Yet another amp project.

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Yet another amp project.

Postby oddioguy » Sat Apr 28, 2007 5:59 pm

I still haven't done anything further with the Sorado, but I'm all keyed up to start Version 2.0 of my Meza 10W (modeled after a famous Californian 22W amp).
I think I may be ADD. :lol:

...really...


One of the features on the original always bothered me....the "lead master" was "cut" only, yet the lead channel was always quieter than the rythm channel. I have a fix for that.


Stay tuned....
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Re: Yet another amp project.

Postby oddioguy » Tue May 01, 2007 8:59 pm

oddioguy wrote:I still haven't done anything further with the Sorado, but I'm all keyed up to start Version 2.0 of my Meza 10W (modeled after a famous Californian 22W amp).
I think I may be ADD. :lol:

...really...


One of the features on the original always bothered me....the "lead master" was "cut" only, yet the lead channel was always quieter than the rythm channel. I have a fix for that.


Stay tuned....

For those of you who want to play along at home, here are the original schematics on which this amp is based...

Preamp and reverb...

Image

Power and footswitch...

Image
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Postby oddioguy » Tue May 01, 2007 9:05 pm

Another vital (I thought) feature omitted from the original was a manual channel switch. Let's face it, sometimes you show up at a gig without the footswitch or a functioning cable (Anybody got a jack cord?!! :twisted: )
My prototype has a front panel channel switch included, but does not address the volume disparity.
This thread will (eventually) chronicle the evolution of my prototype into the full featured combo that I envision.
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Postby oddioguy » Wed May 02, 2007 4:16 pm

If anybody IS planning on playing the home version of this little on-line game, here are the usual cautions:

This project contains high voltages....around 700VAC! You could just kill yourself.
Not familiar with electronics / electricity in general, and tube amps specificly? Then this project is best left alone.



For all others who would like to build this, I have an almost complete bill of materials in .XLS form that will be available on request.
Last edited by oddioguy on Wed May 09, 2007 6:33 am, edited 1 time in total.
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Postby oddioguy » Thu May 03, 2007 7:10 am

Keep in mind that these re-designs are all theoretical for the moment. I will be implementing them into my existing prototype first, tweaking and redesigning as needed, and then integrating into the final design and build.

First, the master volume issue. Here is the section of schematic that I am dealing with...

Image

And here is the theoretical new design...

Image

I'm off to work for now, but will be back to post the theory of operation behind this new design.
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Postby oddioguy » Thu May 03, 2007 9:04 am

As shown above, the original master volume, 250K, is replaced with two 500K pots in parallel. This presents a total resistance of 250K, but now there are 2 signal paths which are potentially capable of being controlled independantly. In practice, I am wondering if this arrangement will affect high frequency response, but that is another post.
Only thing left at this stage is to find an elegant way of choosing one or the other signal path with the flick of a switch (or the stomp of a box). In the interest of keeping things as simple and quiet as possible, I wanted to stay away from relays and such, and didn't want to have to introduce all sorts of control circuitry to an otherwise simple analog design.
Originally, channel switching was accomplished with the use of four LDR's (Light Depedent Resistors) which were either all active, or all inactive. What I need is a fifth LDR which is inactive while the original four LDRs are active, and vise-versa....essentially creating a SPDT switch between 2 LDRs. Still with me?
Excellent...

Next post will outline how I plan to do that...using existing circuitry already in the schematic.
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Postby oddioguy » Thu May 03, 2007 5:41 pm

If you're not familiar with LDRs, here is a schematic...

Image

When the LED emits light, the resistance of the resistor element drops, which in essence makes it work like a switch, but without audible noise like the "pops" associated with relays.

Looking back at my theoretical master volume schematic, you can see where I tried to illustrate 2 contacts (I'm drawing with a mouse in Photoshop. Cut me some slack)...one open and the other closed. This is my representation of the 2 resistive elements from the 2 LDRs I will be using.

Next...how I will make them work.
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Postby oddioguy » Fri May 04, 2007 6:18 pm

Here's the footswitch supply from the original schematic. There is one slight modification already...
I have chopped the EQ supply off. I'm not using it, so out it goes.

Image

....but I have left the LDR which controlled the EQ. Now you're getting it, hmmm?
The EQ had its own toggle switch to turn it off or on manually, as well as an "auto" mode which tied it to the channel switching circuit. I am going to permanently tie it to the channel switching circuit in a manner which will turn it on when the other four LDRs ore off, and vise versa.
Here is the redrawn schematic using existing components, merely moved around a bit for clarity.

Image
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Postby oddioguy » Fri May 04, 2007 8:18 pm

From the post above, you can see that I modified the footswitch supplys' 1/4" phone jack from an open circuit mono type to a double closed circuit stereo type. I like Neutik or Re'an for this type of job, and will probably end up using a Neutrik NYS216 simply because I have a surplus...
Image

This also means that I now need to use a balanced ("stereo") type patch cord to connect to the actual footswitch.
No biggie, except that the original footswitch I chose had a mono jack as well, which of course means that it now requires some attention as well.

The footswitch I chose was the Hosa FSC-384...
Image
...which, although it has a mono jack, does have a SPDT switch, which is what I am going to need. Another attribute is the fact that it comes apart easily, and is wired point-to-point....making modification easy.

More on this later.
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Postby oddioguy » Sat May 05, 2007 7:40 am

Now a bit of practical lab work.
Before I can start applying any of the proposed mods, I need to make sure that the final version of the power supply for the amp is going to provide the correct voltages.
My prototype uses an original Mesa power transformer...

Image

...but my final version is going to use a Hammond transformer. I have selected Hammonds #373X for the job...

