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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 26 Mar 2019, 11:37 
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Joined: 12 Jan 2018, 21:17
Posts: 10
Location: Dublin. Ireland
Thanks Bruce
Thats what I will do.

Steven


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PostPosted: 18 Oct 2019, 06:43 
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Joined: 31 Jan 2015, 17:59
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What is the best value for cathode resistors with ecc81? I have a lot of them.
Thanks


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PostPosted: 18 Oct 2019, 15:26 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, At the given voltages (180-225 B+) IMO there is no best value. There are several issues with ECC81s in this circuit. First the gain is way too much (instead of 7 it is 25). Too much for a line stage. Second the output impedance will be high....about 6.5K. It would be OK for a tube amp input of say 50K or more but might be unsuitable for a lot of solid state gear. All that said if you wish the best values I can determine are for both cathode resistors to be 100 ohms. This gives an idle current of 5.2 ma at 220volts. Output voltage into 50K is about +47/-31. Since you really need only about 1-3 volts it ought to be OK. The non-symmetrical range is likely to cause some harmonic distortion, but I have no way to tell short of building it. My guess would be that 1-3 volts output it should be in the sub 1% range. You can not bypass the lower cathode as the symmetry really gets bad there. I didn't check for higher B+ but it would seem to put the tube in a better range. Let me know what you have available and I re check it.

Good listening
Bruce

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PostPosted: 19 Oct 2019, 15:44 
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Joined: 31 Jan 2015, 17:59
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Many thanks.
:up:


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PostPosted: 22 Nov 2019, 08:51 
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Joined: 08 Jul 2019, 06:05
Posts: 2
Location: Ottawa, ON, Canada
Good Morning Everyone!

I would like to introduce myself and my new Forewatt build! First off a big thank you to Bruce for sharing this design work and all his knowledge, and to all who have posted here. This is an amazing thread. I read all of these pages prior to venturing into my this project. This is my first attempt at building an amp and I would never have tried this without access to the abundant info in these pages.

Forgive me for not posting until now that my project is complete (or almost). I have a hectic work life and I often need to put my hobby endeavors on hold. Any personal job I start is at some risk of not seeing completion. I am finally at the point where I have results to share.

I needed a tube pre-amplifier and decided to build this one because I wanted high performance, several inputs, and remote control - all at an affordable price. To satisfy those criteria on the new or used audio market was obviously not possible. Thankfully I am an incurable DIYer.

In order to keep costs down, my goal was to minimize shipping. Living in Canada, the imperative of sourcing most material from international sources it was important to limit the number of suppliers. I wanted the Edcor transformer but the shipping would have been more than the transformer itself. Another problem was the chassis which seem to be very expensive in Canada and the US. To solve these problems I was able to source an affordable chassis (heavy aluminum construction), the transformer, and the MV-02 remote controller with input board and ALPs 100k pot all from one Chinese supplier.

The reason this was possible was that this one ebayer from China offered custom winding of the transformer in their fine print. I inquired, and it turned out this customization was actually no charge. The transformer cost the same as the Edcor. This is an E-I design with a screen and copper cladding. Windings are 200V/.050A and 9V/1.5A secondaries with 115V/50Hz primary. The day I unpacked it, the enamel was so fresh it reeked of paint solvent. It took weeks to dry out. It was obviously made to order. No worries, the build took me about 6 months. I am also very satisfied with the chassis. Nice anodized finish, IEC connector with fuse, big rubber feet, front face machined from one billet with brushed finish and actually 8 mm thick. This case was a little large for my purposes but about the half the price, and better looking than all of what I could find locally (or in the US).

As for the MV02 control well you would think by now they would have translated some intelligible English instructions. Not a chance. It was a considerable job to figure out how the thing works but it does seem to be of acceptable quality. My only disappointment was to discover the front of the two rotary controls are continuous with the board signal ground. Wonderful! With all that effort to follow Bruce’s grounding guidelines – at the very end of my build I discover two unavoidable ground loops to the chassis - virtually impossible to avoid! I considered painting the front face of these controls with epoxy and using plastic washers to isolate them. Unfortunately I was in the home stretch when I made this discovery and just wanted to get the unit powered up! I removed the 6th relay and used all 5 of the inputs.

For output coupling caps, the Russian PIOs are plentiful and cheap. The most affordable at the time I was looking were 0.68 uF/400V/10%. These monsters were inexpensive and look as though they could survive a nuclear bomb attack. I used the low-end Solen metallized polyethylene caps and Panisonic electrolytics in the locations per the schematics. The Vishay 0.1 uF metallized polys were unfortunately all that was available in that size at the time I placed my Digikey order (I always use Digikey instead of Mouser simply for low shipping charges). For RCA connectors, I chose the most expensive ones I could afford. All the wire is cheap stuff except for the signal carrying ones where I used Teflon insulated shielded wire. The heater portion of the power supply is pretty heavy gauge.

For tubes I really wanted the JJ ECC802S, but with the exchange and the US dollar these were quite an expensive option (will need to save up for a pair later). So I settled on some used RCA cleartops 12AU7A long-plate. These were sold as ‘tested strong’ from an ebay seller who specializing in used tubes and with a high feedback score. I am not sure about this, but it seems the only difference between the ECC802S and the ECC82 is the longer plate. So this is a guess but I think the 12AU7A is basically an ECC802S. Anyway I have read in several places that audio enthusiasts get good results with these cleartops ( I paid about $20 US for the pair).

The results? I had a few hurdles! First I had the LM317 hooked up backwards and could not get proper voltage to the heaters. While trying to fix that, I blew one of the LR8s. I finally tested the project last weekend with 215V on the B+ (left channel) and about 250V (unregulated right channel). There was a significant hum for sure – quite loud and objectionable :bawling: . However I could tell immediately by turning up the volume and drowning out the hum that the sound is superb. Much, much better sounding than my old SS preamp. I am absolutely delighted with the sound.

I have replaced the blown LR8 and now both channels are adjusted to B+ at 215.6V (I used the 20 turn trim pots). The sound is even better now. But the hum persists. I have tried all sorts of things to see what could be causing this. I need to get off to work now so I will post a few pics of the build and just describe my observations about the hum...

• The hum is independent of volume setting.

• The hum is independent channel selected.

• If I power on the preamp with all caps discharged signal present, and power amp on, it takes about 7 seconds for the hum to appear – during this time there is no noise that I can hear whatsoever. The music is just beautiful.

• If I power on the preamp warm with caps charged it takes about 3-5 seconds for the hum tro appear.

• The hum appears prior to the MV02 completing its power-on sequence, and seems unrelated.

• I have tried removing the font face and laying the MVO2 controls and board loosely on an insulated board (no ground loo), no improvement whatsoever.

• I have tried the unit in two different systems (two different power outlets), no improvement whatsoever.

• I have tried removing the screen from the chassis ground, no improvement whatsoever.


If anyone has any suggestions, I would be most grateful for any pointers!

Thanks in advance!

David
Ottawa, ON
Canada

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PostPosted: 23 Nov 2019, 14:09 
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Joined: 08 Jul 2019, 06:05
Posts: 2
Location: Ottawa, ON, Canada
Update - hum has been erradicated. I had the grounds to the lower triode wrong. I wired them them directly to the star ground. Problem solved.

This thing sounds fantastic.

Best,

D


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PostPosted: 24 Nov 2019, 21:18 
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Joined: 04 Jun 2008, 20:59
Posts: 4052
Location: Arizona, USA
Great, I find that relatively simple things can cause a lot of problems. Grounds are one of the most common.

Good listening
Bruce

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