Guitar Amp rebuild.

[served]

Hi.

I'm just practicing and collecting ideas.
My plan is this.
So i don't like how one of my amps sound. I thought that if I keep the same frame and speakers and powersupply, i could easily build something else in it.
Its 60W transistor guitar amp with two 80W speakers. It looks good and is in great condition, so i think its worth it.
At the moment i have no idea where to start. I wondered if some one has done something like that?

What amps are worth building? I mean simple enough to build and the final result is good too.
It should be something around 60W then i don't have to replace the powersupply and speakers. I could keep the costs low and get maximum experience of it.

I like Mesa sounding amps, also I really enjoy clean sounds and bluesy sound. But mostly i play metal and that should be the goal.

Any suggestions or directions? I could really use some experienced advice on this one.
I promise to keep the progress posted and add pictures for every one to see. All so I'm thinking to keep a diary. So that next people who are going to build something like that, could benefit from it.

[jessej]

Build a main-in jack in the back (or two if it's stereo) and build some stompboxes that mimic your favourite amps. That way you can choose what kind of preamp stage you want to use and are not bound to one built-in sound...

[served]

Wow. Thats a good idea. That way I only have to build a good Poweramp.

[BAARON]

You should probably still build a clean preamp that has tone controls you can bypass.  The job of a preamp is to bring the input signal up to a high enough level to drive the power amp to the full extent of its capabilities so you can get all the volume out of it instead of wasting its potential.

The job of the preamp is also to shape the sound a bit.  Most guitar pedals (even the amp-style pedals from RunOffGroove) are voiced through use through a guitar amp, and a guitar amp's preamp applies some serious tone shaping to your signal even if the EQ knobs are all set to 5.  If you just run straight into a power amp without similar tone shaping, you could end up with some very strange sounds from some pedals, while others might sound best that way.  (Hence my suggestion for a bypassable EQ in your preamp.)  Download Duncan's Tone Stack Calculator to see what I mean about the frequency response of a guitar's amplifier's preamp.

[anchovie]

Wow. Thats a good idea. That way I only have to build a good Poweramp.

Not necessarily. The power section that you currently have could be fine - the preamp should be having the biggest effect on the tone.

[davidallancole]

These guys are making some good points.

Can you tell us what amp you have so we can find a schematic and take a look at it?

[served]

Well. Okay. I get the point. This was in my mind before. The problem is my preamp, and i should Bypass it.
I did bypass my preamp and i didnt get what i was looking for. So I deffenetly need a new preamp.
I have this one  http://www.retrevo.com/support/Yamaha-VX-65D-manual/id/961ci565/t/2/
It's actually not the correct one. I have amp with Two inputs. One input is in the schematic, but the other one is not. I hav a picture of the other input cahnnel's preamp.


I actually did came up with this one earlyer in the other thread, when i tried to Make a EFX to the amp, but it was not good enough or i could not make it work for me.

[davidallancole]

If you cut in an effects loop, you could then use various pedals to get the sound you want and bypass the pre-amp stage all together.

[petemoore]

Its 60W transistor guitar amp with two 80W speakers.
  The sounds an amp makes is often largely defined by the power amp / speakers.
  I guess this is a transistor power amp [designed for clean power] driving 80w speakers, in a combo cabinet.
  It looks to have a relatively well featured preamp.
  An effect loop is relatively inexpensive and may afford for use of a distorter which gets closer to ''Mesa''.
  I would start with a schematic and understand the mod before proceeding, your high voltage safety studies may be tested at any time the amp is open, ask questions.
  The mod will allow you to compare and understand whether effects work better than in front of the amp, or as expected through the effects loop.
   

[served]

But is EFX loop a mod that allows to by pass the whole Preamp section? Or it still runs through some part of pre amp features?
The thing is that the channel I would like to mod is a little bit difficult to understand. As i have only been dealing with stompboxes that has Tone control after everything else, this ones Treb+Bass controll is somewhere before the last preamplification process. So I cant find the points that i have to shortcut to get what I want.

I ll make a little drawing to help you to help me.
I didnot add values, as i think is not important if bypassing, i guess. But i can always add if necessary

[served]

Actually im mistaking. The line leads to 6th pin of 4558. Sorry.
I now try to connect K30A directly to pin 6.

EDIT: So i did it. Something is still wrong, not working as I hoped. Major issue is that Volume control is not working as it should. When I turn the volume knob to 9 o clock then its too loud. And im missing some brightness. Hopefully there is a cure.

[slideman82]

My 2 cents: you can buil a mosfet power amp (it will deliver less than 60W with same voltage) and that will make a huge difference in tone. Also a Jfet or mosfet preamp (with regulated 24V supply) will sound better! Try this configuration: first stage - vol pot - second stage and buffer - tonestack and master volume - output buffer. And then power amp.

You could change your power transformer for a bigger one, so you can use two power amps, like a Jazz Chorus does!

Can you measure the power transformer voltage and post a picture of it with some objet to compare it's size?

[davidallancole]

If you go with an effects loop, you can bypass the whole pre-amp.  Just send the signal in from your pre-amp or pedal to the effects return jack and it will go straight into the power amp.

[served]

I tried that, but I think I got it wrong. I ll recheck everything.

Meanwhile. Some pictures of my amp.

First picture shows the whole thing. As you can see, the little board on the left side is the board that Is on the previous picture.
To bypass the preamp I just connected my Boogey to the Blue wire you see running infront of the whole thing. In my opinion this was logical thing to try.



