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18watt marshall clone with 15 watt output transformer?
Old 11th September 2012
  #1
Gear Maniac
 
🎧 10 years
18watt marshall clone with 15 watt output transformer?

I don't know a whole lot about electronics so I'm just looking for a general answer. I purchased an 18watt Marshall clone recently, and I noticed that the description of the output transformer says its a 15 watt transformer. Shouldn't it have an 18 watt transformer if it really is an 18 watter clone?

All things being equal, can I swap it out for an 18 watt tranny? Or is this something that would have to be taken into account as far as all the other components.

Ultimatly, this amp does have a great tone, but it has a crappy buzzy tone when pushed past crunch. I'm going to try the "snubber" mod but I was just curious about the output transformer. THANKS!
Old 11th September 2012
  #2
Lives for gear
 
Richard Crowley's Avatar
 
🎧 10 years
1) The "wattage" rating of amplifiers traditionally has been 90% fantasy and 10% technology. There are way more different ways of specifying "wattage" than you even want to know about. I doubt very much whether your amplifier puts out a halfway clean signal at 18 watts (or even 15 watts).
2) Just finding a proper replacement "15W" transformer would be a challenge. Finding an "18W" transformer with the proper windings, impedances, and physical structure is 100% fantasy. You will spend the rest of your life looking for such a thing. Transformers as a class are a slowly-dying breed. Even in their heyday, there were only transformers of particular specs available (unless you wanted to wind your own). And these days, be grateful that you have one that is still working as designed.
3) If you don't like how the amplifier sounds when overdriven, then get a more powerful amp.
Old 11th September 2012 | Show parent
  #3
Gear Maniac
 
🎧 10 years
Quote:
Originally Posted by rcrowley ➑️
1) The "wattage" rating of amplifiers traditionally has been 90% fantasy and 10% technology. There are way more different ways of specifying "wattage" than you even want to know about. I doubt very much whether your amplifier puts out a halfway clean signal at 18 watts (or even 15 watts).
2) Just finding a proper replacement "15W" transformer would be a challenge. Finding an "18W" transformer with the proper windings, impedances, and physical structure is 100% fantasy. You will spend the rest of your life looking for such a thing. Transformers as a class are a slowly-dying breed. Even in their heyday, there were only transformers of particular specs available (unless you wanted to wind your own). And these days, be grateful that you have one that is still working as designed.
3) If you don't like how the amplifier sounds when overdriven, then get a more powerful amp.
Thanks for the input. Although I'm not sure what you mean by saying that finding an 18W transformer...is 100% fantasy. I don't think I was clear on my question.

What I mean is that I don't think this buzzy tone it gets when overdriven is typical of your common 18watt marshall amp. The research I've done is that it can be a symptom of an underpowered or cheap output transformer. That's what led me to look more closely at the output transformer in my amp, and that's when I realized that its a 15watt output transformer.

So my quest isn't for the holy grail of amps. I'm just wondering if anyone had any input on this situation. So put more simply...what would be the symptoms of an amp (or specifically an 18watter) that had an under powered output transformer?
Old 11th September 2012
  #4
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Richard Crowley's Avatar
 
🎧 10 years
I doubt very much whether the problem is the transformer. It is far more likely to be some other part of the whole system. It is not clear, for example, whether you are even talking about the (electrical) SIGNAL out of the amplifier/transformer, or whether you are talking about the (acoustic) SOUND out of the speaker. It is not valid to judge the amplifier circuit (including the output transformer) by what you hear from the speaker. One equation, two unknowns.
Old 11th September 2012 | Show parent
  #5
Gear Maniac
 
🎧 10 years
Quote:
Originally Posted by rcrowley ➑️
It is not valid to judge the amplifier circuit (including the output transformer) by what you hear from the speaker. One equation, two unknowns.
Should I judge the circuit by how colorful the transistors are? Maybe by how bright the tubes light up? LOL, I'm just kidding. I do get what you're saying.
So that's one vote for not the transformer. Although, you have to wonder then, why use an 18watt output transformer? It's cheaper to use a 15watt.
Old 11th September 2012
  #6
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Richard Crowley's Avatar
 
🎧 10 years
I wouldn't put nearly as much faith in the "15W" rating of the transformer, and almost NO faith in the "18W" rating of the amplifier. Those numbers are far more fanciful than you seem to believe.

Do you know what is the actual capacity (in watts) of the power supply, for example?
Old 11th September 2012
  #7
Gear Maniac
 
🎧 10 years
Nope, no idea. Let me simplify the question further. If one had an 18watt marshall clone, and the 18W output transformer was swapped for one that was rated at 15W. What might be the general sonic difference? That's all I want to know.
Old 11th September 2012
  #8
Gear Guru
 
tINY's Avatar
 
1 Review written
🎧 15 years

You could measure the output power (or at least V*A) pretty easily.

Measure voltage across the speaker and RMS current through a shunt (or current clamp) while feeding the input from a signal generator....

Really, though, a guitar amp is supposed to overdrive. All of the specs for power that mean anything are at a certain % distortion. Does the output transformer get non-linear before the output stage? Will raising the bias voltage on the output tubes get more of what you want? Perhaps lowering the bias is what you really want to do....

...or how about putting in a new, higher impedance speaker is what you need.

It's all a matter of taste at some point anyway. If you don't like the sound, avoid hitting it that hard or get a new amp. If you want to learn how to "tune up" an amp, great! You are going to have to try a few things - and review Tube amp safety.




