7189 vs 6BQ5/ EL84 tubes
 

I was looking at the tube sellers ads regarding 7189/EL84/6BQ5 tubes.
They list this type of tube as being interchangeable. It seems to me that the 6BQ5/EL84 itself cannot handle the higher plate and screen voltages used in some 7189 designs. I'm just wondering if these foreign made tubes would stand up to this application. The left over US sourced 7189's are really high priced. :thumbsdn:

The situation is tricky. A NOS American made 6BQ5 will likely tolerate the higher dissipation and voltages often encountered in 7189 and 7189A amplifiers. In fact, some of the early Fisher amps pushed run of the mill 6BQ5s well beyond even the 7189 ratings with no ill effect, aside from more frequent output tube changes.

As for modern tubes, the Russian EL84s will stand up to the abuse, but the Chinese tubes will not.

NOS European tubes are another ball of wax; in general, I've found them to be much less capable of tolerating the abuse that an American 6BQ5 will take.

If you don't mind rewiring some sockets, 6GK6s great substitutes for 7189s. Most of them are internally identical to a GE 7189A.

If you're building a new amp from the ground up, my recommendation would be to skip the EL84 family altogether and use the 6CZ5. They're cheaper and they offer much lower distortion at very similar operating points.

I know Zenith used 6GK6's as a vertical output tube. Years ago I worked on a Philco HI-FI amp that used 6CZ5's, impressive! In the late 50's, RCA used a 6CZ5 as a vert out. The next years models used a 6EM5. Some juke boxes used 6CZ5's.
The application I'm referring to is an amplifier used in a early 60's Fisher console stereo.

If B+ on the 6BQ5s 360 volts or less I'd say feel free to use any 6BQ5 variant (except the Chinese tubes and the NOS Euro trash).

If the B+ is greater than 360 volts, I'd say either buy new Russian 6BQ5s, or pony up the dough for some NOS GE 7189As.

I bought an octet of JJ EL84's, which seemed it be alright. Made in the Slovak republic.
The Russian tubes I'm not too crazy about are the Sovtechs. One tube from a so-called matched quad red-plates, where the other three seem to be alright.
The amplifier is a Harmon Kardon Citation II, totally recapped. I even replaced the bias pots with Allen-Bradleys. That tube red-plated no matter which socket it was in. :thumbsdn: