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Old 13th October 2011, 01:46 AM   #1
mpc is offline mpc
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Default 3 years in the process of building Simple SE..

Hello!

As the title says I am in a process of building simple SE for roughly 3 years now. It is time to wrap this up and finally to finish it. So I need some help/advice to do so.

So at the moment I have GXSE15 5k:8 output transformers (I also have CXSE 3.2K:8 but I won't be using them in this build). My power transformer doesn't have center tap, so I will be limited to full solid state rectifier. Power traffo is 180 VA with an output of 205 and 115 V windings @ 300 mA and 6V @ 5 A. So the B+ will be above 400 V. I have all the small parts and also I have acquired some 500V caps - I have a large can of 470 uF BHC, 390 uF Vishay, 100 uF Epcos. 10M45 is impossible to get in Europe so I have a replacement DN2540s. Also I am not sure how to select proper capacitors for C10, C12 - As I understand those are pretty essential - low ESR and what not is required?

So the first big question is regarding capacitor selection - Is it a good idea to hook up the large can (470) or is it sufficient to try out the circuit with a lot smaller 390 uF Vishay?
The second question is regarding B+ delay circuitry - is it worth implementing it and which particular solutions do you prefer and would recommend?

What about the choke - will I be able to live without it (CRC) ? If the project will be successful I might order something from Edcor again (for other projects)-so I might get the choke eventually...

Next thing is the heater supply. Is it worth making a regulated DC supply?

Coming up - what about the capacitor discharge circuitry? Is it essential to implement one - to discharge high voltage caps upon power-off? If yes - how would you prefer to accomplish this?

As I don't have the valves yet - what would you suggest? Well I do have NOS 6П3С from 68 but they won't be any good above 350V. So what would be worth trying? 6L6GC, EL34, KT66, KT77, KT88 ? Which particular brand is worth considering? I will probably order on German ebay - so the shop I eyeballed stocks Electro Harmonix, Svetlana, Sovtek, Tung-Sol, Gold Lion, JJ and Shuguang. I will ask this: In your opinion which is the best for the buck? Please remember that this is my first build and it is more like an experiment for me - Like Is it even possible to get some sound out of what I will come up with.

Yup - so these are the questions which are holding me back for now.
If you have any comments, suggestions, experience you want to share - please do so.

Thanks!
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Old 13th October 2011, 04:22 PM   #2
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Hi,
I have built two Tubelab SSEs so far. Both run with tube rectifier, so I can't comment on your B+ delay circuit question, but if you search on this forum, you will find more than one answer...

Choke and motor run cap are highly recommended upgrades, not only by George, but many others. I didn't even try without it, but both amps are DEAD quiet (I can't hear anything even with my ear right on the speaker).

All capacitors that influence sound are called out on the tubelab.com assembly page for the SSE; I would follow the recommendations made there. I used the suggested parts and for the coupling capacitors, I chose AuriCaps. There are other good options; you'll find them on the forum using search.

One of my SSEs runs JJ EL34, the other one KT88EH. The EL34's are fantastic for jazz & vocals, the KT88s have a bit more punch in the bottom end without compromising clarity noticeably. I personally favor the KT88 right now, because most of my listening happens with more 'dynamic' music.

In general, my best advice (these were my first two tube amp builds) is to build according to George's manual and get it to work, even without choke and motor run cap in the PS. Then you can start tweaking and hear the differences. Maybe the basic build is perfect for you already. Oh, and a bleeder resistor is already on the board to drain the caps. See the circuit diagram/description.

Hope this helps.
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Old 13th October 2011, 06:59 PM   #3
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mpc: If you are using George's PCB, the non-center-tapped power transformer will be a problem unless you are planning on doing an entire power supply off of the PCB.

Assuming you are doing a P-2-P build, are you planning on connecting both power transformer secondaries in series to achieve your target B+? Are both windings rated for 300ma?

To assure that your HV caps discharge upon power-down, use a 2W or 3W 220K to 470K resistor across the power supply caps. The caps should bleed down in 10-20 seconds or so.

In the schematic R2, R3 and R4 take care of that (R3 and R4 also serve to elevate the heater voltage as well). Another tip from George, if you are using a CLC supply and it's easy to disconnect the choke while tinkering, then make sure you have a bleeder R across each of the C's (the schematic shows this; R2 across C1 and R3 & R4 across C2).
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Old 14th October 2011, 12:14 AM   #4
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No I am not using the original PCB. I am already halfway finished with the chassis - (which BTW was why I left this project untended for this long). So the holes I have drilled wouldn't match the PCB.

I think I will be making my own PCB that suits the chassis. Maybe point to point - not 100% at this point.

Yes - both windings are 300mA and I will connect them in series.
About this discharge thing... I was thinking of using a relay to connect the resistor on the power-down (I somewhere read that people are using such an approach)... But maybe it should be left as it is.
Also I am not fully getting the potential divider thing R4/R3.. What is the catch? I have seen various designs - some leave the heaters hanging, some connect to ground some to some portion of B+...

But still - I need some info on tubes. At least from the listed brands which would be worth considering and which should be definitely avoided?
Personally I would like to go with KT88s but they are quite expensive. So I am left with 6L6GC, KT66, maybe KT77?

Thank you!
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Old 14th October 2011, 12:20 AM   #5
Pano is offline Pano  United States
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A matched pair of KT88 can be bought new in the USA for about $80. Are they much more in Latvia?
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Old 14th October 2011, 07:58 AM   #6
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Not really... The cheapest EH or JJ kt88s can be ordered and shipped from Germany for around $110. But the 6L6GCs go for approximately $50.
Considering that this is a first time build-I feel the large KTs isn't the right match. On a next build (As I wrote I have a second pair of CXSE-8-3.2K) I would definitely try KT88s. But in this build I consider 88s a bit too pricey.
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Old 14th October 2011, 05:30 PM   #7
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Default maybe it should be left as it is..........

Quote:
Originally Posted by mpc View Post

About this discharge thing... I was thinking of using a relay to connect the resistor on the power-down (I somewhere read that people are using such an approach)... But maybe it should be left as it is.

Also I am not fully getting the potential divider thing R4/R3.. What is the catch? I have seen various designs - some leave the heaters hanging, some connect to ground some to some portion of B+...

Thank you!
If it were me, I would leave it as is, as adding a relay doesn't do much other than add complexity and reduce safety. If that relay fails to close, the caps remain charged, and a tube amp hobby is not worth risking your life over. Note that the resistor values are large (100K+), so they will draw a mouse-farts worth of current when the amp is operating. ...do the math, ohm's law is your friend here.....

Elevating the filaments reverse biases the voltage a bit of the pseudo-diode formed by the cathode and the heater, which can reduce noise. In this application, the output tubes are cathode biased, so the cathode voltages are elevated by the cathode R's, and you want to get the filament voltage above this. Again, do the math, pick a typical cathode R and output tube bias current and use Ohm's law to determine what the voltages are.....or just analyze the schematic as it is to see what the voltages are.
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