Initial testing of a Fisher 400

[tunedslug]

Hi all

New the to the forum.

I've just replaced the electrolytics in the power supply, rectifiers and related components of a stock Fisher 400.
I used the Hayseed Hamfest kit to replace the 4 cans. I think I am ready to do some initial testing, but don't have any working output 7868s to test it with.

I'd like to do some voltage checks to see if I am on track before I buy new 7868s

I have a good variac.

How to proceed? Can the unit be slowly brought up to 110vac and checked without tubes in place?

Thanks for your help!

David

 

[Tim D]

Hi David,

Others will be along, but you'll only get so far without the output tubes. You could bring it up to some lower voltage on the variac (90VAC?) just to check that there are no obvious problems. You could check that the heaters all have voltage and that B+ is heading northward. Other than that you won't get accurate HVDC B+ measurements throughout until you have the output tubes in place. It's important for accurate voltage measurements to have everything in place because tubes will draw down voltages - hopefully to near the schematic voltages.

If it were me I'd be hunting for the 7868 tubes now. It makes the listening experience better with them. (Kidding) But your checkout will go better with them as well. There are a few new manufacture choices available. Good NOS is also out there, but getting harder to find a match set of 4 (quad). Nothing is going to be cheap, but it's an investment in the future of your receiver.

 

[dcgillespie]

Hi David -- Welcome to the AK Fisher forum!

It is safe to operate your 400 without the output tubes installed at full line voltage, but realize that all of the B+ voltages will be somewhat higher than that indicated on the schematic -- on top of the already higher voltages due to today's higher line voltages. Expect your main B+ voltage to be more on the order of about 445 volts or so without any output tubes installed, but all others in place. Also (from an output tube safety stand point for when they are installed), check the voltage at pin 2 of each output tube. This should be at least -17 vdc -- and you likely may need to make internal adjustments to this voltage to allow modern production output tubes to draw the correct quiescent (no signal) current (usually, increase it to a more negative value). As a result, I strongly encourage you to also install 10Ω cathode current sampling resistors and 100Ω screen stability resistors at each output socket before installing the new tubes. The former will allow you to monitor the current flow through each tube (allowing you make the aforementioned adjustments), while the latter will eliminate any tendency towards output tube arcing due to the replacement of the old power supply caps with modern low ESR type caps. There is plenty of information on this forum as to had to do all of these things.

Finally, realize that modern production tubes have slightly larger pins than the original tubes did (if modern tubes are what you are going to install). They will work, but must be installed VERY carefully so as not to damage the tubes -- and once installed, with the socket's receptor pins stretched to accommodate the new tubes, the sockets will no longer work with original American produced tubes again.

Good luck with your 400!

Dave