To work with 6BQ5/7189, a minor wiring change is needed. Pins 1 and 6 are ¡°unspecified internal connection¡± while the corresponding 7189A pins are explicitly G1 and G2 respectively.
On all four sockets, move the G2 wire from pin 6 to pin 9. That is all.
EL84 pins 1 and 6 are no-connect so this tube doesn¡¯t need the mod.
The output tubes yank their plates toward their cathodes (which are being held at a negative voltage determined by the AMPLITUDE control), then release them and the load resistance pulls the line back up to ground.
As the tubes wear out, two things happen. (1) Cathode emission decreases. This ultimately limits the maximum negative excursion. (2) Sometimes the cathode develops an ¡°interface layer¡± which acts like an RC time constant. Since only the positive corner¡¯s quality is specified in HI AMPLITUDE mode, this doesn¡¯t matter unless it becomes so bad that it compromises the negative output in FAST RISE mode.
Dave Wise
From:
[email protected] <
[email protected]> On Behalf Of Stephen via groups.io
Sent: Tuesday, March 01, 2022 10:20 AM
To:
[email protected]Subject: Re: [TekScopes] Type 106 Saga (Again)
On Tue, Mar 1, 2022 at 06:39 PM, Dave Wise wrote:
In Fast Rise mode, the output tubes provide base drive to the Fast Rise
circuit. See SW242A on the schematic.
SW242B bypasses the AMPLITUDE control in Fast Rise mode, jamming it to
maximum. So don¡¯t leave it running for hours in Fast Rise mode unless you
have lots of spare 7189A¡¯s.
Dave Wise
Hmmm. I have left it on a few times for quite a long time¡
Mine doesn¡¯t have 7189A¡¯s. It has old Philips EL84¡¯s which read very strong. But I¡¯ll check them again when I get back.
I¡¯ll report back when I can get check all that tou guys suggested.
And thanks for the tip regarding preserving the tube life. I¡¯ll certainly be more careful now¡.
