[Central-Electronics] 20A project

[Curt]

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 <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> 
> [mailto: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 
> <mailto: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 <mailto: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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