4 posters
Well, I found a pair of MK III's
pjp3- Posts : 39
Join date : 2013-03-19
Age : 62
Location : Pelham, AL
- Post n°1
Well, I found a pair of MK III's
peterh- Posts : 1823
Join date : 2012-12-25
Location : gothenburg, sweden
- Post n°2
Re: Well, I found a pair of MK III's
There seems to still be a selenium rectifier
Change this to a SI ( 1n4007 will be excellent ) ASAP. If it fails it might ruin your tubes !
(It's the rectangular thing in the middle that has a bolt and nut through it. )
Change this to a SI ( 1n4007 will be excellent ) ASAP. If it fails it might ruin your tubes !
(It's the rectangular thing in the middle that has a bolt and nut through it. )
pjp3- Posts : 39
Join date : 2013-03-19
Age : 62
Location : Pelham, AL
- Post n°3
Re: Well, I found a pair of MK III's
Got it! Thanks!
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
- Post n°4
Re: Well, I found a pair of MK III's
Lots of 330V photoflash capacitors, connected in series for 660 working volts.
pjp3- Posts : 39
Join date : 2013-03-19
Age : 62
Location : Pelham, AL
- Post n°5
Re: Well, I found a pair of MK III's
and that's a good thing... right?GP49 wrote:Lots of 330V photoflash capacitors, connected in series for 660 working volts.
I plan on replacing the boards with the octal upgrade from Roy and that should eliminate
the selenium rectifier.
peterh- Posts : 1823
Join date : 2012-12-25
Location : gothenburg, sweden
- Post n°6
Re: Well, I found a pair of MK III's
wize move. You also get individual bias adjustment possibilities.pjp3 wrote:and that's a good thing... right?GP49 wrote:Lots of 330V photoflash capacitors, connected in series for 660 working volts.
I plan on replacing the boards with the octal upgrade from Roy and that should eliminate
the selenium rectifier.
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
- Post n°7
Re: Well, I found a pair of MK III's
Photoflash capacitors were a popular way to add lots of capacitance in a small space. Since most consumerpjp3 wrote:and that's a good thing... right?GP49 wrote:Lots of 330V photoflash capacitors, connected in series for 660 working volts.
photoflash tubes operate at 300 volts DC, the capacitors are made at a working voltage of 10% higher, or 330 DC.
For the B+ voltages in amplifiers such as the Mark III this is not enough so they get connected in SERIES
(with a voltage equalizing resistor across each one). The resultant capacitance is HALF what's printed on
the capacitor. So the two big ones in the photo are the equivalent of 375μF, 660 DC working volts.
The bottoms of my modified Mark II amplifiers have photoflash capacitors in them, too. The original quad
cans are disconnected. They are there just to maintain stock appearance at the top of the chassis.
hawaii.ken- Posts : 157
Join date : 2012-01-31
- Post n°8
Re: Well, I found a pair of MK III's
Photoflash capacitors were never intended for continuous ripple current such as found in power supply applications..
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
- Post n°9
Re: Well, I found a pair of MK III's
That may be true. In their normal applications, they didn't see 120Hz as put out by a full-wave rectifier, but instead the high-frequency pulsating DC produced by the chopper power supplies in photoflash units. Mine have worked just fine and have been absolutely reliable...not one failure. I've bypassed them with film capacitors, as I would any power supply filter capacitor.
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