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

[waldner]

Slightly long story....I was playing a gig last night and using my modified 5E3 amp.  This venue is upscale and we don't get crazy loud, so the amp wasn't quite getting into the point of starting to break up on it's own.  No big deal, I had an overdrive pedal, but like it better when the amp gets there on it's own.  I brought a back up amp to the gig just in case something blows up, a little Wangs Mini 5 watt amp that sounds absolutely fantastic.  It uses a single 12BH7 as a power tube in push-pull with a couple of 12AX7s and your choice of diode or tube (6Z4) rectifier.  It also has a TMB tone stack and a bright switch.  I cannot emphasize enough how good this amp sounds compared to single ended things like Champs, etc...  I have a lot of amps, and this is easily one of the more impressive sounding things I have encountered!


So I thought.. wouldn't it be nice to be able to build these as they aren't being made by Wangs anymore.  It would be a fun and highly useful amp for home, smaller gigs and portability.  I think it could also be nice to add in some DIY tweaks to suit personal taste and, let's be honest, some of us just like to tinker. 

I found someone online who apparently came up with a schematic but I don't have the knowledge to A) know if it's right or workable and B) how to translate it into a layout and spec transformers.  So the only guys I know who might be able to do that are here on the Hoffman forum!  Anyone want to help come up with a design? 

[SEL49]

That schematic does NOT match the amp in your pics.

[DeepBias]

By searching on Google, this schematic can be found. It was posted by a user (vzart) on another forum, mentioning that he has issues with this amplifier, the secondary side of the transformer had fried due to a failure of the 12BH7 tube. He also traced the PCB layout but since I’m not a member on that forum I couldn’t view it clearly at a decent size. He mentioned simulating the volume with two fixed resistors because he's learning to use LTSpice.

That being said, there are some components simulated or missing on the schematic, such as:

1M volume potentiometer simulated by R6/R7 resistors and also tonestack potentiometers simulated with resistors (R11 to R16).
Bright/Warm switch is missing (C4)
The output transformer is missing the 16Ω tap
The 6Z4 rectifier is missing (and the swicth to swap between tube or diodes)

Otherwise, it looks like a typical amplifier schematic, typical 470k/470p attenuator, tonestack fed by cathode follower just like many amps, cathode-biased cathodyne inverter, seems close enough to consider building it, but there’s no certainty about its accuracy. You could contact that guy to ask if he has an updated version of the schematic / layout and transformers specs.
 

[waldner]

Thanks DeepBias.  I joined that forum to get more info.  Here are some more pics:

[kagliostro]

A bit more readable version of the schematic



Franco

[tubenit]

Here is something that perhaps you can use as a guide to create your own layout.  ExpressSCH is a free program and very easy to work with.  I have NOT built this ........... so you're on your own. :-)   I attached a SCH schematic that you can modify.

Compare schematics to what is similar or different.  Yours vs. this one.

With respect, Tubenit

[DeepBias]

PCB is a mess for a tube amp involving thermal management and the potential for signal interference if not designed correctly.

I love the little amps that can be cranked without disturbing the neighbourhood so I drew the schematic, it's in the todo list for later but I'm gonna do it on turrets or eyelets board.

I found the transformers listed on the schematic for cheap here :

https://www.hawkusa.com/manufacturers/hammond-mfg/power-products/power-chassis-mount/272bx - 87$

https://mojotone.com/products/clone-of-the-wright-1580-10k-to-4-8-16-12bh7-12au7-pp-output-transformer - 53$

Edit : This is a rough draft; perhaps it would be better to have elevated heater, and C13 should be 500V.

 

[waldner]

Excellent DeepBias!  You are the man     I don't have the skills to do that, so thanks for your effort. 
I would really like to build one.  Turret or eyelet board layout would be great for me as that's all I have ever built.


Thanks!

[waldner]

Edit : This is a rough draft; perhaps it would be better to have elevated heater, and C13 should be 500V.

Yes, elevated heaters and whatever else you think would be worth trying!

[mountainhick]

As far as layout, if you are not trying to cram it into a tiny package like the original, it is essentially a 5F6A circuit with a few component differences. and that 470K/470pf voltage divider/ bright bypass after the first stage. All you need is a 5F6A eyelet/turret board, and can cut off the normal channel end off the board, plus a cap board or terminal strip setup for power supply caps and dropper resistors.

