Cathode Bias for SimpleSE

[whitelabrat]

Hi all,

I've completed stuffing a SimpleSE board with nice results so far. The only thing I'm hung on is what to set the Cathode Bias resistor to. I'm debating if I should set it to a fixed 560 ohm or to be switchable between 750 ohm, 560 ohm, and 430 ohm via 3 position toggle. The settings would be for EL34, 6L6GC, and KT88 respectively.

Question 1.
Is a switchable bias worthwhile or will 560 do?

Question 2.
Am I likely to fry myself having the bias attached to a toggle switch? The specs on the Philmore switch look good, but I may be missing something.

I'll be using the following iron:
Hammond 1628SEA OPT's
Hammond 274BX Power Transformer
Hammond 193H Choke

[korneluk]

1. yes, switchable cathode resistors are a good idea if you want to quickly test/listen to different tubes on the fly.

2. The best way of doing this is to always leave one resistor in the cathode circuit; i.e. the 750 ohm one, and then parallel other resistors that when connected across the 750 ohm one will give you the desired value - i.e. 560 ohms, etc. Use a toggle switch and you can switch in/out all three values.

Hope this helps.

-- josé k.

[w5jag]

The issue with a switch is that some are break before make, and some are make before break. Consequently, for an instant you will have no cathode resistor, or parallel cathode resistors. Whether this is of any consequence or not, I don't know.

Big wirewpund ceramic pots are still available at hamfests. I got a handful of 25 watt 1K Clarostats or Centralabs for a dollar each at the last hamfest I attended. I think something like that would be a better solution.

After burning out my Bendix transformer, the present iteration of my SimpleSE uses a 330-0-330 transformer, and with this transformer, I use a 470 ohm cathode resistor to set the bias.

I just change rectifier tubes to control the voltage drop and consequent dissipation from big tubes (KT88/6550) to not so big tubes (6L6). I acknowledge this is a cheesy way of doing things, and I'm not advocating it, but for my particular situation, it works. The amp is the PA in our home theater system so it needs to be able to run all day and most of the night while staying very cool to the touch (5Y3 and 6L6), but if I just want to listen to tunes then I can change to something like 5AR4 and 6550 and it will still work reasonably well without going inside the amp.

George's SimpleSE is really a great amp project. I bet that optimized for a specific tube, and with really good transformers, it sounds spectacular. Even with my highly compromised setup, and average (Transcendar) transformers, it sounds awfully good.

Win W5JAG

[whitelabrat]

I suppose I could also take the rectifier road. I could replace the 5AR4 with a 5U4GB and drop the voltage for the EL34's using a 560 ohm bias. I'm afraid that I won't get full potential from the KT88's with 560.

I'm generally very cautious when making changes to thing. I shut down the amp, make the change, allow it to cool and then fire it up again.

Another question I have is where to place the resistors in relation to the switch. I figure having the resistors before the switch would put the least amount of stress on the switch.

[korneluk]

Originally posted by whitelabrat

Another question I have is where to place the resistors in relation to the switch. I figure having the resistors before the switch would put the least amount of stress on the switch.

Look at the attached circuit to get an idea. Use a double pole 3 throw switch or a separate switch for each channel.

If the fixed cathode resistor is 750 Ohms (switch in the open position) , then use 1.1kOhms and 2.2kOhms to get 430 Ohms and 560 Ohms respectively.

-- josé k.

[tubelab.com]

A 560 ohm resistor is a good compromize for all 3 tubes. It runs the EL34 at about 23 watts dissipation which is 80% of maximum. The 6L6GC runs at about 25 watts which is closer to maximum. Use good 6L6GC tubes that can handle it. The KT88 runs about 29 watts which is slightly low.

These measurements were made with 430 volts of B+, and will vary with applied voltage and the individual tube characteristics.

I have a 560 ohm resistor in my amplifier, and parallel it with a 1K ohm resistor for KT88 use. This runs the KT88's at about 40 watts which is no problem for the Electro Harmonix tubes. I use the low tech approach. The board is stuffed with 560 ohm resistors, and I have two 1K ohm resistors with aligator clips on the leads for use with KT88's.

You can put 560 ohm (or higher) resistors in your board, and use a switch to add a parallel resistor as needed. A simple DPST switch can be used. The voltage is about 50 volts max and the current is under 100 mA.

Three different values can be accomodated in the same manner. Put a 750 ohm resistor in the PC board. Use a DPDT center off switch to switch either a 1K ohm or a 2.2K ohm resistor in parallel with the 750 ohm resistor. In the center "OFF" position, you are using the 750 ohm resistor by itself. In one "ON" position you have the 750 ohm resistor in parallel with a 1K resistor giving 428 ohms. In the other "ON" position you have the 750 ohm resistor in parallel with the 2.2 K ohm resistor giving 559 ohms. Again the switch requirements are minimal.

[whitelabrat]

Very good ideas! I am enlightened.

I think that the 560ohm resistor with a parallel 1K ohm resistor will do the trick! Sort of a turbo switch for the KT88. I'll probably start out with Shuguang 6L6GC's that I've already got, and later put in the old used Chinese KT88's I've got. I've also got a quad of SED EL34's but those are for another amp.

Thanks all!

[tubelab.com]

The Shuguang 6L6GC's seem to take all the abuse that I can throw at them. No problem with 560 ohm. I have a lot of old Chinese KT88's from about 10 years ago. These will remind you that the Chinese invented fireworks. I have seen some of them red plate at 20 watts, and arc over internally at less than 400 volts. If your old KT88's are this old, be wary of them. Run them with the 560 ohm for a while before hitting the Turbo button.

I use a 1K ohm resistor with the EH tubes because they can take it. I would pick a higher value (say 1.2 or 1.5 K) with other tubes. The EH tubes have additional heat rediating fins welded to the plates.

It is interesting to note that cranking up the tube current provides only a very small increase in power output (less than 1/2 watt). It does however lower the distortion and increase the damping factor. On my big speakers it provides better bass.

[kvk]

I was looking at the SimpleSE and thought if I built one it would be fun to try different tubes and therefore need to roll cathode Rs.

I was wondering if they made those 2 and 3 pin terminal strips in one pin so you could just screw in different cathode Rs.

I didn't see a one pin in the Digikey catalog. Anyone know if there is a part that would do this?

There is obviously the switch option mentioned above. I suppose that would be worth a try but something about a PCB makes me not want to run lots of extra wires off the board.

[hotbottle]

something about a PCB makes me not want to run lots of extra wires off the board

Best reason I ever heard for not using a PCB - with point to point, you can run wires wherever you like without guilt !