Use a 6L6 or EL34 in place of the 6V6

[brodude]

I am new to amp building but have some knowledge. I have just built my version of a single ended 5F2 Princeton with high end components, switch selectable output impedance, and an adjustable biasing circuit so that I can use different power tubes. I am using the basic Angela's Single 6V6 schematic- Single 6V6 Guitar Amplifier Project.
Can anyone assist me with the necessary wiring mods needed to use a 6L6 or El34 in place of the single 6V6? I want to be able to swap tubes, similar to an Emery Sound SuperBaby, and I cannot seem to find it anywhere on the internet for a single output tube amp.
More information:
Currently the 6v6 socket has pins 1 & 2 jumpered and tied to ground. I am assuming I need to disconnect pin 1 from pin 2 and connect pins 1 & 8.
I am using an David Allen PT model TP35 and a custom Heyboer OT with taps for 6K, 4K, and 2.5K input and 16, 8, and 4 Ohm output.
I believe I will also need to replace the screen resistor but I want to make sure. It currently has a 22K on pin 4 and a 270K on pin 5 to ground.
Thanks
Darrell

[tubeswell]

Hi Darrell, welcome to the forum

Currently the 6v6 socket has pins 1 & 2 jumpered and tied to ground. I am assuming I need to disconnect pin 1 from pin 2 and connect pins 1 & 8.

Correct, Pin 2 is one of the filament pins, pin 8 is the cathode. In a 6L6 (and 6V6 for that matter), Pin 1 is not connected to anything. But in an EL34, Pin 1 is connected to the suppressor grid. (whereas in a 6L6 or a 6V6, the suppressor grid is internally wired to the cathode (pin 8)).

Take a look at the respective tube pin-outs on some datasheets.

You ideally want the suppressor grid connected in the EL34 in order to get the benefit of it.

BTW a better way of wiring your heaters in this amp (to minimise hum) would be to run one side of the heater PT winding to pin 2 of the output tube and thence to pins 4-5 (tied together) of the pre-amp tube, whilst the other side of the heater winding runs to pin 7 of the output tube and thence to pin 9 of the pre-amp tube. Then keep the wires from the respective sides of the heater winding twisted together as much as possible. Then (assuming the heater winding does not have a centre tap) put a 100R resistor from each side of the heater winding, either back to ground (with the rest of the power amp grounding), or to the cathode (pin 8) of the output tube (assuming the amp you are building is cathode biased).

And on another matter, the EL34 will draw 1.5A of heater current, and the 6L6 will draw 0.9A, versus a 6V6 which will only draw 0.45A. So for a SE single EL34 amp with one 12A-7 type pre-amp tube (drawing 0.3A), and a pilot light drawing between 0.15A and 0.5A, your PT heater winding will need to be rated at least 2.5A or more to be on the safe side.

I am using an David Allen PT model TP35 and a custom Heyboer OT with taps for 6K, 4K, and 2.5K input and 16, 8, and 4 Ohm output.

for the 6V6, you need to run the OT at between 5k and 8k reflected load, but for the 6L6 or EL34, you should run it at 2k5 to 4k reflected load. So wire it to the "6k" primary and then use an 8R speaker in the 16R output for the 6L6 or EL34. YMMV on exactly what is an ideal load resistance for an EL34 or a 6L6.

I believe I will also need to replace the screen resistor but I want to make sure. It currently has a 22K on pin 4 and a 270K on pin 5 to ground.

Pin 4 is the screen grid on all these types of output tubes. The EL34 screen needs a high-power screen resistor. The value needs to be such that you safely keep the screen current under 10mA (some would say 5mA is an ideal).

The screen voltage sets the overall power output of the tube, and the screen resistor needs to be sized so that the screen voltage doesn't get higher than the plate voltage under idle conditions. (Typically somewhere between 50V and 1 or 2V below the plate voltage). Merlin b has helpful articles here on screen voltages:

http://www.freewebs.com/valvewizard1/se.html



Use your E=IxR and your W=IxE equations to work out the power handling. The resistor's power handling should be at least twice what the expected dissipation of the resistor works out to be.

Pin 5 is the control grid pin, and the 270k resistor is the grid load resistor. Just leave that where it is.

[brodude]

Use a 6L6 or EL34 in place of the 6V6

Tubeswell:

Thank you for welcoming me to the forum and I can't thank you enough for your detailed response.

