"Champ" power tube options and load requirements

[greg]

Hello,

I have a little amp which started out as a tweed champ, then became a 5E2 Princeton and now is basically kind of a Matchless Spitfire w/ a 6V6 - it is starting to impress me...

My question now is where else I can take it. It was built from a dual 6V6 Motorola record player (so I feel I'll have no issue with running a single El34 or KT66 as far as current draw is concerned.

I'm using a SE Heyboer OT (that is a beefed up Champ OT w/ 8 ohm and 16 ohm taps) into an 8 ohm load. My plate voltage with a 5Y3 is 340 vdc, my cathode resistor is 470 ohm, my power supply caps are all 450v rated.

I'm interested in trying this out with an EL34 and a KT66.

My questions follow:

A) am I correct that to use either an EL34 or a KT66 (or, for that matter, dual parallel 6V6s), that I should use the 16 ohm tap?

B) what should my cathode bias resistor be for either of these tubes?

I have a KT66 and I've tried it with the existing bias (into the 8 ohm tap) and it sounds really good - but I was surprised as I expected it to be biased poorly and that it would sound pretty crappy.

Thanks in advance for any guideance

[Chuck_Farlee]

I'm using a SE Heyboer OT (that is a beefed up Champ OT w/ 8 ohm and 16 ohm taps) into an 8 ohm load. My plate voltage with a 5Y3 is 340 vdc, my cathode resistor is 470 ohm, my power supply caps are all 450v rated.

What are the specs for that OT? Wattage? DC mA through the core rating? Primary impedance?

I'm interested in trying this out with an EL34 and a KT66.

My questions follow:

A) am I correct that to use either an EL34 or a KT66 (or, for that matter, dual parallel 6V6s), that I should use the 16 ohm tap?

If you connect an 8 ohm speaker to the 16 ohm tap on the OT, you will effective halve the primary impedance.

B) what should my cathode bias resistor be for either of these tubes?

I have a KT66 and I've tried it with the existing bias (into the 8 ohm tap) and it sounds really good - but I was surprised as I expected it to be biased poorly and that it would sound pretty crappy.

Here is a quick and dirty way to figure out what you need to do to run other power tubes in your amp:

Find the maximum plate (anode) dissipation for Class A on the data sheet. For an EL34, this is 25 watts. Divide this by your plate to cathode voltage as measured from the plate (anode) to the cathode pins on the socket with the tube installed. You can also subtract your cathode voltage (as measured to ground) from your plate voltage (also measured to ground) to get your plate to cathode voltage.

For the purposes of illustration for this example, 25 watts / 340 v = 73.5mA. This would be your current draw to shoot for in SE Class A. Note when using a tube that draws more current than the 6V6, like an EL34 you plate voltage may go down depending on how efficient your PT is. If you want to play it safe, you could bias your amp at 90% of 73.5mA or 66mA. To increase your mA draw, decrease the value of the cathode bias resistor. If your amp is drawing too much current, increase the value of the cathode bias resistor. You may want to get a few 5W, cement resistors in the 220 ohm to 680 ohm standard range to experiment with.

Once you install your resistor, you have to check the bias again along with your plate voltages, cathode voltages and current draw, then recalculate. If your current draw is is within the 90%-100% you're done.

HTH,

chuck

[greg]

Thanks, Chuck -

I see that the max plate dissipation for the KT66 is also 25 watts, so it should then require the same cathode resistor to obtain the proper target draw. The strange thing is that when I put in the KT66 (while biased for 6V6), it sounded good - I'm certain it isn't optimized, but I was pleasantly surprised. When I put in a JJ EL34, it sounded like it was way underbiased - weak and distorted in a bad way. Perhaps it is just a bad tube.

I'll do this right and see the results.

As for the OT, it has an 8000 ohm primary impedance, so it will go down to 4K primary if used into the 16 ohm tap into an 8 ohm load. I don't see on the KT66 or EL34 datasheet any mention of impedance (but I know that I should be using the 4K primary (again, by using the 16 ohm tap into an 8 ohm load) for 6L6, EL34) - but how is this determined?

The OT is rated at 95ma contiguous.

Thanks again,
Greg

[stokes]

El34's require less bias voltage than a KT66 or 6L6,so it stands to reason it would need a different cathode resistor to bias up optimaly,not likely a bad tube.

[greg]

Thanks, guys - thus armed, I'll get in there and do it right.

Greg