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Upgrade output transfomers, EL34 single-ended

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Hi everyone,

I've recently put together a 6CA7 (EL34) + 6SL7 (6N9P), 15wpc DIY bought from the internet, I'm looking at improving the sound as much as possible mostly for my own learning and enjoyment :)

I want to upgrade the output transformers. The ones that came with the project are 370 ohm on the primary (measured with power off), with 4 and 8 ohm outputs (see attached pictures).

I've been looking at Edcor and Hammond, I read the Hammond are better but I'm not sure how to choose them.

How important is the impedance and how close should I match it? Is is measured with power On or off? Do you have any other suggestion for output transformers than Edcor and Hammond? What about inductance (henry's), how important is that?

Sorry for so many questions, as you have figured, I do not have an background in electrical science...


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  • Aiqin EL34 output trans. question.pdf
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The attached files on top of the pictures:

1. Aiqin EL34 output trans questions: Schematic fo the amp

2. Hammond output transf. single ended: The PDF of the single ended HIFI output trans by hammond

3. Edcor Seemingly most appropriate transformer.pdf: Edcor gives a lot less info about their output transformers it seems, this is their website
Whoa, hold on cowboy!

The primary AC impedance of the output transformer is not the same as the DC resistance (AKA DCR) of the primary. When you measure across the primary with a multi-meter, you're getting the latter and it isn't all that useful when transformer shopping because they're rated with the former. You're looking at OPTs based on the wrong spec.

In the schematic, your primary impedance appears to be marked as 3,500. You're putting about 340 V on the output stage at 50 mA (based on the schematic, tube is biased at 15V with a 300 ohm resistor). It is also connected as Ultralinear.

This is what you want in a replacement:
Primary Impedance 3,500-5,000
Includes Ultralinear tap (AKA UL tap)
Single-ended output transformer (AKA air-gapped)
DC current rating 75 mA+
Multi-tap secondary if you want to use 8 and 4 ohm speakers
Wattage rating 20W+

Appropriate from the Hammond data sheet:

Edcors don't usually include multiple secondary taps, so you'll want to pick the CXSE that matches your speakers. The model numbers follow the pattern CXSE(wattage) - secondary impedance - primary impedance. If you have 8 ohm speakers, look at CXSE25-8-3.75k (or 5k). If you have 4 ohm speakers, CXSE25-4-3.75k (or 5k).

Also, check out Transcendar outputs. They are very nice and reasonably priced. He probably has a 15W that would work for $100 each.

That's a pretty looking amp. Welcome to the hobby!
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Wow Thanks Sodacose!! This is a really great answer!! Really excited about changing those transfos!

You say the impedance I'm looking for goes from 3.5K to 5k ohm, if I have the choice, should I stick to 3.5k or is there any advantages to going to 5k?

Also, quickly, on another unrelated topic, do you know how to calculate the bias current on the preamp tube from the schematic? I'm changing the rectifier tube for a solid state and since it might increase voltage 10-20%, I wanted to know what bias the design was intended for.


Generally, lower primary impedance is going to increase output and higher is going to reduce distortion (as long as you're keeping the tube within operating specs). I have run EL34's on 5k OPTs and I enjoyed the sound. If you like the sound from 3.5k outputs though, stick to it. No right answers, just trade-offs.

You see the little 1.4V next to the pin 6 of the input tube on the schematic? That's the bias voltage that the cathode is sitting at above ground. The bias voltage is created by a voltage drop across the resistors 'under' it (this is also how we determine the current flowing through the tube). Do you know Ohm's Law yet? Yes, I'm making you figure this out yourself :)

Also, when changing the rectifier, you can very easily model the resulting voltage with Duncan's PSU Designer II (free software available online). You'll see a considerable increase in B+ by going solid state and this will change the operating points of the tubes. Get ready to learn how to read a curve chart :)
I find it distorts a bit too much, I might go for 5k then, thanks for that

Also really sorry for sucking, but whatever information you're giving me is pure gold!!

Yes I proudly know ohm's law!! I did electrical physics briefly at college... :) I calculate my actual bias by using the voltage drop on the 82K resistor that's on the preamp tube anode.

But, I'm interested in the intended bias for this model and the 2K and 100 resistor are not really in parallel (the 2K is not going to ground) so I can't calculate an equivalent resistor to calculate the bias current the model is intended for... that's what confuses me

Thanks for the Duncan's PSU designer, I'll try to master it for sure!
I calculate my actual bias by using the voltage drop on the 82K resistor that's on the preamp tube anode.

