OT input impedance question

[Guus]

Hi Guys,
I have a technical question.

I've build a single end guitar amp with an EL84 endtube.
I use a Hammond 125 ESE OT with diffenrent taps to match the correct speaker impedance.
To match an 8 ohm speaker I selected the OT tap in such way to get 5k input OT impedance.
This because I read somewhere you need ~5k for a single EL84.
The amplifier sounds pretty good but...

Because I use a relative high B+ (400 Volts on the plate of the EL84) and doubting about the correct impedance match I made the following calculation:

Input impedance OT = Plate voltage (400V)^2 / max plate dissipation (12 W) This gives > 10k. This is the double of the 5k I used.

When I connected the 10k related tap on the OT to match the 8 Ohm speaker I found out that the amp sounded much better. More lows, more volume. So imo much better.

Questions:
- did I make the correct calculation?
- is it true that if you have a higher Voltage on the plate, you should need a higher OT impedance to get the perfect match between tube,, OT and speaker?

grz
Guss

[Guus]

Hi Guys,
I have a technical question.

I've build a single end guitar amp with an EL84 endtube.
I use a Hammond 125 ESE OT with diffenrent taps to match the correct speaker impedance.
To match an 8 ohm speaker I selected the OT tap in such way to get 5k input OT impedance.
This because I read somewhere you need ~5k for a single EL84.
The amplifier sounds pretty good but...

Because I use a relative high B+ (400 Volts on the plate of the EL84) and doubting about the correct impedance match I made the following calculation:

Input impedance OT = Plate voltage (400V)^2 / max plate dissipation (12 W) This gives > 10k. Pfff, this is the double of the 5k I used.

When I connected the 10k related tap on the OT to match the 8 Ohm speaker I found out that the amp sounded much better. More lows, more volume, more open. So imo much better.

Questions:
- did I make the correct calculation?
- is it true that if you have a higher Voltage on the plate, you should need a higher OT impedance to get the perfect match between tube,, OT and speaker?

grz
Guss

[martin manning]

Yes that is correct. The equation you used will find the slope of the load line such that the operating point will be centered at the chosen B+. Previously you were clipping on the negative grid voltage swing.

[pdf64]

400V seems too high for an EL84. Consider abiding to the manufacturer's limiting values, at least until there's a deep understanding of all the constraints at play.

Merlin has written an excellent summary of the design process, see http://www.valvewizard.co.uk/se.html

Your methodology seems wrong:-
a/ what is the power tube limiting plate dissipation rating doing in there?
b/ a transformer coupled plate swings to twice the VB+.

However, I agree that in general, higher VB+ require higher load impedance, in order limit plate dissipation.

Hope that helps - Pete

[Guus]

Hi Martin,

thanks for the quick respons/lesson.
Again something learned.
Now I see how and why it is so important to get the right operation point of a tube. The amp sings now with a nice gradual distortion.
Also, it wil help me with new builds.

To PDF:
Yes I know, 400 Volts on the plate is very high. To prevent stress of the tube I carefully balanced the screen voltage, which imo is much more critical.

Guys, again thanks for the reply.

gtz
Guss