Bass Power Amp with GU50

[es345]

I couldnt resist, there is too much of an engineer in my DNA:

Having spoken with my OT supplier we have made another try still fitting to the housing:
Raa 2K, Rcu anode to center 50 Ohm, only 4 Ohm secondary to have more room for the primary winding.

Here is the result: 400W down to 20 Hz, saturation now starting at ~16 Hz.
This is a far better compromise. But nows its done, definitely.

Attached is a small excel which illustrates the impact of the OT's winding copper resistance to the reachable output power.

Hans- Georg

[Steve Conner]

Very impressive 8)

[imaradiostar]

I hadn't had a chance to say but yeah, that's awesome! I'm very impressed.

I bought a 6k6 TF to use as 3k3 and this concept really bit me in the butt- I didn't get nearly the power I wanted because of the anode resistance.

I think I'm going to have to make a really high voltage amp to use the transformer now...what a shame.

Jamie

[Ted]

Has the amp had much use?

I'm just wondering how the characteristics of those GU50s drift with age.

You mentioned earlier that they have to be matched as there is no individual bias control and the toroidal O/P transformer is not as tolerant of DC imbalance as an iron core.

[es345]

I am using the machine 2-3 times a week for about 3 hours each day. I didn't observe any drift of the GU50. They seem to be very stable - as expected for military equipment. I am doing regular measurements about one time a month (test pin for each tube is at the rear side of the amp)

Regarding DC imbalance: one time I have measured at one tube only a 10th of the idle current. The reason was a bad soldering point at the plate (shame on me). Interesting: I havent recognized that during play. I see at least 2 possible reasons: 1. I didn't use all the power or 2. I have some problems with my ears . By the way: after fixing the tube was still ok.

Some words about my "matching process": I have bought some 50 tubes for an acceptable price, measured them and sorted them into "sixpacks" with closest measurement results, e.g 1x21mA,3x22mA, 2x23mA. Then take 1x21mA and 2x22mA at one side, the other three at the second side. Adjust the bias and balance, thats it. No big science and the result is ok.

Hans- Georg

[es345]

I was asked to share the latest schematic. Here it is.

[Tage]

Hey good to see that this amp is going strong. I have implemented many of your strategies into my own ~200 watt amp with a kt88 quad. I should post my schem up here too, I haven't done much measurement with it to see if I am getting the power I hoped to but I don't think it would have been as operational as it is without referencing your GU50 amp.

[es345]

Some status update including updated schematic:

the machine is running quite well, the GU50's are extremely stable, no drift at all.

I have built in 2 additional features:

1. Limiter/compressor

This is a passive one around a VTL5C1. Basically it is a volume controlled attenuator just in front of the master volume. It is in the schematic of the preamp (right half). You can see the compressor working indicated with the LED in the mid of the knob second from right hand (amp picture). I have used a pot with clearshaft illuminated via LED and have inserted a Fresnel lense in the center of the knob to pass the light (knob picture).

2. Cathode current display on the back beside the bias adjustment pots

This is a typical gimmick of an engineer : you can select to show the cathode current of each power tube, the average current of group 1, group2, and of all 6 power tubes. As I have external access to bias and bias balance the adjustment can be done now on the fly. Schematic is attached in a separate page.

Hans- Georg

PS: Still working on the labeling of the front plane....

[MarkusBass]

Very nice project. Do I understand correctly that there is no STANDBY switch in the amp?
Can you share some details on the output transformer (was it changed to toroid)?

Mark

[es345]

Very nice project. Do I understand correctly that there is no STANDBY switch in the amp?

Indeed there is no standby switch. I personally don't miss it.

Can you share some details on the output transformer (was it changed to toroid)?

PT and OT are toroids from the beginning. That was the only way to limit the total weight of the amp to ~22Kg.

These are the main specs of the OT:
* Raa = 2K
* RL = 4 Ohm
* -3db limit at 20Hz and 20kHz when running with output power of 350W
* size 165 x 80 mm + 2mm (max)
It was designed by this company Ringkerntransformator, Transformatoren, Ringkerntransformatoren | Ringkern, Ringkern-Transformator, 100V-Übertrager - BADEL Elektronik GmbH and manufactured by their Polish company in Świdnica Transformator du?ej mocy, toroidalny, sieciowy, na zamówienie, zamiennik - BADEL Elektronik Sp. z o.o.

Hans- Georg

[MarkusBass]

I'm not familiar with toroid output transformers. Has it the interleaved windings, or any special solutions were applied? Did the high supply voltage influenced somehow the design?

Mark

[es345]

Well , I have set the requirements and they have done the design. I forgot to mention that inside the specs I have also put the supply voltage of 950V.
Unfortunately I cannot help you with more details about the implementation. Nevertheless the result fits to my requirements:

400W at 20 Hz, thats better than expected
400W at 10 kHz
200W at 20 kHz, (-3 dB)

Hans- Georg

[Roberto]

Hi Hans Georg,

I know it's an old thread, but this is exactly what I would like to build, except I'm looking for an octet instead of an hextet of GU50. I've already contacted a company for the toroidal transformers, that's polish as well but a competitor of the one you linked here.

The specs are close to yours, I will paraphrase them here like yours:

OT
-3 dB at 20 Hz and 20 kHz at 450 Wrms;
Raa 2 kOhm with separate 30% UL and 10% CFB;
B+ 900 V idle (not dropping below 850 at full power);
4 and 8 Ohm taps

PT
300V 100 mA Preamp+PI
420V 1A Ub Rail 3
24V 200 mA MosFET Stage supply
220V 1,2A Ub Rail 1 + G2 supply
200V 100 mA Bias and PI CCS suppy
2x 6,3V 7A Heater GU50
6,3V 3A Heater pre +PI (dc elevated)

Do you have any suggestion on the design?

Thank you in advance.
Roberto