Or, Rob robinnette has already done almost the whole thing here: https://robrobinette.com/images/Guitar/Bassman_Micro/Bassman_Micro_Eyelet_Layout_With_Rectifier_Tube.png

Also, if you are OK foregoing a full tube rectifier, ANTEK toroid PT for about $30. They have a 50va 320V that has a 300V tap or to me a 280V would be fine as well. Can be construed with a hybrid SS/tube rectifier if you wish.

[waldner]

Interesting Mountainhick.  I see enough differences between the Robinette Mico Bassman schematic and this Wangs Mini 5 that for me it's really useful that DeepBias is transcribing it more accurately.  For example, the Bassman Micro is a long tail pair into a 12AU7 where the Mini 5 is a cathodyne PI into a 12BH7.  Also the power supply with switchable diode vs tube rectification. 


I do see a lot of similarities in other parts of the schematic.  Funny because I built Hoffman's version of the classic Bassman and it's way more gnarly and rough sounding than the character of this Mini 5.  Maybe this is a good opportunity for me to study and compare the schematics and learn something!  Thanks for your input.


Also, for anyone who wants to build this project, the switchable rectification is neat but not a necessity for good sound with the Mini 5. 
Surprisingly I actually like it better most of the time with the diodes.  It tightens it up a little, which is helpful with a lower powered amp.

[tubeswell]

Interesting Mountainhick.  I see enough differences between the Robinette Mico Bassman schematic and this Wangs Mini 5. Bassman Micro is a long tail pair into a 12AU7 where the Mini 5 is a cathodyne PI into a 12BH7.

So copy the 5F6A layout up to the end of the TS (but only with 1 input channel). And copy the low power tweed Bassman layout for the cathodyne stage, and splice them together.

[DeepBias]

Despite not being a very complex circuit, the layouts are good starting points and references for organizing the components and saving time, thanks for that. The Hammond 125B/C as per Rob schematic was the first OT I though about, also a good choice.

@mountainhick I agree 280V would be also fine, we can adjust R29 anytime.

Do you guys think it would be worth to add 2 fuses to the secondary AC side, if so what is the best way to integrate them, on the board or to the chassis.

I have now modified the schematic with elevated heater.

@waldner Yes the rectifier selector can be optionnal, simply by not adding the socket and switch, then connect the diodes cathodes directly to the rail or keeping the switch for stanby.
 

[mountainhick]

Interesting Mountainhick.  I see enough differences between the Robinette Mico Bassman schematic and this Wangs Mini 5 that for me it's really useful that DeepBias is transcribing it more accurately.  For example, the Bassman Micro is a long tail pair into a 12AU7 where the Mini 5 is a cathodyne PI into a 12BH7.  Also the power supply with switchable diode vs tube rectification. 


I do see a lot of similarities in other parts of the schematic.  Funny because I built Hoffman's version of the classic Bassman and it's way more gnarly and rough sounding than the character of this Mini 5.  Maybe this is a good opportunity for me to study and compare the schematics and learn something!  Thanks for your input.


Also, for anyone who wants to build this project, the switchable rectification is neat but not a necessity for good sound with the Mini 5. 
Surprisingly I actually like it better most of the time with the diodes.  It tightens it up a little, which is helpful with a lower powered amp.

The link I provided  of Rob's layout is a cathodyne version.

Yes, i agree, there are those differences, but what I am getting at is the layout is already taken care of. You just need to account for the small differences. Your thread title is "layout" after all.

Regardless, enjoy the process!

[waldner]

The link I provided  of Rob's layout is a cathodyne version.

Ah, sorry.  I went to find the schematic and ended up at the LTP version.  Thanks.

[SEL49]

Have you seen Mojos Studio One 1 watt amp? Look at the documentation.

[JPK]

Have you seen Mojos Studio One 1 watt amp? Look at the documentation.

Was going to suggest the same thing. I built one of those. Awesome sounding amp. Similar design except it's two channel. I'd leave out the clean since I rarely used it.

[DeepBias]

I didn't know this Mojotone little amp, watched some videos on YT and it really is a good sounding amp.

Not using an inverter allows for the use of an extra triode in the preamp.

I also rarely use the clean channel so the relay circuit is no longer needed.

