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Post subject: High Bias Plate & Cathode in DRRI
Posted: Fri Mar 29, 2013 3:41 pm
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I bought my DRRI new in 2000, and it has been a great amp for 13 years.

It has had a pair of JAN Phillips NOS 6V6s for several years. I mainly play at home at lower volumes and the tubes were starting to develop an annoying mechanical metallic buzz or rattle. When I tapped on the tubes, it was definitely them. Other than that, the amp sounds awesome. Strong, punchy and clean until around 5 - 6.

I played a rare gig a couple weeks ago and checked the bias on the tubes before the gig. 420vdc plate, 20ma cathode. Perfect. I use a Bias King probe and check plate across pins 3 and 8 with my multimeter.

After the gig, I decided to replace the noisy output tubes. I purchased matched Tung Sol reissue 6V6s. But when I installed them and checked bias, plate voltage was 424vdc and cathode was 25ma. Lowering cathode increased the plate volts, so I couldn't do much.

I put the JAN Phillips back in and now they were also reading high. I replaced the rectifier with a new Sovtek 5AR4 and voltage was still high.

Any thoughts?


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Post subject: Re: High Bias Plate & Cathode in DRRI
Posted: Fri Mar 29, 2013 4:31 pm
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Have you checked the components under the doghouse?

Could be it's time for new electrolytics or perhaps one of the power-rail resistors is beginning to drift.

Arjay

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Post subject: Re: High Bias Plate & Cathode in DRRI
Posted: Fri Mar 29, 2013 5:11 pm
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Hi wildbilly,

Did you let everything warm up sufficiently before measuring? It's not unusual for the readings to be a little high when cold. I think that you'll find that the voltage and current will come down after things get up to temperature. Another thought, are you checking both tubes or just one, and making sure that the current and Plate voltage is for the same tube? The readings can be a little different on each tube, which is OK if they aren't too far apart.

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Post subject: Re: High Bias Plate & Cathode in DRRI
Posted: Fri Mar 29, 2013 5:40 pm
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Changing tubes and biasing is about as involved I'll get, so I have no idea what a dog house is. If things drifted, they drifted in 2 weeks.

I'm checking each tube individually. The readings on both tubes are very close.

I leave it in standby for a few minutes, then take it out of standby and wait until things have stabilized - 5 - 10 min.

All pots are turned off.


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Post subject: Re: High Bias Plate & Cathode in DRRI
Posted: Fri Mar 29, 2013 5:58 pm
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OK... everyone here (well just about everyone-- :lol: ) knows about my exploits with the new reissued TungSol 6V6GT. I've had several thermal runaways and one that dead shorted on me (out of, say 8 matched pairs). They really draw current, in a nearly uncontrolled manner --- at times.

When the tubes do this they aften take the screen resistors and/or grid stopper out. Or send them way off spec. And this will effect any new tubes replacing those TungSols.

You should prolly have a good tech inspect these resistors, the bias supply components, and the sockets for damage. And I'd stay away from those tubes. The burned in TAD 6V6GT seem much more reliable.


Sorry about that.... :(


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Post subject: Re: High Bias Plate & Cathode in DRRI
Posted: Fri Mar 29, 2013 7:01 pm
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I just checked the values of the screen resistors and grid stoppers. Screen resistors measured 473/475 ohm (470 on the schematic). Grid stoppers measured 1490/1488 ohm (1.5k on the schematic). Seems like they're pretty close, but I wouldn't know if that's within spec.

I had just checked the bias of the JAN Phillips tubes a week before the St. Patrick's Day weekend gig and was seeing ~420vdc on the plates and ~20ma cathode. I did not check again until I installed the new tubes and saw that those numbers were higher than I've ever seen.

Household voltage is 123vac.


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Post subject: Re: High Bias Plate & Cathode in DRRI
Posted: Fri Mar 29, 2013 8:25 pm
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It doesn't seem as though the tubes are drawing excess current, if so the Plate voltage would go down also, not up. The difference mentioned was only 4 volts higher on the Plate. You should still be able to lower the Cathode current enough to find a good balance if all else is OK. The wattage dissipated is what's important anyway, not just the current.

If you lower the current to 20 mA, what voltage is the Plate then? Even if the Plate went up to 430, at 20 mA, that would still be only 8.6 watts, which is very nearly the same as before (8.4 watts), and perfectly acceptable. I would bet that it is a simple matter of some sort of inaccuracy of the meter, or the meter reading reading before, or this time. I know that there have been times when I swore that my Plate voltage was different than when I read it again, so I had to start keeping notes to show that it wasn't actually changing.

BMW2002Ti,
IIRC, the trouble that you had was with a particular amp, not a DRRI, was it not?

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Post subject: Re: High Bias Plate & Cathode in DRRI
Posted: Sat Mar 30, 2013 4:30 am
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BMW2002Ti wrote:
When the tubes do this they aften take the screen resistors and/or grid stopper out. Or send them way off spec. And this will effect any new tubes replacing those TungSols.


+1!

Arjay

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