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Post subject: 90's HRDx - Bias Problem
Posted: Tue Jan 28, 2014 3:29 pm
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I have a stock HRDx and one of the power tubes was going bad so I bought a matched set of Groove Tube GT-6L6-GE labed Red10. They are replacing a set of GT 6L6 B stock tubes. The stock tubes biased down to 50mV but when I put the new tubes in, the lowest I could get the bias wheel to was 99mV. Did I miss something? Sorry if it's a stupid question.


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Wed Jan 29, 2014 2:44 am
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Hi DeiterJason,

Are you turning the bias adjust pot the right way? Do you mean that 99 was the "highest" that it would go?

Turning the pot fully CCW should be the lowest setting, and it could be hanging up at a certain spot in the travel (where the red stuff is). Don't use a gorilla grip but you don't have to be gentle when turning the pot.

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Post subject: Re: 90's HRDx - Bias Problem
Posted: Wed Jan 29, 2014 5:05 am
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shimmilou wrote:
Hi DeiterJason,

Are you turning the bias adjust pot the right way? Do you mean that 99 was the "highest" that it would go?

.



He write lowest


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Wed Jan 29, 2014 2:36 pm
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DeiterJason wrote:
I have a stock HRDx and one of the power tubes was going bad so I bought a matched set of Groove Tube GT-6L6-GE labed Red10. They are replacing a set of GT 6L6 B stock tubes. The stock tubes biased down to 50mV but when I put the new tubes in, the lowest I could get the bias wheel to was 99mV. Did I miss something? Sorry if it's a stupid question.


My problem is the revers, I can only get reading from 35.8 to 73.2 and if I set it to the recommended 60mv I get red plating on one of my power tubes. To get rid of this I have to turn the bias down to 43.5

My plate voltage reading is 485v DC which according to some other sites, is a bit high? Each time my tubes needed changing it's always the first tube that goes, the one nearest the pre-amp tubes. any ideas.

Here's a youtube clip showing the red hot spot and readings.


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Wed Jan 29, 2014 2:37 pm
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DeiterJason wrote:
The stock tubes biased down to 50mV but when I put the new tubes in, the lowest I could get the bias wheel to was 99mV. Did I miss something? Sorry if it's a stupid question.


Welcome to the Forum! :D

No legit question is "stupid."

Try cleaning the bias pot, the tube pins, & the sockets with good contact cleaner. I like DeOxit D5 or GN5. Are you using a bias probe of some such adapter to measure the bias? Clean its adapter with contact cleaner, too.

With amp off, work the cleaner into the pot by rotating it several times full full range. Now, rotate the bias pot FULL CCW. You should be at the coldest possible setting of the bias range. If the reading is still way too hot --- TURN AMP OFF.

Did this amp have a "blown fuse issue?" Or early output tube demise? You could have a faulty component in the bias power supply portion of the amp. We can go into this, after you rule-out dirty socket/pin contact or dirty pot.



Good luck! :D :mrgreen: :D


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Wed Jan 29, 2014 2:40 pm
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Kluson56,
Kluson56 wrote:
DeiterJason wrote:
My problem is the revers, I can only get reading from 35.8 to 73.2 and if I set it to the recommended 60mv I get red plating on one of my power tubes. To get rid of this I have to turn the bias down to 43.5

My plate voltage reading is 485v DC which according to some other sites, is a bit high? Each time my tubes needed changing it's always the first tube that goes, the one nearest the pre-amp tubes. any ideas.


Sounds like you may have a set of bad tubes or bad contact to each pin in the socket. You have any other 6L6GC to try? Need to rule-out dirty socket/pins versus bad 6L6GC's.


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Wed Jan 29, 2014 3:00 pm
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Follow what BMW write .

Where do you buy tubes ? Honest seller ? If your meter work well and you know how to use it AND tubes are good , you may need to modify Bias circuit . R76 , 1.5 K to lower the value .


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Thu Feb 06, 2014 2:41 am
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Thanks for your responses, I have tried a couple of old 6L6's and checked the bias and plate voltage. Reading for bias is back to 64 and PV is around 450 so it does appear to be the valves. i didn't think this would be the case as they are only around 6 months old. :cry:


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Thu Feb 06, 2014 2:50 am
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Kluson56 wrote:
Thanks for your responses, I have tried a couple of old 6L6's and checked the bias and plate voltage. Reading for bias is back to 64 and PV is around 450 so it does appear to be the valves. i didn't think this would be the case as they are only around 6 months old. :cry:


Today's tubes are not reliable , you never know what you buy .

But be sure your bias is not too hot m reading error or false meter . Some 6L6 can't hold high voltage than other . 6L6 GC are the stronger one . GW second and 6L6 the weak one


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Thu Feb 06, 2014 10:01 am
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For the recent batch of 6L6GC's, I've had best luck with Electro-Harmonix 6L6GC and the Tungsol STR-6L6GC.


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Post subject: Re: 90's HRDx - Bias Problem
Posted: Thu Feb 06, 2014 9:34 pm
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I always have good luck with JJ tubes from Eurotubes http://www.eurotubes.com/ Disclaimer I am just a satisfied customer and have nothing financially to do with Eurotubes or JJ tubes. I have simply been a very satisfied customer in the tone and quality of the tubes I buy there. I get the anti rattle power tubes and put Eurotubes anti rattle tube damper rings on the power tubes.

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Post subject: Re: 90's HRDx - Bias Problem
Posted: Thu Feb 06, 2014 10:47 pm
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Tubes that require anti-rattle damper rings do not exactly inspire my confidence......

:roll:

Arjay

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Post subject: Re: 90's HRDx - Bias Problem
Posted: Fri Feb 07, 2014 3:22 am
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Retroverbial wrote:
Tubes that require anti-rattle damper rings do not exactly inspire my confidence......

:roll:

Arjay



+1

For me it is a lack of quality or building in parts in the tube.


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