Completed SP14 not working :(

Basically all I am getting is a rather loud hum from the speakers and no sound.

Here is some testing instructions i was given on another forum.

"Hi Duncan,
Hopefully you have a detailed build manual. If not, start-up procedure probably should go something like this...

- Fire it up with no tubes & if you have a voltmeter, check that you have 6.3v at the heater pins of ea tube - 6.3vdc betw 6SN7 pins 7-8 & pins 2-7 on the rectifier.
- Plug in rectifier only & it should light up orange.
- It looks like you have regulated HT (hi voltage) too, so you can check that you have ~265vdc betw ea of 6SN7 anode pins 2, 5 & a Ground point.
- Plug in 6SN7s & you should see them light up orange.
- If you want to be thorough, you can also check that the 6SN7 heaters are 'elevated' - you should have greater than ~140vdc betw R24-R25 & Gnd."



Im getting about 6.1vdc between 7-8 on the 6sn7 and 2-7 on the 6x5.
getting 260vdc between 2 and ground. No reading between 5 and ground.

Getting about 70vdc between r24-25 and ground.

In terms of the "glow" I wouldn't say they are glowing at all. there is a small red/orange light coming out of the top (looks like a bit of fibre optic) 1 on the 6x5 and 2 on the 6sn7. Other than this small light I can report no such glow.

Compared to the picture of the M125 at full glow on the m125 page the tubes in this are definately not glowing.

Here is some pictures of the build so far.

While waiting for Roy to get back to me from whatever time zone he is relative to me I've been running through all I can to figure out what the problem is.

I gave all the connections a solid look.

Rewired 2 of the output rca jacks which were the wrong way around, not related to the ongoing issue but needed doing nonetheless.

I plugged the tubes back in and hooked it all back in. As soon as the power amp is turned on there is a hum even without the preamp turned on. When turned on, the hum dissapears then reappears as the tubes warm up.

I only get sound from one channel that is very loud. The hum is present in both channels.

One tube is lose in its base and when my finger goes near the tube during operation the hum changes. It also changes if I move the glass envelope within its base.

From all of this I can only guess that this is an issue with one or several of the tubes. My first step from here will be to see what tubes are immediately available in Auckland.

Hi Sadface,

I have not finished my SP14 yet, so I can not help much. Have you tried to swap the tubes around? If he hum follows the tube, the issue is probably related to a bad tube.

Kind regards

Michael

Sadface wrote:I plugged the tubes back in and hooked it all back in. As soon as the power amp is turned on there is a hum even without the preamp turned on. When turned on, the hum dissapears then reappears as the tubes warm up.

Sorry to hear your SP14 isn't behaving for you. It does sound like you have some tube problems there.

The 6SN7 tubes barely glow though, don't expect them to light up like you see power tubes or even some small signal tubes.

For a matter of practive you should be turning your power amp on last and off first. It will protect your speakers from any 'thump' caused by the pre or any other upstream component being switched on.

Fixed now,

It turns out it was a dry join on R5 that was causing the issue.

There was also some other issues around the attenuator wiring. It was wired up almost entirely wrong.

I also tidyed up the wiring on the primary side of the transformers. My does this amp sing, it sounds absolutely stunning.

I am considering rearranging the internal layout and mounting the source selection etc on shaft extension kits. I am using a wooden front panel with is about 3/4 inch thick so the short shafts on the source switch/attenuator wont protrude enough without otherwising mounting them into the back of the wood which would be a huge pain.

Glad to hear it is working! Looking forward to complete mine.