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Old 8th October 2010, 10:39 AM   #1
Ian444 is offline Ian444  Australia
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Default EL34 PP output tranny questions

I want to build an EL34 PP guitar amp with parts I have on hand and decided to strip a Music Angel hifi amp (stereo EL34 PP amp) for the power and output trannies. I measured the output trannies and they have a reflected load impedance of 14,700 ohms for the 4 ohm tap and 11,900 for the 8 ohm tap. What is this? This was way higher than I expected, even allowing for the Chinese fudge factor. I removed the tranny cover and tranny and was surprised by it's very large size and weight.

I guess the only way to use this output tranny is to use a 4 ohm load on the 8 ohm tap for a reflected load impedance of around 6000 ohms. The Mullard datasheet mostly gives examples of 3.5K a-a, and one example of 6K a-a with cathode bias. I was hoping to do fixed bias, but cathode bias would be OK if necessary. The B+ on this amp, if it goes ahead, will be around 440V at the first filter cap. The other alternative is to use an output tranny from a Ming Da MC34B (6L6 PP), it has a reflected impedance of around 3900 ohms for the 4 ohm tap and 5300 ohms for the 8 ohm tap, but it is around half the size of the Music Angel monster. For a sanity check I measured the reflected impedance of an 8K Hashimoto output transformer and got 7K2 for the 4 ohm tap and 7K5 for the 8 ohm tap, so I'm sort of in the ballpark with the measurements. The amp will be for a friend and will amplify an acoustic guitar, so needs to be reasonably clean and power output around 25W is all that is needed. I believe he has a 4 ohm speaker box but need to check up on that.

So I guess I'm a bit confused. Any tips, suggestions, comments, or hare-brained schemes welcome.
Are there any other tubes that would work well with the Music Angel OT's high primary impedance?

pic of tranny

tranny schematic
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Old 8th October 2010, 01:45 PM   #2
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The highly-regarded Ken Fischer "Trainwreck" guitar amplifiers used a pair of EL34's with 400 volts B+, fixed bias and 6600 ohms plate-to-plate. Output power was about 30 watts.

There are more details at The Amp Garage forum.
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Old 8th October 2010, 02:13 PM   #3
DF96 is online now DF96  England
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Wild guess: the Chinese factory accidentally wound the secondary with the same wire size as the primary. With a few assumptions and simultaneous equations you can show that they intended to produce a 5200 ohm impedance, or thereabouts - this is quite plausible for a pair of EL34. Unfortunately they ended up with a secondary with wire resistance of 7.44R (4R tap) and 10.5R (8R tap). Was the original amp a bit quiet?

To test my theory, measure the DC resistance. I have given the secondaries above. The primary will be around 267R, which is perfectly respectable.
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Old 8th October 2010, 02:42 PM   #4
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run a 5-20 volt 60hz into the secondaries. Get mister multi meter. Measure voltage going in, and voltage going out, that should give you your ratio.
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Old 9th October 2010, 02:13 AM   #5
Ian444 is offline Ian444  Australia
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nightanole, I did do that, in the schematic attached to the first post.

I measured the DC resistances, there is 90 ohms a-a on the primary, and the four secondary windings are all 0.1 to 0.2 ohm. One of each of the parallel pair of windings on the secondary is stranded wire, unusual.

I haven't listened to the Music Angel amp for a couple of years, I don't remember it to be low in power but didn't listen to it more than a week or so. Strange primary impedances, amazing what you can get away with, I guess it *must* have been low on power, around 27W from the loadlines I drew up this morning. It would have had around 430V on the plates and similar on the screens.

I guess the 8 ohm tap would work for 4 and 8 ohm loads since 25W is all that is required, just going to get more power with 4 ohms. The loadline for 12K (8 ohm load) passes way below the knee of the -5V grid curve, even with G2 at 250V, I don't know what the ramifications are, all I have read is that it is "bad practise". What does actually happen in this situation?

Darryl, the Trainwreck looks interesting, "right on the verge of instability" sounds like my cup of tea for a guitar amp! 6K6 certainly looks like a decent safe loadline for class AB1 compared to the 4K that the Marshall's use.
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Old 9th October 2010, 05:28 PM   #6
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Seem you are fearlessly butchering chinese hifi's to make your own stuff. That is cool! Makes me look at my Ming-Da with a fresh new eye!
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Old 9th October 2010, 11:04 PM   #7
45 is offline 45  Italy
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IAN444 you don't need a big output transformer for a guitar amp because you don't need to go below 80Hz. Any hifi OPT rated for 25W (usually at 50Hz) will be good even for 50W output at 80 Hz.
4-5K max is the way to go for me so use the Ming Da MC34B transformer with a 8 ohm driver.
Pentode connection without feedback, of course.

45
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Old 10th October 2010, 08:39 AM   #8
Ian444 is offline Ian444  Australia
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With 4K a-a, and 360V B+, I get a load line like this. How high can the B+ go, what is the limit?
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Old 10th October 2010, 11:20 AM   #9
45 is offline 45  Italy
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Quote:
Originally Posted by Ian444 View Post
With 4K a-a, and 360V B+, I get a load line like this. How high can the B+ go, what is the limit?
The 1K load line is fine for class AB1. The max power curve is meant for the quiescent conditions. When the device is modulating power the average anode dissipation will be not that much.... otherwise you wouldn't get anything at output!
You can go up to 400V 60 mA (i.e. 24W) safely. The only exception I know is the NOS EI with tipped glass that was safe to run up to 20-21 W but was by far the best sounding EL34 I have ever listened to. Better the NOS Mullards, Telefunken etc....
At voltages higher than 400V you should look at the specifications of your actual tube. At the moment "my" EL34 is the Golden Dragon. Pretty good, I would say.
One experiment you can do is: try two or three rather different anode voltages/biases and you will be able to see how the sound will be affected. It's a really good way to find your sound, also according to the speaker you have. As a line of guide, typically, it will be darker for lower B+ and brighter for higher B+. Much better than just swapping tubes of different types and brands....
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Old 10th October 2010, 11:25 AM   #10
45 is offline 45  Italy
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...also try with a G2 voltage lower than the anode B+.
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