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Post subject: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reissue
Posted: Wed Jul 23, 2014 12:18 am
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I have a new Fender 68 Custom Deluxe Reverb Reissue and I have a set of nos RCA 6V6 tubes en route to me. A well known amp tech will be adjusting the bias for me when they arrive but I am trying to educate myself on what the maximum and minimum bias settings in theory could be. What confuses uses me is plate voltage dissipation. I know that the pv dis is whatever it is measured to be at the time of measurement using a bias probe or whatever. But what is the MAXIMUM dissipation in theory that it could be? The Fender schematic for my amp says that the 6v6 GT power tubes that came with the amp were biased at 24mA at the factory. What I want to know is what percentage does 24mA represent? Is it 60% of max or 70% or max etc? And to know this I would need to know what the max mA could be for this amp/tube? Can anyone help me with this? Thanks


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 3:13 am
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Most Fender are biased cold 50 to 60 % IMO . The many I see where at "cold " bias .

They sound very good at "cold " bias , not better ( most of the time ) with hot bias and you save tube life .

6V6 have a 14 watts maximum plate dissipation

If fender bias is 24 ma , plate dissipation is ;

24 ma X plate voltage = plate dissipation


Be careful to see a qualified tech if you want to keep your RCA tubes long time , there are too many unqualified ones on the market


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 3:19 am
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You can simulate some bias here;

http://www.webervst.com/tubes1/calcbias.htm


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 3:25 am
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stratele52 wrote:
Most Fender are biased cold 50 to 60 % IMO . The many I see where at "cold " bias .

They sound very good at "cold " bias , not better ( most of the time ) with hot bias and you save tube life .

6V6 have a 14 watts maximum plate dissipation

If fender bias is 24 ma , plate dissipation is ;

24 ma X plate voltage = plate dissipation

Be careful to see a qualified tech if you want to keep your RCA tubes long time , there are too many unqualified ones on the market



+1

At 410 VDC plate voltage, I don't think 24 mA would be out of line. Use that figure as a datum point and tweak up or down to suit your ears.

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: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 6:31 am
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So is plate voltage specific to the amp or the specific tube type?


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 6:34 am
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Plate voltage is more of a function of the specific amp. Though the voltage will drop, a bit, when you insert the output tubes. How much it drops is a function of the specific tubes & the current each pass.


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 6:43 am
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Thank you. Does anyone know a reference which specifies what my amps plate voltage is? If my amps schematic says they bias at 24mA what percentage of total is that?


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 6:58 am
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Mowgli77 wrote:
Thank you. Does anyone know a reference which specifies what my amps plate voltage is? If my amps schematic says they bias at 24mA what percentage of total is that?



IMO amp schematic say plate voltage too ,but you can never trust it to save you reading with voltmeter .

You can't bias a amp with pencil and paper .

I don't have this schematic .

Anyway , you tech will read plate voltage with your tubes plug in and he must bias it properly if he his qualified .


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 10:18 am
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stratele52 wrote:
IMO amp schematic say plate voltage too ,but you can never trust it to save you reading with voltmeter .

You can't bias a amp with pencil and paper .


+1

Plate voltage for the original AB763 is 410 VDC by the schematic -- the B+ for the re-issue indicates 391 VDC. However, in both these instances the plate voltage is typically a bit higher -- say, 5 to 25 volts.

Arjay

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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Wed Jul 23, 2014 10:35 am
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Hi thanks so much. Im looking at the schematic but I can't seem to find where the values you mentioned are referenced. Can you please tell me where ion the schematic I can find them?


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Thu Jul 24, 2014 2:30 am
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Mowgli77 wrote:
Hi thanks so much. Im looking at the schematic but I can't seem to find where the values you mentioned are referenced. Can you please tell me where ion the schematic I can find them?


I don't have you schematic , Custom deluxe reverb , a new amp by fender can't t find on the web I wish to have one .

Where to find voltage on schematic ? Look at the plate or follow the wire from tube's plate to power supply . You must see it . If fender write it on this new schematic

Look here a 65 Deluxe schematic ; plate volt ; 391 volts

Is your schematic look same ?

http://support.fender.com/schematics/gu ... ematic.pdf


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Thu Jul 24, 2014 5:44 am
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Hi
Yes I see, thank you. If you send me your email I would be happy to send you the schematic.


Last edited by Mowgli77 on Thu Jul 24, 2014 3:29 pm, edited 1 time in total.

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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Thu Jul 24, 2014 3:06 pm
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The voltages can shift around a bit as well, I've seen my voltages at home change noticeably from day to day on occasion and noticed they can shift around depending on what else in the house is running such as heating or air cons switching on/off, I've actually noticed this happen when checking bias. If you move the amp to different venues the voltages will swing around a bit.


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Thu Jul 24, 2014 3:37 pm
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My RCA 6V6GT tubes arrive any day and I don't know if they are 12 or 14 watt tubes as many references say different things. They are the grey plated 1950s tubes so I'm guessing they are 12?

I have 2 sets coming: one matched set are around 30mA and one set are around 50mA. Will the 30mA give me a lower ceiling/earlier break up?


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Post subject: Re: Max plate dissipation/bias? 68 Custom Deluxe Reverb Reis
Posted: Thu Jul 24, 2014 3:53 pm
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Not sure at 100 % but I would say yes , the 50 ma will break up earlier for the same bias as the 30 ma tubes.

With bias adjustment you can do what you want ; lowering 50 ma tubes set at 30 ma or adjust 30 ma tubes set at 50 ma or any bias ( ma ) reading you want


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