I hope the object is enouch universal to get the idea of size.


Two 8ohm 80W speakers connected paralel.

[slideman82]

Seems to be a nice solid state amp, but I think it's not too much power for those big speakers.

For what I could see, it's a single supply design (with the smaller of those big electrolitic caps in series with the output, I think). You can measure AC voltage if you place a voltimeter right at both red wires that goes from the power transformer to the PCB. It seems it's not a quite big power transformer.

There's a fuse right beside the diode bridge, can you tell us its value?

You have reverb springs too... you could do a nice amp with all that!

[davidallancole]

You have to find the input to the power amp itself and make sure it has no tone shapping around it (Which it shouldn't). 

[served]

Well, Yes, sorry, I forgot to measure the voltages. Ill do it right away when I get a moment.
I ll do some more digging about that poweramp input. At the moment im suspecting that there is still something left un bypassed.
Ill keep you posted!

[served]

Hi!
I have been extremely busy with school and upgrading my guitar (warlock) so I haven't had time to work with this one.
But, I had time today and i figured out where the Power Amp starts and the Pre Amp ends. Basically the Blue wire goes directly to Power Amp in.
I will do all the measurements that you guys required, I hope you will have time to read it some day.

So.
The Fuse: 3.15A
Between the two Red wires, I got 47V
If you are interested in these Caps, they are 3300uF/80V and 2200uF/80V but they don't seem to be connected series, but I am not entirely sure, there might be a jumper.

I am going to change this Pull Gain Pot switch to a tumbler, and add bypass socket to test various stompboxes.
I hope i can hear any news from you guys soon!

I also added picture of block diagram so you don't have down load that pdf file.



With best

Ardi.

[Ibanezfoo]

If you cut in an effects loop, you could then use various pedals to get the sound you want and bypass the pre-amp stage all together.

I used to do this with a Carvin amp.  I'd plug the output of a POD into the effects return on the amp.  Just have to be aware that the POD controls everything at that point so make sure the POD volume is way down when you hook it all up and turn it up slowly.

[regvarna]

I have the same yamaha jx65d . I have 2 Celestion G12S-50 speakers in it. With modded cabinet rear panel.  Love this amps (poweramp/cabinet/speakers) sound.
I have created my own schematic for the amp so I can do preamp mods.

The tone circuit on Ch1 and Ch2 are easy to alter and mods can be tested with duncan amps tone stack calculator----the yamaha tone stack is the same
circuit as fender.  Just change the input, cap and resistor values to those of yamaha.jx65d. 
Then the standard tone stack curve can be seen and altered to get tone required or no tone shaping by changing cap and resistor values.
Also range and curve of bass/mid/treble pot control can be changed by replacing pots with different values and tapers(linear/log/antilog).

I have set up clean channel(Ch1) with tone stack switching.
1. Modded yamaha tone stack  (close to flat response with Bass/Treble on "5" --- for pedal board processors to retain output tone and use Ch1 volume to control level sent to poweramp)
2. Fender tone stack. ( mid scoop around 500Hz -- same tone shape as fender with Bass/Treble on 5. --- mid scoop moves from 300Hz to 1000Hz as treble is moved from 1 to 10 ).
3. Vox tone stack.  (mid scoop around 800Hz --- same tone shape as VOX with Bass/Treble on 5. --- mid scoop pulls down around -12dB more at 900Hz when bass dial is moved from 9 to 10)

I find Ch2 distortion is horrible, so I have modded "Ch2 gain circuit and tone stack" and added switchable SRV modded ibanez ts808 tube screamer circuit. This amp already uses the same
OP amp(chip) as the famous "Ibanez tube screamer TS808" --- JRC 4558D --- so is easy to mod.
Also this amps reverb only works on Ch2 . Reverb circuit can be altered and patched to Ch1 PCB to give Ch1 reverb. A separate pot can be added to allow each channel different reverb setting if needed.
Tube screamer Control pots and Ch1 reverb pot can be mounted on the rear panel to preserve face plate.
If 3-Way channel switching mod is done ( Ch1 / Ch2 / Ch1+Ch2 ) and Ch2 input jacks removed,  the tube screamer controls can be mounted on the front in the empty input holes.
A footswitch can be wired to switch( channel mode / Ch1 tone stack mode/ Ch2 overdrive mode) and connector(multi pin DIN) socket can be fitted next to Ch1 inputs or phones jack on the front panel or on the back panel.

If you need to know where on circuit PCB to make connections for mods or preamp bypass or effects loop connection,   post a request for what mods or diagrams you want and I'll try to help with info.
My schematic is hand drawn from hours of reverse engineering and tracing circuit. Took a long while to get my head around this yamaha's  preamps.   Wired very different to others I have worked on.
The tone stacks are the first thing after the input buffer(j-fet) and before 1st OP-amp stage.
Now that I have modded this amp(65W 2x12') I will keep it forever, best clean and overdrive and more versatile now
than of all my combos.
marshall 8080v -- 80W 1x12"
randall rg75d -- 75W/100W 1x12"
laney linebacker SC -- 100W 2x10".
My 1981 Marshall factory prototype JCM800 lead series "1959" 4 input two channel (complete exact plexi JMP guts) 100w Head RULES everything.  Only a handfull of these around the world.
The legend sound of the late 60's/70's and into the 80's used by Ritchie Blackmore/deep purple, Jimi Hendrix, Eric Clapton, ACDC,  Randy Rhodes etc.