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Post subject: Do bias probes go bad?
Posted: Mon Aug 04, 2014 9:04 pm
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So this evening I decided to check the bias on my DRRI for drift after having biased it a few days ago. For some reason I couldn't get the correct reading utilizing the probe.

I'm only reading 11ma of current on my meter with the bias probe yet @ TP5 I get -49 VDC and at TP36 I'm reading -37vdc, whick is what it should be. Plate voltage read at 407Vdc and all the other TP's at the tubes and phase inverter section check out ok. There's nothing wrong with the amp, it sounds and play great. So is it possible that the bias probe is bad?

I'm using the single octal base bias probe from eurotubes in conjunction with a fluke meter.

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Post subject: Re: Do bias probes go bad?
Posted: Mon Aug 04, 2014 9:16 pm
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Is that the probe that reads actual current and not mV? If so, are you using the current jacks on the meter?

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Post subject: Re: Do bias probes go bad?
Posted: Mon Aug 04, 2014 9:41 pm
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A bias probe is usually just a big one ohm resistor between the tube's cathode and the amp ground, there's not much to go wrong in there. When you measure the voltage drop across that resistor you get a reading that equates to the number of Amps flowing through it.

Difficult to say what's wrong. Did you let the amp warm up for a while before testing? Do all the output tubes read the same? You should be getting nearer 25mA on that amp, I'd guess you need to bias it hotter. Or maybe you've got a bad tube, how long have they been in the amp?

The bias test point readings on the schematics aren't absolute values, they're starting points that allow the amp to run safely before you start using the bias probe. Try running the amp hotter, you'll probably find it springs to life immediately, even if you thought it was sounding ok to begin with.

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Post subject: Re: Do bias probes go bad?
Posted: Mon Aug 04, 2014 9:47 pm
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The schematic shows TP5 -49V, TP6 -37V as you mentioned. Those values are directly after the first bias filter cap and after the bias adjuster pot respectively. As long as there's a significant bias voltage available then you should just be able to back off the bias voltage and see the tube current jump up.

The second TP is after the pot, that's the value that will change when you decrease the bias. It doesn't have to be exactly -37 Volts, don't worry about the voltage there, worry about the current through the tube.

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 8:18 am
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shimmilou wrote:
Is that the probe that reads actual current and not mV? If so, are you using the current jacks on the meter?


Yes it's is, I made sure to plug it in to the mA slot. I've had the probe for several years now and just recently started having issues with it.

GilgaFrank wrote:
A bias probe is usually just a big one ohm resistor between the tube's cathode and the amp ground, there's not much to go wrong in there. When you measure the voltage drop across that resistor you get a reading that equates to the number of Amps flowing through it.


Does that apply to my probe which is read in mA?

Difficult to say what's wrong. Did you let the amp warm up for a while before testing? Do all the output tubes read the same? You should be getting nearer 25mA on that amp, I'd guess you need to bias it hotter. Or maybe you've got a bad tube, how long have they been in the amp?

I let the amp warm up for an extensive amount of time. I haven't tried it on the second tube so I'll give that a shot. Tubes are brand new, less than four hours of use on a different amp.

The bias test point readings on the schematics aren't absolute values, they're starting points that allow the amp to run safely before you start using the bias probe. Try running the amp hotter, you'll probably find it springs to life immediately, even if you thought it was sounding ok to begin with.[/quote]

If I can't get the probe to read correctly I might just go that route. Problem is I wouldn't know how much current is running through the tubes. What would be the absolute highest voltage reading across TP36 without putting too much strain on the tubes, -34VDC?

Thanks for the input guys.

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 9:18 am
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socal323 wrote:

Does that apply to my probe which is read in mA?


See my next post, that's complicated. But the answer is YES

socal323 wrote:
If I can't get the probe to read correctly I might just go that route. Problem is I wouldn't know how much current is running through the tubes.


Of course you would! That is precisely what the bias probe is telling you. If it says 11mA then that's the current running through the tube.

Again, don't get hung up on the -37 Volts test point. Remember that reading comes AFTER the bias trim pot. The trim pot is there to let you vary this bias voltage to suit the particular tubes in your amp, it does not have to be precisely that -37 Volts figure. You need to vary the bias voltage closer to zero Volts to allow more current to run through the tubes. That's what biasing an amp is all about.

