[Central-Electronics] 20A project

Wed Feb 19 14:59:06 EST 2020

Nice to have you on board, Curt! 

While you’re busy with cosmetics and passive component replacements, you might find it helpful to purchase a new 9.0 MHz crystal.  There are numerous vendors selling them on eBay for very short money.  Most of the crystals in CE exciters still work, but they can and do drift, and will drive you crazy. 

Keep going, you’ll get there.

73, George – W1LSB

From: Curt [mailto:radio.ku8l at gmail.com] 
Sent: Wednesday, February 19, 2020 12:04 PM
To: Jim Candela; central-electronics at mailman.qth.net; George Maier
Subject: Re: [Central-Electronics] 20A project

Hi Jim and group:

I am waiting on all the new parts to come in.  replacing all the silver micas around the osc, the B+ feed resistors and the precision coupling resistors that feed the PS-1.   In the meantime, I am doing some of the mechanical cleanup and a scrubbing bubbles treatment and dry cycle in the bake-out oven.  I got all the melted pitch out of the area and mapped out the re-wiring.  Haven't pulled the old stuff out yet--will do that once I have all the parts on hand.  Its nice to work at a commercial industrial electronics service center!    I have a 6MU8 on hand to use.   All wafers look good and all pots are free.   I  will start it all up with the finals pulled to be sure it all works up to that point without risking the PA's

Curt,   KU8L

On 2/18/2020 4:13 PM, Jim Candela wrote:

George,

   Regarding that 6MU8, I only refer to it being used in the 20A where there was a 6U8/A. Other applications might be a crap shoot. On my Gonset G76, they use a 6U8A in the receiver front end (osc-mixer..I think), and in that application my dial calibration jumped quite a bit. Similar in the lakeshore VFO, I replaced the 6CB6 VFO tube with a 6BH6 to save on filament power. Worked fine, but frequency went up 50Khz or so...

hello Curt...let us know where your at with your project. 

Jim

Wd5JKO

On Monday, February 17, 2020, 8:37:18 PM CST, George Maier via Central-Electronics  <mailto:central-electronics at mailman.qth.net> <central-electronics at mailman.qth.net> wrote: 

Hey Jim:

That 6MU8 sounds realy interesting. As you know, both CE and Collins switched from 6U8's to 6EA8's in most cases as the latter was a better tube.  How would you compare the 6EA8 to the 6MU8?

Thanks, 73, and Happy New Year!

George - W1LSB

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net [mailto:central-electronics-bounces at mailman.qth.net] On Behalf Of Jim Candela
Sent: Friday, February 14, 2020 2:07 PM
To: central-electronics at mailman.qth.net; Curt
Subject: Re: [Central-Electronics] 20A project

Curt,  Yes, replacing the socket and related components might be best. I don't have the schematic handy, but most B+ circuits on the 20A have a 560 ohm CC 1/2 resistor in line with whatever is being fed. I believe those are supposed to be the "sacrificial fuse".

As an FYI, a 6MU8 is a pin for pin near equivalent to the 6U8. The main difference is the glass envelope is a little bit taller and the pentode plate dissipation is higher than the 6U8/A. I have been using the 6MU8 for years and years now in my 20A. Most tube sellers have the 6MU8 stockpiled...CHEAP PRICE!

When the B+ is high, like with 125vac line voltage, or worse yet, high line and solid stated 5U4, then the 6U8/A really cooks...the weak link in the chain.
JimWd5JKO

    On Friday, February 14, 2020, 10:54:59 AM CST, Curt <radio.ku8l at gmail.com> wrote:  

Got to look inside the 20A.  This is the rack mounted one, "2nd" gen with the 6U8 Master Osc/aud driver and had tar around TR-1. Well, it had suffered a catastrophic failure around the 6U8. Cathode and Osc plate Res burned open.  the 2 coupling Res to the PS-1 burned.  The 6U8 had a suck-hole thru the side and was massively shorted internally.    TR-1 measures OK so it may have survived long enough to burn out the cathode resistor in the audio half of the tube.  I found some disconnected parts and loose repair/mods at that tube and suspect  the tube ran away and melted down.  (Found the grid resistor on the osc half disconnected). Stuff got real hot for quite some time to melt the pitch out the bottom of TR-1.  PS-1 looks OK as does the coils after it. Crystal is OK.  I might bite the bullet and remove the whole tube socket etc because the socket pins might be trashed.    I don't think I can post pics here but I will post updates with pics on the Vintage Amateur Equipme
nt BA Facebook Group.

Curt,  KU8L
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