 1580 is usable and also a 250V AC PT is fine, may be a measure of the effective Z of the Mini 5 will be useful to establish the exact value of the primary Z but the data that were collectded by DeepBias and SEL49 they would be enough to get close to a clone that is quite close to the original

Franco

 

[waldner]

Ok, I think I'm just going to build it and see what happens!  I've got the 125H OT on the way and will get the 270CAX on order.  I've got a lot of the small parts in stock and the missing ones on order so I can build the board.  It sounds like Steve's updated version is enough in the ballpark.  I will update as I go. 


Thanks!

[SEL49]

I just updated the layout to revision 3 yesterday. It's all just cosmetic. No wiring has changed. Just a bit neater.

[waldner]

Ok, the Mini 5 build is underway!  Just as an aside, I don't have a drill press and it's been years since I had to drill a board and install turrets.  I found a way to do it that is quick and works well using an automatic center punch.   I made a quick video to remind my future self how I did it.  Just in case it helps anyone else, here is a link to it: 

[waldner]

Build progress continues...  Still waiting for the PT but have pretty much everything else.  Turret board is complete.  Front panel controls are mounted.  I'm doing this build in a walnut cigar box, so everything will have to be grounded with a wire.  I think I'm going to try the slope control and put the pot on the top along with the bright/warm switch and the tube/SS rectifier switch.  Here's what I've got so far.

[waldner]

More progress and a couple of questions....  I just got the PT and am about to find room to squeeze it into this layout. Just that and the heaters left to do.  I didn't notice till it arrived that it has a 6.3v center tap.  The original transformer I planned for didn't.  My assumption would be to always use the center tap if it's available, but I though to ask if there is any reason to prefer the artificial center tap instead?

The other question is regarding grounding.  This is obviously an unusual chassis being that it's entirely made of wood.  I've done several like it before and just arrived at the best grounding through trial and error, but thought I'd ask if anyone has a theory.  I've created a grounding point near the AC inlet using a terminal strip with all three legs tied together.  Right now it's wired with the preamp ground bus (near pots) having it's own wire carried all the way to this strip.  The ground lugs that are all tied together on the left side of the turret board are connected to the ground lug of the dual 32mfd big cap, which is then tied to the ground terminal strip.  Once the PT is installed it will be grounded directly to the terminal strip.  Not sure if any of this makes a difference given that everything works it's way back to the AC inlet ground?  Any thoughts?

[kagliostro]

I'll don't use the CT, better to use a Humdinger connected to the cathode of the PowerTubes or to an elevation node (around 40V / 60V)

Franco

[waldner]

I just finished wiring everything up, installed the tubes, a fuse, and fired up the amp for the first time.  I plugged it into a light bulb current limiter, which was then plugged into a Variac.  I connected a speaker and guitar, crossed my fingers and brought it up slowly.  The pilot light and tubes started to glow around 75V, but as I got just over 100V- SMOKE!!!  The 36K resistor pointed to in the attach pic started to burn up.  Anybody have any ideas what might be burning up that resistor?


I also had no sound when I got up near 100V prior to the resistor starting to burn.  I thought I'd have started to hear noise through the speaker, but I had nothing. 
 
Thanks for any insights.

[SEL49]

There is an under-board jumper on each end of that resistor. Verify both jumpers are wired correctly. Is that turret board screwed directly to that wood? IOW, are all the turrets in contact with the wood? If so, fix that by using a fiberglass backer board to insulate the turret board from the wood. Or just use four standoffs to raise the turret board above the wood.

BTW, wood is not a good insulator for this stuff, not even your heater buss.

[waldner]

Good to know about the wood.  I will raise the board up and double check the under board connections.  I have a few free hours tomorrow before Thanksgiving turkey. 


Thanks Steve.

[waldner]

The forum was down.  Thank God it's back! 