The Wangs mini 5 have such a high B+ voltage, some suggest to be the cause of many problems with this amp like output tube or power transformer failure, that 15% lower will bring the output tube to the safe territory but the tone will not be the same. Maybe the extra preamp triode is the key for a good driven tone at lower B+
 

[kagliostro]

If I was building a clone of the Wangs Mini 5 I'll give a try to the parallel V1 triodes like in the Matchless DC30 instead to use the idle triode as a separate gain stage





https://el34world.com/charts/Schematics/files/Matchless/Matchless_dc30_old.pdf

Franco

[waldner]

If I was building a clone of the Wangs Mini 5 I'll give a try to the parallel V1 triodes like in the Matchless DC30 instead to use the idle triode as a separate gain stage
Franco

That could be cool, as could some other variations.  My original idea was to take the Mini 5 as-it-is and add a negative feedback switch to make it capable of even cleaner-cleans and taking pedals better.  It actually takes pedals very well as is, but once it's cranked up above halfway you don't really need an overdrive in front of it.


If we have the original layout/design for the circuit as-is, then several variations could be interesting.  Rob Robinette does a nice job of designing layouts for several versions of many classic amps and micro-versions of those amps.

[kagliostro]

... My original idea was to take the Mini 5 as-it-is ...

I can understand, if you want you can build the amplifier according to the diagram and use a switch to parallel the unused triode to V1

On the VOX AC30CC2 there is something similar



as the schematic is a bit confusing on reading, here the semplified version that is on the Service Manual

EDIT:

I've find better example (the exact solution if youn want apply it)



or this one

Franco

[tubenit]

There seems to be enough interest in this amp that I decided to draw up a layout for it.  I did this quickly and have only checked over it briefly.  It certainly could contain errors!


SO CHECK FOR ERRORS AND COMPARE WITH SCHEMATIC


If someone finds an error ........... open up the SCH file and correct it and repost. It is unlikely that I will repost any corrections because my time is very limited.


Especially on "experiment" builds, I strongly prefer parallel terminals or turrets. It's very easy to change components out using that approach.  And my experience is my amp builds are remarkably quiet and sound pleasing to me.


IF I were building this ............... I'd use a mosfet for the cathode follower and then use an LTPI phase invertor instead.  You can look up the HoSo56  The Minimalist for an example of that.


With respect, Tubenit

[kagliostro]

VERY COOL TUBENIT


---

I think my brain is going out of service

I was suggesting something like if in the schematic there was an unused triode but all triodes are used (me STUPID)

to which schematic was I thinking ????? 

 


Franco

[tubenit]

IF I were going to build this amp ............... I'd use a mosfet CF and then use an LTPI phase invertor.  It would make for
an easier layout also, IMO.


With respect, Tubenit

[kagliostro]

Thanks for sharing Jeff

I was able to edit the .sch schematic and export it as an image but the copy I saved as to be reposted of the .sch file result corrupted, may be because I use Windows 11 Pro 

However here the image with 1 correction and 2 small rectified values

defferences are in red, the Vol pot to me is 1Ma not 500Ka, C9 is .022 not .02 and Treble pot is 220Ka not 250Ka

E&OE

Franco

p.s.: I tried one other time, I downloaded newly the .sch file, edited and saved it but it results in a corrupted .sch   

[SEL49]

Here's another layout for this amp.

     https://sluckeyamps.com/misc/Visio-wangs_mini_5.pdf

[tubenit]

Thanks for sharing all the great schematic and VISIO  drawings and information!  That all looks fantastic.


with respect, Tubenit

[waldner]

Wow, this is awesome!  Thanks for participating guys.    
Steve's super high quality and complete schematic, color layout and wiring info is out of this world! 

What does the optional slope control do?

[tubenit]

What does the optional slope control do?

Download the Duncan Tone Stack calculator on your computer and run different slope values and you will see what it does.


With respect, Tubenit

[waldner]

Interesting... but how well does it work in practice and sonically?  I've never seen it before.  Pretty useful generally?

[SEL49]

I can't say how well the slope control works. I always wanted to try it. Rob Robinette talks about it on his amp voicing page. Scroll down to the "Tone Stack" section...

     https://robrobinette.com/Voicing_an_Amp.htm#Tone_Stack

[mountainhick]

Here's another layout for this amp.

     https://sluckeyamps.com/misc/Visio-wangs_mini_5.pdf

Nice one!

[DeepBias]

Here's a schematic including some interesting mods

[kagliostro]

I have a question for waldner (who could measure the real value having the amplifier)

​​or for anyone else who knows the real value of the B+ on the Wangs Mini 5 (in 6z4 mode, not diodes)

Many Thanks

EDIT:
 
about the OT, I've read: "It is labeled secondary @ 0-8-16ohms and primary @ 5k-0-5k" but if measured the resulting primary impedance is 25K ( )

about TA: "The HT supply should be AC 300VA-0-300VB, this specs is printed in the circuit board."

