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Old 8th September 2010, 07:02 PM   #11
TheGimp is offline TheGimp  United States
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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.
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Old 9th September 2010, 01:59 AM   #12
TheGimp is offline TheGimp  United States
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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.
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Old 9th September 2010, 02:43 AM   #13
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Glad the ol' Monarch is humming along. I would imagine it will sound better after a bit more tweaking.

I'll put in with you on the Sccade feedback. That little RH84 made a believer out of me!
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Old 10th September 2010, 03:47 AM   #14
TheGimp is offline TheGimp  United States
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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.
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Old 18th September 2010, 08:57 PM   #15
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Default 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
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Old 21st September 2010, 05:10 PM   #16
TheGimp is offline TheGimp  United States
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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.
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Old 22nd August 2013, 08:30 PM   #17
TheGimp is offline TheGimp  United States
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Well, that time is approaching again and the good news is that the Hamfest has moved back to the Cleavland County Fairground in Shelby NC.

Shelby Hamfest 2013 - Cleveland County Fairground, Shelby, NC sponsored by Shelby Amateur Radio Club

Anyone going?
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Old 22nd August 2013, 09:01 PM   #18
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Quote:
Originally Posted by TheGimp View Post
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.

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Old 22nd August 2013, 09:02 PM   #19
TheGimp is offline TheGimp  United States
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I'll probably stay with the 6GW8 as I don't want to add another transformer.

Stan from ESRC will be at Shelby.
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Old 22nd August 2013, 09:22 PM   #20
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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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