# Single ended 6V6 to EL34. Impedance?

#### homesick345

##### Poster Extraordinaire
I put an EL34 (instead of the 6V6) in my 5F1 Champ (yes, the trannies can take it, it's all safe & sound)

the amp runs the 6V6 at 8 ohms.

What impedance does the EL34 want to see?

thanks

#### 2L man

##### Tele-Afflicted
It depends of operative voltage. For EL34 I have used 2,5k for about 300VDC and 5k for about 360VDC. Perhaps about 3,5k is optimum for RL34? Often we first get OT and can double or half its impedance changing loudspeaker and where it is connected.

Basically OT impedance, voltage and bias current define operating point on loadline and it define output power and distortion.

Open Universal Loadline Calculator page and input data there and you can see the loadline and test different biases. On it the V+ voltage is voltage between Anode and Cathode so input value which is deducted from your B+1 what voltage loss comes on OT primary coil and carhode resistor. Now when EL34 uses higher current there comes more voltage loss to OT as well. Screen voltage effect to g1 voltage. You can start using your current 6V6 values first.

When you input headroom there comes also distortion boxes. For single ended I set headroom so that 2nd and 3rd distortion are about the same so comparing different setups is easier. Cathode resistor value comes: Grid Bias Voltage : divided by : Quiescent Operating Point.

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#### homesick345

##### Poster Extraordinaire
It depends of operative voltage. For EL34 I have used 2,5k for about 300VDC and 5k for about 360VDC. Perhaps about 3,5k is optimum for RL34? Often we first get OT and can double or half its impedance changing loudspeaker and where it is connected.

Basically OT impedance, voltage and bias current define operating point on loadline and it define output power and distortion.

Open Universal Loadline Calculator page and input data there and you can see the loadline and test different biases. On it the V+ voltage is voltage between Anode and Cathode so input value which is deducted from your B+1 what voltage loss comes on OT primary coil and carhode resistor. Now when EL34 uses higher current there comes more voltage loss to OT as well. Screen voltage effect to g1 voltage. You can start using your current 6V6 values first.

When you input headroom there comes also distortion boxes. For single ended I set headroom so that 2nd and 3rd distortion are about the same so comparing different setups is easier. Cathode resistor value comes: Grid Bias Voltage : divided by : Quiescent Operating Point.

thank you! is there a short version for the math-impaired? Like just stick a 4 ohms or 16 ohm speaker??

#### 2L man

##### Tele-Afflicted
Loudspeaker impedance has direct effect to OT primary impedance. When you replace 16 ohm speaker with 8 ohm speaker the OT primary impedance gets half and 4 ohm speaker make impedance 1/4. If you put 16 ohm speaker to 8 ohm output the impedance doubles and 4 ohm speaker halves.

#### Lynxtrap

##### Tele-Afflicted
thank you! is there a short version for the math-impaired? Like just stick a 4 ohms or 16 ohm speaker??

I think people are hesitant to give such recommendations because the matter is not simple, and consequences can be serious.
Faced with the question above, I'd say 4 ohms. Absolutely not 16. 8 ohms might still be OK.

Would @2L man agree?

#### PDL JahmosAxe

##### NEW MEMBER!
Try it , look for smoke

#### 2L man

##### Tele-Afflicted
I think people are hesitant to give such recommendations because the matter is not simple, and consequences can be serious.
Faced with the question above, I'd say 4 ohms. Absolutely not 16. 8 ohms might still be OK.

Would @2L man agree?

I have changed OT impedances both ways "one step" and plan to use 4 ohm speaker on 16 ohm output for very high tube load on future push pull build.

I always use Universal Loadline Calculator to set operating point I want which means I use lower B+1 when I increase load. I think it is great when there is no need to search tube datasheets and draw loadlines when it takes half a minute to set a new tube and few seconds to change setting. Giuseppe Amato has done great job there!!! His e-book is good and was only \$5.

So far I have not burn any OT although I have dimed all my builds running them to reactive attenuators. I don't have a tube tester so I can't say anything of tube wear yet when my amps do not get enough use.

Silver Supporter

#### Lynxtrap

##### Tele-Afflicted
I have changed OT impedances both ways "one step" and plan to use 4 ohm speaker on 16 ohm output for very high tube load on future push pull build.

I always use Universal Loadline Calculator to set operating point I want which means I use lower B+1 when I increase load. I think it is great when there is no need to search tube datasheets and draw loadlines when it takes half a minute to set a new tube and few seconds to change setting. Giuseppe Amato has done great job there!!! His e-book is good and was only \$5.

So far I have not burn any OT although I have dimed all my builds running them to reactive attenuators. I don't have a tube tester so I can't say anything of tube wear yet when my amps do not get enough use.

Good tip, I will look that up!

In this case, I think the OP just wants a simple answer that gets him to the ballpark safely.

Most of us still tend to be careful when it comes to impedance matching, even if the matter is complex and the output stage might stand a mismatch. You might need to know the math to ignore the math.

#### printer2

##### Poster Extraordinaire
As 2L has said, it is a ratio. If you increase by 2X the normal impedance by putting a 16 ohm speaker where you normally used an 8 ohm then the impedance to the tube doubles (such as, it was 8k before, with a 16 ohm it is now 16k.). If you have a 4 ohm speaker on a 8 ohm tap you are cutting the impedance in half, the 8k gets cut in half to 4k.

#### Ed Storer

##### Tele-Afflicted
Gold Supporter
Though I don't know if a single ended amp would behave by different rules, but I once took a '69 Super Reverb and re-cabbed it into a 2x10. I changed the Rectifier to a 5R4 to lower the B+ voltage. I replaced the 6L6's with JJ 6V6's and changed the fixed resistor in the bias circuit from 27K to 10K to get the adjustment range to suit the 6V6's.

Most single ended designs I've seen are cathode biased. I don't know what changes would be needed in the bias circuit, but I would expect that a change would be needed.

6V6's want to see twice the speaker impedance as 6L6's (I went from 2 Ohms to 4 Ohms by using two 8 Ohm speakers instead of four).

I don't know if a single ended amp would behave differently, but I don't think it would.

So, to answer the OP, if you change from 6V6 to EL34 your speaker impedance should be half with the EL34. This is based only on my experience with a push-pull amp.

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