Transformer selection for stereo EL34?

I knew this was going to get brought up, but they are the same. I just dont know WHAT they are, but whatever they are, they both are identical.

You sure? Do they look the same, or do they work the same? Let me be the devil's advocate here. Do you know the impedance of the original speakers?
 
Just want to through this out:

At 50 watts, you are pushing the EL34's to maximum output. Designing to a somewhat lower output, like 35 watts (a common design goal for others using this tube) you will get longer service life and give up only about 1.5 dB of headroom. Just some food for thought.

Shelly_D
 
Ok, lets make this easy for the sake of conversation.

What If I decided to go with the Edcor cxpp60-42k for OPTs. Even though they say 60W, I only want around beetween 40W to 50W.

How would this calculate out?

Those are similar enough to the Dynaco 4300 ohm output impedance, you could use them more or less under the same conditions as the Dynaco ST-70, or the Mark II. The fact that they're larger than necessary is a good thing, it means you get less distortion at maximum power.
 
The Dynaco clones are more expensive than the Edcors, though, but you WILL get better performance out of the Dynaco's. Given those two choices (Dynaco clones or Edcors) I would pick the Dynaco clones.

More food for thought: Here's a 50 watt dual channel (stereo) amp I built with common off the shelf parts. Engineering/design work is done. Performance is verified. It's not using EL34 tubes though.

I get it if you want to design your own amp. That is an extremely worthy goal. OTOH, if your goal is to build/assemble a very decent 50 watt stereo amp, take a look at my thread above.
 
The Dynaco clones are more expensive than the Edcors, though, but you WILL get better performance out of the Dynaco's. Given those two choices (Dynaco clones or Edcors) I would pick the Dynaco clones.

More food for thought: Here's a 50 watt dual channel (stereo) amp I built with common off the shelf parts. Engineering/design work is done. Performance is verified. It's not using EL34 tubes though.

I get it if you want to design your own amp. That is an extremely worthy goal. OTOH, if your goal is to build/assemble a very decent 50 watt stereo amp, take a look at my thread above.

I took a look at your amp circuit, and I like it... conservative but useful amount of feedback, good choice of tubes for the roles, and classic topology. I really like how you used the -60V to extend the tail of the long tail pair phase inverter/driver.

I agree with you, that the Dynaco transformers will offer better performance - in fact, if this is a dream amp, it deserves the better performance the real Dyna iron offers.
 
The Dynaco clones are more expensive than the Edcors, though, but you WILL get better performance out of the Dynaco's. Given those two choices (Dynaco clones or Edcors) I would pick the Dynaco clones.

More food for thought: Here's a 50 watt dual channel (stereo) amp I built with common off the shelf parts. Engineering/design work is done. Performance is verified. It's not using EL34 tubes though.

I get it if you want to design your own amp. That is an extremely worthy goal. OTOH, if your goal is to build/assemble a very decent 50 watt stereo amp, take a look at my thread above.

You definitely know what your doing when it comes to tube stuff. Thats for sure. You must be much better at math than I because I noticed especially in the front-end alot of "odd-ball" value resistors.
 
I already have the tubes on hand, and some of the parts on hand which is why I wanted to start this project in the first place. I have the OPTs, although I will debate on whether I used them or not.

If I have to settle for a little less output power, well thats fine I suppose. 35 to 40W would probably be ok with not THAT much of a difference. Thats probably a bit more than what my CONN hack with the E34Ls as it is, is able to provide.
 
You sure? Do they look the same, or do they work the same? Let me be the devil's advocate here. Do you know the impedance of the original speakers?

They both have the same CONN number. They are both the physically exact same size, weight, and shape.

So unless the turns are different, then they are the same. Plus they sound ok in stereo.
 
They both have the same CONN number. They are both the physically exact same size, weight, and shape.

So unless the turns are different, then they are the same. Plus they sound ok in stereo.

I wonder if someone stuck the wrong tubes in one channel before you got it, this doesn't make any sense.. Although very good to have.. Do you have a model and schematic?
 
mbates---do you have the means to measure the output transformers for primary impedance? You need a digital meter and ideally a variac, but if you don't have a variac, any low voltage power transformer will do the job, say one with a 25V secondary.


What If I decided to go with the Edcor cxpp60-42k for OPTs. Even though they say 60W, I only want around beetween 40W to 50W.
I'll run some numbers tonight (unless someone beats me to it).
 
I dont have the means easily unfortunately, I still need to do the exact same thing with the Heathkits in my other thread that we talked in.

So, I would like to use all Octal style tubes, so EL34, 6XX7 type, or 12XX7 or something along those lines. So for 35ish, maybe 40W peak, what is a good schematic? I looked at the Dynaco ST-70 but its a different animal.
 
Soo....

I found an old Westell DSL modem wall wart which is actually 12VAC. it outputs 15VAC though unloaded.

