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    Installing 80/40/30/20 quad cap in MK IV amp with stock board

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    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Mon Sep 14, 2015 9:49 am

    How would I wire in a new 80/40/30/20 quad cap in a stock Dynakit MK IV amp I just received? I know the 80 mfd section can't be wired to the rectifier tube - how do I orient the cap itself? Is the front of the amp considered the side with no connections? The amp had its original cap removed and the previous owner decided they didn't want to mess with the amp any further - I now have a lone MK IV that has some dangling wiring and no handy snaps to wire in the new quad cap. Anyne have pictures of the wiring under the hood of a MK IV with the 80/40/30/20 cap?

    Bob Latino
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    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by Bob Latino on Mon Sep 14, 2015 11:18 am

    Arrange the 80, 40, 30, 20 cap so that the lead coming from pin #8 of the rectifier tube and one choke lead goes to the 40 section. The 80 section will be where the red output transformer wire, the other choke lead and one lead of the 6800 ohm resistor will be. The 20 and 30 sections will be in the correct location now to make connections to eyelets 6 and 8 on the driver board.

    Bob

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Mon Sep 14, 2015 2:02 pm

    Thanks!

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Wed Sep 16, 2015 9:57 pm

    Got everything rewired, fired up the amp and got some mighty bright orange glow - bias had zoomed up to 6+ volts! Killed the juice, walked away and came back the next day. Rechecked the wiring to the cap - 80 mfd away from the rectifier tube, all other sections wired correctly; made sure the diode was ok - that was fine. Checked all the caps - good all the way around. Grrrrrr - running out of ideas, then I found the culprit - a poor soldering job on the wire from the bias pot to the board had given way. Out came the soldering pencil, retacked the pesky wire and plugged the amp back in - normal looking orange glow from all tubes, a check showed the bias at 1.25 volts and holding steady. A quick listen to some music - sounded normal, no distorto sound like the first try. Left the amp play quietly for half an hour - bias was the same. Reset the bias to 1.45 volts and it's held steady (better than before in fact - the bias on that amp had been a little on the wobbly side - I assume the crappy solder joint was just loose enough to make the bias a bit unstable).

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Thu Sep 17, 2015 1:45 am

    Grrrrr - after about an hour the bias took off again. Any thoughts on what to check? Could the bias pot itself be faulty?

    peterh

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    Location : gothenburg, sweden

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by peterh on Thu Sep 17, 2015 3:58 am

    As you already have experienced one bad solderjoint, maybe checking all of them coul dbe next.
    Are your soldering tool of sufficient capacity ? Are you using good solder( leaded ) ?

    deepee99

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    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by deepee99 on Thu Sep 17, 2015 1:55 pm

    Does the problem follow the tube or the socket? If it's the socket, check tension on the bias pin and corresponding solder joint below. I don't know the pin-out of the EL-34 but on the KT/6550 series it's pin 6.

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by Bob Latino on Thu Sep 17, 2015 5:13 pm

    deepee99 wrote:Does the problem follow the tube or the socket? If it's the socket, check tension on the bias pin and corresponding solder joint below. I don't know the pin-out of the EL-34 but on the KT/6550 series it's pin 6.

    On EL34 or KT88/6550 tubes pin #6 is not used and not connected to anything inside the tube. On the VTA amps and all the Dynaco amps that use octal output tubes, pin #6 is used as a tie off point to hold one end of a 1000 ohm 1 watt resistor which goes over to pin #5. It is actually at pin #5 where the negative DC bias voltage is injected into the tube. If you have no contact from tube socket pin #5 to the tube itself, then that tube will have no bias voltage available.

    Bob

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Thu Sep 17, 2015 10:49 pm

    Red plating on the last attempt last night - I think I need to step back for a few days and then start checking the solder joints...

    Bob Latino
    Admin

    Posts : 2411
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    Location : Massachusetts

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by Bob Latino on Fri Sep 18, 2015 8:19 am

    Check the bias system to see if it is working.

    Try this > Remove all the tubes. Set the bias pots at the center of their rotation. Take off the bottom cover and turn the amp upside down. Turn the amp on and measure the negative DC voltage to chassis ground on pin #5 of all four output tubes. See if you get about -28 to -40 VDC off pin 5 on ALL four measuring points. If you do get the proper voltage, then you may have a tube socket to tube pin contact issue if you get no bias voltage and red plating on one or more tubes.

    See if you can post a photo of the inside wiring on the forum here. Check the sticky above on how to post a photo on the forum.

    Bob

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Fri Sep 18, 2015 9:51 pm

    Unplugged tubes, turned over the amp and looked for #5 pin on one of the sockets. One end had a resistor soldered to it - the other end of the resisir had worked loose from its connection on the board. Just resoldered the resistor - let's see what happens...

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Fri Sep 18, 2015 10:07 pm

    Bias still zoomed up after the MK IV was on for about a minute or so. Checked the votage on pin 5 and got -49.2 volts on both.

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Fri Sep 18, 2015 10:15 pm

    I'll take that back - forgot to set the bias at midpoint; -34.5 volts at pin 5 on both sockets...

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: Installing 80/40/30/20 quad cap in MK IV amp with stock board

    Post by howlin' hoosier on Sun Sep 20, 2015 1:19 pm

    Under closer inspection it looks like some traces were reworked with what looks like leads from resistors (grrrrrrr) - I'm beginning to think it's time for some replacement boards... Crying or Very sad

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