Shelby NC Ham Fest

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Smaller than I remember.

I got a few sockets, and picked up a Monarch STA-400x for $40 which I couldn't pass by.

Fair condition. Broken tuning string, all tubes look good as in have good getter flash, except it is missing the 5AR4.

As it uses a rather rare output tube, the 6GW8/ECL86 (pentode/Triode combination) which may be why I got it so cheap.

I need to get a paper tube manual and take it with me. RC-30?
 
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Wonder why it isn't at the Fairgrounds?
It was moved 2 or 3 years ago because there was a conflict with some of the officials in Cleveland County, where Shelby is actually located. Seems the county didn't want to deal with the hassle of providing all of the services necessary for such an event (i.e. law enforcement, traffic control, EMS, etc.) so the organizers moved it to the Gaston County fairgrounds to keep it going. Frankly, I think it's a nicer venue even if there's less indoor space.

It did seem smaller to me this year than in years past, but I was able to pick up a few things like ceramic sockets, grid caps and a bunch of 807's for cheap. The BIG find was a couple of JAN 2A3's for $3 each. They tested good; strong emission, non-microphonic, not gassy - Woo Hoo!!! :D

Gimp: good luck on the Monarch. The 6GW8 is a weird little tube that also finds use as the audio driver/output section in the Heathkit SB-series and the HW-101. I finally sold an SB-101 parts chassis after several folks complained about the lack of tubes. Most didn't care if the finals were present, they were looking for the 6GW8 and the 6HS6's!
 
Good snag on the 2A3s. I saw a lot of tubes that I was not familiar with and realized a paper tube manual would be nice to have with me.

One possibility would be to rewire the output sockets for 6BM8 in place of the 6GW8 since they are still available. Slightly less power out, lower transconductance so gain will be less, but the voltages look compatible, load impedance looks compatible and biasing is close.
 
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The 2A3's were definitely a good find. I've been wanting to build a small SE amp with those tubes for years, but that may be put on hold for a while longer as I'm having too much fun with the RH84 I just built. That's a nice little amp!

The thing I always forget to take to these swap meets is a small voltmeter. There's lots of transformers and chokes that I've left behind because I simply had no means of checking continuity.

After looking at the specs for the 6GW8, I'd say you're definitely on the right track with the 'BM8's - if you just want to do a restoration. Depending on the shape it was in, I myself would be tempted to part it out and build something else from the carcass! :D
 
I'm sure the transformers would work for an EL84 amp, but I'd like to try getting it back up an running. I'm hoping the FM section is still good, but need to get some nylon string to restring it. The 5AR4 is no problem as they are still made and go for $11-15 new. I'm surprised I don't have one in my rectifier box. I must have 30 tube rectifiers.

I have a few 6BM8s so I may try a switch once I re-cap it and see if it will play. No physical evidence of leaky (pio) caps but I understand from other threads that these old Japanese units are noted for cap failures. So, I may have my hands full with more than I barganed for.

By the way, If you saw an old guy at the hamfest with a gray ponytail, cane, and hat with sale lists on it, it was me.
 
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I recall seeing that hat quite a few times; the list made it look kinda like an "Uncle Sam" hat. It was actually a pretty cool idea. I'd have come over and said hey If I'd known you were a fellow forum member.

I was the guy in the orange DOT cap walking 'round with a good-looking brunette. :)

It's probably not going to be too hard to swap out the coupling and bypass caps in that old Monarch. I am interested in finding out how much you'll have to change the cathode resistors to get the best bias in those 6BM8's...
 
I'll probably order the caps this week. I need to pull the cover off and look at the two cans again to see how they are rated. Looks like four sections in one large can and three sections in one small can. Luckily they are not solder in twist tab cans, but are clamped in place so removal is much easier.

I hate firing up the 160W soldering iron and digging around a bunch of wires.

I really need to find a schematic before I make any changes.

Based on tube data sheets, it looks like I have to do the following pin swaps just for pin compatibility:

Pin 2 wire to pin 8
Pin 8 wire to pin 3
Pin 3 wire to pin 7
Pin 7 wire to pin 2

Design center cathode current for two 6GW8s (ECL86s) in PP AB accroding to Phillips is only 37mA ea. That is within the limits of the Svetlana 6BM8 which calls for a max of 50mA vs Phillips 55mA.

