Bass Power Amp with GU50

[es345]

As a member in this forum now for some time - but more reading than contributing - I want to share with you the results of my prototype of a Bass Power Amp created over the last month.

The target for the design was more than 250W, weight around 20Kg with housing, small size. Here in Germany I have found a company who is delivering customized toroidal transformers for power and output which is a prerequisit to reach the design goal.

Here are the results up to now, schematic and a picture is in the attachments:
For the power stage I have selected the GU50, they are reliable and cheap.

Power at 20Hz: 295W
Power at 5 KHz: 295W
Power at 20KHz: 200W
Weight without housing 15Kg, Size without housing: 53cm x 19cm x 19cm

Plate voltage at idle is around 950V, at full load around 850V. The operating mode up to now is AB, the next step is to evaluate AB₂

Hans- Georg

[Jazz P Bass]

Wow!
Does it produce the target 250 watts?

[es345]

As stated: 295Watts starting at 20Hz

[Steve Conner]

Very nice!

Have you tried playing bass through it? Any thoughts on the sound quality?

[es345]

Yes, I have tested it with my 5 string American Jazzbass using two Reflex boxes with 15 inch speaker.
From my perception very clean, direct ,with fast response and a lot of headroom.
Using the active tone control I haven't missed any additional tone control.
I stopped after playing 10 minutes of full power to save my ears for the future.

[g1]

That's a lot of PT secondaries! I was looking at a 300W bass amp that uses a dozen 6L6's (new Traynor), and wondering how your tube costs compare. I only found one seller, but they are only $3.50 each! Are they common or hard to find?

[exclamationmark]

That's a lot of PT secondaries! I was looking at a 300W bass amp that uses a dozen 6L6's (new Traynor), and wondering how your tube costs compare. I only found one seller, but they are only $3.50 each! Are they common or hard to find?

There's usually tons of russian surplus sellers around for these tubes. The sites are kind of hard to find using google though, unless you know what to look for. One site I frequent had something like 900 of them in stock.

[imaradiostar]

Awesome! What is your output transformer primary impedance?

What voltage are the screens running at?

How are you driving the output grids?

I ask a lot of questions.

jamie

[es345]

Hi Jamie,

here are the answers:

1. The primary out is set to 2K. I have done some tests with the same OT at 4K (switch the output impedance) and another OT at 2K5, 2K75 and 3K (OT with 3 possible input impedances ). The summary is shown in the diagramm below

2. The voltage at the screen is 325V at idle mode , about 300V at full output power mode.

3. I assume that "output grid" is the control grid - g1 - of the GU50.
Please have a look to the schematic at the top of the thread: the control grids are driven via MOSFET IRF 840, direct coupling.

Hans- Georg

[imaradiostar]

Hi Jamie...
3. I assume that "output grid" is the control grid - g1 - of the GU50.
Please have a look to the schematic at the top of the thread: the control grids are driven via MOSFET IRF 840, direct coupling.

Hans- Georg

Yeah, that's what I meant. I should have used the correct terminology. What is the voltage of the supply rails for the IRF840 relative to ground? (+- voltages)

Am I correct to assume the phase inverter is capacitively coupled to the mosfet gates (6 of them) which drive the gu50 grids directly and allow for individual bias settings?

This is a really neat build. I've been wanting to build something like this for a while but haven't yet had the time.

I think it's interesting to see that the max power was at a slightly higher reflected impedance. How are you measuring the power? (Sine volts rms^2)/load impedance?

I recently bought a 6.6K:4,8,16 transformer from Edcor with the intent of mismatching it was a 3.3k:2,4,8 transformer for a bass amp. The TF weighs about 12 pounds and should be capable of 200 watts down to around 30Hz. Your amp inspires me to get to work and build something!

jamie

[es345]

... if you look to the schematic (pdf file at the beginning of the thread):

- I have taken one MOSFET for each push pull group to save space taken into account that - as a drawback of that choice - I need then to match the tubes.
- the MOSFETs are capacitively coupled to the phase inverter, the negative rail for them is - 140V, the positive is +140V. The marging is ok because the bias for that tubes is around -40V to -44V.
- I have done the measurement with resistive load and sine wave as input. With an oscilloscope I have measured the peak voltage just before the clipping starts.
- from matching point of view you OT should be OK. Please check the maximum allowed current at the primary. In addition it might be that you will go into saturation at low frequency and full power.

Hans- Georg

[imaradiostar]

Yeah, I'm planning to use the Kt120's I've already purchased and I'm shooting for 150 watts. You're right- the output transformer probably couldn't take the current on the bottom end. I am going to start with a few large power transformers I have laying around. I'm tempted to used a Fender Twin Reverb transformer but with bridge rectified secondaries, giving a B+ around a thousand volts. I also have a few old industrial and o-scope PT's giving voltages between 600 and 800 volts. The only issue is that none of them have proper bias windings so I may have to use an addition transformer to generate those voltages, most likely using plus/minus rails and mosfets like you did.

jamie

[Ted]

That's interesting. I have a large box of GU50s and sockets and I'd been thinking about building something similar.

Where did you get the transformers? And how much did they cost?

How did you get on with AB2?

[Steve Conner]

I've got all the parts to build this except the GU50s, want to sell some?

[es345]

That's interesting. I have a large box of GU50s and sockets and I'd been thinking about building something similar.

Where did you get the transformers? And how much did they cost?

How did you get on with AB2?

The transformers I have got from this company in Germany.
Ringkerntransformator, Transformatoren, Ringkerntransformatoren | Ringkern, Ringkern-Transformator, 100V-bertrager - BADEL Elektronik GmbH
They deliver customer defined transformers. The price for both NT and OT together is about 250 Euro.

Now after tweaking this is the result:

clean power: 350W (Ub at idle 940V, at full power 840V)
predicted weight including missing "optical" finish: 22Kg
to be mentioned: forced cooling by 3 slow speed fans from the bottom through the chassis

schematic and a picture is included.

How about the sound?

I have tested the machine with my 2 EVM15L boxes (vented)
- with my Jazzbass : unbelievable "pressure", clean fast reaction both in active and passive mode
- with my ES345 and my Blade basically the same. Obviously I was not trying to test the power stage overdrive...

In summary: I am satisfied with the result.

Hans- Georg