Image

...but need to be sure that it can do what is expected of it. To that end, I will be setting up a temporary power supply and verifying all voltages.
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Postby oddioguy » Sun May 06, 2007 9:43 am

Here is the temporary setup...

Image

Transformer is wired for 120VAC and connected to the terminal strip, with a circuit breaker wired to the primary side just in case I do something stupid. :oops:
Secondary side is wired to the terminal strip and organized by voltage. Each section is labeled, and the associated wires are wire-tied together.
Unused wires are capped with heatshrink and wire-tied together so as not to be in the way.
I have a few EMI/RFI filtered IEC receptacles (the silver looking dealie to the right of the terminal strip), so I'm using one to see if there is any appreciable difference in the noise floor with and without. Can't hurt...
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Postby oddioguy » Sun May 06, 2007 10:59 am

Looks like the Hammond transformer is going to be the shit for this job. AC voltages are a bit high, as are the DC voltages, but they were measured without any load on the DC side.
I will calculate the load and simulate it in my next test just to be certain that the DC side is going to pull down to within 20% of the design voltages.
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Postby oddioguy » Sun May 06, 2007 12:59 pm

Safety note:
You should probably bleed the filter caps after each test like this. Even after power was disconnected for a half hour, there was over 270VDC stored in the caps. If you forget that it is there, you could get a nasty surprise if you start tinkering around with the power supply.
I use a 5W 330 ohm resistor for the job. (5w because it's big enough to hold without touching either lead.) Bleeds the caps in 5-10 seconds or so.
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Postby oddioguy » Tue May 08, 2007 8:03 pm

Based on plate currents from all the tubes, and ignoring the push/pull arrangement of the EL84s ( I assumed maximum current draw of 55mA per tube simultaneously) I have calculated 3 load resistors that will reasonably simulate the actual operating conditions of the amp.
From the original schematic, I see "A" needing a 3K5 50W load resistor, "B" needing an 82K 2W load, and "C" needing a 28K 2W load. These loads include generous margins in the wattage department, just to keep things cool to the touch.
If I consider the class AB output arrangement, I can assume the the maximum plate current at a given time from the EL84s will drop to roughly 75mA instead of 110mA. This adjusts my "A" load upwards to 5K, so somewhere between those two values should suffice.

My total measured current in the high voltage section was 80mA, so the calculations are.....

Close enough for R'n'R.
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Postby oddioguy » Wed May 09, 2007 5:29 pm

My test loads are connected, and I'm about to apply power.

However...

It's 5:30 p.m. and I am off to job #2 in a few minutes, so pics and results will be posted later...tonight?....
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Postby oddioguy » Wed May 09, 2007 9:04 pm

Okay...I'm ready to roll...

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Postby oddioguy » Wed May 09, 2007 9:08 pm

Using a load of 4K4 on the "A" terminal, I am getting 454VDC which is still 60V too high. It is, however close enough that I am willing to graft the Hammond onto my prototype and run it in the real world to see what happens. My feeling is that it will get pulled down to around 400VDC, which is as close as you could ask for.

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Postby oddioguy » Wed May 09, 2007 9:21 pm

The results above do not take into account the current drawn from the bias circuit which is connected directly to one side of the high voltage windings, nor does it consider that the footswitch supply is derived from the real bias tap, also taken from the high voltage windings. When these are factored in, the voltage should be right where I need it to be.
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Postby oddioguy » Thu May 17, 2007 8:09 pm

My prototype as it is now...

Image

Image
Last edited by oddioguy on Thu May 17, 2007 8:14 pm, edited 1 time in total.
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Postby oddioguy » Thu May 17, 2007 8:12 pm

I'm also swapping the output tranny for one with a higher primary impedance. The original I used has an 8K primary....

Image

The new one has a 10K primary. Both are suggested for use with EL84's, but I'm wondering if the 10K primary will help reduce plate current...for reasons that I will explore later on.
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Postby oddioguy » Fri May 18, 2007 8:26 pm

The new 1609 output tranny graft was a success. It has reduced the EL34's plate current, and now the original Mesa power tranny runs cooler.

Image

I'm not going to u/l a pic of the operation, as it is a temp arrangement and as such, I left all the leads intact, which is to say they look like hell inside the chassis. While I was there, I also moved some more wiring around and consolidated a few grounds, so the noise floor is slowly being reduced to the point of being acceptable.
Last edited by oddioguy on Fri May 18, 2007 8:33 pm, edited 1 time in total.
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Postby oddioguy » Fri May 18, 2007 8:32 pm

I have heard comments that Hammond tranny's are too "high-fi" for guitar amps. I dunno....
They certainly reveal the full potential of the amp, but frequency reproduction is still limited by the guitar and speaker. I rather like the Hammonds so far...

This is the only major tweek for the weekend.
There is a rumour that I may end up at some sort of jam session on Sunday, and if so, I want to take this amp and give it a real-world workout.
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Postby oddioguy » Sun May 20, 2007 11:07 am

Just for your amusement...
Some of the major components for the new amp.

Image
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Postby Alain Benoit » Sun May 20, 2007 11:52 am

oddioguy wrote:Just for your amusement...
Some of the major components for the new amp.


You should have checked with me, I could have supplied you with the footswitch, reverb tank and orange drops all brand new.

I think I like your choice of knobs though.
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Postby oddioguy » Sun May 20, 2007 12:46 pm

U1176 wrote:
oddioguy wrote:Just for your amusement...
Some of the major components for the new amp.


You should have checked with me, I could have supplied you with the footswitch, reverb tank and orange drops all brand new.

I think I like your choice of knobs though.

Thanx man...this won't be the last amp, so I'll take you up on that offer.

Knobs have got to be the hardest thing to pick, along with an indicator lamp. Glad you like the choice. They're MIL-SPEC with 2 set screws each, machined aluminum with a flat black finish.
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