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Post subject: DRRI Bias - from Stock GT to EH
Posted: Mon Jul 04, 2011 4:09 pm
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I've had my DRRI since the end of November last year. So a good 7 months old.
In the meantime been reading all the posts I find on DRRI and making mental and sometimes written notes of what has been discussed.

Finally, decided to buy a Dual Bias Probe and look at the bias of my power tubes as I've read that these can vary wildly from the factory. Wasn't dissapointed!

What I had in the amp were Groove Tube 6v6-R "Matched 10 Red" pair of tubes. I know nothing of what the "Matched 10 Red" means but include it here for those that do.

Along with a Dual-Bias Probe I also ordered a pair of Electro-Harmonix 6v6 GT matched set.

I setup my bias probe and took the following measurements:
V7 V8
Plate Volt 412 411
Cathode I 23ma 30.5ma

Using the calculator @ http://www.webervst.com/tubes1/calcbias.htm this translated to:
9Watts 12Watts

Remembering from reading of other posts, the 30.5ma seemed pretty high so I adjusted the bias to get:
420V 419V
16ma 23ma
Couldn't get them to 'match' and so I swapped the tubes to see if it would make a difference. The readings moved with the tubes so the "Matched" GT are 7ma apart. In some research I found that "Matched" tubes can be as much as 7 ma difference and still be called a "Matched Set". I was surprised to see that!

So, I installed the EH tubes which I paid a small premium, $1-2, for a "Matched Pair".
I let the amp warm up for a few minutes and then took readings.

V7 V8
Plate V 429V 429V
Cathode I 12ma 12ma
Plate Dissipation 5W 5W

I then adjusted the bias to get
414V 414V
23ma 23ma
Plate Dissipation 9W 9W

A couple of things came to mind...
1) I was surprised to see the Cathode current so different from the Factory Tubes. Especially the 30.5ma reading vs 23ma. If I remember correctly, the end result is a shorter lifespan of the tube being driven harder?

2)The sound I hear is different now from the amp. I guess I knew that would happen but in some ways, I liked the sound of the original tubes! Seemed "crunchier" and I'm thinking it was due to the higher bias of one tube. But I need some confirmation of that from you folks. Could that really make that much difference?

3)I'm going to let it sit for today and tomorrow I'll power it back up and listen again. It might be I'll need to get used to the sound of a properly biased "matched set" of tubes!

4)I know I can increase the bias and that will change the sound at the cost of shorter lifespan. I may do that as an experiment to see what happens. Not really sure though, how much higher I can go and still maintain long tube life. Any other components to be concerned with?

5)I'm also going to try another set of "matched" tubes. Money is a bit tight right now so I can't afford the NOS stuff. I'm open to suggestions if anyone has any.

6)In using the Weber calc I noticed that when telling it to compute the Plate Dissipation, after hitting the "calculate power" button, the Cathode current decrements by 1. Is that normal? Or should I "add" 1 to the value before calculating to get the real Watt number?

Why do I feel I opened a Pandora's Box now? :lol:
Happy 4th!

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Mon Jul 04, 2011 4:27 pm
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7 mA is a lot for a DRRI IMO, but that's not necessarily a bad thing... I had a pair of 6L6 in my SR that were off by 10 mA but the amp sounded so sweet with them. I remember reading what Cesar Diaz had to say:

"And tube matching – all of those Fender amps were unbalanced, and they’re push/pulls to begin with, so sure, it would help to have matched tubes, but it wasn’t super necessary. That’s not something they did at the factory, sitting there matching tubes and resistors, testing each part. They’d just put them on and every once in awhile one was magical and the next one would be suck-ass."

You might like the GT tone better cause the EH tubes aren't for you or cause the GT were biased hot. But if you can't get a good tone with a bias at 20 mA I'd try something else. If you're looking for something else to try the TAD 6V6GT-STR sound wonderful in the DRRI.

http://thetubestore.com/tad6v6.html


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 5:56 am
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That first set wasn't matched....and you'll find that in buying "matched sets" from different folks at times also.
I don't have much advice on the DRRI,I don't own one,but in my vintage DRs,I find that around 10-11 watts on the 6V6s sound the best,12 watts is running pretty hot...if you do run 'em up some,check under dark conditions that they dont cherry plate.

btw....on what Diaz or whoever it was that said that about matched tubes from the factory....way back when I was a fledgling musician and could go to the drugstore and check my tubes in the Tube-Tester,if one was bad you would buy a replacement,yep I nor any of the guys I played music with,(some had started in the late 50s),knew anything about "matched sets"......you just popped them in and away you went....so it can be done,but nowadays,I would rather set the bias and get the tubes close.


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 9:34 am
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Rebelsoul wrote:
btw....on what Diaz or whoever it was that said that about matched tubes from the factory....way back when I was a fledgling musician and could go to the drugstore and check my tubes in the Tube-Tester,if one was bad you would buy a replacement,yep I nor any of the guys I played music with,(some had started in the late 50s),knew anything about "matched sets"......you just popped them in and away you went....so it can be done,but nowadays,I would rather set the bias and get the tubes close.


+1!

Who could afford two tubes back then? At the time we were playing for union scale ($52 for a three-hour gig, split four ways). Thirteen bucks didn't go far.

