Guitar Amp rebuild.

[served]

Hi.

Thank you! This is good information!

But saddly I have to say that I ruined my amp couple of months ago. Well I ruined the speakers. I used it as a cab for a while and then one day one of my heads broke and it feed current to the cabinet. So the speaker windings are half gone. I am still keeping the amp, but I have to figgure out what I will do with it.
Its a great amp though. It gets along with pedals very well. Was thinking to sell it some time ago but then I realised that there is nothing like that on the market.

In stock its just a really good amp! Because it has no charracteristics. Its just so flat and booring and it doesn't sparkle. But if you are a pedal guy then its your favourite amp because allmost everything will sound good on it. For example Randal amps, they add so much tone to the sound that most of the distortion pedals sound like a zizzzling crap.

So I have to give this amp some credit!
I got it for 100$ and have not found a better one that I would swap it with. Exept tubes but I am keeping theese topics separate and will not compare them.

[bukas]

are you sure speakers have 80W RMS each ?

if that is the case you need a 200W amp in my humble opinion. speakers them self don't have power, declared power on speakers is actually needed power of amp to drive them properly. it sounds silly, but if amp is weaker it will burn speakers. you want your amp to play around with speakers not to struggle

[served]

I have never heard of that. That feels very unrealistic. I think you got it wrong.

Speakers can be as powerful as you like. Amp can't be more powerful than speakers, well it can but then you can't max the amp out.

I have used 1W amp with 200W cabinet. No problem.

[bukas]

http://en.wikipedia.org/wiki/Audio_power

read this for beginning, especially "Matching amplifier to loudspeaker"  part

[amptramp]

http://en.wikipedia.org/wiki/Audio_power

read this for beginning, especially "Matching amplifier to loudspeaker"  part

Note the different units in the measurement:

"Matching amplifier to loudspeakerCharles "Chuck" McGregor, while serving as senior technologist for Eastern Acoustic Works, wrote a guideline for professional audio purchasers wishing to select properly sized amplifiers for their loudspeakers. Chuck McGregor recommended a rule of thumb in which the amplifier's maximum power output rating was twice the loudspeaker's continuous (so-called "RMS") rating, give or take 20%. In his example, a loudspeaker with a continuous power rating of 250 watts would be well-matched by an amplifier with a maximum power output within the range of 400 to 625 watts.[6]"

The amplifier peak power is larger than the speaker RMS power.  This is to be expected.  The speaker RMS power should be equal or more than the amplifier RMS power.

[PRR]

There are two cases:

#1) "Clean" reproduction -- amplifier is "Never Clipped" (<5% of time): select speakers for desired output, amplifier considerably larger

#2) "Guitar amp": aplifier is "Often Clipped": select amplifier for desired sound level, speaker *must* be rated for much higher thermal power.

Yes, in "clean reproduction" it is reasonable to run "500W" speaker with a 1,000W amp which is NOT regularly clipped.

But guitar amps are *usually* clipped, semi-constantly. This thread's "60W" amp can deliver 110+W of grossly distorted square waves. The speakers must accept the *heat* of 110+ Watts.

Yamaha's "60W" rating is probably conservative, low THD, nominal wall voltage. Take the clipping power, plus a high wall voltage, 160W (two 80W) speaker is not excessive.

[served]

I would say that this Yamaha was running on the edge if maxed out.

I had to turn the amp all the way for few times and I can say that it was not pleased. Speakers didn't handle the maximum volume very well. Low end started to go loose.
So in guitar amp situation I always take more powerful speaker.
For tube amps it should be like 3 times bigger or something. Maxed  Fender Tweed Deluxe (DIY around 15W) will beat a crap out of Eminence Red Coat The Governor 12" 75W
I built one and I was happy that I took more powerful speaker because it was dancing like hell when the amp was maxed out.

[regvarna]

For volume in dB from speakers to increase by only 3dB the power in watts from amp would need to double.
A 92dB sensitivity speaker would need a 200watt amp to sound as loud as 98dB sensitivity speaker with 50watt amp.
The Volume of the amp (combo) is controlled by the sensitivity(dB/1w@1m) of the speakers more than watts of speaker or amp.

[regvarna]

I tried attaching a pic of tone mod but its not working

How do you attach image

[served]

ok.
Yes ofcourse the Sensitivity is what we are looking here.
Eminence Red Coat The Governor 12" 75W is a very sensitive speaker 102dB.

But still it confuses me. What does he mean by " it sounds silly, but if amp is weaker it will burn speakers."?

[regvarna]

If amp is weaker, when people turn up volume(to achieve high power) and the amp goes in distortion it sends square waves to speaker and kills it.
If the amp has high wattage it doesn't need to be driven in to distortion and drives speakers to maximum efficiency by matching power(watts).
Better to have amp with more power(headroom) and just drive it to the wattage of speakers.

[PRR]

> "if amp is weaker it will burn speakers" ??

As regvarna says: if users turn-UP to get the desired racket, the gross distortion is extra heat in the woofer, and particularly the distortion overtones burn the fragile tweeter.

This is another reason guitar amps don't have tweeters.

[served]

Still doesn't make any sense to me. But I can see your point of veiw now. I didn't know to look at it this way.

So what you are saying is that take a bigger amp instead of a low wattage amp but keep the speaker.
That way you get more louder clean sound and speaker to amp relationship is better.

But it doesnt make any sense to change a speaker if the amp is low wattage.In this case I have a two 80W speakers and a 65W amp, nothing happens.
I just use a low watt amp and very powerful speaker. It doesn't always mean that I have to turn the amp until it distorts. I can also play it clean, who cares what speakers I have behind it. So there is no speaker burn or anything.
If I turn a 500W amp into square distortion then it will kill a 400W speaker even if the amp is more powerful than a speaker.

And I am not going to talk about tubeamp poweramp distortin magic because its not a tube amp.

Don't read it as a rude post, I was just so carried away by your statement that it didn't make any sense to me. So I was thinking about it. Now I can see what your intension is and why it felt so wrong to me in the beginning.

Its a different need and a different world:::    Hi-Fi vs. square-fi-Guitar-amp-thing

[regvarna]

[regvarna]

[regvarna]

I forgot to draw the capacitor(1 uf) in the tone-bypass switch circuit in previous pic.
The capacitor is needed to stop DC leaking from JFET stage, and ensures correct operation of JFET.
Here is the correct tone-bypass mod circuit pic.

[APX500IIEW]

I realize this thread is old...
I stumbled upon this discussion while Searching the Internet for a Service Manual for the Yamaha VX 65D Amplifier. Over last weekend I visited a local Flea Market and purchased a VX 65D for $40.00, working condition unknown. I am waiting for my first Guitar ever to arrive Friday, a Yamaha APX500IIEW . I will then be able to Test the Amplifier... I will use a "Poor Man's Variac" or Dim Bulb Tester as it is also referred to, for the initial Power Up. I will only briefly test it, as I intend to restore it. I will most definitely change all the electrolytic caps due to old age alone. To me, it is just cheap insurance...
I was able to locate a Factory Service Manual, which no one here seems to have... It is on it's way here to Massachusetts... All 146 Pages of it...  Got it here:
http://ronsound.com/index.php?main_page=product_info&cPath=37_124&products_id=836

Let me know if anyone needs anything from the Service Manual... I'd rather not have to Scan 146 Pages... You can always buy your own too...

Bill