On 05 Aug 2012 23:10:03 GMT, Jim Mueller wrote:

On Mon, 30 Jul 2012 13:59:54 -0500, legg wrote:


6BK7A/ECC83

RL

I assume this is a typo. An ECC83 is a 12AX7. It doesn't have the same
pin connections as a 6BK7 and isn't particularly suited to VHF uses.

A lot of typos that day, including the header, which should have read
Counterpoint, as in the message body.

Should have read 6AQ8/ECC85 and 6BK7A/ECC81

Also, again 6AQ8/ECC85

So no ECC83/12AX7's there.

As well, first IF is 6AU6, vs the 6BA6 in the documentation. Didn't
figure that this was major, because the other two IFs are 6AU6, as
indicated in SAM's. and no major problems appear in this section.

Anecdotal info suggests that ECC85 (6AQ8) was developed and marketed
after ECC81/12AT7, and that ECC85 was developed specifically for VHF
input/oscillator combinations. An improved inter-device screen
supposedly reduced oscillator stage radiation. This isn't an
input/oscillator schematic application, in any event.Not sure what
6BK7A does that's special.

When converting from 6BK7 to 6AQ8 or 12AT7, the cathode bias resistor
in self-bias is supposed to increase from 56 to 200R, to maintain
similar plate currents.

Looks like this is actually a later revision.

Just wish they'd documented the changes.

The IF strip was functional, but seriously out of tune. Would accurate
notes have indicated a different IF? Anyways, I swung it back to
10.7MHz.

The main problem was the silent local oscillator. This turned out to
be a persistently bad tube pin contact, which finally responded to
vigorous polishing in the socket connector.

The whole commercial band didn't really wake up until the base and
cover were reattached. Luckily, most fine tuning can be performed with
the case fully assembled, as the appropriate access points are present
in the metalwork.

All OK for now. Wish there was a real schematic floating around, or at
least some explanation of the revision history.