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Old 13th August 2011, 01:06 PM   #121
regal is offline regal  United States
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Quote:
Originally Posted by mogliaa View Post
I ended up with three chassis as I ran out of space on the HT chassis :-)
HT chassis has also the bias supply and 8 core cable to the main chassis.
LT supply chassis is housing two e-core trannies for the 45s and one toroidal trannie I had at hand for the driver filament supply. On this chassis I have three raw supply boards and another multipole connector with 4 core cable from this chassis to the main one.


Cheers,
Ale
Do you run a separate ground for each channel back to the HT chassis ? If you have the time I'd be interested in your grounding scheme, I'm having issues with just a separate HV transformer/HVSS for each channel being in different chassis's.
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Old 13th August 2011, 02:45 PM   #122
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Hi!

Quote:
Originally Posted by regal View Post
I'm having issues with just a separate HV transformer/HVSS for each channel being in different chassis's.
Seems you have a ground loop. Care needs to be taken with grounds when you have a separate PSU. All grounds (if you have separate transformers) should come together at a single point, preferably in the amp chassis.

Thomas
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Old 14th August 2011, 05:19 PM   #123
mogliaa is offline mogliaa  United Kingdom
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Hi!



Seems you have a ground loop. Care needs to be taken with grounds when you have a separate PSU. All grounds (if you have separate transformers) should come together at a single point, preferably in the amp chassis.

Thomas
Indeed. I have one single point in the main chassis where all grounds are connected. This point is the only contact with the chassis to avoid ground loops. Also is the input of the driver input potentiometer. The line input signal is wired from the back connectors with shielded cable to this point as well.

BTW: I tested the amplifier last night with the fostex speakers. Very surprised and pleased with the sound, fantastic bass response which evidently my EL34 PP was lacking off. Tone is clear and rounded. For my current setup the nearly 2W this amplifier can deliver is more than enough! I was worried about the power required, but this really proves that 2W can be quite a lot!

Well, this glory moment had to end with a bit of smoke! The darned multipole connector is making false contact - I knew this, but I still went ahead with the test despite the warning and risk. Obviously the bias supply connection failed and the poor 45s were at the mercy of full current. Luckily my RC filter (1k2 + 47uF) to drop HT voltage from 350V to 300V felt the pain first. The resistors burnt and I managed to switch off the amplifier....

Lesson learnt: will add a fuse to the cathode return for the 45s and also will change these multipole connectors!


Cheers,
Ale
Attached Images
File Type: jpg 45 SET first test.jpg (78.2 KB, 338 views)
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Old 14th August 2011, 05:32 PM   #124
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Hi Ale,

first congrats for the amp! Glad that you like the sound! many people are surprised how far 2W can go when they hear it the first time!

Sorry about the incident with the bias failure. If you add a fuse to the cathode circuit, it is a good idea to put a resistor in parallel, say a few kOhms. This way the current will not be totally interrupted when the fuse blows which can cause a nasty voltage spike in the output transformer, when the inductance tries to discharge. With the resistor the tube will go in kind of a cathode bias mode. 4,7k would be ok.

It is worth trying cathode bias, with the ultrapath scheme I prefer that to fixed bias. Makes the amp simpler, avoids such problems and sounds just as good IMHO

Best regards


Thomas
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Old 14th August 2011, 05:37 PM   #125
mogliaa is offline mogliaa  United Kingdom
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Hi Thomas,
Many thanks to you, as you helped me quite a lot along the way. I'm absolutely surprised with the sound and happy to see that 2W is more than enough for me!
I will do as you said. I still need to finish the bottom part of the chassis and feet.

I may try cathode bias. What is the ultrapath scheme?

I also will try the 6N7 driver once I build my 26 pre-amp which is my next project!

Thanks again,
Ale
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Old 14th August 2011, 05:45 PM   #126
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Hi!

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Originally Posted by mogliaa View Post
I may try cathode bias. What is the ultrapath scheme?
That is a capacitor from B+ side of the transformer to cathode, this effectively bypasses the cathoderesistor and it's bypass cap. In many cases the bypass cap can be left out with ultrpath.

