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Post subject: Re: 1965 Pro Reverb
Posted: Tue Nov 05, 2013 6:01 am
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Rock Star
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BMW2002Ti wrote:
I believe that the total dissipation depends on the configuration of the tube. Triode strapped tube will make different power than in a pentode mode. A push-pull amp will max-out at a different amount than a single-ended. Class "A" differs from Class B, etc.


While what you say is true, it has absolutely nothing to do with calculating the bias level based on the percentage of maximum plate dissipation. As shown in the data sheets for both the 5881/6L6GWB and 6L6GC, the 5881 maximum plate dissipation is 25 watts and a 6L6GC is 30 watts.

The Pro Reverb is rated at 40 watts, not 19 watts as in the AB1 push-pull circuit for two tubes that you referenced. In fact, the Pro Reverb is spec'd to use 6L6GC tubes, not 5881s.

http://ampwares.com/amplifiers/fender-b ... ro-reverb/

http://ampwares.com/schematics/pro_reverb_aa165.pdf

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Post subject: Re: 1965 Pro Reverb
Posted: Tue Nov 05, 2013 6:03 am
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Nikola Tesla wrote:
:oops:

Oooops .. I thought all 6L6 tubes are 25W, but obviously not. :D


Nikola, you should be biasing the tubes in your amp based on 25 watts maximum plate dissipation for a 5881 as shown on page one of the provided data sheets.

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Post subject: Re: 1965 Pro Reverb
Posted: Tue Nov 05, 2013 6:17 pm
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Thank you guys, your help is much appreciated.

Now that I still have the bias mismatch:

Image


... and



Image


.. and having in mind I'm not very technical (and unable to check on unmatched grid-to-ground and/or grid stopper resistors), I will lower the value on the bias pot and install a set of 6L6GC's from another amp to see if the mismatch is still there.

If not, the culprit will be the tubes (5881 and 6L6WGB).


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Post subject: Re: 1965 Pro Reverb
Posted: Tue Nov 05, 2013 6:25 pm
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Bill,

Me mistake. The data I have is 19 watts per TUBE. I looked the specs up in an old article by Eric Barbour (VTV Volume1, issue 4). The Tungsol test max'ed at 23 watts per tube. This is the dissipation data using a Design-Center tube tester.

The 6L6GC (p-p) = 30 watts PER tube.

The 7581A and 7027A = 35 watts PER tube.

The 6L6 metal and 6L6GA/GB glass = 19 watts PER tube.


Sorry for the confusion. VTV Issue 4. Page 8:

http://www.jumpjet.info/Pioneering-Wire ... /VTV04.pdf


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Post subject: Re: 1965 Pro Reverb
Posted: Wed Nov 06, 2013 12:15 am
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Aspiring Musician
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Here is my new acquisition: :D
A set of brand new Stetlanas Winged C, made in St Petersburg, Russia.

How would these perform in my Pro :?: :?:

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Post subject: Re: 1965 Pro Reverb
Posted: Wed Nov 06, 2013 12:25 am
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:arrow:

After the first reading both tubes displayed the same (or close) plate current/voltage :lol:

(As per my previous posts both 5881 and 6L6WGB were quite different in that regard).

Here are Svetlana's readings:

1st tube

Image



2nd tube

Image


Looks like the bias is too cold :?: :!: :?:

As per webervst calculator the bias should be 47.1mA

.. or 30:445 x 0.7 x 1000 =47.1mA

:mrgreen:


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Post subject: Re: 1965 Pro Reverb
Posted: Wed Nov 06, 2013 5:38 am
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Nikola,

A bit cold at 34mA (with 445VDC on plates) = 15 watts per tube = 50% of max.

Try 40mA (445VDC) = 18 watts (around 60%). You can prolly go to 45mA (esp if these are good Svetlanas) = 20 watts per tube (66.6% of max).

I hope that these are one of those made in the old St Petersburg plant. Well matched and good grid alignment. Watch for red plating and overheating tubes.

Good luck! :D


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Post subject: Re: 1965 Pro Reverb
Posted: Wed Nov 06, 2013 4:50 pm
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:D

I'm pretty sure these are made in Russia in Svetlana factory in St Petersburg.

Would you know the meaning (if any) of the gold and brown dots on the packaging?! :?:


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Post subject: Re: 1965 Pro Reverb
Posted: Wed Nov 06, 2013 6:33 pm
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Nikola, no idea what the dots on the packages mean. The Svetlana made in St Pete will have four digit date codes (eg: 07 05 = 2007, 5th month). And the "OTK" approval stamp.

Anode plate are spot welded (not stapled).


http://www.dougstubes.com/media/catalog ... -6l6gc.jpg


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Post subject: Re: 1965 Pro Reverb
Posted: Wed Nov 06, 2013 10:09 pm
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BMW2002Ti wrote:
Watch for red plating and overheating tubes.Good luck! :D



I will watch, but once they go very red, that's it .. finito la musica :lol: :lol: :lol:


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Post subject: Re: 1965 Pro Reverb
Posted: Thu Nov 07, 2013 6:07 pm
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Ok, the amp is on the bench. :mrgreen:

The result is as per photo - with the pot all the way up, so no way I can increase the bias at this stage and go up to 40mA.

What to do to increase the bias value??

Also I have noticed the amps power is not as it should be. Looks like the tubes are only working with 50% of their potential (if that' the word) producing approx. 15W per tube.

Opinions please!


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Post subject: Re: 1965 Pro Reverb
Posted: Thu Nov 07, 2013 6:16 pm
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Try 70% of 30 watts per tube. Which is 20-21 watts per tube. Then, check the tone. Red plating, etc.


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Post subject: Re: 1965 Pro Reverb
Posted: Thu Nov 07, 2013 6:31 pm
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That's not possible, the bias pot is at the max, it doesn't go any further. :lol:


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Post subject: Re: 1965 Pro Reverb
Posted: Thu Nov 07, 2013 10:18 pm
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Opps! Was this amp originally designed for the 5881/6L6WGB? If this is the case, you may not be able to bias a 6L6GC into range.

Or the tubes are weak and can pass more than what your pot maxes out at.


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Post subject: Re: 1965 Pro Reverb
Posted: Thu Nov 07, 2013 10:38 pm
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AA165 is designed for 6L6GC tubes, the original schematics has 6L6GC's.
Every where I look its the 6L6GC :?: :?: :?:


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