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Post subject: Musicman clone
Posted: Sat Jul 27, 2019 3:24 pm
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This project started cheap, then got deep. I got the frontman cabinet and a 70-watt 4 ohm speaker at the auction for a dollar. I added a chassis that almost fit and 24 volt power transformers from my boneyard for nada.
Since I retired last year, I've been busy relocating and haven't been at the bench much. So it was great to sit down and build something from scratch.
I chose the Musicman RP65 to clone for two reasons. First is the analog phaser. I always wanted to build one since I first heard one in the 70s. Second was the sound. We had a shootout of amps a few years back. A visiting newbie was getting into electric guitars and wanted to demo some amps. four pickers fired up eight amps in my shop and the winners were a delta blues and the Musicman RP65. No Fenders were available in working condition.
Now, for those of you unfamiliar with these amps, they are solid-state op-amp with a pair of power tubes. The EL34s are in series with the output transistors and there is no signal applied to the tubes. They are biased ON fully and just go along for the ride with the transistor controlling the signal.
Well, I wanted to build a SS power stage for the final output. Big problem there are no low voltage push-pull output transformers available for the 40 volt PS.
So I took an old Weber OT that was rated for 15 watts and took it apart to try to rewind it. Since its low voltage, I wound the primaries bi-filar. The secondary I left alone. I needed a 3.5:1 ratio and sucessfully wound a full bobbin. However, it measured only 4 ohms DC each wind.
If I tried to use it, it would burn up. I could add a limiting resistor, but then it would be a a big toasty honker and the tranny only good for a few watts. So I gave that dead end up and went looking for sumthin else.
I found a 70-volt line transformer with power taps for different speaker levels. It wat rated 50 watts for 75 volt and 25 watts for 25-volt operation.I figured I could split the taps and use it as a 30 watt push-pull. But when the tranny arrived, it was soon clear that wasn't gonna work.
The DC resistance was unexpectedly different between taps and the results of driving
the tranny couldn't get a good balance.

So I sat on that and went to building the rest, waiting for inspiration.
I got fiberboard and two sizes of eyelets. I used four 16 pin gold plated breadboard sockets. These have 1 inch leads that I could use to bend out to accomodate the smaller eyelets. There is enough space between the eyelets for quarter watt resistors to lay flat.
I drew up the layout of each circuit on paper without scale, then laid the components on the board, marked the spot and centerpunched the locations at the proper spacing.
I inserted the bent leads into the holes and photographed the layout. Then I removed and bagged the parts, with labels.
After laying out all the circuits, I installed the eyelets. The staking tool does a poor job by itself. The eyelets drift to one side and swedge unevenly. A centerpunch spreads them evenly for the first blows. Then the staking tool flattens them very nicely.
The backing for the eyelets during staking also makes a difference. Staking over steel flattens the rounded shoulder of the eyelet. Aluminum works better and oak is hard enough, but wears out.
I built 4 photocouplers for the phaser. The schematic a vague, but through trial and error, i got them working using the 1M dark rating on the photoresistors. While the print shows 1 led for two photoresistors, I used 1 LED per.
I used small eyelets throughout the signal path. I should have used the large one for the power busses and IC sockets. It fits but its tight. I used turrets to terminate the connections out to the jacks and controls.
I built the power supplies on the other end of the 14 x 3" board. There's enough room for a bias circuit in between.
But what am I biasing? Then it hit me. Use that tranny and run it Class A.
No, Nobody builds a 25 watt Class A. They may sound great, but it'd get too hot.
Then I remembered the big new heatsinks in my boneyard. And the low voltage fans ... And if I switched in the multitap for low power, it could keep its cool.....until it needed to burn.
Long story short, it now sports a 6" x 7" heatsink, a fan, and one brave transistor. Its a TO220 Nexfet. Its a third generation mosfet that, while it is only rated at 60 volts DS, it can carry 124 amps. While most power fets have very low gain, @6-20S, this unit is rated 187S.
The bias circuit will be powered by the preamp power supply, which has a separate transformer than the power stage. So when the power stage sags and hums at high power, the bias will remain stable.
The power tap switch is a 2-pole 5-position. one side is for the transformer. The other pole will be for bias adjustment for changing loads. The bias will be fixed. Any adjuster would be Dial-A-Bomb.

