GM70 PP

[pre65]

I did my GK71 amps as monoblocks using (mainly) MDF.

Each monoblock was in two sections with jumper cables to aid disassembly and save my poor old back.

[izzy wizzy]

Have revised the circuit to be a 3 stage amp as originally planned when starting the 813 version. In the end I needed the gain as my line has none. Choice was to either put the gain in the line or in the amp. Testing revealled that to get full power out, not that I need it, the input transformer would be overloaded. I was apprehensive about adding another stage.

I had the Lundahl LL1635. It is an older design so not ideal but my LL1660PP is on the phono and although I have an SE version lined up, it wasn't free to use. Plans were also to try a direct coupled version like Kevin Carters front end; choke loaded pair DC coupled to the driver sitting on a long tail. That could be next.

My worries about adding the extra stage were largely unfounded as I think the amp works much better now having relieved the 4P1L of input duty and the 2P29L stage driving it properly. I'm super happy with the result.

Although it uses all Russian valves, I console myself that I bought them from Ukraine. Received another 10 x 2P29L from Ukraine today. These 1958 versions I like better then the 1986 I had first.

I also want to change the bias to be fail safe after my previous accident without fusing; losing the power tx and choke. The OPT survived somehow as did the valves. I now have the HT fused.

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[izzy wizzy]

Some pics. Hasn't got clothes yet.

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Ply is all baltic birch coz had some lying around. 1000V rated 2mm plugs and sockets carry HTs from PSU to audio chassis. An IEC plug carries the mains from PSU to audio and plugs into a socket under the HT caps on the finals filament module.

Brass furniture castors on the front as they can be seen with regular seat castors at the back.

[izzy wizzy]

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Even separated, the PSU is very heavy so tend to work on it while on floor. Audio chassis is easily manageable but still heavy.

Modules are left, GM70 filament supplies, centre, GM70 bias and right, front end filaments. Packaging is tight but the audio can be worked on with modules demounted and wiring still hooked up but it isn't ideal. I should have used PCB plug and sockets rather than the screw terminals. I may have to do this in the fine tuning phase otherwise it might be too painful. This was the price for making it relatively compact.

Terminals on the side are to measure bias with the trimmers accessible via the small holes in the top. I had planned these would be in the side but turned out to be very hard and would have been difficult to adjust. The bias remains very stable over time.

The part I'm most happy with is the audio layout but it can't be seen now buried. Overall, it has all worked out as planned and the Sketchup modelling part, while taking ages to work everything out, was well worth it for a final build.

With it's clothes on, I hope it will look like the vision I have for it.

[simon]

Impressive! Good to see you haven't spared any expense and used birch ply

[pre65]

Very well done.

It's given me a few ideas for when (or if) the GU-48 (833a) amps get to a final version.

[andrew Ivimey]

Some might say 'Shoddy!'. -very impressive indeed.

It's good to see peeps using wheels again.

[IslandPink]

Very cool !

[andrew Ivimey]

Hardly cool as a cucumber though!

[Dave the bass]

A veritable Tower of Power.

I doff my cap.

[izzy wizzy]

Did some basic tests to see if it was doing something like planned.

Full power is about 50W before the waveform starts to flatten a bit, maybe a bit more. It clips very gracefully and gradually only rounding off the edges top and bottom so pleased with that. It needs about 2.4V to achieve that.

1W bandwidth is 12Hz to 75kHz. I didn't really want to push power bandwidth as my load isn't really up to that for long. It does full power at about 40Hz but the waveform isn't very pretty; fairly steep sided sine with pointy-ish top. Quite where it goes wrong in the amp not too sure but I can look harder at that when it makes it onto the bench.

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10kHz.jpg (171.09 KiB) Viewed 4329 times

Square waves look OK to me but I'm no expert. Ringing seems under control but I didn't expect a 10k square wave would ever look great through so many transformers. I imagine with a bit of work, I could get that under control as there's effectively 3 undamped transformer secondaries in there but at what cost?

[Ray P]

Impressive!

[Mike H]

Yes indeed.

Square waves look as expected, you might be able to reduce the ringing with Zobel networks connected across the anodes pair, but that might just open another rabbit hole ...

[izzy wizzy]

Yet to check if the ringing is reflected in a rising response. It did in the line stage and correcting that produced a good result. I'd have to improve ease of access first.

[slowmotion]

Looks very nice, I think!