tube biasing

[darrkwing]

Im kinda a newbie to tube gear, and i would appreciate some imput. I’m fortunate to have the gear that i have, i have a few questions concerning the (4) new electro-harmonix 7591 tubes i purchased from Viva Tubes. They are a bit larger than tubes that came with the amp which are (2) RCA’s (1) Sylvania (1) Phillips Here are my Questions

Two of the new Electro-Harmonix have a very faint blue glow near the getter, is this cause for concern?
Here are the voltage readings i get from each tube in the order they are in the amp.
#1, 294V, # 2, 390V #3, .472V #4 .442V i double checked #3 #4 I’m not sure why the decimal point moved.
there is a big voltage difference between #1 and #2 is that a concern ?
when i set my multi-mete to miliamps. i get no current readings at all, why?
i connected analog milliamp meters to the ports to check bais readings, and they show current readings, for each tube, #1, 20ma #2, 28ma #3, 30ma and #4. 35ma (These meters may not be accurate, but they at least show whats going on with the tubes ). on the box it reads #1, 45.2ma #2, 45.6ma #3, 46.2 and #4, 47ma
I have not yet checked the voltage readings of the tubes that came with the amp, i am waiting to install a new JJ rectifier friday, although there doesn’t seem to be anything wrong with the one in there now. i just want to see if there is any difference in the voltage of the new tubes with the new rectifier over the older one. stay tuned, below is the url of videos of the amp on youtube

 

[gadget73]

blue glow isn't a problem

Those should typically bias somewhere under 1/2 (0.5) volts. Not sure what exactly its meant to be set to, but you should definitely not have hundreds of volts. Hundreds of volts sounds like an open current sample resistor.

Usually this is measured across a 10 ohm resistor, so using the MA scale won't read accurately. Volts / 10 (assuming 10 ohms) will give you amps, so 0.3v 0.03 amps or 30ma. If you get no reading at all, check the fuse in the meter and confirm the leads are connected correctly. Some need the connections moved around for it to work.

 

[Schmidlapper]

“Two of the new Electro-Harmonix have a very faint blue glow near the getter, is this cause for concern?”
No

Your voltage readings have no real context as presented.

Regarding the no milliamperes question, I can only assume your measuring incorrectly, not in circuit.

Is there individual or pairs bias adjustment for the tubes on this amp?

The transformer cooling is interesting to look at. It would really be interesting to perform performance measurements with and without the output transformer ones operating, not temp measurements, but rather full power frequency response and square wave tests just to determine impact.

Bill

 

[kward]

A schematic for this amp would tell the story on how the bias is to be measured. Any chance you have it? But In the range of 30 mA bias is not unrealistic for 7591’s.

very interesting setup you’ve got with some nice additions to your turntable.

 

[Schmidlapper]

The four amp meters showing in the photo all show the same current reading, ~21 mA.

Bill

 

[mkane]

 

[mkane]

 

[GordonW]

A schematic for this amp would tell the story on how the bias is to be measured. Any chance you have it? But In the range of 30 mA bias is not unrealistic for 7591’s.

very interesting setup you’ve got with some nice additions to your turntable.

If it's the amp in the picture, that looks to be a Scott 208.

Regards,
Gordon.