But having read about the OT impedance I'm no more sure of the accuracy on the info on the amp 

So, if waldner or anyone knows TA e TU spec for sure, please report it here

MANY THANKS

Franco

[waldner]

I have a question for waldner (who could measure the real value having the amplifier)

​​or for anyone else who knows the real value of the B+ on the Wangs Mini 5 (in 6z4 mode, not diodes)

Many Thanks

EDIT:
 
about the OT, I've read: "It is labeled secondary @ 0-8-16ohms and primary @ 5k-0-5k" but if measured the resulting primary impedance is 25K ( )

about TA: "The HT supply should be AC 300VA-0-300VB, this specs is printed in the circuit board."

But having read about the OT impedance I'm no more sure of the accuracy on the info on the amp 

So, if waldner or anyone knows TA e TU spec for sure, please report it here

MANY THANKS

Franco

Help me figure out where to measure B+ on the amp and I'll do it.  See pic attached.

[DeepBias]

Set your multimeter at the highest DC voltage or DC voltage auto range (not under 500V DC), negative probe of the meter clipped on the chassis (metal) and positive probe at one of the pad (circled in red), never use both hands to probe such high voltage. Please report voltages of both tube and diodes rectifier.

Edit: Be sure to connect a speaker (or resistive load) and sets all pots to minimum

[waldner]

Ok with the tube rectifier I get 329V.  With diodes 342V.

[DeepBias]

Ok with the tube rectifier I get 329V.  With diodes 342V.

So the transformer secondary is not 300-0-300 but in the range of 250-0-250 or 260-0-260 V AC
 

[kagliostro]

Ciao Waldner

Better to use this image that is annotated



Be careful on what you do, don't touch any thing on the amp with your hands

Set your Tester to measure DC Voltages 500V or higher and remember to annote all the voltages you measure with indication of the Test Point you measured

then measure voltage putting terminals of your tester one on Ground (see the big screw on the top left of the photo you just published anthe the other on poits labeled B+ - X - Y)

when done give a measure to the HT AC that comes from the PT, I don't see the PT but I can suppose the two grey wires and the black one that are connected to the 300VA - 0 - 300VB comes directly from the PT, verify and if so measure the voltage putting the terminals of your tester one on 300VA and the other on 0 then repeat between 300VB and 0 (you can also measure the voltage between 300VA and 300VB and you must find the double of the previous measures

To do a good job I prefer to have both AC PT voltage and DC B+ voltages

Do you know how to measure the Output Transformer and establish the real Z of the primary in accomplish with the secondary ?

If you don't know I can explain, is not difficult if you know how to do it

Franco


p.s.: 3K - 0 - 3K is real impossible (the same for 5K - 0 - 5K) for an OT to be used with a 12BH7 as output tube in PP

[SEL49]

So the transformer secondary is not 300-0-300 but in the range of 250-0-250 or 260-0-260 V AC

I'm thinking it probably is a teeny-weeny 300-0-300 PT and the amp load just pulls the voltage down.

[waldner]

Kagliostro,
 
I measured B+ in two locations; the one Deepbias instructed and I also took it again at the center tap of the primary side of the OT. I got the same results in each case.  I will go and measure the other stuff you mentioned tomorrow.


Thanks!

[DeepBias]

So the transformer secondary is not 300-0-300 but in the range of 250-0-250 or 260-0-260 V AC

I'm thinking it probably is a teeny-weeny 300-0-300 PT and the amp load just pulls the voltage down.

It’s quite possible ... According to reviews, that transformer seems to burn out just by looking at it. There also appear to be several points in this amp that don’t match the markings. Maybe it would be best to just use Slucky’s excellent layout and make whatever modifications you want to it. A transformer with 500V C.T. (250-0-250) should be suitable. Unless I’m mistaken, the values suggested by Slucky are appropriate annotation-wise but according to the Hammond 261G6 datasheet, the secondary is 125-0-125 (138,4 NLV) @ 115V AC primary, but still can be used with a bridge rectifier ignoring the center tap... As for the output transformer, you often see an impedance of 11.5K (plate to plate) for the 12BH7, though I’ve seen higher values depending on the circuit.

In the worst case, just come back to the forum for adjustment questions.

Kagliostro,
 
I measured B+ in two locations; the one Deepbias instructed and I also took it again at the center tap of the primary side of the OT. I got the same results in each case.  I will go and measure the other stuff you mentioned tomorrow.


Thanks!

You got the same result since those 2 points are directly linked by a trace on the PCB.
 

