K5881 PSE Heater connections.

[prakash]

Hi All,
Ihave completed a K5881 PSE build but have not switched on.
There are certain points that i am troubled with.This unit was
partially built by someone and given up.
1.The mains transformer has a 6.3v with a centre tap going nowhere.
2.The heater wiring to the tube bases were already wired.
The tube bases for the 6922 tubes have pins 4 & 5 wired together and
going to one side of the 6.3v .The other side of the 6.3v going to pin 9.
Is this correct.
3.The output transformer has K5881 written on them.
A to A -360ohms
A - C -A :180 -0- 180 ohms
Please Help.
Prakash.

[bob orbell]

Hi Prakash, I am not familier with your amp. but pin 9 of a 6922 is a screen or earth and pin 4 and 5 are 6.3v connections, so please wait for another member to guide you, the C/T may go to earth. BOB

[Richard]

Hi Prakash, all,

Firstly confirm which amp it is.
There were 3 5881 variants which used similar chassis and layout,
5881 Push Pull
5881 Mk2 Push Pull
5881 PSE

Your existing heater wiring is consistent with ECC83 used in Push Pull not 6922 used in PSE.

Your measured primary ohms are consistent with equal sides of CT as on Push Pull not 44% tapped UL on PSE where you'd expect to find a difference between sides of the tap.

Are they WAD transformers?
Are the top caps gold/brass or silver/chrome?
Are the output transformer lamination stacks as tall as the mains transformer lams or less tall?

The heater CT goes to signal ground. This may be solidly bonded to chassis or floated inside chassis mains earth via 10R resistor depending on your chosen construction.

[prakash]

Hi Bob,
Thank you for your reply.I switched the unit on changing the h-wiring via
the viaric.There was a small sound coming from the speakers.Disconnected
and removed heater wiring.Reconnected 6.3v to pins 4 and 5,leaving pin 9
open.I grounded ct. to chassis via 10ohm/.1mF.Switched on and the amp
came to life.Lovely sound with a tiny hum.
Prakash.

[prakash]

Hi Richard,
I was asked to build this unit from a friend.He supplied me with the chassis,
K5881 PSE circuit diagrams and tubes.
I did a complete PSE build and got concerned after noticing heater wiring
for the 6922's.
1.O/P transformers the lebel :SP WOUND COMPONENTS.
1.O/P transformers lower in height.
3.Top covers are all black.
4.Back of chassis :Modek K5881
20 Watt Stereo
Power Amplifier
The amp sounds nice with the Pse components.Just a slight hum.
Do I have to rip all the stuff out and build it as a K5881 ?
Thanks,
Prakash.

[Richard]

Hi Prakash,

Glad you've got the heaters and amp going. Re the hum, check the earthing arrangement you've used;

In all cases the metal amp chassis should be solidly bonded to mains earth, then you can either,

1) construct the amp with it's ground connected directly to the chassis, (usually using a bus bar connected to chassis at one point) or

2) construct the amp inside the chassis but isolated from it by using insulated RCA sockets etc. You would connect the heater CT to that isolated ground as well. Check isolation from chassis with a meter. Then make one connection to chassis earth via a large 3W w/w or similar 10R resistor and put a 0.1uF film cap across it to give a path for RF.

So that may help the hum but going by the description of the output transformers it sounds like you have Push Pull ones. The SE ones were as tall as the Mains TX. The daft thing is that yes it will work as an SE but you'll have a 50% UL tap and TX load and un-gapped cores that aren't optimised. The OPTXs would saturate easily and distort if that is the case but you may get 4W or so and that can sound quite convincing. The easiest way to check now is to see if it's giving a clean 17W if you have test gear.

[prakash]

Hi Richard,
The hum came from the pre amp I was using.Thats sorted out.
I have no test gear to check the output power.The amp seems to
put out a lot of power because it is driving a pair of HiVie 6.8
dual voice coil speakers which require a lot of power.
After 2Hrs of test I noticed one of the output transformers warming
up.Tested a few times and sound is still awesome.
I think I will have to rebuild the unit as the K5881 Push Pull so as
not to damage the output transformers.
Please comment.
Regards,
Prakash

[Richard]

Hi Prakash,
That's good news on the hum
I wouldn't run it too much unless you can confirm they are PSE TXs as they will be passing twice the designed current of PP through the windings even at idle and may burn out.
Like I say, 4W can sound quite convincing if it clips gently and is only a few dB away from 17W in loudness terms anyway.
Here are the only pics I can find on the web at the moment, see the taller stack height for PSE OPTXs top, compared with PP bottom.
When my isp stops playing up I can post a bigger pic.
If they are PP you could rebuild to PP circuit or ask your friend if he wants to supply or get suitable SE TXs but check core size so they fit as it would be a shame to chop the chassis.

[prakash]

hI,Richard,
The OPTX's are lower in height.The unit also came with a
50mF/50mF-500v cap attached inside the chassis for the psu.
After checking the various schematics I am now convinced
that the chassis was for the K5881 MK11 PP.
I am now going to rebuild the unit as a MK11.
I will connect with all after I have completed the rebuild.
Thanks,
Prakash.

[Richard]

Hi Prakash,
OK good luck.
Do you have the info you need, if not drop me a PM with your email so I can forward attachments if you need them