Tube amp question

[absrec]

I know the title is vague. I didn’t know what to call it. I’ve been trying to figure out how to find the appropriate specs for some lesser used tubes like the EF80 to be able to use in push pull designs. I know Rob Robinette has figured out that a 22k5 primary works well for a (2) EF80 setup but how does one come to that conclusion? When I research that tube, I can only find very basic specs like internal resistance and limiting values for plate voltage and dissipation. On most power pentode datasheets, you find ‘equivalent resistance plate to plate’ which helps you pick a transformer for your design. I realize the EF80 was designed to be an RF amplifier and as such, not intended for push pull amplifiers. Rob has proven that they can be used for that purpose though. I just don’t know how he came up with the proper impedance ratio, etc. Is there another way to compute this from such limited data?

Thanks,
-Aaron

 

[rock soderstrom]

I know Rob Robinette has figured out that a 22k5 primary works well for a (2) EF80 setup but how does one come to that conclusion?

I am not an expert, but I have seen this several times:

Quote...."The general rule of thumb for a one-tube single ended output stage is to use a transformer with a primary impedance
approximately 1/10th of the specified plate resistance."

Plate resistance = internal resistance is for an EF80 roughly 500K, so take the 1/10th of it and divide it with 2 and you are pretty close to the 22k5 for the PP EF80 amp.

Let's see what the experts have to say on this subject

 

[C.B. - Boudio]

http://server.idemdito.org/blog/histo/radio/tube/tube-amp3p.htm

Maybe this can be of some inspiration? It's in Dutch (or, if you understand French better, it's just the same: http://server.idemdito.org/electro/histo/audio/tube/tube-amp3p.htm), but you'll get there.

It's an unbelievably interesting website, in my opinion. I have nowhere else learned as much so far, I think.

 

[Ó Draighnáin]

It is an interesting question and I'm glad you asked it. I quite like low power push pull tube amps. I found the below discussion which might be useful:

https://el34world.com/Forum/index.php?topic=25747.0

You could buy a little Hammond 125B and play with the connectivities (there are a lot impedance options) listen and watch for red-plating?

 

[rock soderstrom]

Maybe this can be of some inspiration? It's in Dutch (or, if you understand French better, it's just the same: http://server.idemdito.org/electro/histo/audio/tube/tube-amp3p.htm), but you'll get there.

Thank you, interesting. But this statement is not entirely valid:
(The EF80 can not be used in preamplifier...)
"De buis kan niet als voorversterker gebruikt worden, maar wel als stuurbuis van een eindtrap en als eindtrap zelf van een kleine versterker."

The EF80 was also used as a preamplifier tube and today it is even used in microphones as a replacement for the VF47.

But it is true that the tube was designed for other applications.

 

[C.B. - Boudio]

Thank you, interesting. But this statement is not entirely valid:
(The EF80 can not be used in preamplifier...)
"De buis kan niet als voorversterker gebruikt worden, maar wel als stuurbuis van een eindtrap en als eindtrap zelf van een kleine versterker."

The EF80 was also used as a preamplifier tube and today it is even used in microphones as a replacement for the VF47.

But it is true that the tube was designed for other applications.

Strange, seems like I've read over that. But I have not written it, nor do I know the writer

 

[rock soderstrom]

Strange, seems like I've read over that. But I have not written it, nor do I know the writer

You are innocent!

 

[Disco Volante]

Ohms law should give you a ballpark figure. 22K5 sounds reasonable enough, for the size/dissipation of a small pentode. Run them in parallel and you could probably get away with using a "standard" EL84-size OPT. And get a bit more power. Maybe use a couple of dirt-cheap ECC91 (with grids and anodes connected) for the PI...
And end up with a lot of little tubes doing the work of a few bigger ones. Fun or masochism?

Cheers, V!

 

[Tubetec]

Theres a lot to be said for lower powered push pull tube amps ,
they give that same characteristic bounce just like 50 and 100 w models ,
much less headaches from the engineering point of view , with mic spill etc

 

[absrec]

Ohms law should give you a ballpark figure. 22K5 sounds reasonable enough, for the size/dissipation of a small pentode. Run them in parallel and you could probably get away with using a "standard" EL84-size OPT. And get a bit more power. Maybe use a couple of dirt-cheap ECC91 (with grids and anodes connected) for the PI...
And end up with a lot of little tubes doing the work of a few bigger ones. Fun or masochism?

Cheers, V!

The idea of using EF80s over a 12AU7 (or similar) is that a pentode is supposed to break up more like the amps using 6L6s, EL84s, EL34s, etc. Of course, I don’t expect a free lunch but the concept is intriguing.

 

[absrec]

I am not an expert, but I have seen this several times:

Quote...."The general rule of thumb for a one-tube single ended output stage is to use a transformer with a primary impedance
approximately 1/10th of the specified plate resistance."

Plate resistance = internal resistance is for an EF80 roughly 500K, so take the 1/10th of it and divide it with 2 and you are pretty close to the 22k5 for the PP EF80 amp.

Let's see what the experts have to say on this subject

That does make sense. I’ve heard the 1/10th (or 10x) rule when designing headphone amps or preamp input sections.

 

[Ó Draighnáin]

Theres a lot to be said for lower powered push pull tube amps ,
they give that same characteristic bounce just like 50 and 100 w models ,
much less headaches from the engineering point of view , with mic spill etc

I quite agree Tubetec, I don't use any of my 30W - 50W amps (tinnitus and I don't perform live any more).

My main amp is a little push-pull amp with radio output valves. I use military type Mullards (7 pin valves) and run them at about 270Vs (250V recommended). They haven't died yet. I based the amp on the AC30 and played around with dropping resistors for the voltages (dropped the PI and output compared to the V1 voltages). Amp is valve rectified with the same rectifier tube as the EQP1A I think. I also added a Matchless type capacitor switch for tone control. It has two slightly different channels (both essentially lead channels). It suits me fine. Still very loud.

I found I had to tune the headroom with the output cathode resistor.

 

[Tubetec]

Pentodes typically have a little more edge than triodes when driven ,

We learned early on a 10-15W tube amp was enough , you didnt need to lift the roof off the venue with 50 and 100W tube amps cranked ,and it messed up my job as sound engineer and roadie.

 

[merlin]

I just don’t know how he came up with the proper impedance ratio, etc. Is there another way to compute this from such limited data?

Generally you find a copy of the anode curves then draw a load line, same as with any tube.