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Old 14th September 2011, 02:16 PM   #11001
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Quote:
Originally Posted by albertNL View Post
If I understand correctly, the F5X is better that using a stock F5 as a balanced monoblock (F5X is designed as a balanced monoblock; better than a bridging option for a single ended stereo amp), but my (lack of) knowledge of electronics means the discussions are beyond my comprehension. I would not feel comfortable starting on an F5X at all.

So I go back to my original question. As I am afraid that a stock F5 will probably not be enough to drive Magnepan MGIIIa's, is it an option to build 2 and use them as balanced/bridged monoblocks? Or is the design such that the "F5 magic" is lost when two channels are bridged?
Yes you're true, I have read that apart from giving a higher output swing (because of being bridged), it too parallel mosfets devices, increasing output current. That's bridge parallel.
Two bridged F5 just provide more voltage, not more current.
apart from that, joining their grounds in that special manner make it able to reduce distortion further.


The F5 "softstarts" as it is, I made mine with the stock PSU from Nelson's PDF and it starts up and shuts down WITHOUT ANY noise or pop-up, that's one of the best features of this amp in my opinion.
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Old 14th September 2011, 03:13 PM   #11002
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Hi all,
I have just finished a Pass F5 with Toshiba output transistors.

I know I am stupid, but I just turned on the power, without this bulb thing. Some friends will join me this evening for a HiFi meeting, so I were in a hurry, which is a bad thing.

I have borrowed another amp for this evening, but I have used a lot of time building this one, and I am very exited to have it play.

I think I had the pots maxed in the wrong direction, and R11 and 12 looks burned.

The power supply is giving out +/- 25,5 and it seems that it is not drawing any power, because nothing gets hot anymore.

Can anybody estimate what needs to be exchanged ?

Thanks in advance
Arthur.
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Old 14th September 2011, 05:45 PM   #11003
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Quote:
Originally Posted by Russellc View Post
They can be dangerous, but this thing doesnt make me near as nervous as the tube amps I've built, with B+ over over 600 volts in my 6550/KT88 PP amp!

Russellc
Russel...... Imagine what it's like to do a major revision to a large spacetrack radar, like we do....... that's 32 Megawatts of power. Our back-up generators are powered by two jet engines used on Boeing 727's..... I'm not so certain the old "work with one hand in your pocket" is adequate, around these power supplies....!
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Old 14th September 2011, 06:58 PM   #11004
6L6 is online now 6L6  United States
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It's advisable to take off your wristwatch and wedding band as well...

And possibly remove your fillings!
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Old 14th September 2011, 07:20 PM   #11005
anilva is offline anilva  India
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Quote:
Originally Posted by ArthurDK View Post
Hi all,
I have just finished a Pass F5 with Toshiba output transistors.

I know I am stupid, but I just turned on the power, without this bulb thing. Some friends will join me this evening for a HiFi meeting, so I were in a hurry, which is a bad thing.

I have borrowed another amp for this evening, but I have used a lot of time building this one, and I am very exited to have it play.

I think I had the pots maxed in the wrong direction, and R11 and 12 looks burned.

The power supply is giving out +/- 25,5 and it seems that it is not drawing any power, because nothing gets hot anymore.

Can anybody estimate what needs to be exchanged ?

Thanks in advance
Arthur.
Output transistors would have blown...I will start there with light bulb in place and pots replaced and in the right direction.

Cheers.
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Old 14th September 2011, 08:30 PM   #11006
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Quote:
Originally Posted by ArthurDK View Post
Hi all,
I know I am stupid, but I just turned on the power, without this bulb thing.
I think I had the pots maxed in the wrong direction, and R11 and 12 looks burned.

Arthur.
Arthur....... sounds as though your power supply is still good. Your voltage levels suggest that......

Something I'd recommend, as you troubleshoot (and I'm surprised more folks don't do this.....). Build/rebuild only ONE amp PCB (double and triple check it, as you go), and then connect it to the power supply. Use the bulb tester! If you do this, you only place ONE amp PCB "at risk".

Once the first channel is working, you can compare component values, rough bias settings, etc, between the first and second PCBs..... Reduces the risk of problems in the second PCB when you fire it up.

Now connect your second amp PCB to the power supply (leaving the first one connected is OK). Then test and set bias for the second amp PCB.

This way, if you have a common problem in your boards (miswiring, wrong components, wrong bias), you'll only be sacrificing ONE PCB, and not both!

(Trust me. I have learned this the hard way, over the last 50 years....!)

Ken
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Old 15th September 2011, 10:55 AM   #11007
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Thank you so far for your recommandations. I will try with one channel, and report back if I find out something that others can use.

I am sure I am not the first, and not the last one to make this mistake.
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Old 15th September 2011, 01:58 PM   #11008
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Quote:
Originally Posted by CanAm Man View Post
Russel...... Imagine what it's like to do a major revision to a large spacetrack radar, like we do....... that's 32 Megawatts of power. Our back-up generators are powered by two jet engines used on Boeing 727's..... I'm not so certain the old "work with one hand in your pocket" is adequate, around these power supplies....!
The hand in the pocket would likely be the only thing left!

Russellc
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Old 15th September 2011, 02:10 PM   #11009
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Quote:
Originally Posted by 6L6 View Post
It's advisable to take off your wristwatch and wedding band as well...

And possibly remove your fillings!
I once had a Mcintosh MR 67 putting 120 volts AC on the chassis. It was transferred via the interconnects ground cable to the ground on the RCA of the preamp, and to its chassis. While reaching behind the pre to where the cable attached to the RCA, with one hand, my knee came in contact with the seperate case of the tube amps power supply. Fortunately I only felt a tickling sensation as my knee started to contact the chassis....sometimes this can really sneak up on you!

Russellc
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Old 15th September 2011, 02:43 PM   #11010
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Speaking of safety -- just this past year there was an undergrad at Yale who got her hair tangled in a lathe and she died of her injuries. I worked in a die-casting plant in the summer back in the 1960's and NONE of the punch press operators had all their fingers!

Getting back to topic -- you can use other JFETs than the K170/J74 -- you may have to parallel P-Channel JFETs to get the gm you need on the gain stage and current you need to set the bias). It's a matter of necessity filling the void. You may have to add a little resistance in series with the trimpots to get the bias correct, but this is DIY after all. (You can, as well, build a "Starving Student" headphone preamp with tubes other than that specified!)
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