In addition, Phillips ECL82 shows two tubes class AB at 31mA each.

Both are rated above 300V, but the ECL82 look like it is normally operated at 250V vs 300V for he ECL86.

In addition, the bias voltage for the operating points looks off a bit with the ECL82 at -15.5V (Vg2@200V) and the ECL86 at -6.5V (Vg2@250V). Again, the schematic will really be helpful in determining the bias configuration since I don't know how they are getting the screen supply and that will be critical.

I expect to need to change the screen bias as well as the cathode bias.
 
Refusing to give up I did some more testing.

I found the 'good' transformer had primary dc resistance of 189 and 204 ohms. The 'bad' channel transformer had primary dc resistance of 198 and 305 ohms. It is a pp amp.

This discrepancy is why I figured I had a bad transformer.

SQUANKKKKKKKKKKKKKKKK!!!!!!!

Gee, that sounds like a positive feedback oscillation!

GNFB reversed. I checked the circuit and it is wired per the schematic pin for pin. They must have wired the transformer feedback winging backwards or the primary although they look the same to me.

Amp is now humming along. Although I don't think it sounds as good as my SE 6P1P amps.

Time to start tweaking.

Schade feedback maybe.
 
Oops wrong thread. That was the Webcor that I got going and found the feedback reversed.

After much research and digging through my rectifier tube box, I got up the nerve to sub a 5Y3GT for the 5AR4 for a quick check out. The 5Y3 is stressed but should hold up for a little while.

I foundone bad 12AX7 and FM stereo is sounding nice.

Tuning indicator works. Picked up nearly a dozen stations with a set of rabbit ears.

Stereo indicator does not work.

Stereo separation is evident so it is de-multiplexing properly.

I used a 5Y3GT for an initial check out, and shut it down before I blow anything.

I found I have 5 6GW8s in my 'large triode/pentode' cigar box. Two look like new. One looks very good. Two show emission staining. I'll stay with the 6GW8s for now.

Multi-section capacitor selection is not good. I need :

40-40-40 @ 350V 1.4" dia
20-20-20 @ 300V 1.085 dia
40-20-20 @ 300V 1.085 dia

I've not found any of these in clamp mount, and will probably have to do some creative engineering to replace the caps.

Till I get new caps in there is no sense in powering it up again.
 
Need Schematic for Monarch STA-400X

Hi,

I just recently purchased a working Monarch STA-400X that was serviced and had 22 caps replaced, I would like to make sure that it was done properly. Does anyone have a schematic for this unit, I've googled it with no luck.

Also PS caps are a little bloated. Did anyone have any luck finding some to fit. All my Spragues are physically too large.



Regards,

Bonanza
 
Al caps should not look bloated. That is normally an indication of gross overheating and outgasing.

I'm looking for a schematic as well. If I find one I'll let you know.

I got my caps in from AES, but haven't had time to see if they will fit or not. I figure I will probably have to rearrange the cap segments to make something fit.
 
One possibility would be to rewire the output sockets for 6BM8 in place of the 6GW8 since they are still available. Slightly less power out, lower transconductance so gain will be less, but the voltages look compatible, load impedance looks compatible and biasing is close.

Old post, however this might be useful.


14GW8 fed from a voltage doubler will work just fine. Those are available for about 2-3 dollars each.

V4lve.
 
Not me I'm afraid. I went there a couple of times in the late 80's or early 90's (?). It's a long trip from NJ. Back then it was worth it in the sense that I bought a carload of stuff at cheap prices. Industrial tubes, test equipment and some audio goodies. The Southerners were very nice and the sausage & egg breakfast there was fantastic. I didn't eat the grits though. The local motels were all filled and the restaurants were not to my liking. They didn't know what half and half or real butter was.

A few years ago I heard that the cars would line up days before the place even opened to get in. That's not for me anymore. And eBay has hurt most of the hamfests very badly, including Dayton. The pickin's are too lean, especially with dozens of people running around like chickens with their heads cut off all after the same stuff. But you're probably close enough that it's worth it. So good hunting and take pictures and post them here.
 
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