[kagliostro]

Measuring the Primary Impedance of a Push-Pull Output Transformer (How To)

We assumes that the secondary winding will be connected to an 8 Ω load (use the proper speaker connection)

To determine the primary impedance of an output transformer (OT) for a push-pull amplifier, proceed as follows:
 

Preparation

It is not strictly necessary to remove the OT from the chassis.

However, all tubes must be pulled and the speaker cable disconnected to ensure that the transformer is fully isolated from the rest of the circuit.

   
Test Setup

Apply a known AC voltage across the two ends of the primary winding (do not connect the center tap).

A test voltage of approximately 100 V AC (or more) is recommended.
If the voltage applied to the primary is too low, the induced voltage on the secondary will be very small and difficult to measure accurately with a standard multimeter.


Measurement


Measure the exact primary voltage (Vp) you applied and the resulting secondary voltage (Vs)


Calculation

Calculate the turns ratio:
 
Vp (the voltage you measured at the primary - we assume 100V) / Vs (the voltage you measured at secondary - we assume 5V) = Turn Ratio = 100 / 5 = 20


Calculate the reflected primary impedance:
 
Zp = (Turn Ratio x Turn Ratio) x (Output Load)

Zp = (20 x 20) x 8(ohm) = 400 x 8 = 3,200ohm = Primary impedance


Result

With an 8 Ω load connected to the secondary, the primary will present a 3.2 kΩ plate-to-plate impedance to the push-pull output stage.


I expect a way higher primary impedance for the 12BH7 tube (in the orther of 18K aa to 21K aa)

(no the 3K-0-3K or 5K-0-5K labeled on the OT ---- 3K-0-3K = 6K aa ---- 5K-0-5K = 10K aa)

Franco

[SEL49]

...according to the Hammond 261G6 datasheet, the secondary is 125-0-125 (138,4 NLV) @ 115V AC primary

Good catch on the PT. I've updated the documents to use Hammond 270CAX instead.

[DeepBias]

Yeah it would be nice to know the exact primary impedance of that output transformer! If you have some AC voltage, just a ~12V AC transformer will do to inject at primary and follow the instructions by kagliostro.
 

[kagliostro]

The math I did say near a 240-0-240 volt but a 250-0-250 will be fine

About the use of a low voltage on the primary to perform the test, 12V will result in a very low voltage on the secondary, to be measured a high efficent multimeter is required, if improperly measured the secondary voltage it will result in a not accurate Turn Ratio number and then a non accurate Z primary impecence value, try a higher voltage if you want to be sure about the result

You can go a little better measuring at the 16ohm intake instead of the 8ohm intake but I'll prefer to use a higher voltage in both the output intakes

EDIT:


One thing you can try Is to connect a small 6,3V transformer to the secondary and measure the voltage obtained at the primay (I'll espect around 150/200V)

Measure exactly the two voltages and do the math

Franco

[kagliostro]

The link to the Layout drawn by Sluckey is out of service (https://sluckeyamps.com/misc/Visio-wangs_mini_5.pdf)

and, the more, I'm not able to find the copy I saved on my PC

please, if someone has saved the .pdf file post it here

I tried some time and now the link is working     

MANY THANKS

Franco

[waldner]

I'd like to take the transformer measurements you ask for but don't know that I have a way to provide a 100V AC to do the test.  What might I have laying around that would provide this?

[DeepBias]

I'd like to take the transformer measurements you ask for but don't know that I have a way to provide a 100V AC to do the test.  What might I have laying around that would provide this?

If you don’t have a high-voltage AC source, you can use a low-voltage AC source instead, but inject it into the secondary rather than the primary. For example, you can use a 12V AC (lower is better) transformer. The actual voltage doesn’t really matter, what’s important is to write it down. If needed, the always-friendly Uncle Doug has a clear demonstration of the procedure on YT. Be careful of the high voltage that will appear on the primary.
 

[kagliostro]

I agree with DeepBias

connect the 12V AC to the 16ohm speaker jack, it will result in a lower voltage on the primary
(lower respect if you connect it to the 8ohm jack)

The important thing is you measure with your multimeter the exact voltages you have on the primary and secondary

then annotate it and do your math or post it and we'll do it for you

remember, please, to meaure also the other voltages I asked to you

AC HT Voltage of the PT

DC B+ (you have already measured it and indicated it as 329V and 342V

DC Voltage on test point X

DC Voltage on test point Y

if you want give a control also to the voltage on heaters, test point 6.3A and 6.3B

Franco