-tINY

Old 11th September 2012 | Show parent
  #9
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tINY's Avatar
 
1 Review written
🎧 15 years
Quote:
Originally Posted by Tennis ➑️
Nope, no idea. Let me simplify the question further. If one had an 18watt marshall clone, and the 18W output transformer was swapped for one that was rated at 15W. What might be the general sonic difference? That's all I want to know.


It depends. The winding ratio, core saturation point, and copper parasitics are going to change the sound. The 18w may saturate more than the 15w depending on the specifics.

Generally, a saturating core on a transformer is going to reduce the bass output and introduce distortion components of the lower frequencies.



-tINY

Old 11th September 2012 | Show parent
  #10
Gear Maniac
 
🎧 10 years
Quote:
Originally Posted by tINY ➑️


It depends. The winding ratio, core saturation point, and copper parasitics are going to change the sound. The 18w may saturate more than the 15w depending on the specifics.

Generally, a saturating core on a transformer is going to reduce the bass output and introduce distortion components of the lower frequencies.



-tINY


Thank You! This is more along the lines of what I was looking for. Do you care to hear a recording of what I'm talking about just for kicks? I think if you heard it you might agree that it shouldn't sound like that, but maybe not. I was thinking of just strumming the guitar while I turned up the volume in stages. (no gain or master volume, just volume and tone control). Would you take a listen if I went through the trouble of tracking it?
Old 11th September 2012
  #11
Gear Nut
 
🎧 10 years
What size and type of speaker are u playing through. I found that makes a massive difference in sound too.
Old 12th September 2012
  #12
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AndyFromDenver's Avatar
 
1 Review written
🎧 10 years
Quote:
Originally Posted by Tennis ➑️
All things being equal, can I swap it out for an 18 watt tranny? Or is this something that would have to be taken into account as far as all the other components.
A. Yes you can.
B. No, or to cover all bases, unlikely.
Not sure why you got those responses (except tiny ).
Here's an excellent replacement (scroll down to either the standard or vintage outpt trans):
http://gdsamps.com/?page_id=7
As an aside, I used their transformers to make an excellent ac15 clone. So it may be as another has said and a poor speaker, failed part/connection, tube, etc.
On the rare coincidence that you're near Athens GA, I can do this for you.
Old 14th September 2012
  #13
Gear Maniac
 
🎧 10 years
Thanks for the info. That was really helpful. Yea, I wasn't sure if folks just weren't reading my question, but the posts didn't seem to make sense, LOL. If I was in GA, I would be hitting you up for sure. But I will not need to mess with the tranny, unless I just want to see what difference it would make.

Well, the "snubber kit" came in and I installed it last night. WOW, what a difference. That amp went from decent, to holy crap this amp is amazing! THis is the 18watter tone I've been reading raves about.

I'm sure if you google snubber mod or something like that you can find it, it's a really easy and cheap mod, and it took that nasty buzzy overdrive away entirely. It really is a gorgeous sounding amp now. I wish I had tracked it before the mod so I could post the difference.

I also have a 6 watt VHT Ultra head that has a similar problem, just not as extreme as my 18watter clone. The amp is not bad, especially for the money ($200 used), and its more of a Fender tone, so its a nice contrast to my 18watter. I'm going to open her up this weekend. Since it's hand wired, I think I might be able to use the snubber mod on it also.
Old 14th September 2012
  #14
Gear Guru
 
tINY's Avatar
 
1 Review written
🎧 15 years

Sounds like the amp was too cheap by a couple of caps and resisors....

I'm guessing that the HF oscilations in the output stage of the amp were quashed by the snubber and now it doesn't get the IM distortion from that when it's pushed hard. This could sound really bad.

Most circuits using an output transformer have this kind of HF compensation designed in. Since the inductance of the transformer makes the out-of-band impedance high, the parallel RC snubber keeps the HF impedance that the tubes see low.




-tINY

Old 14th September 2012
  #15
Gear Maniac
 
🎧 10 years
lol, yea, I like how you put that. Too cheap by like $3. Is this snubber cuircuit something I could experiment with? Maybe using different values of one of the components? What about using a variable pot (resister?) so that I can control that filter, is that something worth doing in your opinion?
Old 15th September 2012
  #16
Gear Guru
 
tINY's Avatar
 
1 Review written
🎧 15 years


The problem with a pot on the output of a tube amp is the high voltage. Most pots don't like wiping around with 100s of volts applied...

For this case, you can use the square of the turns ratio times the nominal speaker impedance to determine the effective impedance seen by the output stage. Then figure out the capacitance you need to get a 8kHz (or so) turnover point. The resistor should be set to the reflected impedance of the speaker through the transformer.



-tINY

Old 4th October 2012 | Show parent
  #17
Gear Maniac
 
🎧 10 years
Quote:
Originally Posted by tINY ➑️


For this case, you can use the square of the turns ratio times the nominal speaker impedance to determine the effective impedance seen by the output stage. Then figure out the capacitance you need to get a 8kHz (or so) turnover point. The resistor should be set to the reflected impedance of the speaker through the transformer.
-tINY
I don't really follow on this. Is this to determine the best value for parts on the snubber circuit?
Old 4th October 2012 | Show parent
  #18
Gear Guru
 
tINY's Avatar
 
1 Review written
🎧 15 years
Quote:
Originally Posted by Tennis ➑️
I don't really follow on this. Is this to determine the best value for parts on the snubber circuit?


Yes. If you have a better way of determining AC impeadance seen by the tubes (looking into the transformer with a speaker attached), then use that.

Determine capacitance:

C = 1/(2*pi*(8000Hz)*(impedance))



-tINY

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