The -37 Volts reading is irrelevant, it doesn't need to be exactly that figure. The bias exists to control the tube current. Set the bias voltage to whatever it needs to be in order to give you 25mA (or thereabouts) through the tubes.

The whole reason you have a bias adjuster pot is to compensate for variations in the electrical properties of different tubes. If you insist on setting -37 Volts then you're not compensating for the variations in different tubes. Turn that pot til you get 25mA, don't bother with the bias voltage test point, your amp will spring to life and you'll be a very happy bunny.

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 9:23 am
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Regarding the meter measuring current or voltage - ALL current meters really measure voltage internally. Or more specifically they measure voltage drop across a known resistance then calculate the current.

Ohm's Law says V = IR, voltage equals current times resistance. If 11mA of current passes through a 1 ohm resistor then the voltage drop will be 11mV. By measuring that voltage drop the meter calculates a current of 11mA. That's how your bias probe works.

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 9:27 am
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socal323 wrote:
What would be the absolute highest voltage reading across TP36 without putting too much strain on the tubes, -34VDC?


That's the whole point. No one can tell you. Not Fender, not JJ, not any amp tech. You don't set the bias by measuring that voltage!

All tubes are different, they all react slightly differently in an amp. Tiny variations in the spacing of the grids and plates make them vary electrically and that's why you need to just measure the current at the cathode (11 mA) and set the bias voltage wherever it needs to be in order to raise that to around 25 mA.

The bias trim pot is only there to adjust the current through the tubes when you change them. THE VOLTAGE AT THE TEST POINT DOES NOT MATTER. ONLY THE CURRENT THROUGH THE TUBES MATTERS.

I'll say it again - the -37 Volt figure is irrelevant. I hate to say it but if you don't understand this then you'll never get the amp biased correctly.

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 11:02 am
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Thanks for all the help and knowledge. I understand exactly what your saying, I was just trying to get a better understanding of the TP's. I've been able to bias my Bandmaster, Bandmaster Reverb, and Twin reverb amp with no problems. The deluxe reverb is a new aquisition and I was just giving it a little tlc as it had all original tubes from 1991. I had a set of like new Electro Harmonix 6v6's and slapped them in. I just couldn't get it to bias above 14ma with bias pot maxed out. That's why I started with the test points to make sure my voltages were correct. I apologize for not pointing this out earlier.

Well I finally got it to where it needs to be. 23ma with 407 plate voltage. Funny thing is I had to have the volume control set to 10 to be able to set that particular bias. I followed the schematic instructions which dictate that I set the trem intensity to max and all other controls to 50% with the trem and reverb off. But I was never able to get more than 14ma with the bias pot maxed out. Then I increased the volume knob to 10 and viola, current jumped up and I was able to back it down to 23ma. It sounds good where it is and thats where I'm gonna leave it.....for now. :wink:

Should it matter where the volume pot is set when biasing? I see alot of bias video's out there but very few, if any, mention how the controls are set.

Thanks again.

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 1:28 pm
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I always bias amps with the volume up just a little, not sure it makes any difference - but it certainly seemed to on your amp. I should probably give the Marshall a test this week, not sure I can motivate myself to pull it out of the rack, open it up and fiddle with the tiny pots hidden away inside though.

Anyway, we got there in the end, is the amp snarling and growling like a bulldog now?

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 4:20 pm
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This amp is growling like a rabid dog that just had its dinner taken away :twisted:

There is nothing like pushing an amp to the verge of breakup and then giving it just a hair more. It sounds amazing.

Never owned a Marshall, but always dreamed of owning a JCM800. That's the one I'm holding out for. :D

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 4:52 pm
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I'm English, I grew up listening to Motorhead and Deep Purple so really there's no other amp for me. There's something about a set of steaming EL34 tubes that just kicks you in the chest and forces midrange into your brain until your brain explodes and trickles out of your ears. And I mean that in the nicest way possible.

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Post subject: Re: Do bias probes go bad?
Posted: Tue Aug 05, 2014 9:45 pm
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If you have the Eurotubes probe that reads current directly, there is not a 1-ohm resistor in the probe.

The volume of the amp has nothing to do with bias current. The tremolo can have an affect on the bias, so the tremolo should be disabled when biasing. That is probably why your bias reading was off, you had the tremolo on.

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