Vintage Ampeg BT15 Help

[J M Fahey]

100V caps, same value, are fine.
But first build a lamp bulb limiter and plug this amp there, until pronounced safe.

PS: post a couple pictures, both full combo, amp only and guts.
Also transformer and heatsink area.
Not a very common amp.

[52 Bill]

When you replaced the diodes did you replace all 4 or just the two melted ones?

The bulging on the bottom vents is a sign of a bad cap. Try reading the resistance across the filter cap, it should not stay low. Normally they will start low and then rise as the meter charges the cap.

You can read the resistance along the power supply bus to ground to try and find the short.

[g1]

Dang good thing you got rid of that 30A fuse before you fired it up, I must say.
As I've probably mentioned before, I'm trying to make it a habit of checking for correct fuse BEFORE firing up any unit I get for repair. I've caught a few overfused items that way, although they may not have been a problem until the next time there was a fault. An ounce of prevention...

[erock0138]

When you replaced the diodes did you replace all 4 or just the two melted ones?

I replaced all of them, the other 2 still in tact were mismatched

[erock0138]

[erock0138]

I will definitely use the light bulb limiter before I try and power it up again, and post results

There is very little info on this amp, The only other gutshots i've found online show a green pcb on both sides, as opposed to brown on one side like mine. (Which I've been told can date it a little better)

In an interview with Jess Oliver, (I believe from the story behind the sound book) He said he had told the designer of this amp that the heat sinks were too small, and wouldn't work, right at the time he came out with the portaflex. Interesting.....

Anyways, thanks again everyone!

[52 Bill]

There is very little info on this amp.....

They weren't made for that long, as I remember it. They just didn't sound as good as the tube versions so nobody bought them. And I guess that they had their share of technical problems like the under sized heat sinks and the power/driver transistors rated too low for the power supply.

The front panel lights up and has a bluish glow which does look cool in a darkened room. Kinda UFO-ish.

[J M Fahey]

Thanks for posting.
I was very interested because that's an important model for me, yet I had *never* seen pictures of it ..
It's the basis of my first commercially successful product; 1 BT15 preamp channel (built on Veroboard) driving a homemade 100W Motorola Power amp where I replaced MJ802/4502 with 2N3055/MJ2955.

[J M Fahey]

Thanks for posting.
I was very interested because that's an important model for me, yet I had *never* seen pictures of it ..
It's the basis of my first commercially successful product; 1 BT15 preamp channel (built on Veroboard) driving a homemade 100W Motorola Power amp where I replaced MJ802/4502 with 2N3055/MJ2955.

[Amp Kat]

Thanks for posting.
I was very interested because that's an important model for me, yet I had *never* seen pictures of it ..
It's the basis of my first commercially successful product; 1 BT15 preamp channel (built on Veroboard) driving a homemade 100W Motorola Power amp where I replaced MJ802/4502 with 2N3055/MJ2955.

I'm glad I'm not the only one who uses those trannies. Have all of those at the work shop that we use on the old Executone PBK 6150'S and they are monsters. For some reason the MJ802's don't sound as well as say a NTE181 which is really a nice sounding Xistor of course the MJ-15024/25 compliment are nice also.

[erock0138]

Thanks for posting.
I was very interested because that's an important model for me, yet I had *never* seen pictures of it ..
It's the basis of my first commercially successful product; 1 BT15 preamp channel (built on Veroboard) driving a homemade 100W Motorola Power amp where I replaced MJ802/4502 with 2N3055/MJ2955.

That's awesome! The schematic inside the head has a date of either '67 or '69, hard to make out.

[erock0138]

My recent findings:

I used the light bulb limiter, as suggested, I powered it on, and the bulb stayed very bright.
I then noticed the pilot light on the amp was not on at all, so I quickly measured the voltage, and got nothing coming in to the amp. Then quickly turned it off. Worrying that the fuse, (somehow) May have blown, but it didn't.
Which means, from my understanding, that the amp was getting no voltage/current at all, it was consumed by the bulb.

So I removed the board all together, leaving just the transformer, and power light assembly in the chassis.
I measured the 2 blue wires of the secondary to verify again that the transformer was working correctly, and got 60 volts AC between the 2, which is correct. I then measured each blue wire to ground, One of which was 55+/- a few V AC. The other, was about 30 V AC. Does that seem suspicious? But the 2 combined are correct.

I also noticed that the blue light that lights up the front panel was not working, schematic says the 2 red wires are 300 V AC off the tranny. After measuring, I was only getting 200 V off of one, (The other one was to ground)

Should I still suspect the transformer, and then move onto the board after I get that straightened out?
Thanks again, everyone.
You've been a great help.

[loudthud]

I had an amp like this back in the 70's. I think it was a BT-25 rated 100W at 4 ohms. Same circuit but with heat sinks hanging down from the chassis and no elecro-luminescent panel. The fuse blew at a gig and I replaced it with a chewing gum wrapper. The amp made a strange sound when I turned it on. It worked at reduced power, and I finished the gig with it. Two of the power transistors blew out. It only made a half wave output.

Those input JFETs are drawn wrong. They are P-channel source followers.

Dates on schematics can be misleading when dating an amp. The date is from when the schematic was drawn, not when the amp was produced.

[52 Bill]

So I removed the board all together, leaving just the transformer, and power light assembly in the chassis.
I measured the 2 blue wires of the secondary to verify again that the transformer was working correctly, and got 60 volts AC between the 2, which is correct. I then measured each blue wire to ground, One of which was 55+/- a few V AC. The other, was about 30 V AC. Does that seem suspicious? But the 2 combined are correct.

How you go about working on the amp is up to you, but I try and remove as little as possible to check things out. It makes for less work in putting things back together again. There should be no connection to ground from the transformer secondary. You can test this with your ohmmeter. The ac voltages that you are reading may be leakage voltages.

I also noticed that the blue light that lights up the front panel was not working, schematic says the 2 red wires are 300 V AC off the tranny. After measuring, I was only getting 200 V off of one, (The other one was to ground).

Are these readings with the light bulb limiter?

[erock0138]

Are these readings with the light bulb limiter?

No, I removed the lightbulb limiter and the board to test the transformer on its own.
Which I know that the transformer isn't necessarily shorted, because fuse doesn't blow, direct short is on the board. But I'm curious as to if the transformer can be "Damaged" as opposed to being shorted, in reference with these voltage readings.