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Post subject: Another Bias Question
Posted: Tue Apr 05, 2016 2:15 am
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Whilst I am very happy with the Tung-Sol 6V6GT power tubes in my DRRI I am curious to try the TAD 6V6GT-STR.

On the TAD website it says to bias them at 28mA when installing in a DRRI. Does this mean there is no need to check the plate voltage (which I cannot do and would need the help of a techie) and can I just use my simple bias probe to set the current?

My Tung-Sols are biased at 22mA so I am thinking that TADs biased at 28mA might sound very different.


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Post subject: Re: Another Bias Question
Posted: Tue Apr 05, 2016 5:20 am
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How to bias is always same;
Other way is a guess.
1-You must check plate voltage .
2-You must check bias each time you put new tubes

As you adjust bias, plate volts change.
Check plate volts for each bias adjustment pot turn, re-adjust bias.

You may do listening test with some different bias and keep the one you like more with safe bias.


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Post subject: Re: Another Bias Question
Posted: Tue Apr 05, 2016 6:36 am
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stratele52 wrote:
How to bias is always same;
Other way is a guess.
1-You must check plate voltage .
2-You must check bias each time you put new tubes

As you adjust bias, plate volts change.
Check plate volts for each bias adjustment pot turn, re-adjust bias.

You may do listening test with some different bias and keep the one you like more with safe bias.

That has always been my understanding but when the tube manufacturer states "bias these at 28mA" what am I to do once I know the voltage? My understanding was that you needed to know the voltage so that the current could be calculated for 60% (or some other percentage) dissipation at idle. But, if the manufacturer is telling you what the current should be then what does knowing the voltage add? It seems very simple but am I missing something?


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Post subject: Re: Another Bias Question
Posted: Tue Apr 05, 2016 7:04 am
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Bias calculation came from cathode/ plate current and plate voltage.

Why amp manufacturer suggest a specific bias current ? How did he choose it ?

Some answers ? I guess it is ;
1- Voltage on schematic
2- Brand of tubes

1-Schematic voltage may not be the same with your house wall outlet. Or over the years, amps's parts aged.
2- You may use another tubes's brand than original or same brand but not same production .

To resume; you are safe if you use manufacturer suggested bias. It may not a fine tuning but amp is safe and sound should be good.
If you can't read plate voltage, this is the best and only way for you.


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Post subject: Re: Another Bias Question
Posted: Tue Apr 05, 2016 8:16 am
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Thanks for the comments.

I am prepared to trust TAD as they are a major tube designer/seller and I will use my simple bias probe to set the bias as they recommend if I do decide to buy the tubes.

According to the TAD website their 6V6GT-STR was put into production after 3 years of intensive research so I am hoping they got it right!


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Post subject: Re: Another Bias Question
Posted: Tue Apr 05, 2016 9:01 am
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From 22 to 28 you may won't hear any difference in tone, Try it with your Tung Sol.

65 Deluxe Reverb Reissue schematic show 391 plate voltage
At 70% plate dissipation it is a 25 ma for bias.
At 22 ma your Tung Sol are little colder if plate voltage is same.
You tube may be weak too because hour of playing
22 or 25 ma do not make any difference in tone or tube life. You will not hear it.
What you may hear is the different tube brand like TAD

Most of Fender amps I fix, work very good at colder bias, no hearing difference with more hot ( or less colder ) bias.

If you bias TAD at 28 do a listening test with your Tung Sol at 28 too


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Post subject: Re: Another Bias Question
Posted: Tue Apr 05, 2016 9:27 am
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Well, I decided to place the order for a matched pair of TAD 6V6GT-STR so will be interested to hear what they sound like. If I don't like them I will reset the bias to 22mA and put the Tung-Sols back in.

Am I correct in assuming the bias can be adjusted on the DRRI from the underside of the chassis without any dismantling?

I bought the Tung-Sols to replace the factory fitted GT-6V6S (one of which went microphonic) which I think are JJs. I used a local techie to install the tubes and bias them so I don't know what the factory tubes were biased at. What I do know is that the tone changed hugely for the better with the Tung-Sols compared to the GTs.


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Post subject: Re: Another Bias Question
Posted: Tue Apr 05, 2016 11:24 am
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fifty4 wrote:

Am I correct in assuming the bias can be adjusted on the DRRI from the underside of the chassis without any dismantling?

.


I don't know , I never work on this reissue. Look on the chassis close to power transformer if you see a hole like a input jack, this is the bias pot access hole for a screwdriver.


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Post subject: Re: Another Bias Question
Posted: Wed Apr 06, 2016 10:38 pm
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Yes, you can bias the DRRI with the chassis in the cab. Position is as described by Stratele.


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Post subject: Re: Another Bias Question
Posted: Wed Apr 06, 2016 11:10 pm
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stratele52 wrote:
Look on the chassis close to power transformer if you see a hole like a input jack, this is the bias pot access hole for a screwdriver.


On the re-issues, the bias pot is accessible from either the top or bottom of the chassis......

Image

If you were a contortionist in a previous life and don't mind waving a screwdriver around the paper cone of a loudspeaker, you'll have no problem.

However in the interests of safety, the wisest and most experienced techs will always opt to remove the chassis from the cabinet (a 5-minute job at most).

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Another Bias Question
Posted: Thu Apr 07, 2016 1:08 am
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Retroverbial wrote:
stratele52 wrote:
Look on the chassis close to power transformer if you see a hole like a input jack, this is the bias pot access hole for a screwdriver.


On the re-issues, the bias pot is accessible from either the top or bottom of the chassis......


Arjay


Same as vintage Fender


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Post subject: Re: Another Bias Question
Posted: Thu Apr 07, 2016 6:52 am
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My new TAD 6V6GT-STR tubes arrived today. Very easy to adjust the bias without removing the chassis. Only been able to play at house volume so far so cannot say how they compare to the Tung-Sols. It'll be a few weeks before I will be able to crank the amp.

One thing I did find interesting that confirms just how little I know about amp electronics and tubes. Before replacing them I re-checked the bias on the Tung-Sols which came up as 22mA as expected. When I put the TAD tubes in the amp and before I made any adjustments to the bias, they measured 34mA so I had to adjust the bias control in the opposite way to what I was expecting to set it to the recommended 28mA. Obviously the internals of the TAD and Tung-Sol tubes are very different.


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Post subject: Re: Another Bias Question
Posted: Thu Apr 07, 2016 6:57 am
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IIRC, TAD's 6V6GT-STR is built more like a 6L6 than a conventional 6V6 so the higher recommended bias-current rating is not surprising.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Another Bias Question
Posted: Thu Apr 07, 2016 7:02 am
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Retroverbial wrote:
IIRC, TAD's 6V6GT-STR is built more like a 6L6 than a conventional 6V6 so the higher recommended bias-current rating is not surprising.

Arjay

From 25ma as plate voltage on DRRI schematic to 28ma for TAD tubes , that is not what I call a higher bias. You can't see or heard a difference.


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Post subject: Re: Another Bias Question
Posted: Thu Apr 07, 2016 7:10 am
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Retroverbial wrote:
IIRC, TAD's 6V6GT-STR is built more like a 6L6 than a conventional 6V6 so the higher recommended bias-current rating is not surprising.

Arjay

Does that explain why the Tung-Sols had a current of 22mA and the TADs 34mA without adjusting the bias control?


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