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F6 Illustrated Build Guide
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Old 7th March 2020, 06:11 PM   #1711
6L6 is offline 6L6  United States
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F6 Illustrated Build Guide
Must be X2 rated for across the line. - ECQ-U2A332ML Panasonic | Mouser
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Old 7th March 2020, 06:19 PM   #1712
avdesignguru is offline avdesignguru  United States
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Quote:
Originally Posted by donhughes111 View Post
That little blue disc capacitor between connector port 2 and 3 on the build guide pics, that is described on the first watt power supply diagram as a .0033 line cap.
Would that not actually be .0033uF? As in a ceramic safety cap. A big difference but I don't like to assume anything at my newbie level. Thanks,
Don
A ceramic safety cap is good:
https://www.mouser.com/ProductDetail...uxF5C76Iqaw%3D

I believe an X2 rated film-type safety cap is also OK if you want to save 50 cents. Something like this (for 120V mains):
https://www.mouser.com/ProductDetail/667-ECQ-U2A332KL
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Old 7th March 2020, 07:08 PM   #1713
donhughes111 is offline donhughes111
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thank you both. big help
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Old 8th March 2020, 08:12 PM   #1714
LATB is offline LATB  United States
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Default re-using power supply at 30V

Hello all -- I'm planning to finally start my F6 build. I have the parts (except the MOS FETs which are backordered at Mouser and Digikey!).

But I do have questions regarding the power supply that I already have but which has 30V rails instead of the standard 25V (something similar came up in this thread before, but I don't quite get what the final answer is).

I was planning to re-use my existing massive power supply from an old Gryphon Tabu integrated (see picture). There are 2 monster C-core transformers, and I will throw out all the pre-amp parts to get sufficient space to make it a nice F6 build.

The original Gryphon schematic says 2x50V rails (dual mono), and my idea was to put the Tx primaries in series to halve the VAC, should make about 25V DC, perfect for my F6 build? That was the idea, anyway.

Well, it turns out that the secondary is more like 22VAC and the resulting rails are at ~30V.

I have 2 questions:

1) is it safe, long-term, to run these transformers at 120V in the 230V configuration, in order to half the output VAC, or will I get into problems? Does it just mean I can't get to the full VA spec? I don't know what these guys are capable of delivering anyway, but the old amp was spec'ed at Class AB, 2x100W RMS at 8 Ohm, 200W at 4 Ohm, and 300W at 2Ohm -- shouldn't I be just fine using these for a 25Watt class A?

Or am I missing something? Are there any other issues with running transformers at half the expected primary voltage?

2) Is there a problem with running the F6 rails at 30V, as long as I keep the bias at 25W, the same value as for 25V? I made sure the Caps can stand 30V, are there any other concerns?

There were discussions on this before. But the answers were mostly about changing the MOS FETs to something bigger, better (but the IRFP240 are spec'ed at 200V anyway, so what's the problem?), and about evenutal cooling issues when you go to higher power -- but I'll be very happy with 25 Watt into 8 Ohm, that will be more then enough for my Zu speakers.

Shouldn't I be able to run at 30V and 25Watt just fine?

Any reply, suggestion, insight is much welcome before I just go ahead and try it out... :-)

Thanks, LatB
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Old 8th March 2020, 11:03 PM   #1715
Dennis Hui is online now Dennis Hui  Canada
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F6 Illustrated Build Guide
Please note that 30V rails means running the jfets at higher voltage than their rating.
Mr. Pass has indicated that the Toshiba jfets are conservatively rated so that voltage
should be ok. (See F5 turbo article, for example.) However, dissipation could be an
issue so you should use low-ish Idss jfets (say 7mA) and also
consider putting small TO92 heatsinks on them.
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Old 10th March 2020, 08:39 PM   #1716
LATB is offline LATB  United States
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Quote:
Originally Posted by Dennis Hui View Post
Please note that 30V rails means running the jfets at higher voltage than their rating.
Mr. Pass has indicated that the Toshiba jfets are conservatively rated so that voltage
should be ok. (See F5 turbo article, for example.) However, dissipation could be an
issue so you should use low-ish Idss jfets (say 7mA) and also
consider putting small TO92 heatsinks on them.
Thank you, I had not thought about that! But I'm assuming I'll be fine on the MOSFET side of things?

I listened to Nelson Pass' Burning Amp 2012 talk again. Maybe I should consider to put in degeneration resistors, as he proposes to address the power problem.

However, in his talk he had 10R degeneration resistors (also the M2X Ichikawa input stage has 10R resistors). So I wonder how this makes any difference?

For an Idss of, say, 7mA, which for rails of 30V corresponds to a resistance across the JFET of ~4.3 KOhm -- how can a 10R resistor in series make any difference?

So I don't quite understand Mr. Pass' statement that introducing these resistors would help with bringing the power dissipation down for the JFETs -- what am I missing?

Given that the LSJ74 has max VGDS of only 25V (40V for the LSK170, so that's good) I am considering the Cap multiplier that @2 picoDumbs proposed earlier.

Before I go there, with the complication of having to sink another 5 Watt per channel for the additional MOSFET, I want to be sure I can actually use these transformers in their 230V configuration at only 120V -- will this be a problem?

Thanks again for any help and advice with this!
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Old 10th March 2020, 11:12 PM   #1717
Bare is offline Bare  Canada
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There is always Ettore Bugatti's famous quote / response to an owner who was complaining about his particular Bugatti :
Sell it on to someone who can afford it.
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Old 26th March 2020, 12:31 PM   #1718
jphabc54 is offline jphabc54  United States
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Hot swapped my F5 for an F6. Biased up no sweat (3.3K/6.8v zener). Unlike my F5 build, this one is dead silent. Next up is the M2X.
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Old 26th March 2020, 01:03 PM   #1719
PJN is offline PJN  United States
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Interested in your opinion concerning the F6 vs the F5.
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Old 26th March 2020, 01:15 PM   #1720
6L6 is offline 6L6  United States
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F6 Illustrated Build Guide
LATB, running the transformers with the primaries in series will be completely fine. It’s designed for both series and parallel. The transformer only knows ratios, and is a perfectly good way to use them.
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