Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: waldner on November 01, 2025, 02:39:08 pm
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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?
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That schematic does NOT match the amp in your pics.
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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.
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Thanks DeepBias. I joined that forum to get more info. Here are some more pics:
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A bit more readable version of the schematic
(https://i.imgur.com/levsWtn.jpeg)
Franco
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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
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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.
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Excellent DeepBias! You are the man :worthy1: 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!
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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!
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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.
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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.
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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.
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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.
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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!
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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.
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Have you seen Mojos Studio One 1 watt amp (https://mojotone.com/products/mojotone-studio-one-1-watt-2-channel-guitar-tube-amp-kit?variant=41674212737120)? Look at the documentation.
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Have you seen Mojos Studio One 1 watt amp (https://mojotone.com/products/mojotone-studio-one-1-watt-2-channel-guitar-tube-amp-kit?variant=41674212737120)? 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.
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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+
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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://i.imgur.com/1VCdS0i.png)
(https://i.imgur.com/levsWtn.jpeg)
https://el34world.com/charts/Schematics/files/Matchless/Matchless_dc30_old.pdf (https://el34world.com/charts/Schematics/files/Matchless/Matchless_dc30_old.pdf)
Franco
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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.
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... 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
(https://i.imgur.com/RMSulUg.jpeg)
as the schematic is a bit confusing on reading, here the semplified version that is on the Service Manual
(https://i.imgur.com/gvb6VNB.jpeg)
EDIT:
I've find better example (the exact solution if youn want apply it)
(https://i.imgur.com/uKKWcS9.jpeg)
or this one
(https://i.imgur.com/rB7PKk2.png)
Franco
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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
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VERY COOL TUBENIT :thumbsup:
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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 ????? :cussing:
:BangHead: :BangHead: :BangHead: :BangHead: :BangHead: :BangHead:
Franco
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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
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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 :w2: :w2: :w2:
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 :dontknow: :dontknow: :dontknow:
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Here's another layout for this amp.
https://sluckeyamps.com/misc/Visio-wangs_mini_5.pdf
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Thanks for sharing all the great schematic and VISIO drawings and information! That all looks fantastic.
with respect, Tubenit
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Wow, this is awesome! Thanks for participating guys. :worthy1:
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?
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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
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Interesting... but how well does it work in practice and sonically? I've never seen it before. Pretty useful generally?
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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
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Here's another layout for this amp.
https://sluckeyamps.com/misc/Visio-wangs_mini_5.pdf
Nice one!
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Here's a schematic including some interesting mods
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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 ( :w2: :w2: :w2: )
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 :dontknow: :dontknow: :dontknow:
So, if waldner or anyone knows TA e TU spec for sure, please report it here
MANY THANKS
Franco
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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 ( :w2: :w2: :w2: )
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 :dontknow: :dontknow: :dontknow:
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.
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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
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Ok with the tube rectifier I get 329V. With diodes 342V.
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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
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Ciao Waldner
Better to use this image that is annotated
(https://i.imgur.com/mysOaSP.jpeg)
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
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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.
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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!
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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.
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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
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...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.
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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.
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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
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The link to the Layout drawn by Sluckey is out of service (https://sluckeyamps.com/misc/Visio-wangs_mini_5.pdf (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 :w2: :w2: :w2: :dontknow: :dontknow: :dontknow:
MANY THANKS
Franco
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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?
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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.
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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
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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.
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...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.
:thumbsup:
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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?
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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?
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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.
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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.
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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
(https://i.imgur.com/M4oo3ng.jpeg)
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
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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.
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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.
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Amp back tubed up and stable, measured DC voltage at point X= 262.5V and point Y= 296.6V
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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.
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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?
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With the power tubes removed the voltage is not going anywhere, that method should work. You sure your meter is set to "AC"...
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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!
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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
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Franco, I tried to send you a PM but your inbox is full. So, check your email.
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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.
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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.
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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
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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
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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.
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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 :smiley:
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.
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I would like to perform the measures for you but there is a problem, betwen us there is the pond, I live in Italy
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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
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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 :smiley:
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I'm posting from the phone and have some problems on posting, re read my message, I revised It
Franco
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I would like to perform the measures for you but there is a problem, betwen us there is the pond, I live in Italy
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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 :grin:
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Hey Franco, lots of Italian people in my family, cheers from Canada!
the world is small :icon_biggrin:
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 ?
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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?
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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
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Nice score the Mojo OT
Franco
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... 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.
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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)
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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)
(https://i.imgur.com/F2O9OPM.png)
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
(https://i.imgur.com/fUz25oS.jpeg)
I've find online the schematic of the Studio ONE (rev. 3)
(https://i.imgur.com/pc2e5KV.png)
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
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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!
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I just updated the layout to revision 3 yesterday. It's all just cosmetic. No wiring has changed. Just a bit neater.
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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:
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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.
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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?
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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
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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.
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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.
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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.
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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...
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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.
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Success!!! Good eye Steve! Those were indeed 470ohm instead of 470K resistors. Also that 10K-3watt was a 1K. She now works beautifully. :worthy1:
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
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May be a bad cap, bad switch, or solder connection. Put a 100K across the switch contacts. Any better?
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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?
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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.
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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?
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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.
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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?
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By the way, here is the finished exterior.