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Post subject: DRRI Redplating Problems
Posted: Tue Feb 07, 2017 11:26 pm
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Hey guys it's been a while since I've been on this forum. I'm having problems with a DRRI that I've owned for about three years now. It was manufactured in 1993. Soon after I bought it I had the power supply recapped and it's worked for me very well up until a month ago. The power tubes have started redplating within about 20 seconds of turning the amp on. I've tried different tubes for all preamp and power tubes. Using my BiasRite to check plate voltage and current I get a really high current and low voltage, I'm talking 150 mA and 300V. A key detail is that both tubes (and I've tried a few pairs) redplate at the same time and look to be the same intensity. I'm not sure where to go from here; I have a good multimeter and I'm good at working with circuits but I do not have an oscilloscope. Any help would be greatly appreciated.


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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 2:23 am
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Output tubes need a bias check and adjust if needed .

You check and see 150ma, that'is why they red plate.
Now adjust bias :lol:
Or tubes are bad ; short ? You write amp work for a month, you may adjust bias toooooo hot and destroy tubes ?
How much ma did you adjust bias a month ago ?

1- Remove 6V6's, power amp ON
2- Read negative voltage pin 5, on each 6V6 you should have close to - 37 Vdc
3- If not, adjust bias pot until you are close to -37 Vdc
4- Can't get negative bias voltage ? Check bias circuit, you may cut a "wire" or printed circuit when you work on. Or defective resistor , diode .....


Last edited by stratele52 on Wed Feb 08, 2017 3:58 am, edited 1 time in total.

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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 3:39 am
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+1 stratele52

Seems that you might have lost the bias supply voltage. :idea:

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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 4:12 am
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shimmilou wrote:
+1 stratele52

Seems that you might have lost the bias supply voltage. :idea:



Or tubes where destroyed by a wrong bias ( to hot ) adjustment ?


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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 6:52 am
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shimmilou wrote:
+1 stratele52

Seems that you might have lost the bias supply voltage. :idea:


+2

Time to pull all the tubes except for the rectifier and start taking some voltage measurements. The test points and the corresponding voltages are clearly marked on the re-issue schematic.

Arjay

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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 7:29 am
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If you are only getting 300VDC on the anode plates of the 6V6GT, something is wrong. Now, the excessive draw by the tubes (near short) could lower the plate voltage. Try removing the tubes and read the voltage. If it goes back to 400-425VDC + --- then the bad tubes are dropping the voltage.

But, if the voltage remains at 300VDC... you have other problems. Low voltage with no tube load indicates possible problems with the power supply. Low voltage usually leads to higher current draw by the tubes, to compensate.

Also check tube pin contact with the socket. May need cleaning and tightening. Recent made sockets seems to be a little more prone to bad contact versus the old school US made Amphenol and Belton sockets.

Try a little contact cleaner. Good luck! :)

BTW... mA for typical DRRI should be around 20mA per 6V6GT (with nominal plate voltage of 400-425VDC).


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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 7:50 am
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BMW2002Ti wrote:
Now, the excessive draw by the tubes (near short) could lower the plate voltage.


Not could lower the plate, it do !

It is like each time you do bias adjustment;
Colder bias, less tube draw and higher are the plate voltage.

Hot bias, more tube draw curent, less plate voltage.


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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 7:54 am
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BMW2002Ti wrote:
Recent made sockets seems to be a little more prone to bad contact versus the old school US made Amphenol and Belton sockets.


The amp is twenty-four years old.

I doubt that there's anything amiss with the sockets.

Arjay

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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 8:06 am
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Retroverbial wrote:
BMW2002Ti wrote:
Recent made sockets seems to be a little more prone to bad contact versus the old school US made Amphenol and Belton sockets.


The amp is twenty-four years old.

I doubt that there's anything amiss with the sockets.

Arjay


+1 , both socket broken ( pin 5 ) in same time ? I guess not.
Common printed circuit broken ?

Why recap a not old amp ?


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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 4:58 pm
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BMW2002Ti wrote:
If you are only getting 300VDC on the anode plates of the 6V6GT, something is wrong. Now, the excessive draw by the tubes (near short) could lower the plate voltage. Try removing the tubes and read the voltage. If it goes back to 400-425VDC + --- then the bad tubes are dropping the voltage.

Today I took some new measurements of the power tubes. With the tubes in the amp, pin 3 measures 302VDC on each socket. Without the tubes the voltage goes up to 456VDC, so it seems like the tubes are the problem. But when I swapped them with a known working pair of 6v6 from my Princeton Reverb the voltage is again around 300VDC! Why could this be?

Just a note that the bias was always in an acceptable range until this stuff started happening a month ago. Adjusting the bias pot now will not even get it close to where it needs to be.


Last edited by JZL on Wed Feb 08, 2017 5:16 pm, edited 1 time in total.

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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 5:08 pm
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Pin 5 should read minus 32 volts ( -37 volts dc ) not 300 to 400 volts :shock:

That are plate voltage which is pin 3 !!!

What are you doing ?


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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 5:16 pm
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Sorry I meant pin 3, post will be edited


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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 5:31 pm
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JZL wrote:
Sorry I meant pin 3, post will be edited



We know about 300 to 400 volts you read, we ask you for pin 5 without tubes. The bias voltage.


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Post subject: Re: DRRI Redplating Problems
Posted: Wed Feb 08, 2017 10:43 pm
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JZL wrote:
...Today I took some new measurements of the power tubes. With the tubes in the amp, pin 3 measures 302VDC on each socket. Without the tubes the voltage goes up to 456VDC, so it seems like the tubes are the problem...


As already stated, check the bias supply voltage, seems like the output tubes are only behaving as they should, assuming little or no bias voltage. That would be pin 5 on each of the output tube sockets. Leave the output tubes out, all other tubes can be left in.

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Post subject: Re: DRRI Redplating Problems
Posted: Thu Feb 09, 2017 2:09 am
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Don't put output tubes in the amp until you fix bias voltage issue.


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