JCA22H bias problem

  • Thread starter Thread starter ledvedder
  • Start date Start date
Ok, I tried Rob's resistance method.

1. OT voltage is 357
2. V7 plate voltage is 354
3. V6 plate voltage is 354
4. Resistance from OT to V7 pin 7 is 61.7
5. Resistance from OT to V6 pin 7 is 75.6

With a voltage drop of 3, I did his calculations (voltage drop / ohms)
V7 is 3/61.7 = 48.62mA
V6 is 3/75.6 = 39.68mA

I set the bias calculator to EL84 tube type, entered 354 plate voltage. It gave me 23.7 for 70% dissipation.

When entering the mA values above into the Tube Dissipation Using Plate Current calculator, it gives me 143.4% and 117.1% respectively.

Something isn't right here.
 
ledvedder said:
Ok, I tried Rob's resistance method.

1. OT voltage is 357
2. V7 plate voltage is 354
3. V6 plate voltage is 354
4. Resistance from OT to V7 pin 7 is 61.7
5. Resistance from OT to V6 pin 7 is 75.6

With a voltage drop of 3, I did his calculations (voltage drop / ohms)
V7 is 3/61.7 = 48.62mA
V6 is 3/75.6 = 39.68mA

I set the bias calculator to EL84 tube type, entered 354 plate voltage. It gave me 23.7 for 70% dissipation.

When entering the mA values above into the Tube Dissipation Using Plate Current calculator, it gives me 143.4% and 117.1% respectively.

Something isn't right here.
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i trust robs method. Amperage is usually measured with the current going through the multimeter not in parallel. Can you back the bias down in range using robs method. Cameron used to bias at 100% dissipation. If you aren’t pushing the amp too hard that might be accurate.
 
I really don't want to spend $140 to bias my amp. Are there any alternative units?
 
ledvedder said:
I really don't want to spend $140 to bias my amp. Are there any alternative units?
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The 1 ohm resistor between cathode and ground is exactly what the probe is. It’s a 20 cent mod and you no longer have to use your amperage setting. It still think robs method is accurate and your tubes are just running hot.

EDIT: the euro tubes probe relies on the amperage setting by placing its self inline between the OT and the tube pin 7, so I don’t have a high level of confidence in the amperage setting on your multimeter
 
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ledvedder said:
I really don't want to spend $140 to bias my amp. Are there any alternative units?
Click to expand...
https://reverb.com/item/24981963-bi...9.v2VXRUy1gR2aGJbIj7iedNPBjB_KFHIACjENmBQOf-0
Everyone's making this far more difficult than it is, really.

Get the bias probe above, set it based on your PV of 354, 20-23mA, and call it a day. With the right tool, you'll cut out all this fucking around, and have it done in 10 minutes. Tops. Let it burn in a bit, check it again in 20 minutes or so, and adjust any drift, and done.

EL84's are typically rated 12 watts, so 12 ÷ PV of 354 x .70 gives you 23mA. I typically go a hair under for amps with shitloads of preamp gain for cleaner power, but 70% might be the ticket in that amp.

Seriously, its easy, and safer. I personally refrain from poking around. It's just unnecessary. A probe is your best friend. I have the dual SRS for amps with independent adjustments, and the Pro One for single adjustment. The Pro One still gets me PV, either way.
 
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You could also just...you know...bias it based on tone with your ears. Not trying to be a smartass either. Did you buy a matched pair for a JCA?
 
I bought a pair of Russian 6p14p's, based on another JCA22H owner's suggestion. Every other amp I've ever owned had bias test points. But, of course, this amp doesn't, and I want to get it set correctly. I bought the SRS bias probe. Thanks for everyone's help!
 
ledvedder said:
I bought a pair of Russian 6p14p's, based on another JCA22H owner's suggestion. Every other amp I've ever owned had bias test points. But, of course, this amp doesn't, and I want to get it set correctly. I bought the SRS bias probe. Thanks for everyone's help!
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You have a link to that probe ledvedder?
 
311splawndude said:
You could also just...you know...bias it based on tone with your ears.
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This is what I usually do lol. Never burned up any tubes this way in 20+ years either. I tweak the bias pot, play test, then let the amp idle for a while. If it ain't redplating after like 15 minutes you should be all good
 
Devin said:
This is what I usually do lol. Never burned up any tubes this way in 20+ years either. I tweak the bias pot, play test, then let the amp idle for a while. If it ain't redplating after like 15 minutes you should be all good
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I've never redplated a power tube. Does it literally turn red?

But also, if the bias is too hot, besides instantly redplating, doesn't it shorten the tube life significantly?
 
ledvedder said:
I've never redplated a power tube. Does it literally turn red?

But also, if the bias is too hot, besides instantly redplating, doesn't it shorten the tube life significantly?
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Yeah, you can see the plate? inside the tube glow red like the element on a stove. I reckon if they're biased too hot they will probably die earlier but like I said I've never had that issue. As long as you don't max the pot out you should be fine
 
ledvedder said:
I've never redplated a power tube. Does it literally turn red?

But also, if the bias is too hot, besides instantly redplating, doesn't it shorten the tube life significantly?
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This typically happens if you drop a hot pair in, and the bias is turned up. Or, scenario 2, would be that the tube sockets need tensioned and are loose fitting on the pins. Brands can vary on the pin thickness, and a pair with fatter pins will open them up, and a pair with thinner pins will have poor contact. As part of routine maintenance, (while you're waiting for new tubes), I would re-tension those sockets, along with deox-it.

As for actual replating, I've seen a hot pair drop in, bias shoot up to 90mA, and take out an SGR. I typically dial down the bias all the way on an install, and go up from there. Redplating will get so hot, it'll burn the logos right off. If you see a red JJ logo, for example, turned brown, that suckered got hot!

Of course, there's scenario 3, you get a shit tube out of the box.
 
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I go by ear + what my meter is telling me.
As Napalmdeath said, start low and work your way up.
Yes, the tubes will die faster if hot - but honestly, that could be a difference of 4 years vs 5 years. :dunno:
Yes, let sit awhile after playing hard and watch for bias drift after you plug probe back in.
Usually, most probes do not like to be plugged into amp while playing.
The tone will get 'too hot' when the sound starts to get warbley - as I call it. Bad cross over distortion basically.
IMO. Not a tech here.
 
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