Scott 299B & Headphones

[Zack Sainz]

Hey all,

Still new to this world, so thank you in advance for any insight.

I have a Scott 299B with some later production KLH 6s. It’s incredible. I would love to be able to use headphones, mostly for night use.

I saw this article:

http://audio-room.net/no-headphone-...rs-pioneer-jb-21-heco-ut-3-canton-connect-50/

At the end of the article it says:

“A word of warning: all these boxes have common ground. There are amplifiers, especially T-amps and some tube amps, that will hum, distort or even blow if you short grounds of two channels. When in doubt, check if your amp has common ground. If it does, you can safely use one of these boxes. If it doesn’t, best not risk it.”

If it makes any difference, my friend who I bought the Scott from did some work on it and updated the power cable, so now it has 3 prongs.

I have the Pioneer JB-21 in hand but I am waiting for “okay” to use it.

Thanks again.

 

[primosounds]

Hey all,

Still new to this world, so thank you in advance for any insight.

I have a Scott 299B with some later production KLH 6s. It’s incredible. I would love to be able to use headphones, mostly for night use.

I saw this article:

http://audio-room.net/no-headphone-...rs-pioneer-jb-21-heco-ut-3-canton-connect-50/

At the end of the article it says:

“A word of warning: all these boxes have common ground. There are amplifiers, especially T-amps and some tube amps, that will hum, distort or even blow if you short grounds of two channels. When in doubt, check if your amp has common ground. If it does, you can safely use one of these boxes. If it doesn’t, best not risk it.”

If it makes any difference, my friend who I bought the Scott from did some work on it and updated the power cable, so now it has 3 prongs.

I have the Pioneer JB-21 in hand but I am waiting for “okay” to use it.

Thanks again.

I don't know the background of the person who wrote the article, but "blowing up if you short grounds" seems like an exaggeration. The term itself seems to be a contradiction. Anyway, The headphone jack on these tube amps take their signal from the output transformer secondary and run it through a chosen resistor to get the impedance for a headphone. In the Scott 299C it is 2.2K resistor. IME when things blow up 99% of the time it is NOT because you plugged a headphone into the amp.

 

[Zack Sainz]

I don't know the background of the person who wrote the article, but "blowing up if you short grounds" seems like an exaggeration. The term itself seems to be a contradiction. Anyway, The headphone jack on these tube amps take their signal from the output transformer secondary and run it through a chosen resistor to get the impedance for a headphone. In the Scott 299C it is 2.2K resistor. IME when things blow up 99% of the time it is not because you plugged a headphone into the amp.

Thank you for your fast response.

The thing that seems tricky to me still, is using the external headphone junction box. The reason for using the Pioneer JB-21 is because the 299B doesn’t have a headphone jack.

and I don’t know the authors experience with gear is either, even if his article is exaggerated, I love my amp and I want to keep it safe and sound.

 

[century tek]

Most tube stuff is universal ground or center tapped to the chassis. Also, speaker negatives are sometimes tied to the chassis but in your case, the negative speaker connections are grounded to the chassis.

You're not going to hurt your amp using headphones but yeah, if it didn't have a headphone jack to begin with, you are going to need to pad them down with some resistance or you are just going to blow your head apart when barely cranking it.

Put a couple of 100Ω - 200Ω resistors in series with the right & left headphones positives to start with. Ground the sleeve to the chassis or the "O" speaker connection.

I have seen a lot of headphone circuits with 100Ω resistors so it probably won't hurt to start there. If it is still too loud, then go up to 200Ω's.

 

[larryderouin]

Build a box. Take a DPDT switch and connect one side to speaker out's, the center tap to the output on the 299B. The 3rd tap, do what Century Tek said and pad them down with a 2-3w resistor. The grounds are common and run thru the box from the speakers to the unit being used and the headphone ground. Fisher used a 330Ω on each channel. Anywhere from 100Ω to 400Ω should be acceptable.

 

[Zack Sainz]

Here are the guts of the headphone box that I currently have. (Pioneer JB-21)

 

[larryderouin]

You should be fine with the PIONEER BOX. Some amps don't have a common ground for the speakers and their grounds float above ground potential. Your's is grounded to the chassis so you're ok. Like Century TEK said , If the Volume is too high, go up to a 200Ω or higher until the output is isn't too loud at 9:00 or so on the Volume control. With a pair of KOSS PRO-4AA (360Ω or so impedance) 330 ohm padding is about right. I would think a lower impedance headphone would need more padding, but I might be wrong. Not good at the math part of electronics.

 

[Zack Sainz]

Thank you everyone!

I’ll let you know what the results are!

 

[Zack Sainz]

Thank you so much everyone!

After lots of handholding and head scratching, the headphone box works! And it was only $14.

Yay for vintage gear!

(Scott 299B, KLH 6, and Pioneer JB-21)

 

[Zack Sainz]

Alright Fam!

So I have been having sooo much fun with this headphone box (JB-21), listened to a few albums last night, Dark Side of the Moon is so buttery with the tubes.

With my headphone listening (Sennheiser HD 280 Pros), I have noticed what seems to be a 60hz hum, no scopes for me to use, so I resorted to A/B-ing youtube 60hz. When I switch to "speaker" on the headphone box (JB-21) there is zero of what I assume is a 60hz hum coming out of my KLH 6s.

I wouldn't ask for help without proper research before hand.

My research lead me to:

1. Ground Loop Isolator (Ebtech Hum X)
2. Parallel Power and Sound cables
3. Grouping power cables on the same surge protector
4. Different power outlet
5. Plugging straight into the wall
6. Unplug various house hold items

All of which I have tried, and none have worked. Womp Womp.

Inherent tube phenomena to live with? Something very simple that I have overlooked?

Thank you again,

 

[century tek]

Either your headphones are too sensitive or you may need to pad them down some more with some higher ohm resistors other than those in the JB-21.

No hum through the speakers is great but if you hear just a slight hum when your ear is up next to the speakers, you're going to hear that more loudly (sensitivity) through the headphones so you might need to pad them down more to your amplifier.

 

[larryderouin]

The Sennheisers have a nominal impedance of 64Ω. My Koss Pro-4A's and Pro-4AA's are 260Ω & 250Ω while my Koss Prota-Pro's are 60Ω. On any of my FISHER's (all tubed) with 330Ω padding resistors (their normal padding for headphones) the Porta pro's will be very much brighter and allow more noise in with a slight 60Hz hum, where the Pro-4A & 4AA's tend to be more balanced with less noise/hum. This could very well be the case with the Sennheisers (more like the Porta-pro's sensitivity wise.) I agree with Century Tek that padding down (A/B switch with 270Ω or 330Ω plus your original resistors so you could use different phones) would be effective for the Sennheisers.