Help with found amplifier

[mdmoyer]

I recently moved into a house with a separate garage and was cleaning out the stuff the former owner had left behind. I found an amplifier head made by Bryan Model 760. Its a solid state amp with two instrument inputs, one labeled "bright", a volume and tone control and a speed and strength knob for trem with a footswitch. It has nothing on the back but a jack and I'm not sure how to determine what impedance is necessary. I have tried searching for this amplifier online and have come up with nothing. I have attached pictures as well. Thanks for the help.

[Matt Syson]

Hi
You should be OK with 8 Ohms for starters, or 16 Ohms if you are nervous.
As it is so old there is a possibility of caps failing catastrophically so don't leave it unattended while powered up, especially if it is hooked up to an expensive cab.
It's power ratings could be worked out by looking at the transistors used and knowing it's power supply DC voltage.
Matt S

[mdmoyer]

appreciate the help! im gonna go find a nice 2X12. Im guessing i wouldnt need a 4X12 or anything for this. I might have to pull it apart and replace some of the "prone to failure" parts as its been sitting in a shed not completely isolated from humidity and heat/cold

[Bill@WelcomeHome]

suggestion: take your solder iron and reheat all the joints until they melt. The caps likely need to be replaced. Use a little Deoxit on the jacks, pots, and switches. (Not much. It doesn't take much.) Examine the power cord with an eye toward replacement. If it is dry and cracking, throw in a 3 wire to replace it.

[mdmoyer]

Yeah the power cord is a 2 prong job that looks like it belongs on a lamp. When putting in a 3 prong where will I attach the third wire. Sorry simple question I'm sure, just haven't ever had to put a 3 into a 2

[Matt Syson]

Hi
To attach an earth wire (the green one on a 3 conductor cable) drill a new hole in the chassis and use a solder tag and 'star' washers to connect that ONE green wire to the biggest piece of metal, the one with the mains transformer on it.
It is difficult to see from your photos but I wouldn't expect it to be more than about 30 Watts maximum. If you posted more pics showing the output transistors (unless it is an IC in there) and measure and report the DC supply voltage it is possible to get a better idea.
Any old speaker impedance will actually be fine if you keep it quiet. This would allow you to get an idea of whether it is really worth persuing.
Matt S

[mdmoyer]

Thanks a lot! I'm moving out to Arizona on Monday, so it may be a few weeks before I can post results. Again I thank you for all your help. I will take the board out and post some better pictures in the meantime!

[Papanate]

Do you have a Scope and a Attenuator available? If not then bring it to a Tech who has both. They can give you the Ohms - and the attenuator will allow them to ramp the voltage up slowly so the amp doesn't burn up with a jolt of volts!

[mdmoyer]

No scope or attenuator. I just got my handy multimeter both a digital multifunction and an analog one with a needle, which surprises most that i still have it haha. Are attenuators somewhat expensive? Never really looked at them before

[Matt Syson]

Hi
That's OK because you don't need one.
You do need a sorce of continuous sinewave tone at say 400Hz and a speaker of say 16 or 8 Ohms or if you don't have that, use a speaker with a 8 Ohm (or therabouts) resistor 25 Watt and put that in series with the speaker you do have.
Connect the multimeter set to AC on the speaker output socket, and the speaker of course, then turn up the volume until it starts to sound distorted.
You should also measure the DC supply on the amplifier as you can get an idea of how much power it can put out.
The main problem is deciding what impedance speaker it really 'wants' and this is determined by the rating of the power transistors and the amount of heatsink as it is largely a function of how hot the output stage gets.
A couple more decent pictures posted with the output transistors and the area around the power supply would reveal quite a bit.
Power amp ratings are often specified in suspicious ways to make them appear 'good' so a HiFi amp would tend to state power out at say 0.01 percent distortion whereas a guitar amp may be at 1 or 2 percent. A valve amp can be around 5 percent although the 'quality' of the distortion is different (different set of harmonics).
Matt S

[mdmoyer]

I have pulled out the insides and these are the pictures i have taken.



This is the top left. The Transformer in this picture is a Bendix Transformer N2090266-4 5 150 10 it looks like.



Heres the middle section of the top. Orange dip is for bright input. You can see one of the transistors up on the board between the two resistors and capacitor.



heres the right hand side of the top. another of the same transistor on the left. and on the right hand board you can kind of see three of the black "pill transistors" as i call them haha The two giant red capacitors are Delco Radio 7266376 Capacitors. .47 MFD 100 VDC and it also says 446 0-35 on one and 446 0-45 on the other. The three yellow caps are Sprague 1uF 150VDC. All the same rating but one has a different part number.





Heres the two three section capacitors on the bottom and other transformer. These two capacitors are Delco Radio 7282272 three section capacitors 50/400/4 MFD 16v/16v/11.5 rms The Transformer other than that number has no other markings on it.



And this transistor is on the bottom as well. '

After taking this apart and looking it, it definitely needs some work. But none of the transistors have any visible markings on which could be a huge problem in finding replacements...I think this might be destined for the trash...

[Matt Syson]

Hi
I would not put this amp at more than about 10 Watts as there is insufficient heatsinking available for much more.
It has a strong hint of 'wreck' about it but if you can be bothered you could work out the schematic by sketching out the components as you find them on a large sheet of paper then redraw it more neatly after.
Thee is not a huge variety of amplifier designs so you could look at some 'typical' old transistor designs to get a 'feel' for how it should go.
Simply counting up the transistors will get you some way towards this.
If you intend to play, get an earth wire on it as the first job to make it as safe as possible. Cover up any mains connections too.
A nice quiet project for the winter evenings (assuming that is what you have at the moment).
Matt S

[Bill@WelcomeHome]

The other side of that coin is that it may have a unique sound that you can't use often but is worth having around. I had a whole collection of old tube amps until a couple of years ago, for just that reason. And I have an old solid state Thomas Organ Vox Cambridge and a Vox Berkley from the 1960s that I keep just for their interesting sound. So if there is little to no repair required, ..... but I wouldn't put much money into it.

[mdmoyer]

I was thinking along those lines as a "learning experience", but I'm not so sure this would be a good beginners project. I only have experience with 9, 12, and 24 volt stuff and never really dealt with transformers. I guess the reading shall begin! I really appreciate all the help you gentlemen have provided!

[mdmoyer]

Also, when I draw out the schematic, how do i draw up the tranny as I can't find any documentation for them. And how will I draw up the transistors I.e base, collector, emitter? I was thinking.about just drawing a generic transistor and trying to find a schematic that fits.

[Matt Syson]

Hi
You have a mains transformer there but is the 'other' one a choke (2 wires) or transformer, 4,5 or 6 wires?
Just draw a generic 'transistor, even a circle with the 3 wires going to it would do.
As long as it doesn't 'bite' you you could learn a lot from the excercise of working it out. It might be useful at the end....or it might not.
What voltage ratings were on the big silver capacitors? That is the 'worst' you would have to deal with, assuming you have the mains covered up.
Matt S

[mdmoyer]

The one on the bottom has three wires, two green and a green/yellow wire. The big silver 3 pronged caps are 16v (never saw three pronged caps before). The red ones on top are rated at 100vdc