I have installed a rotary switch for the primary impedance to my Heyboer OT so I can select between 6K, 4K, and 2.5K inputs. I have two independent outputs; one at 4R and one at 8R that is switchable. I am assuming this will be OK depending upon what power tube I use. Do you think I need the 16R connection? The speaker cab I have built has two 12" 8R independently ported top of the line speakers from Warehouse Guitar Speakers. One is American voiced and one is British voiced. I have both a 1/4" jack and speaker binding posts off each speaker so I can use each one independently or patch the two together in parallel.

My heater 6.3V heater off the PT is good for 5A so I believe I am OK there. Does it matter where I ground the 100R resistors off each leg of the heater wires? Are 1/2WR's OK or do I need larger wattage?

I am a little unclear as to how to size the screen resistor since I could use different output tubes. Can you provide more detail on this? Do I take some sort of average or worse case senario?

Thanks again for all your help. Let me know if there is any way I can return the favor.

Darrell

[tubeswell]

Does it matter where I ground the 100R resistors off each leg of the heater wires?

I either:

ground reference the heater winding with the other 'high-current' ground returns (like the output tube ground returns); or

sometimes if that is not quiet enough, I ground the heater ground reference to the cathode(s) of the output tubes (if they are cathode biased, and if the cathode resistor is bypassed - with a cathode bypass cap). This elevates the heaters to whatever the cathode voltage is; or

if I want to elevate the heaters even higher, I create a voltage divider from the filter cap bleeder resistor network, using a suitable electrolytic cap of sufficient voltage rating (so as to be greater than the voltage at the knee of the voltage divider I have created) to decouple the voltage at the knee from possible fluctuations in the B+ (not that you should get any - but this extra cap does theoretically make the elevated heater voltage more stable, thus reducing possibility of unwanted oscillations wreaking havoc back through the various stages)

Are 1/2WR's OK or do I need larger wattage?

1/2W are fine for the 100Rs

I am a little unclear as to how to size the screen resistor since I could use different output tubes. Can you provide more detail on this? Do I take some sort of average or worse case senario?

work out the voltage drop (E) across the resistor as accurately as possible, then divide that by the actual measured resistance (R) value of the resistor. This will give you the current (I) flowing through the resistor. (E/R=I).

Then multiply that current (I) by the voltage dropped (E) to get the power being dissipated (W) by the resistor (E x I =W). Then make sure the resistor is rated at least double what that calculated power dissipation is likely to be.

[MWJB]

Use a 10W cathode resistor at the power tube & a 100v rated bypass cap.

[brodude]

Use a 6L6 or EL34 in place of the 6V6

Thanks tubeswell and MWJB.

I went to college back in the late '60's for my EE when my professor at Purdue had told us that he didn't believe that transistors are never going to be large enough for indudtrial use. Boy was he wrong. All is starting to come back to me now. I haven't used these skills in years.

Undoutedly you have been building amps for a while and really enjoy it.

I appreciate the help.

Tubewell: Are you saying that you sometimes land your heater grounds right at the pin on the cathode when the amp is cathode biased with a resistor and a bypass cap or where the cap and resistor are grounded? It looks like you mean on pin 8.

If I have a center tap for the heaters shouldn't I just ground the center tap and forget about the 100R's?

I knew Ohm's law and how to calculate watts but I thought, at first, you were telling me how to calculate the resistance I needed for the screen resistor and not just the watts. My mistake.

Darrell

[tubeswell]

Tubeswell: Are you saying that you sometimes land your heater grounds right at the pin on the cathode when the amp is cathode biased with a resistor and a bypass cap

Yes (pin 8). Putting the heater ground reference here eliminates potential interaction between the voltage at the cathode and the filament AC voltage by making the filament AC swing all fall on the positive side of ground potential.

If I have a center tap for the heaters shouldn't I just ground the center tap and forget about the 100R's?

Yes you can. Or if you want to use 100R, then forget about the centre tap on 6.3V winding. But either do one or the other - not both. The heater CT way has advantage that here is no current used up by the resistors, but the disadvantage that the CT may not be in the middle of the winding resulting in a small imbalance in the winding ends that could potentially induce hum. The resistors of course are assumed to be identical values and offer a better balance at the expense of a bit of current.

[brodude]

tubeswell:

Thanks again.

I have received my new PT and once I receive a few resistors that I ordered I may actually be able to play it.

One other thing if I may: I am using isolated speaker binding posts for my outputs. When I first wired my amp I had the common on the secondary of the OT grounded but I had a tremendous squeal after the amp warmed up. I lifted the ground and just went straight to the binding post and it went away. Am I still supposed to ground this connection, if so, where is the best place?

Darrell

[tubeswell]

When I first wired my amp I had the common on the secondary of the OT grounded but I had a tremendous squeal after the amp warmed up. I lifted the ground and just went straight to the binding post and it went away. Am I still supposed to ground this connection, if so, where is the best place?

