HH Scott 299c

[Pablodeux]

Beautiful. Tubes show way more color in the dark with the cell phone. Naked eyes, they look much less orange. Sounds fantastic.

I'm happy to see things have worked out for you.

It looks like your collection of Old Stock 7591's is working well together.
Sometimes,even though they test well, the current at full plate voltage will vary, and it's necessary to pair them with different partners, so you can get meet the .35ma spec for both tubes in a pair .
That's not your problem here, since you got the balance down to 0V.
It's a pain without being able to check the bias from the top of the chassis, but I'd check again after 50 or so hours, after things have gotten a chance to settle in.
From observing my line voltage, I've found mine runs from 118.5 to 120.5V most of the time, so when I get around to doing my final adjustment of bias I monitor the voltage.
When it's running near the high side I set the current to .35ma while being sure the DC balance near 0.
This keeps the current near .33 when the voltage swings to the low side.
I can't tell the difference sonically even when the current occasionally strays out of this zone, and the old stock tubes can run at .38ma without catastrophic results, so I feel pretty safe operating the way I set them.

Happy listening !

 

[hogwylde09]

Wow, my first experience with the 7591 tube. It’s impressive. They say the 7868 same tube in a different bottle. I think the 299c has an “edge” on the Fisher 400. Not by much. More listening time will tell.
Just can’t bring myself to put it back into the metal case. Although it’s picking up some stray magnetic hum from being too close to the turntable. I suppose, not noticeable at lower volume.
Hmmmm, maybe I’ll try the wood case from the 130.

 

[Pablodeux]

Wow, my first experience with the 7591 tube. It’s impressive. They say the 7868 same tube in a different bottle. I think the 299c has an “edge” on the Fisher 400. Not by much. More listening time will tell.
Just can’t bring myself to put it back into the metal case. Although it’s picking up some stray magnetic hum from being too close to the turntable. I suppose, not noticeable at lower volume.
Hmmmm, maybe I’ll try the wood case from the 130.View attachment 2467923

Have you got the aluminum bottom cover, tube shields, and the aluminum "box" that covers the back side of the faceplate mounted controls installed ?
They're supposed to be beneficial for minimizing hum.

With my early model LK72, which has the same layout as your 299C, I find the middle (right Channel output) transformer, gets pretty warm due to its proximity to the rectifier and 7591 tubes.
I have been hesitant to put it in a cabinet, since the insulation on the 60+ year old transformer windings might have a case of heat induced aging.
The later model 299D and LK72B amplifiers have the 7591's and the 5AR4 relocated to behind the transformers and there is less of an oven effect from the tubes.

 

[hogwylde09]

Have you got the aluminum bottom cover, tube shields, and the aluminum "box" that covers the back side of the faceplate mounted controls installed ?
They're supposed to be beneficial for minimizing hum.

With my early model LK72, which has the same layout as your 299C, I find the middle (right Channel output) transformer, gets pretty warm due to its proximity to the rectifier and 7591 tubes.
I have been hesitant to put it in a cabinet, since the insulation on the 60+ year old transformer windings might have a case of heat induced aging.
The later model 299D and LK72B amplifiers have the 7591's and the 5AR4 relocated to behind the transformers and there is less of an oven effect from the tubes.

Yes, all covers, tube shields on. No case yet. That’s debatable. I’ve the metal “leatherette” or the very nice one built by AK member Asaalah.
With two CL-80 installed in my unit and plugged into my line bucking 115vac (visible bottom right in photo) I feel it’s running about as cool as possible.
Correction, not motor induced noise at all !
No, using the least expensive table in my possession, the Lab 400 , uses and micro switch to silence the audio output until about needle drop.
It need not apply here, the tube audio forum. I do have had some slight Shure M91 ground noise before now. More so with the Scott. Ground wire from table to amp chassis with alligator clip.
While on topic re: phono stage, I am going to roll something into V1 other than the Telefunken. Also experiment with different phase splitters.
Once again re-tension and clean V1 - V4.
I did encounter some distortion that worried me a bit that I had not heard before on extra input and the test bench speakers.
The big horn speakers tell the truth for real !

 

[mcgjohn]

or select your own caps such as nichicon or other favorites and solder them into an adapt a cap adapter.

https://adapt-a-cap.com/collections/all

 

[therling]

Some years back, Charles Kittleson of Vacuum Tube Valley recommended using 7687 tubes for V5 and V6, stating that they provide much improvement over the stock 6GH8/6BL8 tubes.

The gain of both tube sections is slightly different, but the sonic improvement is incredible. Just adding a pair of 7687s will give you more headroom, better bass, high frequency extension, and a more 3-D soundstage.
https://worldradiohistory.com/Archi...m-Tube-Valley/Vacuum-Tube-Valley-Issue-12.pdf
​

I put a pair in my 299B and agree that they sound better. They’re a bit rare these days but occasionally pop up on eBay but they’re getting more expensive. That section of the power amp is pretty much the same in the 299C.

 

[hogwylde09]

After almost two months and quite a bit of use, I decided to re-check the bias on the 7591’s. Before I was pretty much maxed out with the bias and balance controls. I know at least one of the tubes NOS. I am able to get 23mv access the 3.3 ohm resistors now.
Swapped out the Sylvania 6U8a for a matched pair RCA label Amperex 6BL8/ECF80. Also robbed tube shields from a dilapidated Fisher model 80-T.
I’m putting the Fisher 400 back into rotation at the moment. Don’t need caps to dry up.