Check the single ended amplifier concept series of posts on my blog for schematics, for example this one:

VinylSavor: Single Ended Amplifier Concept, Part 7

Best regards

Thomas
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Old 15th August 2011, 02:18 AM   #127
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Moglia, if you put the star ground in the amp chassis, does this mean the B+ prefiler grounds are wired up to the amp chassis and not connected to their chassis?
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Old 16th August 2011, 09:38 PM   #128
mogliaa is offline mogliaa  United Kingdom
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Moglia, if you put the star ground in the amp chassis, does this mean the B+ prefiler grounds are wired up to the amp chassis and not connected to their chassis?
Hi,
Yes. There's is no -B connection to chassis on the HT chassis, this is wired to the main chassis star connection. Filaments are floating and only the 45 heater + is grounded in my case as I have fixed bias. see Rod Coleman's instructions for more details on this.

Cheers,

Ale
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Old 3rd September 2011, 01:53 PM   #129
mogliaa is offline mogliaa  United Kingdom
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Default At last, it's alive!

Hi everyone,
Finally and after a long 9 month period of weekend work, my 45 SET amplifier has been completed. It sounds brilliant, albeit it has a bit of too much gain when I use my turntable with a preamp.

Thanks everyone who helped me on this journey (Kevin Maier, Thomas, Rod Coleman, Andy Evans amongst others). I managed to learn a lot and still have much more to learn and experience.

This is just fascinating, what else can I say....nothing better than turning on the amplifier and playing one of your favourite LPs. (I did play five years from David Bowie).

A special thank will go to my dad who inspired me all the way and helped me with some building tips and coincidentally yesterday was exactly 3 months since he passed away. I know that he was enjoying this moment with me yesterday when I put the record on and pour a proper glass of single malt to celebrate.... :-)


Here are some additional pictures:
Hi-Fi photos

I will probably change the driver stage to lower the gain with a CCS loaded 6N7, 6J5 or paralleled 6SN7GT. I will do some distortion measures first at full drive voltage.

Cheers,
Ale
Attached Images
File Type: jpg 45 Set final test.jpg (64.4 KB, 240 views)
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Old 3rd September 2011, 05:04 PM   #130
mogliaa is offline mogliaa  United Kingdom
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Well, still trying to get the best out of the current driver with the paralleled 6SL7 triodes and CCS load. Today I tried the following configurations in order to minimise the stage distortion at maximum drive voltage i.e. 100Vpp. I restricted my test to triodes I had at hand, octal base and 6.3V heaters as I'm not willing to start changing my amp too much. These were the best results so far:

VALVE
6J5GT Brimar @ Ia=15mA/Vg=-3.8V THD=0.46%
6SN7GT Brimar blackened @ Ia=20.5mA/Vg=-7.6V THD=0.32%

Then I tested 16 valves 6SL7/6N9S and achieved the lowest distortion at the following operating point:
Ia=4.5-5mA / Vg=-1.8V, Va=251V

6SL7GT/VT-229 Sylvania THD=0.37%
6SL7GT Brimar (black base) THD=0.42%
6N9S Russian THD=0.43%

The above distortion level of around 0.40% is at full drive of the 6SL7 to achieve the voltage swing I need on my 45 output stage. Distortion is significantly lower at lower signal levels of course. However, in order to optimise the stage, I measured all at the same output level so had to adjust input signal as valves had different mu.

Clearly going for a pair of blackened 6SN7GT will be the best option, but my current CCS board doesn't allow me to include a heatsink for the top mosfet. May need to change the board. But, for a minimum difference I will select a nice pair of 6SL7/6N9S and bias them at 251V anode voltage and Vg=-1.8V using the RED LED bypassed with a russian 100nF PIO capacitor that I have at hand...

Learnt the lesson, if I compare these results with the distortion figures of the CCS loaded 4P1L, CX112, 30 and specially 26 valves that I did earlier, next time I may go for two stages of DHT driver (or one being the preamp) instead of the current setup....

Cheers,
Ale
Attached Images
File Type: jpg 6SL7 DRIVER.jpg (104.5 KB, 233 views)
File Type: png 6sl7 bias point2.png (11.3 KB, 223 views)
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