Tube newbie with Scott 299B

[Mike K2]

I am a complete neophyte when it comes to electronics. Ohms law is about the limit for me, but for some time I have been intriqued by tube sound.

I recently dug out a Scott 299B from storage that someone gave me a long time ago. A few years ago I had tried to set the bias and the rectifier tube failed. Since the only rectifiers I found were over $40 I gave up and put it away. I just recently found a $15 Russian made tube so I put it in.

The sound was very nice! I was encouraged and decided to try the bias adjustment again. It was very high, and even with the trimmer turned all the way toward the minimum, it was still way to high. If memory serves me, it was well over 40 volts across the bias resistor. I had read somewhere that the selenium rectifiers can go bad and cause this to happen.

I got the bright idea of replacing it with a silicon bridge rectifier. That must have been a mistake. The bias resistor cracked and got so hot that the ends of my meter leads melted! The voltage drop was well over 50 and climbing fast. One of the EL84s started getting a purple glow that didn't look good at all.

I though I might have wired the rectifier wrong so I double checked. It was right.

What am I missing in my ignorance? If anyone has suggestions, I would really appreciate them.

[EC8010]

The voltage dropped across a selenium rectifier is much higher than that dropped across a silicon rectifier, so when you replaced your rectifier you increased the voltage applied to the output valves. You may have damaged some other components as well. Can you post some photographs of the insides of the amplifier?

[Frank Berry]

Check the electrolytic capacitors in the bias supply.
I'll bet one section is shorted. Please beware .... this electrolytic "can" has a common positive.
You will probably need to replace the output tubes as well.
If you have not replaced the coupling capacitors on the grids of the output tubes, please do it.

Visit www.hhscott.com for more information and schematics.

[Mike K2]

Darn.

It sounds like I am going to be spending some time and money on this.

Why does education have to be so expensive and hard?! I thought the silicon rectifier was a good suggestion. Looking back on it, is bringing the amp up to voltage slowly with a Variac while monitoring the bias voltage the way to try new components?

EC8010- I will see if I can manage a photo. What area in particular are you interested in?

Frank- Is there a convenient diagram showing the location of the caps that you mentioned, or do I need to find them by reading the schematic and tracing the wires?

Thanks for the responses!

[JandG]

My 299B had the silicon rect. allready installed when I got the amp, but I am sure there is a resistor change that goes with the swap out, go to the Scott web site & down load the schematic, also I am sure someone over there could answer questions on this specific amp, be carefull when messin with caps man...I am sure there are caps that need to be replaced.. they don't need to be expensive. just take your time, ...mine is gone thru & is a outstanding sounding amp, especially the phono stage..Well worth spending the time & a liitle money on..

[EC8010]

Well, a few good general shots of the insides may show damaged components. As others have said, you're probably going to have to replace all the capacitors. The electrolytics will have dried out and gone low capacitance and high ESR, whilst the coupling capacitors may have gone leaky. It's not a big deal, modern low ESR electrolytics are far better than the old ones, and polypropylene coupling capacitors will also be far better. Yes, you need to add a series resistor to the output of your silicon rectifier - someone with more experience of selenium should be able to give you a suitable resistor value and rating. And yes, a variac would have been a good idea.

[Mike K2]

OK, I attempted to include some photos of the amp. The resistor closeup is an attempt to show the cracks in the bias resistor that are now present.

Oops.

Uhmm, is it not possible to include multiple images in the same post?
If it is, would someone please explain how?

For now I will limit it to one shot showing the entire bottom and the face panel.

[Mike K2]

Sorry, the photo is actually one of the power supply area. I don't think at this resolution there is much chance of seeing any problems short of something actually being on fire.

[EC8010]

It's not so much resolution that's the problem, it's the lighting (or lack of it). Try to position the amplifier in front of a window and at window height. I also use a scrap of sheet aluminium as a reflector to get extra light where I want it.

[kevinkr]

As someone else pointed out in his post the bias supply electrolytic can is common positive.. Note that this supply also provides power to a series string of 12AX7 in the front end. (Phono stage as memory serves.)

Those cans are now unobtainium because of the inverted polarity. To retain the original look and because of the limited amount of under chassis space you might want to consider gutting this can and installing modern replacement caps inside it - there should be more than enough room to do so if you choose appropriately sized caps.

There may well be a bad cap section in the filter circuit, it is also possible that you swapped connections on the bridge. IIRC the original was a siemens selenium bridge..

I've had several of these and imo it is definitely worth fixing. Hopefully the output tubes weren't really damaged as they are actually 7189A and almost no current production 6BQ5 is going to survive in the output section of this amplifier for more than a couple of minutes. IMO The Russian made NOS 6P14P-EV is the only tube that might stand up long term in this amplifier.

I have a heavily modified 299B, and a Realistic Stereolyne 40 and these both seem to work ok with either 7189A or 6P14P-EV.. JJ 6BQ5 did not work in either of these amplifiers. Both have plate voltage on the order of 380V - 400V depending on line voltage..