Only thing I can think of if that the OT secondaries were the wrong way around and you were getting positive feedback (Sometimes the colour codes on factory transformers are wrong way around) One side of the OT is supposed to be grounded. So maybe try it the way to first had it, but with the secondaries swapped around. 2CW

[Chuck H]

Another note on the CTR vs. 100r resistors for the heater ground reference... If you use the CTR on the winding ther is a generous amount of current potential if a tube shorts to the filament. With the 100r "false" CTR it's more likely that the resistors will fail and save other more expensive components, like transformers. The elevated heater is always a good idea anymore as there are many 12AX7 tubes that can't take high heater to cathode voltages present in so many designs. Elevating the heaters improves that margin by the amount that you elevate them AND it usually reduces hum. And using resistors instead of the actual CTR tap when elevating at the cathode R improves the safety margin should a tube short to the cathode.

In cathode biased amps I ground the heaters on top of the cathode R with a false CTR (pair of 100r) for everything but EL84 tubes (not enough voltage there to matter). And even with EL84 tubes I use the false CTR instead of the winding CTR. The current used by the resistors is really negligable unless your tubes heaters are so far out that they should be changed anyway.

JM2C

Chuck

[brodude]

Thanks all. More great advise.

Darrell

[brodude]

More help?

Happy New Year and thanks again for all the assistance.

I know it has been a while but work and my custom guitars have kept me from concentrating on my amp.

I tried the 1K 5W resistor from pin 4 to the 22K but it made a toaster (smoking resistor). Since then I purchased a 7W and a 10W and they still got hot, at least the 10W did not smoke.

I have 444V on pin 3 with just the 5Y3 installed. When I plug in the 6V6 I have 340V on the plate.

Do I need to put in a 15W or 20W 1K? Does adding this resistor have any effect on the 6V6 or 6L6 when used?

I am also confused on the proper bias mA. If I used Weber's site it tells me to adjust it to 36mA for a Class A 6V6GT with a plate voltage of 340V. Another site tells me it should be around 22mA. Can you assist? I am using a 500R pot and a 20R resistor in series to make my adjustments. I also have the appropriate cap in this circuit.

Darrell

[majwild1]

If your building a class a webers site:


Bias Settings For Safe Plate Dissipation - Class AB 70% Class A 90%
6v6gta 340v= 37 ma class a.

the 22ma is for a class ab

[tubeswell]

I tried the 1K 5W resistor from pin 4 to the 22K but it made a toaster (smoking resistor). Since then I purchased a 7W and a 10W and they still got hot, at least the 10W did not smoke.

Yep 5W won't be enough for an EL34

I have 444V on pin 3 with just the 5Y3 installed. When I plug in the 6V6 I have 340V on the plate.

Yep that's normal - The B+ voltage will drop under load (with the tubes installed)

Do I need to put in a 15W or 20W 1K? Does adding this resistor have any effect on the 6V6 or 6L6 when used?

For what application?

I am also confused on the proper bias mA. If I used Weber's site it tells me to adjust it to 36mA for a Class A 6V6GT with a plate voltage of 340V. Another site tells me it should be around 22mA. Can you assist? I am using a 500R pot and a 20R resistor in series to make my adjustments. I also have the appropriate cap in this circuit.

Darrell

What's your cathode idle voltage? And what is the exact/actual measured resistance of the cathode resistor? You probably don't need a 500R pot, just something like 470R 5W-10W resistor. Is the pot rated for 5W? If its just an ordinary ole 1/2W pot, it will probably smoke.

[brodude]

tubeswell:

Looks like you have become my mentor. Thanks for the speedy reply.

The application for the 1K resistor in place of a direct wire from pin 4 to the 22K decoupling resistor and 2nd filter cap turret is to allow me to use an EL34 in my amp in place of the 6V6, since the EL34 has the extra supressor grid. I have already jumpered pin 1 to pin 8 on the octal socket. If this is the correct way of allowing the amp to use an EL34 then I would guess the resistor should be at least capable of 15W. Is this correct?

The 500R pot is a Bourns and is rated for 2W. Is this sufficient? I am attempting to allow myself a way to adjust the bias for any power tube I may use. Do you have another suggestion? Can this be done? I can purchase a 5W Omite rheostat if necessary. I have not seen any smoke but that doesn't mean it's not getting hot.

Can you explain how to measure the cathode idle voltage? I am assuming it is just the voltage with all tubes in and the volume control at zero. Is this correct?

Thanks again
Darrell (brodude)