So, while its not tube, its still an interesting challenge for me. Its not much to look at. But if it gets off the ground, I'll post pics.


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Post subject: Re: Musicman clone
Posted: Sat Jul 27, 2019 7:10 pm
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You're an intrepid soul, Tim! I'm surprised you were able to shoe-horn all those components onto that chassis. You're right about the dearth of discreet power transistors available -- how I miss the days of those beefy Motorola TO3's.

From what did you retire?

Arjay

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Post subject: Re: Musicman clone
Posted: Sat Jul 27, 2019 7:20 pm
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I had the honor of being an independent repair technician for the pickers of Central Oregon. Carrying those amps around made me 2 inches shorter over 20 years. I had to quit before I disappeared.


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Post subject: Re: Musicman clone
Posted: Sat Jul 27, 2019 9:42 pm
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Maybe you could sell your story to Spielberg so he could turn it into a movie......

THE INCREDIBLE SHRINKING AMP TECH

:wink:
Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Musicman clone
Posted: Mon Jul 29, 2019 5:48 am
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Location: Province de Québec, Canada
The body may have shrink but knowledge has grown :)


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Post subject: Re: Musicman clone
Posted: Mon Jul 29, 2019 8:32 pm
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If I can sell the amp for what I've got into parts, I'll be happy. First I gotta get it working.
I powered the preamp for the first time. It was a DC amps test to figure out the value of the limiting resistors feeding the zeners. They are only 1 watt, but I'm only having to drop from 18v to 16v.
The dc idle was 32mA per side. I don't think it will rise much above that. So if I calculate in a little headroom, say, 50mA and a 2 volt drop, thats only .1watt dissipation.
2 volts drop at 32mA takes 62 ohms. 2volts drop at 50mA needs 40 ohms. OK, I'll try that tomorrow.


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Post subject: Re: Musicman clone
Posted: Thu Aug 01, 2019 7:41 pm
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Well, I got that math right on the limiting resistors. Too bad the zener failed for no good reason. That's why you buy extras.
I got the bias circuit right on the third try. I'm running it off the 5V supply for the digital reverb. Its a simple voltage divider set to 1.6 volts. I need to get some limiting resistors to keep it less than 3 volts. As it stands now, its a dial-a-bomb. 1.5 volts GS = .2A DS. 1.6V GS = .5 amps DS.
I found that at the 40 volt supply i'm using, the transistor is derated to just 1.2 amps DC. So I need to keep a tight lid on the bias to keep it from letting the smoke out.
I ran an oscillator into the gate and pumped it up to 6V P-P under an 8 ohm load.
Then I adjusted the bias to center the waveform. When its too low, it clips the bottom of the wave. When its too high, it rounds off the top of the wave.
It liked it around 250 mA drain current. That's about 10 watts idling. So it should give 20 watts output. I switched in the higher power taps on the output transformer and the power went up about 30% while the bias remained pretty close to centered waveforms.
I connected in the IC driver stage and ran it up to clipping. It showed apparent distortion on the scope.
I hooked up the feedback loop and it lowered the gain, but the waveform was a lot more accurate. I could also drive it harder with less distortion.
Then I plugged in the other ICs to see what else worked.. Short answer, nothing. I need more filtering on the preamp to stabilize it.
Time to order more parts....


