New SP14 build. Really noisy!

Hi,

First post here!

Looks like there is a lot of info about the SP14.

I’ve wanted to build one of these for a long time, and finally got the resources together to get a kit.

I ordered the kit without the chassis, as I wanted to build my own - plus I wanted to try to save some $$$’s

I’m getting a LOT of noise out of this…

All the voltages are as they should be.

B+ is 276vdc
H+/H- is 6.13 and 5.87
Voltage drops from 313 to 276 across D13 (both sides…)
6.43vac on heater from transformer


I have checked, double and triple checked the wires from all RCA inputs/outputs. The ground bus and all the connections to that, the ground to the circuit board from the bus. I’ve gone over the input selector, and all the wires coming and going from that. All the connections from the selector, through to the gain and volume controls. The output from the volume to the board, and ground wire are as outlined in the instructions.

I did not install the tape switch, and ran the wires straight from selector to the gain pots.

There are two things I changed as far as the power supply goes. I used an old school toggle switch in place of the power switch that came with the kit. And I accidentally reversed the red and black primaries to the transformers from a hot/neutral perspective. But I don’t think the transformers care about hot/neutral?

The noise is the same regardless of gain/volume settings. It’s the same in both channels. It gradually comes up in volume as the unit powers up. As soon as it’s powered off, it disappears but music keeps playing until it fades away.

The noise is the same, albeit with different volumes, if I run it into my DIY Hiraga Super 30 Class A SS amp, or my DIYAudio VFet SE SS amp.

I took a measurement of the frequency output, but can’t seem to upload the photo?

Thank you for taking the time to read through this, any help would be greatly appreciated!

I had trouble with noise in my SP14 kit. I bought the complete kit minus the tubes. I did have problems with objectionable hum at any volume setting which was evidently a ground loop problem. I experimented with grounding the PC board at different spots along the board and to the ground at the AC power input. I finally got the hum down to almost nothing as seen on my scope and my trusty Heathkit signal tracer. Unfortunately there isn't much information in the assembly manual about hum problems. You have something seriously wrong if you have 276 volts across D13 as the specs call for between 20 and 40 volts DC across the diode. If you want some help with your problem, send me a PM with your contact information and I will do my best to help you with your problems.

Thank you both for your responses!

I managed to get the hum down to almost nothing. I grounded the top plate and front plate to earth and that seemed to work!

Though I do think I can get it to be more quiet as there is a slight hum on the right channel.

I thought the voltage drop I’m seeing across D13, which is 37volts, is what I am aiming for?

Wish I could figure out how to post photos…

And thank you for you offer! I’m going to try to figure out how to send a pm

Per

20 to 40 volts across D13 is what you're aiming for. I found with my SP14 that the 6X5 will cause the voltage across D13 to vary. Fortunately I had several 6X5s in my tube stash to experiment with. If you are still having problems with your preamp after 7 days, shoot me a PM with your contact info and I will do my best to help you.

Perkri, you have to be a member here for ONE WEEK before you are allowed to post photo. Welcome. Dale

Big Harry wrote:20 to 40 volts across D13 is what you're aiming for. I found with my SP14 that the 6X5 will cause the voltage across D13 to vary. Fortunately I had several 6X5s in my tube stash to experiment with. If you are still having problems with your preamp after 7 days, shoot me a PM with your contact info and I will do my best to help you.

Thanks!

I bought two 6X5’s. NOS Sylvania. Wanted a backup other than the one that came into my possession under questionable circumstances. Pulled from a dilapidated console unit that was rotting in a garage…

Once I have some funds together, I’m going to put some fresh 6SN7’s in it. 

It’s sounding really good, even with the 6SN7 tubes I had on hand (Electroharmonics Gold, and RCA Black Plate)

I like either early chrome dome Sylvania 6SN7 GTA or GTB and GE 6SN7 GTA tubes in my SP14. I also have a bunch of 6SN7 GT's from various manufacturers from the 40's in my 6SN7 stash, but I haven't tried any of them yet. The issue with older 6SN7's is microphonics and it seems that some tubes are worse than others for microphonics.

I’m looking at the Linlai tubes, and some of the other “high end” new build Chinese tubes. Don seems to like them in his preamp.

I’m thinking I’m going to remove the gain pots in order to simplify and reduce all the wires running on the front of the amp

Guess I should measure the gain pots at the setting where I feel it works well with my amp and put some resistors in their place?

And when putting a single volume control in, would I stick with a 50K rated one?

Thanks!

Here's a post I did some time ago - good luck!

Over time, there have been several postings about hum issues with the SP14 preamp, with many "no issue" experiences. Having built many of these fine preamps, I wanted to share a grounding philosophy that has resulted in dead quiet units for me. These observations are implicit in Roy's instructions, but, made explicit, may help those with issues.

Think of a ground path through the preamp, starting at the ground lug at the IEC input jack and ending at the output jacks:

-IEC grounding lug to a grounding tab fastened to the chassis
-Chassis star grounding tab connected to purple leads (shields) from power transformers
-Chassis star grounding tab to circuit board ground
-Circuit board ground to gain/volume control ground tabs
-Circuit board ground from only one shielded output cable to the jack field ground bus - only ground connection point for bus
-For each input jack cable, shield is connected to jack field bus but other end of cable shield is not connected - ground is extended along cable through shield from jack field bus


By thinking about the ground path through the preamp this way, ground loops and resulting hum issues are eliminated, at least have been for me.

Hope this helps,

Bill

Thanks!

-IEC grounding lug to a grounding tab fastened to the chassis

I have the shell of the IEC attached to the metal front, and metal top plate. Rear plate is attached to the shell of the IEC plug. The ground tab on the IEC is connected to the shell of the IEC plug internally.

-Chassis star grounding tab connected to purple leads (shields) from power transformers

Transformer purple leads connected to star ground, which is connected to the ground tab on IEC

-Chassis star grounding tab to circuit board ground

I have a wire going from star ground to the board ground at the input of AC from transformers

-Circuit board ground to gain/volume control ground tabs

The shield of the input cable from volume pot (gain pots removed as I’m working through ground issue…) is used to connect ground of volume pot to circuit board ground at signal input

-Circuit board ground from only one shielded output cable to the jack field ground bus - only ground connection point for bus

One of the output cables has shield connecting rca ground bus wire to circuit board.

-For each input jack cable, shield is connected to jack field bus but other end of cable shield is not connected - ground is extended along cable through shield from jack field bus

Only one end of input wires shield is connected to rca ground bus wire. Other end is not connected to circuit board

Sadly, the hum persists on the right channel only

Thanks again!