Thank you, I see that it is 50mA... i don’t know how to convert that to voltage. Can you help?
You need help reading the manual? Okay. As the marantz owner's manual explains, tube bias is adjusted to 50 mA. There is no reference to voltage. Got it?
I took a look at a "partial" but fairly complete service manual for the 8B, which can be downloaded from hifiengine.com if you register there or are already registered there.
I think that you might be able to get **roughly** into the right ballpark by adjusting the bias pots for the nominal control grid and plate voltages which are specified in the document. Obviously that is not an ideal procedure, as the resulting bias current will depend on the parameters of the specific tubes, among other variables. But FWIW those values are as follows, measured with respect to chassis and at a line voltage of 117 VAC (adjust the following numbers by the ratio of your actual line voltage to 117) , and with inputs shorted. (If you don’t have RCA shorting plugs on hand, a number of eBay sellers offer them at very modest prices):
Control grid of EL34s (pin 5): minus 36 volts
Plate of EL34s (pin 3): 420 volts
A better procedure, of course, would be to temporarily connect a milliammeter in place of the defective bias meter. If you choose to do that, though, BE VERY CAREFUL. The terminals of the meter are at 435 volts relative to chassis ground!!!
Hope that helps. Regards,
Thank you for your efforts, I’m not familiar with the procedure you speak of. it’s over my head, I don’t understand it. I can Only adjust the bias if I know the voltage. I’m really not a tech guy, just want to listen to my tube amp.
I have a Dynaco ST 70 that tells me to set bias at 1.56 volts.I once owned a Cary tube amp and it gave me the bias in volts.
i have a HK tube amp and set the bias at 1.4 volts for each output tube. Maybe this doesn’t apply to a 8B? I don’t know what to do
Maybe this doesn’t apply to a 8B?
That appears to be the case, based on my examination of the schematic that is provided in the service manual I referred to earlier. The meter in the 8B measures the bias current directly. The voltages you referred to for the other amps are presumably the voltages appearing across resistors which are conducting the bias current, those voltages therefore being proportional to the bias current. The 8B is different.
The only alternatives that occur to me, aside from my earlier suggestions, are to try to find a replacement for the meter, or to get the amp to a good tech.
Good luck. Regards,
A trick I used to use was to dim the light down so the amp is in the dark but you can see what you’re doing. Turn them on with test speakers connected. Have adjustment tool ready and as tubes heat up, adjust bias so that the plates start to glow slightly red. Then adjust back so the red fades away. While it’s not accurate, it will ensure they are biased. Be careful not to let the power tubes glow strongly red as they could fail (spectacularly.) You can tell from the heat generated after 10 mins or so how heavily biased they are. Adjust when warmed up for sound quality ensuring they don't "red plate".
Clever suggestion, @noromance . I hadn't thought of that technique. Thanks!
Thank you, that’s pretty much what I’ve been doing. I using some old tele’s and just wanted to make sure I don’t have too
much juice on them
Adjust both output tube cathode current to 50mA and you will get 0.68V across cathode resistor.
Great find, @imhififan That looks like an excellent, easy to use, and safe to use solution.
I believe it will provide results which are **slightly** less accurate than the original meter would have provided, because the cathode current that is measured will take into account screen grid current as well as the plate current which is measured by the original meter. But I suspect the resulting inaccuracy will be small.
The one caution I would cite for the OP, if he purchases that device, is that if the tubes are removed shortly after the amp is powered down he should be sure that his fingers do not come into contact with the pins of the tubes while they are still partially in their sockets. Some of those pins will have potentially lethal voltages on them for some amount of time after the amp is powered down, as long as the pins are making contact with the socket.
The one caution I would cite for the OP, if he purchases that device, is
that if the tubes are removed shortly after the amp is powered down he
should be sure that his fingers do not come into contact with the pins
of the tubes while they are still partially in their sockets. Some of
those pins will have potentially lethal voltages on them for some amount
of time after the amp is powered down, as long as the pins are making
contact with the socket.
Add a DPDT switch in a small plastic enclosure closed to both tube socket adapter can solve the problem, if the meters are not in use, short the red and black wires by the DPDT switch. So no need to remove the output tubes and tube socket adapter.
I ordered one from amazon, thank you for the help