Bass Power Amp with GU50

[jazbo8]

you can find it here Smartest way to 200W? - Page 2

Thanks! Now I gotta read that thread too...

[Steve Conner]

Another advantage of AB2 is that the power stage can be overdriven without charging up the coupling capacitors and getting crossover distortion. That's part of the sound of many classic guitar amps, but it's maybe not so hot for bass.

The Ampeg SVT used a diode limiter like es345's to avoid this. I proposed using a Vactrol compressor with the LED driven by the grid current (or a filament lamp driven by screen current)

[woodyc]

...If you have a OT with a coil resistance center to anode of about 70 Ohm (this is my case) you will get about 270 W output, not more...

I guess that begs the question: why 70 Ohms? It seems like a lot of power to leave in your transformer.

For a practical example, I had an ampeg V4 on my bench yesterday. The center tap to anode measures lower than yours (54 Ohms dc) -- an thats only a 120W amp!

By the way, I don't mean to sound like I'm complaining. I'm just trying to understand your design choices. Your project is very impressive and inspiring.

[es345]

No problem at all

Well thats what I have measured and it has told me something:

1. Don't forget that thing when you estimate the output power.
2. The next time I will put a low Raa resistance to the specifications.

There might be one other reason for that high value: I have defined a maximum size of the OT...

[Steve Conner]

Yes, the physics of transformers won't allow you to have small size, low DC resistance and extended bass response all together.

As far as I know, the 300PS OT is only specified down to 50Hz at full power.

[woodyc]

...2. The next time I will put a low Raa resistance to the specifications....

I just thought of an alternative...

[es345]

They are already mounted at the bottom

[woodyc]

They are already mounted at the bottom

Sneaky.

But nice idea. Do your GU50 sockets have vent holes?

[es345]

There is some distance between the socket and the glass, not much but enough for slow vent. You can see it at the beginning of the thread. The fans are low noise, with 1200 rpm (Slip Stream Slim 120 mm Case Fan, 12 mm)-

[woodyc]

By the way, how do you figure the grid dissipation? I assume you get the rms current across the stopper but what about the voltage? Do you look at the positive blips and make an estimate?

(You've inspired me to revisit an old design and shove some AB2 into it.)

[es345]

I assume you are asking for G1.

You find some scope measurement pictures and my rough estimation done during summer time this year when I was still optimizing. Please not that I have done the measurement in a sequence, the drive adjustment has varied a bit. But its ok for an estimation.

- In the chosen overdrive situation the peak voltage at G1 is +27V (clipping picture)
- the related current is about 25mA peak, estimated up to 25% of the period. That leads to an average power dissipation of 25mA*27V*0,25=~170mW . As said above I have limited the G1 voltage to be on the safe side for the G1 power dissipation. But it helps also in addition to limit the power disipation of G2 , not to forget.

To explain the gridcurrent picture: the upper scope is showing the grid current, the lower scope is showing the output voltage at 3,9 Ohm. For the gridcurrent measurement I have separated the upper scope from ground. If you want to do that measurement, dont forget to use an isolation transformer for the scope measuring the voltage at the grid resistor. The whole scope is floating with the G1 driving voltage, in my case from +27v to about -140V during the measurement. Alternatively use a differential probe.

Hans- Georg

[es345]

A small amendment:

you may remember Steve Connors comment

The physics of transformers won't allow you to have small size, low DC resistance and extended bass response all together. As far as I know, the 300PS OT is only specified down to 50Hz at full power

Taking that into account I have discussed with my OT supplier to find another design compromise.
This was the resulting spec. he agreed to.

- Toroid, designed for 350W
- same size as the former one ( 80mm height, diameter 165mm, max 167mm)
- Raa 2K (changed from Raa 2K2).
- full power at 50 Hz
- primary coil resistance center to anode 18 Ohm. This is a much smaller value compared to the value of the first OPT (~70 Ohm)

Here are the first measurements of possible clean power at 3,9 Ohm Resistor load.

- 460 W at 46Hz or higher, the limitation is clipping. The upper frequency limit for this power is above 20 kHz.

below 46Hz the first limitation is the magnetic saturation of the OPT
- 400 W at 41Hz,
- 250W at 34Hz,

Thats an impressive result. Tomorrow I will do a test with speakers.

[Steve Conner]

I heard reports of a small earthquake in Germany, are you all right?

[12stringbassman]

I heard reports of a small earthquake in Germany, are you all right?

Yes, we are.
My hometown is 170km away of Hans-Georg's, but I could feel the rumble

Cheers
Matthias

[es345]

speaker test done, I am still alive

My subjective asessment: the sound seems to be a little more tighter, the high frequencies more transparent.

Ok, I am fine with this machine now, I will start the next project (definitely not one with more power... )

Hans- Georg