dc-5 el34 mod

[timjtitus]

i've been thinking about doing an adjustable bias mod on my dc-5, would replacing the 22k resistor with a 10k pot and a 10k resistor be enough to adjust the bias to use el34s?

 

[Monsta-Tone]

I don't think that the power transformer can handle EL-34's. You will have to use an additional 6.3 volt transformer to allow for enough filament current.


As for the bias mod, I removed the 22k resistor that goes to ground. I replaced this with a 10k pot that had a 15k resistor tied between pin 3 and ground. I then placed a wire from the circuit board (where the bias voltage went into the 22k resistor, not on the ground side of the resistor) to the wiper or middle pin of the pot.

You could adjust the bias to where it would work with EL34's, but I think that you would do serious damage to your power transformer if you don't add a filament transformer for the power tubes.


The adjustable bias made a world of difference in my amp though, I can now use 6V6 or 6L6 tubes of any brand and make it sound the way that it is supposed to.

 

[timjtitus]

hhhmmmm, would it even be worth it then? would i be able to buy a transformer that could handle both el34 and 6l6 tubes? where could i get one and how much are they?

 

[Monsta-Tone]

Mouser Electronics has a great one. All you need is a tiny transformer that will supply filament voltage to only the power tubes. You simply remove the original power transformer's filament wires from the power tube sockets. If I remember right, they come from the main board to the tubes, but make sure that you are not cutting off the filament voltage from the preamp tubes and make sure that any wires that you may leave in there are secured so that they won't short out.



As for the price, the last one I bought was only $8 or so. They are about 1" on all sides, so they are easy to put somewhere. Since the DC-5 has the widebody chassis, you could put it just about anywhere you wanted. It would be much harder to find a place on the smaller 17" chassis.

 

[timjtitus]

oh, i thought you meant that i would need to replace the transformer, not add one, thanks for the advice

 

[Monsta-Tone]

If you're not familiar with working on tube amps, don't do this yourself.

I've been a tech for many years and have shocked the hell out of myself a few times.


Just the other day, I was adding Cathode Bias to my DC-3 and removed a component with the thing plugged in & turned on. I was in a hurry and could have blown my hand off or blown the amp up.

Your amp will have about 500 volts DC in it in some places, and it keeps a few hundred volts charged up in the capacitors for at least 20 minutes if your caps are good. This really hurts.


Plus, a really good tech could do it in a way that was completely reversible should the amp ever escalate in price or need to be sold.

 

[timjtitus]

i actually took the amp to a tech yesterday. any recomendations on el34 and kt88 tubes to try out?

 

[Monsta-Tone]

I really like the JJ E34L's in my amps. I have an Ampeg/Lee Jackson head in the store that has the regular JJ EL34's in it and they are really nice too, but not as much headroom as the E34L's.

 

[timjtitus]

yeah, i want less headroom, thats one thing i didn't like about the 6l6gcs, which are JJs