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Discussion in 'Amp-a-ridifiers' started by duceditor, May 10, 2018.
Ah! I missed that notation. I thought Don pulled apart the Mini. I stand corrected.
I love the Mini 5, it's a great amp. I've owned mine since January, and it seems to be having some troubles now though. Has anyone else had issues? Wangs has said many of them are experiencing issues. It seems it was a new design for them and there may be a problem with it.
As for mine, it lost 90% of its volume, and it seems like the plate underneath the connector socket (on the amp, not the tube) for the rectifier tube glows. The transformer also stays hot for a long time after powering down. I've tired replacing the power tube but that didn't solve it. I've got a 6Z4 on the way and we'll see if that fixes it. I'm really bummed because I love this amp.
BTW, Wangs has been great and has offered a full return/refund. I'd rather try to fix it because this amp sounds so great.
Mine is still working fine. Although in truth I've not been using it much of late. -Still totally locked into that Sweetbaby!
Maybe it's time to log some hours into it and see if yours exhibits the issue, food for thought ...
Yes. Starting today. (Thanks)
I'd wondered about this amp. Very nice review, glad to hear it's doing amazing things for you. Also, possibly the 1st ST post announcing (and really bragging) about said poster's "Small Wang". (Yeah, I couldn't resist, it was just too easy)
Just had a nice playing session with the Mini 5. No problem at all with mine. (Yea!)
I'd not been using it much because, as mentioned earlier, I am just so entranced with the heavy overdriven sound of the Sweetbaby. -Something I expected to be able to roughly duplicate with this amp, but with the advantage of its having far more controls for tone shaping.
In fact, as per my initial post, the Mini is not that type of an amp at all. It is closer to a baby Twin Reverb. Very articulate. Very little tube "distortion" even when driven hard. Just a beautiful tube voice.
But it is also very responsive to *good* pedals. Emphisis on that word "good" because of the Mini's transparency. Like a Twin Reverb it reveals everything in the signal chain. (Yes, including poor playing!)
The Champ type amps are the opposite. They bury the sound of the signal chain into their own sound. -Sort of a "one trick pony" to be truthful. But oh, what a pony that is!
The various pedals in Joyo's 'sound' series -- the American, the British, the ACTone and the California all sound wonderful through the Mini 5. In fact I know of know better way to get a full complement of of classic amp sounds that by simply buying a Mini 5, a suitable cabinet, and all four four of those pedals.
All analog. Lots and lots of control. and foot switchable. No pedal is pure clean. The American a Fender tone of your choice. The Brit a Marshall -- again, your choice. Plus a Vox AC15 or 30 and a Mesa Boogie (old or newer type).
Tube or SS rectifier. (How many amps at any price give you that choice?)
I'm sorry to hear that others have had issues with these amps. But thrilled that Wangs is backing them up -- just what I'd expect from my dealing with that company.
My Wangs Mini 5 died a few weeks ago. The power transformer fired, maybe caused by 12BH7 failure. The Wangs from China didn't help me, I ask them for power transformer specs and other informations (and they tell me nothing), and now I'm trying to fix it. It's a lot of work. I also traced the printed circuit board and now I have its schematics. Take a look in the image:
Soon I will have a new power transformer and a new 12BH7. So I will do some measurements on the circuit and try to find exactly what happened.
Maybe the power tranformer is burning. In mine the secondary was shorted inside. Take a look, I took out the tranformer from the amp (and its cover) to rewind the High Tension secondary (the greens and black wires with tapes). The primary is ok (black, blue and brown wires), as the heaters windings (grey wires with tapes).
Very nice work, vzart. It looks like you drew the potentiometers as fixed resistors, correct? That's a Marshall tone stack! Its the same as in the Monoprice 15, as well as many Marshalls.
Is R18 1000k? (1 Meg)
But wasn't your amplifier still under warranty?
Yes, it is under warranty, but send it to back to China is more expensive than local repair cost. The refund didn't worth to me, so I arranged a partial refund with the seller. Well, I loved the amp and I will try to fix it myself, also it is an opportunity to learn about it.
I drew the pots as fixed resistors (I'm still learning about LTSpice). And yes, the R18 is 1Mohms.
I'm curious about the output transformer specs. It is labeled 0-8-16ohms in the primary and 5k-0-5k in the secondary, what should means reflected impedance of 10kohms. But I measured about 25kohms. Primary (tube side) to secondary (speaker side) ratio measured is about 40:1 for 16ohms and 56:1 for the 8ohms tap.
The HT supply should be AC 300VA-0-300VB, this specs is printed in the circuit board. Considering this I sent the power transformer to a repair center to rewinding yesterday. Before installing, I will add a fuse in one of its secondary legs. Maybe it would not had burned if it had one.
Hmmm... Maybe they got a bad batch of output transformers and that's the cause of the failures. A higher output impedance will generate greater voltage swings at the output tubes. Perhaps it exceeded the voltage rating of the output tube which caused it to arc, short out, and overload the power transformer.
How did you measure the 25,000 ohms?
With proper access the OT terminals, I left the tubes out and the output opened (plugs in the output jacks to open the connections inside). This way all the windings was isolated from other parts of the circuit. Then I applied a AC voltage to primary (5k to 5k) and read the secondary voltages (0-8ohms and 0-16ohms). The relation between primary and secondary voltages revealed the turns ratio for both taps. The reflected impedance is just math:
(secondary impedance - 8 or 16ohms) x (primary to secondary turns ratio measured for that tap)^2
I was tempted to purchase mine that way. There are some amazing direct-from-China deals out there. But instead I waited for a sale from the American distributor and got almost (not quite) an equally low price.
These folks are GREAT. Musicians. Vets. Truly dedicated!
WAUSA BIYANG DIRECT
But if the transformer failed, some turns may be shorted, as an example. But it really shouldn't matter, since a transformer rebuilder will measure the wire and estimate the turns, so it should be a copy of the original.
Out of curiosity, I started sketching a load line for the output tubes and I can't see how these things aren't red-plating even with a 25k output transformer.
If that is a 300-0-300 power transformer then the B+ has to be up around 400 volts.
Maybe someone who has one of these (that hasn't melted down yet) can measure the B+, Y, and X points shown in the schematic, as well as the bias voltage at the power tube cathodes?
I think the output stage failure first blew the 12BH7, what had increased the DC current supplied from B+ , cause it is connected in series to both output transformer primary windings, 12BH7 plate and cathode, and finally to the common cathode resistor and bypass capacitor. It overloaded the power transformer and toasted it, that's my theory. I'll try to find what caused the 12BH7 failure, maybe the high OT impedance (it can be out of specs, as mriggs said), maybe the bias was too hot. I don't know yet for sure, I have to reassembly the amp with the new parts and see (measure) what happens. I hope to find out and will share it to you.
If the output tranformer primary winding was damaged, it would be reduced its reflected impedance. So I believe there is no relationship between the power stage failure and the apparently high impedance (about 25k). I mean the failure didn't causes the OT impedance increase.
Good idea. Here is the points to get these readings with the amplifier on, no signal in and volume to zero. The chassis should be a good earth reference to clip the multimeter black probe. The bottom cover have to be removed (only 4 screws):
B+: the center tap of output transformer primary
Y: the point shown in the picture
X: the point shown in the picture or second 12AX7 tube (from right to the left) pin 1
12BH7 Cathode: 12BH7 tube pins 3 or 8 (should be the same reading)
12BH7 Plates: 12BH7 pins 1 and 6 (should be slightly different readings)
Here is the PCB upside for the curious: