Tube Amp adjustment.

[N8RMS]

I see rectifiers turned around the wrong way in the top picture.

Not sure how to illustrate where they ought to go with my limited graphic-arts skills.

Only the NON-banded end of diodes in a bridge circuit like this should go to the ground lug. You have the banded end of one pair going to the ground lug.

That's wrong. Just not sure how to illustrate the correct setup clearly.

I'll look for a pic of the "eight-diode" hookup in a D&A amplifier that's clear enough to use as an example.

73

ok we can fix that easy, now what about the other question? the opposite side high voltage of the caps red wire where should it go ?

 

[391redneckradio]

here is the info you need for your amp. Unfortunately there are only pictures on that website. It is made by SOS Electronics RF Amplifiers.
The Scout 20 and mini Scout are the model names.
http://www.cbtricks.com/Amp/sos_electronics/index.htm

I'll bet that NOMAD has a schematic!

Do you know what tube goes in the mini scout? I have one the tube went bad and some how it was taken out and misplaced. thanks.

 

[Tallman]

Do you know what tube goes in the mini scout? I have one the tube went bad and some how it was taken out and misplaced. thanks.

I'm looking for the schematic, no luck yet.
I sent information to Nomad, maybe he has the part you need.

You might have the only amp no body has heard of. I even checked one site in the Netherlands and not a word. Even Nomad was a no go.

 

[391redneckradio]

I'm looking for the schematic, no luck yet.
I sent information to Nomad, maybe he has the part you need.

You might have the only amp no body has heard of. I even checked one site in the Netherlands and not a word. Even Nomad was a no go.

I dug around and found a thread about a super scout that made mention of a mini scout having a 8417 tube. They are super expensive but I also saw something a 6l6 or 6550 tube as a substitute and they are about 40 buck. Im going to try one. Its just a single sweep tube modulator.

 

[391redneckradio]

 

[Shockwave]

The 8417 and 6550 both have 35 watts of plate dissipation and a rather robust cathode that supports a lot of peak emissions. That makes for the good forward swing those modulators are noted for. The 6L6 has almost as much dissipation at 30 watts but a much smaller cathode that tends to "fade" under hard conduction. I'd bet the 6550 is a better replacement for the 8417. Bias will be the key factor in any substitution. I know changes are required in bias between 6L6 and 8417.

People use to change the 6L6 modulator in the Tram D201 to the 8417. Some of them got the bias right by cutting the -31 volts down to about -12. What most didn't realize was that also meant the drive requirement was less than half of the 6L6 and were usually distorted since no attenuation was done to correct the overdrive.

 

[ExitThirteen]

Install a KT88, that's the "modern day" 6550, offering 42W of plate dissipation. Those work nicely in the old 1 tube Scouts and the JB150A modulators. You can get KT88's at your local music/guitar store or online, they are still being made.

~Cheers~

 

[Shockwave]

Even better. I forgot all about the KT-88. It may be a little larger than the others so check to see if it fits before buying one by looking at the datasheet first. Otherwise you may find yourself having to drop the socket below the chassis to get the cover back on.

 

[nomadradio]

If I'm guessing correctly about this amplifier, it's grid driven. Presumably the same "modulator" setup as the Black Cat "JB", or "Jewel Box" amplifiers.

These work by biasing the tube beyond cutoff. This suppresses the carrier power and exaggerates the modulation envelope. This keeps the plate dissipation from getting out of hand. Also produces exaggerated "swing" compared to the radio's barefoot ratio of peak power to carrier power. This also lets you obtain an unnaturally high peak output from a tube that has only a modest power rating.

Might want to check the DC voltage found at pin 5 of the tube socket. This might provide a hint about the tube type that it was originally set up to use. For that matter, if you don't have at least 35 Volts negative DC on that pin don't put a tube in it just yet. This voltage is typically regulated by a zener diode or two in series. If the previous tube failed while it was keyed, the zener may be shorted. When a tube arcs over inside, a surge can come out of the grid and toast that zener. A new tube would not last long without this voltage present to hold the tube current down to a safe level.

The previous tube may not have been misplaced, rather it may have just been blown to Hades by whoever used it last. Cranking the carrier power too high with this kind of amplifier will overheat the tube and permanently hammer it pretty fast.

73

 

[Shockwave]

All amplifiers that use these tubes have to be grid driven. This is because the suppressor grid (grid#3) sometimes called the beam plates are tied directly to the cathode inside the tube. All of the bias problems you mention are common, with the blackcat versions having even more because they had to be converted from receive pre-amps to get past the FCC and many didn't even have a relay contact to switch the bias on and off.

 

[391redneckradio]

I will check pin 5. The amp belonged to my dad and I remember, years ago, when it blew and I priced a new tube for him and opted not to fix it. Neither one of us could remember who took it out or misplaced it but its gone. He gave it to me yesterday and I want to try and get it going. Thanks for all the info.

 

[357]

Guess my scout thread died with quackshack.

heres some pics

 

[357]

 

[JoBob]

The small trimmer on the back of the Mini Scout is to set the dead key. Usually from 2 to 10 watts. That is all the tuning for it....just set the dead key. It will modulate forward from there.
On the Scout 20 set the Hi/Lo switch to Hi and then max the two tuning knobs for a peak dead key without any audio. This will pretune the amp. Once you have this done key the amp and say AHHHHHHH while moving the tuning controls slightly to acheive the highest peak reading. Only one of the controls will make a difference in the peak power, the other one will change the key slightly. Once you have the peak reading continue to move the control (that makes the peak higher) clockwise until you see a drop in wattage of about 30 watts or so. You then can tune the input tuner on the back of the amp for max peak from there. The amp then is tuned and will require no further adjustment. If it is in good shape it should run cool and have great audio.
Also you do not want to drive the Scout 20 with the Mini. The driver tubes in the Scout can't take that much wattage for a long period of time. With the price of tubes now it is not something to do. Both are good amps and are reliable. I have owned both and still have several Mini Scouts. They are keepers.

BC

I have a mini Scout modulator looking for numbers on Transformer mine went up and smoke just trying to find a new one didn't know if you might have or know what goes in it

 

[AudioShockwav]

I have a mini Scout modulator looking for numbers on Transformer mine went up and smoke just trying to find a new one didn't know if you might have or know what goes in it

Hello JoBob, welcome to the forum. You might get a better response if you start a new thread titled" looking for a transformer for a mini Scout" here in the Amplifier section.

73
Jeff