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Post subject: Re: Musicman clone
Posted: Thu Aug 01, 2019 11:49 pm
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Any idea why the zener failed?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Musicman clone
Posted: Fri Aug 02, 2019 7:58 am
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I'm not sure why.
The zener had been run when I did an overnight burn in, so I know it was OK.
There was no visible damage, just shorted.
I had been running it that day, but after some changes, I flipped it On and the 51 ohm limiting resistor for the negative zener started smoking.
A quick turn Off and that was that. Shorted zener.
The usual reason a zener goes is from being forward biased with too much power. They won't carry much current.
I don't know what I did to cause it, but when I replaced them, no problem.
I did find a small piece of clipped wire in the chassis, but I don't think that cause it.
The board was not mounted at the time and its possible that it shorted to the chassis, But how does that forward bias the Zener?
At any rate, I found a wiring mistake in that area while fixing it.
The IC power buss had never been hooked up to the limiting resistors. When I calculated the resistor values, I measured the current from the raw supplies. I never changed the busses to draw through the resistors.
So the zeners were running with only the 100uF filter cap for a load. The raw voltage is only 20VDC there, so I don't think its a case of an overvoltaged zener. Those usually go with smoke.
Now, there is a lot of ripple. I don't have enough filtering. Still, there shouldn't have been enough voltage...

I'm going to work on getting each stage of the preamp working while I'm waiting for parts.
I've gotta modify the cabinet a bit to fit the chassis a little better.


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Post subject: Re: Musicman clone
Posted: Fri Aug 02, 2019 9:35 am
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TimsAudio wrote:
Now, there is a lot of ripple. I don't have enough filtering. Still, there shouldn't have been enough voltage...

:?:
I wonder if an unanticipated ripple surge was responsible?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Musicman clone
Posted: Fri Aug 02, 2019 1:42 pm
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Unanticipated, Yes.
I used the filter capacitor values described in the original GP-3A. 400uf before and 150uf after the limiting resistors. But I have volts of ripple with little load.
I ordered 1000uf for both sides.

I also broke down and ordered a digital effects unit from Belton. Its got digital reverbs as well as other effects. But there is no control for modulation speed of the chorus etc.
Well, I got it mostly for the plate reverbs so...


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Post subject: Re: Musicman clone
Posted: Sun Aug 04, 2019 7:01 pm
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Well, you were right Retro. Its too tight a fit
I fitted the chassis to the cabinet today. Then I went to power up the first IC stages in the preamp. I just jumpered it to the output stage to monitor it on speaker.
It worked OK, but had a low level hum. Then my scope indicated it was coming from the output stage.
It was only 10mV, and I had heard it before. I attributed it to just hum from a class A. You can never get enough filtering to make em happy.
But then I turned the bias off, shutting down the output stage and my hum was still there.
Then I pulled the B+ fuse. Hum was still there.
Sigh... I pull the OT and rotate it around. The hum comes and goes.

I have to move the OT away from the power transformers. The one it's closest to is not shielded, the other one is. I thought about swapping the two and see if it helps, but then I removed the OT wires and jumpered in longer ones. I moved it around and the OT had to be more than 6 inches away before the hum disappeared. So I have to move my heatsink closer to the PT and mount the OT at the other end. There's barely enough room.

I guess the good news is that the inevitable hum I was going to put up with may not be so intractable. The bad news it this chassis is getting a lot of extra holes. Oh well, it was free.


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Post subject: Re: Musicman clone
Posted: Sun Aug 04, 2019 7:05 pm
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Will you be able to modify the chassis to fit?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Musicman clone
Posted: Sun Aug 04, 2019 7:55 pm
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I can move the heatsink over an inch and I only need 3/4" to fit the OT on the opposite end. That gets it away from the power transformer, but puts the OT near the input stage...
I'm trying to make the major components mount so that they can be disassembled individually. If I put the OT towards the rear, it won't be covered up by the eyelet board and I like that.
I almost thought about putting it in the chassis, but I think it would get too hot running Class A.

It'll give me something to do while I'm waiting for capacitors


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Post subject: Re: Musicman clone
Posted: Sun Aug 04, 2019 11:19 pm
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TimsAudio wrote:
I almost thought about putting it in the chassis, but I think it would get too hot running Class A.

I concur. Unless you have room for a fan (problematic at this point) the tranny will likely run most efficiently when convection-cooled.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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