6C33C SE-OTL Project

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IslandPink
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#46 Re: 6C33C SE-OTL Project

Post by IslandPink »

Cool !
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Ray P
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#47 Re: 6C33C SE-OTL Project

Post by Ray P »

I've got a cunning plan for dealing with the heat from the 6C33Cs and have ordered one of the smaller fans so I can adapt the chassis metalwork to include it should it be needed.
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ratbagp
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#48 Re: 6C33C SE-OTL Project

Post by ratbagp »

Greetings from the USA where it is really hot and humid. 95F.

In my 6C33C amp I raised the sockets above the top by about 2/3 inch and that seems to have solved the cooling problem for me. The amp has had a lot of use over a few years. And yes, since it is a resistor loaded parafeed design it really creates some heat.

Look at the 3rd photo down.

http://audioratbag.blogspot.com/2015/01 ... afeed.html

By the way, since I have virtually all the parts needed, I am having a go at this OTL amp as well using the original schematic.

ray
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Ray P
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#49 Re: 6C33C SE-OTL Project

Post by Ray P »

Hi Ray, good to hear from you again and thanks for the link to your cooling (and cat vomit) solution.

We'll be interested in your findings/opinions from your 6C33C SE-OTL project in due course.

Ray
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Ray P
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#50 Re: 6C33C SE-OTL Project

Post by Ray P »

I managed to pick up a used pair of Johnson brand Septar sockets for the 6C33Cs at a reasonable price. They were made in the USA probably more than 50years ago but in spite of their age and the fact that they've been used the contacts for the valve pins are loads better than those of the current manufacture Chinese sockets I have.

They're big, that's a one pound coin next to them for scale.

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andrew Ivimey
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#51 Re: 6C33C SE-OTL Project

Post by andrew Ivimey »

Delicious!
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ratbagp
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#52 Re: 6C33C SE-OTL Project

Post by ratbagp »

Well I fired up my version of the amp this afternoon.

Image

Underneath view.

Image

I just used one of my standard cheap plywood boxes with an aluminium plate. Essentially it is a breadboard with sides and I reused parts that I already had such as the Russian OMBG-2 1 uf caps as well as transformers, a choke and motor run caps for the driver power supply.

I made two significant changes from the schematic. The first is I used 48 vdc 650 ma smps units as I did for my GK71 amp. There are three per channel for a high voltage of 144. You have to make sure not to connect the grounds together and then you daisy chain the outputs. Across each unit you put a 4.7 K 2 watt resistor.

https://www.allelectronics.com/item/ps- ... ply/1.html

I got the idea from this GM70 amp and I do this since I don't want to carry boat anchors any more.

https://greenvalve.wordpress.com/the-maxim-gm70/

I also used a 3 ohm resistor instead of 2.73 ohms reducing the current to 410 ma to compensate from the higher voltage of 144.

It certainly sounds good and very clean but 1 watt or less is not going to do the job with my Tannoy Red 15" speakers so I will think about doing a line array wired in series with some cheap speakers I have. But that will have to wait for a couple of months since I head off to Australia next week.

It was a fun build.

ray
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#53 Re: 6C33C SE-OTL Project

Post by Wolfgang »

I don’t want to sound like a party pooper….but this seems to be a different amp with a different sound.

The original idea was to match an amp design with a speaker impedance/sensitivity (15ohm Lowther DX/EX4) which included the specific CDF of the speaker. From there the amp was set/tuned to sound best.

Even with a CCS load line, setting the OP still plays an important role. Also the idle current /NFB/CDF are connected. If we change all this the amp will sound different, even more so with less sensitive drivers.

Also don’t know how suitable "motor run caps" (ESR?) are for the prestage? The original design also used 2 completely independent psus, one at 350V. A daisy chained wall wart smps HV source might deliver a stable voltage for a SE amp but I don’t know how the HF/hum would affect the noise spectrum in a CCS setup (no big anode resistor value that could filter at least a little of the ripple/noise from the smps).

All these things would be my concerns. Maybe somebody can give some satisfying answers.
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#54 Re: 6C33C SE-OTL Project

Post by ratbagp »

The 350 volts comes from the separate transformer, choke and two motor run caps. The 148 volts comes from the smps units.

I agree that my amp is a different design but I was interested in hearing what the topology and tube component sounded like. I had the parts but also a limited amount of time to put them together before I travel for a couple of months. I often do a preliminary build / breadboard to see if a future version would be worthwhile and in this case I will probably return to the project in the future but with more efficient speakers and a different power supply.

A Mofo is also under way.
http://www.diyaudio.com/forums/pass-lab ... -mofo.html

ray
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Ray P
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#55 Re: 6C33C SE-OTL Project

Post by Ray P »

Gradually collecting the parts for my 6C33C project. I've taken delivery of the PCBs for the power supplies and driver stage;

Image

Image

and the postie has also brought various bags of components. I have some time off next week so I should be able to start assembling the boards.

The build will be close to Wolfgang's with just one or two variations, primarily the use of a Neurochrome Maida regulator for the driver stage. Each 6C33C will have its own CLCLC supply. Power transformers will be in a separate chassis. I have a transformer for the B~ supplies on order from Terry at Canterbury Windings.

I've almost finished setting out the metalwork layout so will be able to get that ordered from Schaeffer (Front Panel Express) soon.
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Ray P
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#56 Re: 6C33C SE-OTL Project

Post by Ray P »

I've just finished the drawings for the chassis metalwork using Front Panel Designer. It's not cheap and that's before I get a quote to have it re-anodised after the machining!
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brommermartin
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#57 Re: 6C33C SE-OTL Project

Post by brommermartin »

I like this project very much :D I have the Oris 150 horns with 16 ohm AER drivers that must be a good match.
From earlier OTL projects ( Tim Mellow ) and the Trancended Mini Beast are a lot of parts at hand , as tubes and sockets and transformers.

Would you share your powersupply design :?:


Grtz....from the Netherlands.
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Ray P
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#58 Re: 6C33C SE-OTL Project

Post by Ray P »

Hello Grtz.

Power supply for the 6C33Cs for my build is lifted straight from my Transcendent 300B SE-OTL build, which itself is from Rozenblit's 'Tubes and Circuits' book. Anyway, it's a simple CLCLC arrangement. Here's my supply;

7200uF >> Triad C40X choke >> 2400uF >> Triad C40X choke >> 2400uF in parallel

https://www.mouser.co.uk/ProductDetail/553-C40X

On the PCBs for the power supplies I'm using paralleled 1200uF 200V capacitors.

For the 6SN7 stage I'm going to use a Neurochrome Maida regulator for the power supply.

Wolfgang, who build the prototype of the amp, posted his power supply schematics if I remember correctly.

Will you build the amplifier?
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brommermartin
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#59 Re: 6C33C SE-OTL Project

Post by brommermartin »

Hello Ray,

I am definately going to build this amp, maybe the 6C8G instead of the 6SN7 but listening would decide that.
For the driver stage I will use the SSHV2 regulator ( diyaudio )
I could not find powersupply details, but it also looks a bit like the Mini Beast power supply.

Probably I will start building around October. First have to collect some parts.

Keep you informed! :mrgreen:

Grtz... Martin.
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Ray P
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#60 Re: 6C33C SE-OTL Project

Post by Ray P »

Hi again Grtz.

I'm just about there with collecting the parts required for the build, apart from the custom wound transformer that won't be with me until October.

I'll order the chassis metalwork over the weekend and start assembling the PCBs in a day or two.

Keep in touch.
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