Turned out the only thing I had handy, So....

15VAC across the plate leads, disconnected the centertap so it wouldnt foul the reading.

Highest reading was 0.651VAC, and 2nd tap was 0.33X, and then the 3rd tap was 0.48X something. Oddball. 651 would be the 16Ohm tap, 33x would be the 8 ohm, but the 48X doesnt belong. maybe a 12ohm tap? i dunno, anyways. Its CONN maybe they did something different.

Both output transformers test identically. Other one was 0.652 instead of 0.651. so negligible.
 
Are you sure it's exactly 15.0 VAC? You will get a little better result if you can measure the 15VAC ouptut to get a more specific reading. Then you can redo this math below for a more accurate result.

For now, using 16 ohm secondary data:
  • Turns ratio = 15VAC / 0.651AC = 23.04
  • Square of turns ratio = 530.91.
  • Primary impedance = 16 ohms * 530.91 = 8.5K Ohms.
Using the 8 ohm secondary data:
  • Turns ratio = 15VAC / 0.48VAC = 31.25
  • Square of turns ratio = 976.56
  • Primary impedance = 8 ohms * 976.56 = 7.8K Ohms
Using 4 ohm secondary data:
  • Turns ratio = 15VAC / 0.33VAC = 45.45
  • Square of turns ratio = 2066.12
  • Primary impedance = 4 ohms * 2066.12 = 8.3K Ohms
The 8 ohm measurement is always an estimate since the tap point is not at an even number of turns, so it shouldn't be used with confidence, but the fact that is is "close" to the other measurements is strong evidence that your primary is 8.5K Ohms. So there you go.

How "big" are these transformers? How do they compare size-wise to Dynaco A470's?

Well anyway, let me now run some numbers for your "8.5K primary" transformers on max power you can reasonably expect using EL34's and what your power supply requirements would need to be to support that output power. But I can tell you right now that this primary impedance is not ideal for EL34's.
 
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Option 1: Squeaking every ounce of power out:
  • EL34 output tubes, PP class AB1, 8.5K primary
  • 460V plate
  • 300V screen
  • 40 mA bias current per tube
  • Power at speakers = 45 watts per channel
This is running the tubes pretty hard, idling them at 92% of their design center max ratings. Tubes will wear out more quickly.

Option 2: If you back off to a more sane operating point for longer term reliability:
  • EL34 output tubes, PP class AB1, 8.5K primary
  • 420V plate
  • 250V screen
  • 40 mA bias current per tube
  • Power at speakers = 35 watts per channel
This is idling the tubes at 68% of design center max ratings. Output tubes are run conservatively for longer tube life.

Taking option 2 as the more sane approach, power transformer high voltage winding requirements with dual 5AR4 tube rectification and capacitor input filtering:
  • 360-0-360 CT
  • 400 mA (could squeak by with 350 mA)
There you go. This could be a decent long-term reliable amp if you go with option 2, and if those output transformers are decently wound and can handle 35 watts output.
 
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I actually use my variac and an isolation transformer to do this. Run the input voltage up until I get basically 1 vac on the output and go from there. I find I get more accurate results that way.
 
How "big" are these transformers? How do they compare size-wise to Dynaco A470's?

Well anyway, let me now run some numbers for your "8.5K primary" transformers on max power you can reasonably expect using EL34's and what your power supply requirements would need to be to support that output power. But I can tell you right now that this primary impedance is not ideal for EL34's.

Option 2 looks better and safer. :)

They are the same physical size when it comes to mounting. They match up to the A470 mounting and hole drill measurements if that means anything?

Also What if I put 8 ohm speakers onthe 16 ohm tap? That makes the primary impedance 4.3K right? or no?
 
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The secondary voltages have a ratio of square root of 2 difference between any two consecutive taps, so evidence is strong the secondaries are 4, 8, 16 ohms.

Hammond tends to state their specs unloaded. If you load the secondary with the rated load size for the tap in question, they will measure a bit higher. Meaning if unloaded they measure at 8.5K, loaded they will measure at something like 8.8K. Either way its close enough I think.
 
If those are the tubes you pulled from your specific unit, I doubt someone sourced and installed Conn-branded 6L6 tubes in a 6V6 spot. Looks like the chassis was marked for the tube type as well, so that would confirm.

It would be possible to run 8 ohm speakers on the 16 ohm tap to effectively lower PI too. The tubes don't really know what the mfg intended a particular tap to be used for, they only know what load they are working in to. Might make an argument for lower turns ratio and no "tail end" of the winding that goes unused to possibly have better stability or response too. I'd still double check those measurements though just to be sure of what you have.

Absolute worst case, use these as an 8K transformer with EL84 tubes and buy something else to run with the EL34. Big iron with an EL84 should make for fantastic frequency response.
 
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