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Post subject: How to find what class of amp this is. 1964 Deluxe Blackface
Posted: Wed Dec 15, 2010 11:57 am
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I have searched all over the internet trying to find out if my amp, a 1964 Fender Deluxe Blackface AB763, is Class A or AB?

Does anyone know or know where I can find out?

The bias calculators I have found online ask whether it is A or AB.

Thanks

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Posted: Wed Dec 15, 2010 12:48 pm
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If it's got 2 6V6 output tubes producing 22 watts it's probably a class AB amp.


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Posted: Wed Dec 15, 2010 1:01 pm
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It definitely has 2 6v6's. I just bought new tubes all around and they came in yesterday. However, when I went to calculate the bias, the calculator asked what class amp it was. I said AB, and it came out sounding great, but I thought I would ask someone. Thanks a ton.

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Posted: Wed Dec 15, 2010 1:03 pm
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Most fender amps are AB


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Posted: Wed Dec 15, 2010 1:10 pm
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Not the ones with 1 output tube!


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Posted: Wed Dec 15, 2010 1:23 pm
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Well, since I've got 2 6V6's, I am going to assume it is AB. Thanks you all.

Last question. When I check the idle current in the tubes, they are about 8mA apart. Is this too far apart.

I am using a Weber Bias-Rite probe. And using the calculator at http://www.webervst.com/tubes1/calcbias.htm

On the bias calculator there, I have class AB, 6V6 tube types, and a plate voltage of 429. The calculator says 19.5mA.

I can either get tube A to 19.5 and tube B will be at 27.5, or set tube B to 19.5 and tube A will be at 11.5.

Where do I go?

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Posted: Wed Dec 15, 2010 5:08 pm
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That amp is a class A/B push pull,which is a study all in itself. :)
Your tubes are not matched,or they would be closer than 8ma,it will probably sound good like you said,and I would bias the one tube to around 28ma rather than the lower settings you mentioned,that would be too cold in my opinion...but others more knowledgeable will add to this I'm sure.
Did you buy those tubes as a "matched pair"?


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Posted: Wed Dec 15, 2010 5:18 pm
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I went a little hotter. About 30.5 on one which brought the other to 22.8.

To me, it sings. Nice warm, clean tones. I can up the volume on the guitar only and get some beautiful breakup. So I will call it good.

No, I did not buy those as a matched pair. I came across a steal on some used RCA 6V6 GT's and could not pass them up. I knew they might be a bit far off. I just didn't know how much of a mismatch would be a big deal. As far as I am concerned, it's perfect where it is, so I am leaving well enough alone for now.

Thanks guys for the input.

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Posted: Wed Dec 15, 2010 5:22 pm
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Hey there MJ, where did that bump put the plate volts ? Art

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Posted: Wed Dec 15, 2010 7:02 pm
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I know it's been said so many times by us old enough to remember,but way back when, we didn't buy "matched pairs" of tubes,we just bought what we needed and stuck them in the amp.
And we usually tested them at the drugstore on a tube tester. :D
Probably most of the classic songs we grew up listening to were played through amps that were treated that way.


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Post subject: Re: How to find what class of amp this is. 1964 Deluxe Black
Posted: Sat Dec 25, 2010 5:51 am
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1950's tweed Deluxes (like famous 5A3 circuit) are cathode biased, Class "A" amps. You can tell by the cathode tied resistor with the bypass cap (250 ohm resistor, 25 mfd/25 VDC bypass cap in schematic). Fender fixed biased amps usually have their pwer tube's cathode tied directly to ground. And they have a BIAS POWER SUPPLY applying negative voltage to the grid. They nominally are set to run in Class AB1 range.

In general, Class AB1 push-pull amps output is higher than Class "A" push-pull amps, using same power tubes. AB1 have cleaner highs, tighter bass. "A" have a sweeter midrange, slower onset of overdrive. All other things (like speakers) being constant.

Class "A" amps are somewhat self-leveling. So, you usually do not need to rebias the amp everytime you change power tubes. Similar reason why you don't reset parameters when you change out gain stage tubes (like 12AX7's). Almost all gain stage/phase inverter stages in guitar amps are set to cathode biased, Class "A" parameters.


http://www.ampwares.com/schematics/deluxe_5d3.pdf


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Post subject: Re: How to find what class of amp this is. 1964 Deluxe Black
Posted: Thu Dec 30, 2010 1:48 am
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From my "souvenirs" of the university, AB1 and AB2 are only for valve stages with "1" without grill current and "2" with grill current. 8)

(Class A refers to 100% signal conduction in active components (e.g. valve) and the idle current is at the middle of the load strait line,
Class B refers to 50% signal conduction in active components (one push the other pull after) and the idle current tends to 0,
Class AB refers to B with an idle current adjusted to minimise the current crossing distortion from the push to the pull.)

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Post subject: Re: How to find what class of amp this is. 1964 Deluxe Black
Posted: Thu Dec 30, 2010 2:30 pm
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Here's an excellent resource for learning about pretty much any old Fender amp.

http://www.ampwares.com/fender.asp

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