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Post subject: Blues Jr screen voltage
Posted: Tue Feb 18, 2014 9:06 am
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I noticed that Fender runs the screen voltage on the EL84 of the Blue Jr a good 22VDC below the anode voltage (X tap versus B+). Then, runs the screen voltage through 100-ohm final resistors, to further lower the voltage.

Maybe, this is why they need to run the EL84 bias so hot. To compensate for this conservative setup on the screens.

Has anyone tried running the screen a bit closer to the anode voltage?

Thanks!


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http://www.webphix.com/schematic%20heav ... ues_jr.pdf


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Post subject: Re: Blues Jr screen voltage
Posted: Tue Feb 18, 2014 10:58 am
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If you increase the screen voltage, the tubes will run even hotter with more current. The lower voltage on the screens is to prevent the screens from becoming a defacto Plate. The screen voltage on the BJr is usually closer to the B+ than that listed on the schematic, and the 100 ohm SGRs will not drop much voltage at all (half a volt?).

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Post subject: Re: Blues Jr screen voltage
Posted: Tue Feb 18, 2014 11:28 am
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Shimmy,

I know that the greater difference between anode plate and screen voltage will reduce runaway issues, but at the cost of total output wattage. So, maybe this is why Fender has the reputation of running these EL84 so hot from the factory. To compensate for such a conservative screen setup. I often wondered why this amp seemed to have the rep of being too hot out-of-the-box. Whereas the 6V6GT and 6L6GC amps seem too cold.

The 100-ohm resistor is a historic footnote. Vintage Vox and Marshall amps that used the EL84 often placed 100-ohm resistors inline between the OPT tap (B+) going to the anodes. Most EL84 are very tough tubes. Modern amps that used the EL84, only use a 100-ohm screen resistor off B+ or no screen resistor, at all. So, a 2 volt difference between the anode and screen works fine with most modern EL84's. Fender didn't need to add this 100-ohm resistor, as it already tapped off a lower voltage point in the power rail.

Anyhow, just a curious question. As a tech I know was working on a Blues Jr that had zener diodes and a regulator placed on the screen. This is not good, as the screen voltage should vary with B+, as the amp is going through its paces. Similar no-no to those ppl that put heavy regulation onto bias supply circuits. This may work for hi-fi amps, but can hurt the tonal response in guitar amps.

It would be interesting to change the 2.2k-ohm rail resistor to say 1k-ohms. Compensate the remaining rail resistors to get proper Y and Z values. Then, run the EL84 cooler. See how the tone compares to stock.


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Post subject: Re: Blues Jr screen voltage
Posted: Tue Feb 18, 2014 12:00 pm
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Zeners/regulators on the screens? Weird! The screen voltage at the tube itself, can only drop if more current flows through the SGRs (as it should), but it will never rise. People do some weird, wild stuff. :?

No shortage of output wattage on the stock BJr. Mine idled at over 13 watts per tube before I put in a bias adjustment pot.

The lower screen voltage will give a bit of compression to the sound, but I doubt if the hot bias is to compensate for lower screen voltage (or vice-versa), that is counter productive, as a higher screen voltage would give a hotter bias anyway. If they wanted to compensate for the lower screen voltage, they could have done just that, and they certainly didn't have to over-compensate. I see no benefit in the hot bias of the BJr.

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