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SE 6L6 power amp, 12AT7 driver (design by Suncalc)
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5731
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Author:  Geek [ 12 Jan 2017, 04:53 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

You could do a HT boost, a-la-Bogen where they use the filament DC in series with the HT ground line.

Author:  mwhouston [ 12 Jan 2017, 05:58 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Geek wrote:
You could do a HT boost, a-la-Bogen where they use the filament DC in series with the HT ground line.

?

Author:  Geek [ 12 Jan 2017, 06:11 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Boosts it up 12V

*checks manuals*

Ah, not Bogen, Magnasonic :blush:

Author:  mwhouston [ 12 Jan 2017, 06:20 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Boy do I have some issues with this amp. When you do a deal with the devil there is a goood chance that in the end he will win.

I cant use my tube preamp (Im not going to tell you what happens when I do try but lets say it is good). I can't use anything with a mains connection. My media players (Pi3 with HiFiDac+) have to run from batteries. It appears the problem is all about the input.

I think I require a coupling capacitor for the input. There is a 100k on the input. I would liek a suggestion what what coupling cap would suti this?

Apart from that the sound of the amp through is dry. But I'm not using my best source but it is playing through the 99fb effcient concentric 15" Beymas. The sound is not bad it is just every different to waht I am use to.

Please some help with the input cap.

Some images:
Attachment:
P1123570 (800x600).jpg
Attachment:
P1123568 (800x600).jpg

Author:  mwhouston [ 12 Jan 2017, 06:31 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Geek wrote:
Boosts it up 12V

*checks manuals*

Ah, not Bogen, Magnasonic :blush:

Because the 6L6 has a cathode usually there is no problem with AC heaters.

This is what I should try to fix the "funny" issues with this amp;

1 - Earth the PS with the mains earth
2 - Try an input cap
3 - An isolation tranni (expensive and takes from the compact build)

What I'm adding is two speakers in the same wine bottle boxes, vertical and singe driver. Should all look good.

Author:  mwhouston [ 12 Jan 2017, 06:39 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Geek wrote:
Boosts it up 12V

*checks manuals*

Ah, not Bogen, Magnasonic :blush:


Not CT also.

Author:  mwhouston [ 12 Jan 2017, 08:44 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

I was looking for some good inter-stage caps. I wanted to use some Jensen coper, paper and oil caps with pure silver leads I had for years but wrong value. I did have two and two only Solen and this is what I used.

Author:  Suncalc [ 12 Jan 2017, 16:01 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

mwhouston wrote:
I used a 1A rec bridge - 47uf - 470ohms - 100uf - 150ohms - 470uf.
Ah... that will be an issue. Likely that your signal ground is held at one Vd (0.7v to 1.0v) above earth ground (or depending on how your 240 is derived up to 120v above earth ground). This path will also be very low impedance so it can sink LOTS of amps when shorted. This is why you can't use your mains powered equipment.

This is why all those old direct mains powered system only used a single diode in the line side of the feed. This arrangement guaranteed that the ground was at the same level as the neutral.

An input cap on the signal line is not going to fix your problem because the real problem is on the ground side. You need to isolate the mains from your ground connection.
mwhouston wrote:
The total current draw is 89mA and Vg is 20V on the 6L6s. These are well under what Matt thought they would be.
This part confuses me as well. For the low power version I had a 430Ω resistor in the 6L6 cathode and 330Ω on the high power version. For you to have a 20v bias voltage and 89mA of current, this would indicate a cathode resistor value of ≈225Ω. This is far below what I thought was in there.

For reference, here is the schematic I thought I sent to you.
Attachment:
6L6 UL Schematic (low power).jpg
Something more than your power problems seem to be afoot.

By the way... I really like the look of the amp.

Author:  Suncalc [ 12 Jan 2017, 16:06 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

And I just looked at the schematic. I made a mistake in the cathode bypass cap on the 6L6. This should be much bigger. More like 470µf not the 100µf I have in the schematic above. :blush:

With 100µf in there, the amp will be light on the bass by around 5dB.

Author:  mwhouston [ 12 Jan 2017, 17:59 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Ok the Ck I can fix. Also my Rk is 470ohms not 430. While thinking of the problems with the amp I realise I have put the 100k input resistor after the 1K blocking resistor and not before. This may explain why the amp goes into super sonic oscillation when there is no input. My 25year old daughter and my dog heard it I couldn't. But also it may explain why it hums when you touch the ground of the RCA.

I'll try and fix the above and see what happens. The mains earth is only connected to the filament tranni metal frame.

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