I followed Steve's advice and raised the turret board up off the wood on standoffs, cut the copper heater wire bus rods down and got them off the wood.  I replaced the burned up 36K resistor and added a ground wire from the output speaker jack to the terminal strip.  I fired the amp back up.  Much better!  Nothing burned and I now have what sounds like a normal amp but it's only putting out 1/10th the power.  It's like it's working completely but only enough output for headphones. The only other glitch is when I flip the warm/bright switch to bright, it goes into crazy oscillation.  Sounds normal but tiny in the warm position.  Here are DC voltages I measured:


B+ = 311V

12AX7(V1)plate (pin 1) = 177V
                  plate (pin 6)= 146V
             cathode (pin 3)= 1.4V
             cathode (pin 8)= 1.03V
12AX7(V2) plate (pin 1)= 271V
                   plate (pin 6)= 261V
             cathode (pin 3)=147V
             cathode (pin 8)= 39V
12BH7(V3) plate (pin 1)= 312V
                   plate (pin 6)= 311V
              cathode(pin 3)= 14V
              cathode(pin 8)= 14V

Any ideas?  So close...

[SEL49]

Look at the attached pic. Please verify these resistor values. Two 470K will probably fix the volume issue, but the other two resistors need to be correct value if not already correct.

We need voltages for nodes X and Y and also all grid voltages even if some are zero.

[waldner]

Success!!! Good eye Steve!  Those were indeed 470ohm instead of 470K resistors.  Also that 10K-3watt was a 1K.  She now works beautifully.   
I think Steve has saved every build I've ever posted about on this forum- please don't get hit by a bus or go anywhere:)

The only thing issue that remains is the bright/warm switch which introduces the 1uf bypass cap.  When in the bright position it causes severe fizzy high frequency oscillation.  Any idea what that might by caused by?


Here are the other measurements taken with the tube rectifier:


Node X= 239V
Node Y=  271V

12AX7 (V1) pin 2 - .4mV
                  pin 7-  0mV

12AX7(V2) pin 2= 203V
                 pin 7= 16V

12BH7(V3) pin 2= 2mV
                 pin 7= 2.5mV






   

[SEL49]

May be a bad cap, bad switch, or solder connection. Put a 100K across the switch contacts. Any better?

[AlNewman]

The only thing issue that remains is the bright/warm switch which introduces the 1uf bypass cap.  When in the bright position it causes severe fizzy high frequency oscillation.  Any idea what that might by caused by?
   

Is that the MOD cap that terminates at the back of the volume pot?  Why not ground it to the same point as the cathode resistor?

[SEL49]

That cap should be connected to the ground buss that runs behind the pots. Nothing should be soldered to the back of the pots.

This may very well be a poor ground issue. Experiment.

[AlNewman]

I meant to say terminate 'behind' the volume pot.

If the blue circle is the switchable cap, the red circle is where the cathode resistor meets ground.  Doesn't it make sense to ground the cap at the same spot?

[SEL49]

They are connected together. Just not directly. But a direct connection may be better. Something is clearly wrong. Could very well be a grounding issue. Easy enough to experiment.

[waldner]

Ok, I played around with trying to solve the fizzy oscillation when the bright switch is engaged.  I tried everything, moving the ground to the location where it would naturally be, getting rid of the switch, and even installing a different 1uF cap directly in the bypass location above the resistor.  None of them sounded great.  The fizzyness wasn't as bad and the oscillation seemed to stop but it was still fizzy and not useable.  I decided to abandon it as the amp sounds well balanced and plenty bright without it.


I thought, maybe I use that switch for a negative feedback loop instead  So I played around with alligator clipping in various values between the output jack 8ohm tap and pin 8 on V1.  I pretty quickly found that around 1K worked nicely.  When I ran wires to that switch, which is right next to the input jack, I started to have a new problem.  The sound would fade out slowly to total silence, then fade back in, eventually fade back out.  I'm guessing the proximity of that strong speaker output signal near the delicate input signal is the cause.  So I'm going to move the feedback switch to the back panel. 


Since that switch is already drilled and mounted in the front/top, maybe I'll use it as a fat/lean switch to select between the stock 22uf cathode cap vs a smaller value like a .68uf or something?

[waldner]

By the way, here is the finished exterior.