Jump to content
Gibson Brands Forums
Sign in to follow this  
bluefoxicy

EL34 impedance vs a Hammond 125ESE

Recommended Posts

The EL34 as far as I can tell...

 

 

So I've got my Marshall 1986 bass plexi here that's loaded with a pair of EL34s that are a few years old. I've also got a pair of NOS smoked glass Genalex KT66s that I'd like to replace the EL34s with which test as follows:

 

Vgrid = -3.02V' date=' Vscreen = 131.0V, Vplate = 138.9V, Iplate= 75.8mA, GM = 7950 where NEW SPEC is 3600

 

Vgrid = -3.03V, Vscreen = 131.8V, Vplate = 139.5V, Iplate= 76.1mA, GM = 7950 where NEW SPEC is 3600

 

[...']

You can double primary impedance as you outlined by setting the amp for one setting below the cabinet impedance. So, 16 ohm cab, amp set at 8 ohms. This doubles primary impedance

 

I've also seen this thing suggest 3.0k primary output for single-ended class A:

 

http://www.sacthailand.com/AmpMinuetEL34.html

 

So the EL34 is 7.9k, the original OT was 7.5k (...wtf?)? Or is it 3K? And the Hammond 125ESE is 5k.

 

I can get 10K by plugging up to the 8 ohm output; possibly if I add a jack with the 16 and 8 ohm output run to it I could get somewhere in between. Or what about the tube is 3K?

 

I'm not sure, I'll find out the numbers somehow and report back here later. It's too late now, I'm using the Hammond.

Share this post


Link to post
Share on other sites

From Aiken amps tech page:

 

"Reflected impedance

An output transformer has no impedance by itself (ignoring primary inductance/resistance for the moment, which is a different subject). It simply reflects the impedance load on the secondary back to the primary.

A transformer has a turns ratio which can be measured by putting an small AC voltage across the primary and measuring the resulting secondary voltage. The resulting voltage ratio is the turns ratio (you can also put a small voltage across the secondary and measure the resulting primary voltage, which is usually easier, because the voltage is higher and there is not much secondary resistance to introduce measurement errors - but watch out for high voltages on the primary if the turns ratio is large!).

 

The impedance ratio is the square of the turns ratio, which is also the square of the voltage ratio, as shown in the following equation:

 

Zp/Zs = (Np/Ns)2 = (Vp/Vs)2

 

If you put 1VAC across the secondary, and measure 20VAC across the primary, you have a turns ratio of 20:1, which corresponds to an impedance ratio of 400:1. This means that if you put an 8 ohm load across the secondary, you will get a reflected impedance of 3.2K ohms across the primary. If you put a 4 ohm load across the secondary, you will get a reflected load of 1.6K ohms.

 

For example, if you have a transformer designed for 4.3K : 8 ohms, you can apply a 1 volt AC signal across the secondary 8 ohm winding, and you should see 23.18VAC across the primary, which corresponds to a 23.18:1 voltage ratio or a 537.5:1 impedance ratio, which would reflect an 8 ohm load back as 4.3K. "

Share this post


Link to post
Share on other sites

I think you may find the EL34 sounds better with the Hammond at a 2.5k load, which means you'd plug your 8ohm cab into the 16ohm jack.

 

The original 7k OT will not sound very good with an EL34. It's simply crappy iron. Besides, it isn't big enough to handle a 10watt tube.

 

Gil...

Share this post


Link to post
Share on other sites

Thanks lay, gil. I have no idea where those numbers come from but I'll give that a try. My PT shipped today, so by the end of the week I should have the amp together.

 

Do you know how I would go about getting a 16 ohm dummy load? If 2.5k does work better (I'll test), I'll solder in some 16/8/4 ohm dummy loads in parallel with the jacks, with a switch to kick them on. I don't think dropping a resistor across it will help... need impedance, not resistance. :(

 

... I think O_o

 

http://www.gearslutz.com/board/instruments-guitar-bass-amps/100129-4ohms-16-ohm-speaker.html

 

These fellows seem to think dropping a 16 ohm resistor across it will make the speaker 8 ohms (I'd need a big fat 15 watt resistor... or two 8 watts in parallel!), one of us doesn't quite understand the difference between impedance and resistance and I'm not sure which x_X

Share this post


Link to post
Share on other sites
Thanks lay' date=' gil. I have no idea where those numbers come from but I'll give that a try. My PT shipped today, so by the end of the week I should have the amp together.

 

Do you know how I would go about getting a 16 ohm dummy load? If 2.5k does work better (I'll test), I'll solder in some 16/8/4 ohm dummy loads in parallel with the jacks, with a switch to kick them on. I don't think dropping a resistor across it will help... need impedance, not resistance. :(

 

... I think O_o

 

http://www.gearslutz.com/board/instruments-guitar-bass-amps/100129-4ohms-16-ohm-speaker.html

 

These fellows seem to think dropping a 16 ohm resistor across it will make the speaker 8 ohms (I'd need a big fat 15 watt resistor... or two 8 watts in parallel!), one of us doesn't quite understand the difference between impedance and resistance and I'm not sure which x_X[/quote']

 

Rat Shack caries an 8ohm 20watter. Two of those in series is 16ohms. Of couse, it won't sound quite as good as a real speaker since there's no reactance.

 

Gil...

Share this post


Link to post
Share on other sites

Yeah. If I knew how to actually measure impedance I could build something... pickup wire windings would do the trick for making a quick coil. But, then I'd have to shield it (screen around it, ground the screen).

Share this post


Link to post
Share on other sites

Basically, we were supposed to be talking about the reflected impedance the power tube sees from the OT. For a better idea of what a particular tube needs to see, check that particular tube's data sheet. Don't expect to get the right answer from Eurotubes unless you ask the right person the right question. He was probably thinking you wanted to know the plate resistance, which is 15k ohms for a JJ EL34L. The law of GIGO most definitely applies to sales clerks, as well as computers.

 

Wanna know what tranformer load an EL34 wants to see? Take a look at Duncan's Data Sheet reference page. Check "Zout". http://tdsl.duncanamps.com/show.php?des=EL34

 

Gil...

Share this post


Link to post
Share on other sites

Thanks gil!

 

I am going to need to get into some serious tube theory soon and stop asking so many questions here. I'm actually starting to break the scope of this forum... I tried working out possibly a 2 tube (EL34, not 6V6) design, and realized there's no Hammond 125xSE that'll handle it (80mA on the DC bias, the 125GSE can do 100, 2 tubes gives 160...) so I need a Class AB push-pull design.

Share this post


Link to post
Share on other sites

The 125GSE is a SE output transformer. I've got one right now I'm about to throw into a former Valve Special. I'm also using, or rather, going to be using a 270FX power transformer and a P-TF22921 reverb transformer from AES. Basically I'm hoping to squeeze maybe 15 watts of this thing with a 6550. The trick with the GSE as with the DSE or ESE, o any other OT for that matter, is to bias the power tube(s) so that the tube isn't drawing too much current across the OT. Think of the OT's limitation as more of a limitation on your max bias current, set the bias accordingly with respect to the limits, and the OT will survive even your grandkid's grandkids.

 

Gil...

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  

×
×
  • Create New...