:lol:

Arjay (former AFM member; Local #175 -- Trenton, Illinois)

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 11:47 am
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Tyronne ,you see by yourself that we must ALWAYS check bias when changing power tubes.

Many tech said 5 to 8 milliamp from tubes are better , more musical ,less sterile than perfectly matched. I agree with that with all the amps I test.

Enjoy your amp


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 12:21 pm
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Tyronne,

WIth the values that you ended up with, you are right at the 70% max plate dissipation for your tubes. I wouldn't go any higher.

When you enter your measured cathode current into the Weber bias calculator, it recalculate plate current which is less than cathode current. This is explained (in red) in the calculator. The bias tables that follow use plate current, but cathode current is close enough.

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 12:49 pm
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WIth the values that you ended up with, you are right at the 70% max plate dissipation for your tubes. I wouldn't go any higher.--bluesky

++++1

Most Fender amp work fine aroud 50%. With a Class AB amp if you crank the volume ,you can go overspecs at 100 % and more tube dissipation if you have a bias too hot.

1- mesure plate voltage and adjust bias; this will change the plate voltage so...
2- mesure plate voltage and adjust bias many times until no plate voltage change and you have correct bias.


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 3:09 pm
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bluesky636 wrote:
Tyronne,

WIth the values that you ended up with, you are right at the 70% max plate dissipation for your tubes. I wouldn't go any higher.

When you enter your measured cathode current into the Weber bias calculator, it recalculate plate current which is less than cathode current. This is explained (in red) in the calculator. The bias tables that follow use plate current, but cathode current is close enough.


Doh! You're right Bill. I read it again and see it.

I played the amp today and my initial reaction has changed. What I hear different is that I'm able to take the volume up higher before it breaks up so I now have more headroom than before. I'll have to adjust my distortion pedal a little.

stratele52 - I never heard that having a mismatch is good! That's good to know going forward. Thank you.

cedarblues - I'll keep the TAD in mind for my next purchase. I'm really curious now as to what I'll hear with another set of tubes. Do you go for matched sets with every purchase or take what comes?

Which brings up a question based on what I saw initially. My tubes were 7ma apart in value. How do you adjust with that difference? Do you make the highest values 23ma and let the lower one fall where it may or do you try to average? Or as I found with my set, the lower values was at 23ma and the higher at 30.5ma. Do I potentially hurt anything else in the amp besides the tube running one at that higher value?

Sorry for all the questions! Just trying to understand what's best.

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 3:35 pm
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At 23 amp / 30 ma your amp is cold so unless you want to go hot ( 70 % ) do what you want,

If you go hot use the highest tube ma ( 30ma ) for maximum mesure.

Your questions are welcome


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 3:43 pm
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stratele52 wrote:
2- mesure plate voltage and adjust bias many times until no plate voltage change and you have correct bias.


Please explain this statement. I have never heard anything like this before.

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 3:50 pm
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tyronne wrote:
cedarblues - I'll keep the TAD in mind for my next purchase. I'm really curious now as to what I'll hear with another set of tubes. Do you go for matched sets with every purchase or take what comes?

That's the fun part about owning tube amps, for me at least... I'm always trying something different and finding out what works with what amp.

Most new production tubes will come matched if you buy from someone that takes the time to test and match 'em... not as easy to find matched NOS pair. BTW, both pairs of TAD (6V6 and 6L6) I got from the tube store are perfectly matched.

stratele52 wrote:
At 23 amp / 30 ma your amp is cold so unless you want to go hot ( 70 % ) do what you want,

How's the amp cold with these #s? Using my Weber Bias and plate voltage of 420 at 20mA I'm already 70% with 6V6GT. I'm confused :?


Last edited by cedarblues on Tue Jul 05, 2011 4:06 pm, edited 1 time in total.

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 3:58 pm
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Sorry cedarblues I'm talking for a 6L6 Yes you are very hot at 23 ma .IMO less ma for a Fender

Use the 23 ma tube fot checking


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 4:03 pm
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cedarblues wrote:
stratele52 wrote:
At 23 amp / 30 ma your amp is cold so unless you want to go hot ( 70 % ) do what you want,

How's the amp cold with these #s? Using my Weber Bias and plate voltage of 420 at 20mA I'm already 70% with 6V6GT. I'm confused :?


Reread the original post.The OP only measured a plate voltage of 412/411 VDC at those current values. One tube is 9 watts and the other 12 watts.

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 4:09 pm
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stratele52 wrote:
Sorry cedarblues I'm talking for a 6L6 Yes you are very hot at 23 ma .IMO less ma for a Fender

Use the 23 ma tube fot checking

No worries, I thought you might've been talking about 6L6...

bluesky636 wrote:
Reread the original post.The OP only measured a plate voltage of 412/411 VDC at those current values. One tube is 9 watts and the other 12 watts.

Still, at 23 and 30 mA that's too hot for 6V6GT in a DRRI.


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 4:17 pm
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cedarblues wrote:
bluesky636 wrote:
Reread the original post.The OP only measured a plate voltage of 412/411 VDC at those current values. One tube is 9 watts and the other 12 watts.

Still, at 23 and 30 mA that's too hot for 6V6GT in a DRRI.


Agree on the whole. Per tube, 23 mA at 412 VDC is right at 70%. 30 mA at 411 VDC is way too hot.

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