Trying to bias my amp need some help...

  • Thread starter Thread starter bobbtoz
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bobbtoz

bobbtoz

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I've been using a Weber Bias Rite for years and never had an issue biasing my amps until today. Just purchased (2) matched tubes from Doug's tubes EL34 BHT ma-43.1 and ma- 42.9. Popped the tubes in 20-30min warm up and here's my readings on my Bias Rite:

Tube A- Vp- 456/ Ik- 39.4
Tube B- Vp- 469/ Ik- 49.4

The measurements are with the trim pot turned all the way down!

Amp- 50 watt JCM- Cameron Jose Mod (owned this amp for years w/ no issues)

Current Amp Tech is 5-6 weeks back logged, so hope I don't need to go that route.

Any thoughts/ suggestions?

Thanks
Bob
 
You're getting different plate voltage readings on each tube? A 10mA mismatch on the tubes is fine IMO. If you paid for "matched" tubes though, that doesn't seem "matched." But you shouldn't have a 10-volt difference on the plates.
 
Can you measure the voltage drop across the screen resistor on each?
 
My tech ability's are pretty much limited to the use of the bias rite and keeping one hand in my pocket.
 
You might be getting some oscillation (that you can't hear). Does your amp have grid stoppers on pin 5 of each power tube socket? If so, please check each one (and the screens) to make sure they all test out ok.
 
bobbtoz":17491vyp said:
My tech ability's are pretty much limited to the use of the bias rite and keeping one hand in my pocket.
Click to expand...

You will need a multi meter to troubleshoot. Otherwise you'll need to bring the amp to a tech.
 
At the very least, you could try swapping the tubes around to see if the voltage discrepancy follows the tubes or if it stays consistent with one of the sockets.
 
If the problem stays with the same socket and doesn't follow the tube, then check the ground of the cathode pin. But you need a multi-meter and know what to look for.
 
FourT6and2":ughdefgm said:
At the very least, you could try swapping the tubes around to see if the voltage discrepancy follows the tubes or if it stays consistent with one of the sockets.
Click to expand...

Thanks for the suggestion- I'm going to check that out.
 
Switched the tubes around:
A- IK- 44.7/ VP- 462
B- IK- 39.4/ VP- 468

*trim pot adjusted to lowest setting
 
Ok so your initial reading was that Tube A was reading a lower Vp as well as a lower bias.
You swapped the tubes and now Tube A is reading a higher bias but lower Vp (although Vp is now much closer to each other)?

It also looks like your bias can't get low enough? Your bias pot is all the way down, but you're reading over 70%.

Without a multimeter to check a few things, it's hard to really help and further. The only thing you can really do is assume it's a tube problem and return/exchange the tubes for a new set.
 
I appreciate your assistance. It sucks I'm so limited w/ the inside of an amp. Might try to throw the old set in there and see if I have similar results.
 
I'm not an expert on this stuff but any chance there is something wrong with the Bias Rite?
 
romanianreaper said:
I'm not an expert on this stuff but any chance there is something wrong with the Bias Rite?[/quo

Well I put the old tubes in and the plate voltage was 455 on both, however still a big difference in tube ma- Tube A 42/ Tube B- 32.3. At least my trim pot wasn't bottoming out at 39.4.
Click to expand...
 
romanianreaper":2r8hjfe8 said:
I'm not an expert on this stuff but any chance there is something wrong with the Bias Rite?
Click to expand...
Yea, check the battery
 
Send the tubes back and tell him you need a cooler quad because you cannot bias them. The different voltage reading is more than likely your bias rite not functioning fully proper.
 
bobbtoz":3ano0r0p said:
romanianreaper":3ano0r0p said:
I'm not an expert on this stuff but any chance there is something wrong with the Bias Rite?[/quo

Well I put the old tubes in and the plate voltage was 455 on both, however still a big difference in tube ma- Tube A 42/ Tube B- 32.3. At least my trim pot wasn't bottoming out at 39.4.
Click to expand...
Click to expand...

The difference in ma draw isn't that bad. Set the bias where you want it on the one that draws the most current, the other will just draw less current no biggie. People obsess too much over perfectly matched tubes.

I would venture to say that the new set of tubes are just hotter tubes drawing more current than your old set and/or they may have QC issues, that happens sometimes with tubes. Your house voltage can move +/- 5-10 volts sometimes and your plate voltage will go up and down accordingly that is normal. If the new tubes have a partial short becoming resistive that could also cause your plate voltage to go up or cause different DCV on tube socket.

If the plate voltage is good at 455DCV on both sockets with the old tubes ma current draw 32/39 is fine, if the amp sounds good play the old tubes and maybe ask for a replacement set. Since you know the bias range the old tube are in you should ask to see if you can get a set that match the current draw range of the old tube set. Tell your tube guy what current draw range is with the plate voltage and they should be able to get close so you will have enough bias range to adjust them without having to start swapping resistors in the bias supply to accommodate tubes that have a higher current draw to the ones you are running in there now.
 
Going return/exchange new tubes and try another set. Lots of great suggestions covering all angles. Once again thanks for the help.
 
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