[AMRadio] 6U8 Subststute?


jeremy-ca km1h at jeremy.mv.com
Sat Nov 10 20:13:40 EST 2007


Actually the AC mains before a standard was set varied from 110-125V 
depending on location. At that particular house I had a pole pig right 
outside and my line was 121V most of the time. I also had a CE 100V which 
was loaded with 6U8A's and they lasted just fine. The 20A was used to drive 
VHF transverters.

Before jumping to conclusions that a tube swap has to be made check the 
coupling and bypass caps for leakage. Do the voltages agree? The 290V plate 
spec is a bit high and could be dropped down to 240-250 with little or no 
effect. Has the PS been changed to SS?

The 6MU8 has a bit more pentode section dissapation at the expense of more 
filament current; but the 6EA8 has more in the triode and even a tad more in 
the pentode than the 6U8A. IMO it is a better plug-n-play fit.

If line voltage is an issue then install a small 12V transformer in a 
bucking configuration with the AC line. Ive built bucking transformers into 
dual duplex metal outlets at all my boatanchor operating positions and work 
benches. Takes up a lot less room than a Variac which can then be used for 
other purposes such as powering up BA's for the first time.

Glad to hear someone is still flogging those old CE's. I was always partial 
to phasing rigs and would love to find a 100V again.

Carl
KM1H




----- Original Message ----- 
From: "Jim Candela" <jcandela at prodigy.net>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Saturday, November 10, 2007 5:59 PM
Subject: Re: [AMRadio] 6U8 Subststute?


>
>
> ----- Original Message ----
> From: jeremy-ca <km1h at jeremy.mv.com>
>
> If the 20A is running them too hot then I'd check out the circuit for
> something causing excessive current drain. I used a 20A decades ago and
>
> never had that problem.
>
> Carl
> KM1H
>
>
>
> Reply by Jim WD5JKO:
>
> Carl decades ago the line voltage was closer to 110-115 vac whereas now it 
> is closer to 125 vac. A stock 20a plugged into todays AC mains will 
> literally "cook" the tubes. Sure a variac is the right choice here, but 
> the poor 6u8A is still the weak link. The 6MU8 is just a beefier 
> alternative.
>
> Regards,
> Jim
> JKO
>
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