Delta Blues Half Watt Mod Help

[ncusack]

Hey Everyone,

A friend of mine just tried to do the 15Watt mod to his Delta Blues amp by snipping all but the heat pins on two of the EL84's. He managed to break an EL84 and has since ordered a new matched quad for his amp.

This got me to thinking could the heater be simulated using a couple of resistors in pins 4&5 instead of ruining the power tubes?

My thoughts were 6.3VAC at 0.76A per EL84 would be roughly 8ohms for the resistors. I'd say at least 10W rating since each one would be dissipating close to 5W.

Let me know if im completely off the mark here.

Cheers,

Neill

[jjasilli]

   But, if someone inadvertently plugs-in tubes, that may cause a problem.  Another solution is to buy some cheap used tubes on ebay.

[ncusack]

I was thinking about making the resistors external using something like a 9-pin socket saver so the option to go full power is just a matter of pulling the socketed resistors and popping the tubes back in.

[jjasilli]

You might be able to fashion something like that inside a "dog house" like some Fender amps use to house filter caps.  You could even drill holes for ventilation, which may be needed to dissipate 10W of heat energy.

[HotBluePlates]

A friend of mine just tried to do the 15Watt mod to his Delta Blues amp by snipping all but the heat pins on two of the EL84's. ...

WTF??

What was he trying to do? The schematic shows this is a 4-output tube fixed-bias amp. If he was trying to cut power in half, why didn't he just remove 2 output tubes? Why cut anything?

This got me to thinking could the heater be simulated using a couple of resistors ...

Yes, resistors could dissipate heat instead of a tube heater heating the cathode. But why is it you need to do this at all?

[sluckey]

Why cut anything?

The filaments for the output tubes are series connected. Pull any one and they all go out.

[HotBluePlates]

Thanks for pointing that out to me Sluckey! Now I get it! 

I was thinking about making the resistors external using something like a 9-pin socket saver so the option to go full power is just a matter of pulling the socketed resistors and popping the tubes back in.

I think this is an excellent idea!

[printer2]

The whole point of using modified tubes is so you do not have to modify the amp. If you are going to put in a socket and a pair of resistors why not change the heaters so that each P-P pair is in series with each other? I do not know how hard it would be to rewire not knowing the insides of the amp, but if it were possible...

Also the plate load is too low for the pair if you do the mod. Just assuming the Delta Blues is the same as the Classic 30, to get the right primary impedance you had to put an open plug into the extension speaker jack. This switches the impedance of the transformer to match one pair of output tubes.

[HotBluePlates]

The whole point of using modified tubes is so you do not have to modify the amp. If you are going to put in a socket and a pair of resistors why not change the heaters ...

Ncusak mentioned "socket savers" which is a 9-pin plug; he can add the resistor and have something to plug into the original socket to act as a tube heater.

My problem before was I didn't look at the schematic. Peavey got clever: they needed 36vdc, 30vdc and 15vdc to power ICs and relays. Getting these from a 6.3vac winding is awkward. But they could put 4x 6.3v heaters in series for 25.2vac, and rectify that to get the other needed voltages.

Actually, they started with more like 36v / 1.414 = 25.5vac (plus some change for diode drops). The upside is it's generally unusual for a tube heater to open up; even if the tube is dead as far as cathode current, it seems the heater will most often still have continuity.

So my original post was simply that I wasn't aware of the series heater arrangement for the output tubes.

Also unusual is that the -36v supply is powering all 3 12AX7 heaters in series (each wired for 12.6v operation).