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Old 11th May 2004, 08:18 PM   #1
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Default Best PP transformer for 2A3 amp?

Primary impedance 5k? I ask because I have seen a couple of posts recently that usually higher impedance means lower distortion...but I have seen reports that sometimes lower than usual impedance can also produce lower distortion for certain tubes. (Thus here appears to be no rule of thumb). Hence my question.

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Old 11th May 2004, 08:53 PM   #2
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Hi,

Quote:
Primary impedance 5k?
That's pretty much the norm for a PP 2A3, 2K5 SE, 5K PP.

I use 6K6 which is a little high but it allows me to use only local feedback at the expense of less output power.

The result is a fast 2A3 PP amp with relatively low distortion and extended frequency response.
I quite like it that way...

Quote:
(Thus here appears to be no rule of thumb).
There should be one...

Cheers,
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Old 11th May 2004, 09:24 PM   #3
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Thanks Frank!
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Old 11th May 2004, 10:39 PM   #4
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Bas,

A possibility for your "iron" is MagneQuest's upgraded replacement for the Dyna ST-70. The ST-70 trafo has been used reliably with Sovtek 6B4Gs at power levels that destroy NOS and Chinese tubes. Except for basing and filament voltage, the Sovtek 2A3 is the same tube.
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Old 11th May 2004, 11:03 PM   #5
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Hallo Eli,

Thanks for the tip...I noticed they were on sale at Triode Electronics.

Sale price: $119.00 ....very tempting. That would have to be the 4300ohms jobby. Since the 6600 one is to be discontinued.

However transport for these babies is runs pretty high.

I might just ask AE-Europe or another winder/friend I have to wind me some nice c-core 5k pp's :-)

Regards,
Bas

PS...How do you rate the MQ Dyna stuff?
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Old 11th May 2004, 11:17 PM   #6
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Hi,

Quote:
I might just ask AE-Europe or another winder/friend I have to wind me some nice c-core 5k pp's :-)
I can already see Wil's face....What? Push-pull??

Is Tribute any good BTW?

Cheers,
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Old 11th May 2004, 11:53 PM   #7
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Quote:
I can already see Wil's face....What? Push-pull??
Ha..ha..you are right...Wil is not very fond of PP..

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Is Tribute any good BTW?
Apparently yes. But are more expensive than AE.

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Bas
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Old 12th May 2004, 03:13 AM   #8
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Quote:
PS...How do you rate the MQ Dyna stuff?
The OEM O/P "iron" in a ST-70 is DECENT. "Mikey" uses improved materials and builds to the original blueprints. You figure it out.

Transatlantic shipping costs are definitely a PITA. A possible plus to the MQ "iron" is the ability to build a "2 for 1 special" amp. Uncle Ned at TE has a way to set up an amp so that both Sovtek 6B4Gs and EL34s work, simply by changing the tubes in the unit. Being able to change from PP DHTs to PP UL IDH pentodes, when the mood strikes, could be fun.

Hum levels using the 6B4G with AC heating are less of an issue in a PP circuit than they are in a SE circuit, due to cancellation in the O/P trafo. If you build with dedicated floating windings for each 6B4G filament and use individual hum balance pots, the residual hum level should be pretty low.
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Old 13th May 2004, 03:42 AM   #9
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Quote:
Uncle Ned at TE has a way to set up an amp so that both Sovtek 6B4Gs and EL34s work, simply by changing the tubes in the unit. Being able to change from PP DHTs to PP UL IDH pentodes, when the mood strikes, could be fun.
I've done this and it works pretty good. Since the pinouts overlap correctly you can just plug and play mostly. Separate cathode bias (one for el34, one for 6b4g) is the easiest to use, but you can do fixed bias if it is adjustable over the proper range. 25W plate dissipation doesn't seem to be a problem for the sovteks either.

Quote:
Hum levels using the 6B4G with AC heating are less of an issue in a PP circuit than they are in a SE circuit, due to cancellation in the O/P trafo. If you build with dedicated floating windings for each 6B4G filament and use individual hum balance pots, the residual hum level should be pretty low.
Thats the best bet. I am using a shared winding for each pair w/ one balance pot per pair w/ wiper going to a bypassed Rk (hybrid bias). This makes zeroing the hum a bit more difficult because the null point is not identical for each tube. I adjust 4 fixed bias pots and 2 hum pots in a tedious attempt to balance idle current and minimize hum.
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