Ampeg SB12 Bass Amp Build

[martin manning]

Adding power tube current will drop more voltage, but It sounds like you might need even more. Got a 5U4GB?

[dawsonaudio]

Same readings with 5u4gb installed. Tried another 5u4gb tube with the same results. Still trying to figure out why the plate readings are that high.

[martin manning]

Got a schematic showing how you have it wired now?

[dawsonaudio]

[img:800:600]http://www.serviceairsocal.com/amp/ampeg2.png[/img]

[sluckey]

Changing rectifier tube types will not change the B+ if you don't put a load on the power supply. Easiest way is to plug in the output tubes. You should verify the negative bias voltage before you even plug in the rectifier tube. Make sure each output tube control grid has sufficient negative bias voltage right on the socket pin.

[martin manning]

Yes, you're right Steve, it needs to need to have some (more) load on it to get the voltage down. I'd set ca. -25V on the grid pins with no rectifier plugged in to start. I would still keep the 5U4 in there for now too.

[dawsonaudio]

I'm getting a -21 vdc max negative bias voltage without anything plugged in. Will increasing the 47k resistor give me more negative bias voltage or is that fine to start?

[martin manning]

I'd want a little more than that. Do you mean the 27k? Maybe make that 33k or 37k.

[dawsonaudio]

Correct, the 27k. Changing the 27k back to the 39k gave me the needed -25vdc at the control grid pins with a swing from -17.5vdc to -25vdc using the bias adjust.

What would the voltage drop be with all tubes in place? I'm more concerned with my electrolytic cap choice. Maybe I should have made those rated at 525 volts instead of 450vdc. Just didn't have any higher ones at my local store.

[sluckey]

If increasing the value of that 27K still does not get you to a good range of bias voltage you can decrease the 220K before the diode. I have my 7591 amp set up with a bias range of -20v to -32v. It's currently adjusted for -27V.

BTW, our amps are very, very similar. When I got this AO-63 I was already thinking Ampeg because of the 7591s. I'm very pleased with the sound of this circuit. I bet you will be too if you're looking for a warm clean sound with ample bottom end. Here's my conversion schematic so you can see how similar they look. I left the AO-63 power amp circuit intact just because it looks so much like Ampeg (except for that center tapped OT).

[dawsonaudio]

Would decreasing the 220k resistor also increase the range between high and low bias points or just an overall increase in negative bias voltage? How could I increase the range if changing the 220k resistor won't do it?

Not sure if 7.5vdc difference is adequate.

Thanks for the help.

Nate

[dawsonaudio]

So putting the power tubes in along with the rectifier, still no 12ax7's in yet, the plate voltage's read around 220vdc, but the 1k 10watt resistor started to smoke...really hot! So I turned off the power...

[sluckey]

Would decreasing the 220k resistor also increase the range between high and low bias points or just an overall increase in negative bias voltage? How could I increase the range if changing the 220k resistor won't do it?

Decreasing the 220K will increase the level of bias voltage but it will not change the spread between hi and lo. The spread is determined by the ratio of the value of the pot and the resistor connected to the wiper of the pot. Making the pot larger will give you a wider range, or making the resistor smaller will give a wider range.

Using a 50K linear pot and a 27K resistor should give you plenty of range. Then diddle the 220K to set the overall voltage level. I would set the pot to mid range then experiment with the 220K to give about -26V to pin 6 of the 7591s. Then you'll be able to swing above and below -26v.

You can safely do all this with your rectifier tube unplugged.

[dawsonaudio]

Ok, I'll get a few different components tomorrow and see if I can get some more negative voltage happening. So I went ahead and installed the power tubes, 7951's, and the plate voltage is reading about 220vdc and the 1k 10watt resistor was getting real hot and started to smoke....so I shut things down. Seemed like quite a large drop in voltage from the previous reading I had with just the two power tubes installed.

On my previous supro amp build, similar thing happened and the resistor was eliminated. The field coil instead was responsible for dropping the voltage quite a bit. Have to take a look back at that thread.

[sluckey]

So I went ahead and installed the power tubes, 7951's, and the plate voltage is reading about 220vdc and the 1k 10watt resistor was getting real hot and started to smoke.

Well, that's not good! I suspect something is not wired IAW your schematic, or a filter cap has failed since you had 500 plus volts on them.

Pull the 7591s nut leave the rectifier tube plugged in. Recheck your B+ on ALL filter caps (should all be high). If that 1K still gets hot you likely have a failed filter cap. If the resistor does not get hot, measure the voltages on pins 3, 4, 5, and 6 of both 7591s. Plates and screens should be high again.

Now would be a good time to post some hi rez pics so we can see what you see.

Now I've got a couple questions... Why do you want to use that field coil speaker? Has it been reconed? FC speakers were phased out in the late 40s and early 50s. Any survivors will likely need a recone, otherwise the paper will likely disintegrate when you play a bass guitar thru them. I would recommend a speaker designed for bass guitar.

And a much smaller issue... The secondary of the OT must be referenced to ground, otherwise the NFB will not work and will likely give you some hum/buzzy sounds. I would wire the secondary exactly as it was in that M3 amplifier, ie, green and yellow to speaker and black to ground. If you use a 1/4" phone jack it must be isolated from chassis. Leave the NFB wire disconnected for now until you get the amp up and running. Then reconnect the NFB wire to either the green or yellow wire. It'll probably be obvious which is correct. It may be possible to change this wiring later on, ie, float the black wire, connect green to ground, yellow to speaker, connect the NFB wire to the yellow. If the NFB is wrong phase, just reverse the green and yellow.