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Old 1st October 2002, 10:55 PM   #1
arnach is offline arnach  United States
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Default Check my first design, 6900/2A3 IT

Hello,

I'm looking to build the ultimate 2A3 amplifier for my Lowther PM6As in medallion cabinets. I've selected the 6900 version of the 417A triode as the input (due to it's high plate voltage) and an interstage coupled design. This is my first time drawing a tube schematic so the resistor and capacitor values are basically made up, please help me with what will work! I know how to build the PS, I will be using a 5AR4 regulator and AC filaments.

Let me know what you think.
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Old 2nd October 2002, 01:17 PM   #2
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Certainly looks good, could be an absolute killer judging the parts you'll be using. Proper PS needed of course.

One small point: with the high-gm tube such as the 6900, a grid resistor should not be omitted. I'd opt for 1kOhm, mounted directly to the tube socket. The 2A3, when not driven into A2, might like one as well (100 ohms or so).

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Remco
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Old 2nd October 2002, 02:51 PM   #3
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Excuse the shortness of my reply, but, I only stopped in for a moment whilst waiting for a download and saw this post.
1: there is no such thing as an ultimate amplifier. I am constantly amazed by the variation in how different designs sound when playing into a real reactive load. But it and tweak the sound until you love it.
2: 2A3: B+ is a bit high. About 300V on the plate, say 310 at the top of the transformer. Aiming for 250V plate to cathode, -45V bias, Rk =750R and Ik = 60mA. This is a well known and used design point and apparently sounds excellent. Whether it is the best for you or not I dunno, but it will be close. Tweak from there.
3: If you adopt the operating point I mentioned, the Tribute transformer will be a fraction short on current capacity. With 60mA as a centre, you really need double that for full power. Check the specs on the transformer. Some are underrated a little deilberately yo allow some adjustment in bias. 2k5 Zp will be fine
4: 6900 driver. I know nothing about it except that it was designed for pulse applications in weapons etc. Supposed to be good, but the data I have <a href="http://frank.nostalgiaair.org/sheets/131/6/6900.pdf">from here</a> is sketchy on audio applications. From a quick look at the curves I'm WAGing Vp=180V, Ik=20mA, Vg=-8V, Rk=400R. Should swing 150Vp-p, about 3 x what you need. Good, linear and lots of headroom. You need to decide on a bias for the 6900 so you can have the IT gapped correctly.
5: PSU: Full wave tube rectified, LC -> OPT -> RC -> 6900. I'd use a good shunt regulator too. Try Lundahl LL1673/20H PSU chokes.
6: Driver filament: AC, use CT transformer and bias filaments to 40V above ground.
7: Grid Rs: As ultranalog said, grid R of 100-1000R on grid of 6900, soldered with almost no lead, none if possible, right at the socket. Low power metal film or carbon comp. 2A3 shouldn't need one with an IT.

Gotta run
Ciao
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Old 2nd October 2002, 02:58 PM   #4
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Default Forgot.

- Cathode bypass on 2A3: I like em big, say 100u PP, no crappy electros.
- Cathode bypass on 6900. Get rid of it. they sound awful.
- PSU caps should be PP or PP/oil
- 6900's are really expensive and a bit hard to get. There are alternatives like ECC99, 6H30, 6c45 (if you really need the gain), 7119, 7044 etc. If you have them, great use em.
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Old 2nd October 2002, 05:57 PM   #5
arnach is offline arnach  United States
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Default Ok

Ok, thanks so far for the help!

I'm going to go back and work in the new values for the circuit and draw up the PS.

Anyway, should I use an input transformer instead of any resistors? Or should I go ahead and add the extra resistor?

Thanks,

Aaron
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Old 3rd October 2002, 12:09 AM   #6
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Default ARNACH

No need for yet another input Xformer.Just solder the series input resistor close to the socket pins,not much point having it there otherwise.

Happy soldering,
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Old 3rd October 2002, 07:28 PM   #7
arnach is offline arnach  United States
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Default choke/cap input

Which do you guys prefer sonically, choke or cap input?

I see that Sakuma, Rankin, Kondo all use cap input, but I have read choke input offers better (have also read more analytical) sonics.

Thanks,

Aaron
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Old 3rd October 2002, 07:43 PM   #8
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Default CHOKED

Aaron,

I could be half asleep.
Can you elaborate a bit.
Are talking about input coupling at the input cable end or about power supply filtering?

Rgds,
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Old 3rd October 2002, 08:34 PM   #9
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Default Re: choke/cap input

Quote:
Which do you guys prefer sonically, choke or cap input?
Choke input. Definitely. Better regulation, less peak current draws from the rectifier (read: less HF from rectifier). Smaller caps needed. PSU will be cleaner; the amp will sound more open and relaxed.

Regards,
Remco
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Old 3rd October 2002, 09:03 PM   #10
arnach is offline arnach  United States
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fdegrove, PSU filtering. what do you think?
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