You're close there. The resistor at the anode in this design is determining the load, but not the bias (at least not directly). Tubes amplify based on the difference in voltage between the grid and the cathode. Biasing creates this difference in voltage by making the grid negative in relation to the cathode (to understand why, you'll have to learn a little more about how tubes work). Anyways, you can either make the grid negative in relation to the cathode (AKA fixed bias) or you can make the cathode positive in relation to the grid (cathode/self bias). As long as that difference in 'idle' DC voltage is there, the tube don't care how you do it.

Your amp is cathode bias. The cathode has been raised in voltage in relation to the grid with the resistor(s) that connect the cathode to ground. These resistors raise the voltage at the cathode and also determine how much current is flowing through your tube at 'idle'. Find those resistors connected between the cathode and ground on your schematic. Now you can use the 1.4V bias voltage and the resistor values to determine how much current you have. (hint: it's not very much current)

But, I'm interested in the intended bias for this model and the 2K and 100 resistor are not really in parallel (the 2K is not going to ground) so I can't calculate an equivalent resistor to calculate the bias current the model is intended for... that's what confuses me

(bonus hint: ignore the 2k and 100 ohm resistors. Those are there for feedback, not biasing)

Also really sorry for sucking, but whatever information you're giving me is pure gold!!

It's absolutely my pleasure. Everyone is a beginner at some point (I'm still a beginner, in my opinion).
Joined 2004
Paid Member
It isn't necessarily the case that there is anything wrong with the provided output transformers, and it isn't necessarily the case that replacing them with something else is going to net you any significant improvement in performance, and the reverse could well be the case.

You need to learn a little more before running off and making such a significant change to the amplifier. At the very least you ought to know what the maximum output power is at clipping for a range of frequencies, what the actual frequency response of the amplifier is at a couple of power levels commensurate with how you use the amplifier, and what the distortion performance is like.

Changing the output transformer is not going to increase output power capability very significantly, if the transformer quality is poor it might give you a little more power at the extremes and less distortion overall, but just changing it does not guarantee that outcome.

If you have insufficient power for the current speaker in use increasing primary Z to 5K will just reduce output power while potentially improving average linearity until you run out of headroom.

I seriously doubt that most Hammond output transformers are going to be significantly better than what is in there now. Something from Electra-Print, James, or Transcendar might be worth the trouble if your unknown OPT is as mediocre as you think it is.

I don't follow your comment about bias measurement at all. The output stage bias is calculated by measuring the voltage across the 300 ohm EL34 cathode bias resistor, at the given 15V that would be equivalent to 50mA. (I=E/R)
Oh it's because my question about bias was relating to the preamp tube, not the EL34, hence the voltage confusion.

Thank you both, I'll definitely give a look to the James, they seem to have good prices and really good reviews!!

Sodacose: thanks for you hints ;), I got it, you really helped me!!

KevinKr: Thank you very much for you insights, I don't really know how cheap the output transformers are. I'm with a bunch of friends, and the aim is eventually to build a more expensive tube amp but meanwhile we want to find how different parts of the amp change sound quantitatively and qualitatively. I've ordered a bunch of audio grade caps, some tubes, a solid state rectifier etc.. We plan on changing parts by parts and listen to changes. If the OPT don't change anything, we wont spend as much on them on the next project.

Thank you both again!

Though it seems to contradict Kevin's statement (and his advice is always spot on), the output transformer is often considered to be the most critical component in a single-ended amplifier if everything else is up to snuff. You may hear considerable change with a new transformer and it's a good (albeit expensive) place to experiment.

I think what Kevin may have been getting at is that the rest of the circuit should be considered and optimized as well. New output transformers aren't a panacea for everything/anything that one finds lacking, but they can help a good design squeeze out every last drop of tubey goodness.

Sodacose: thanks for you hints , I got it, you really helped me!!

I'm glad you figured it out:) I noticed that there are two 6N9p tubes in the picture and that the schematic makes it look like both triodes in each tube are connected in parallel. So you have a full octal socket dedicated for the input on each channel. That leaves room for a lot of fun experimentation and optimization down the line. Have fun!
haha I'm not yet at improving that part of the circuit, but that will be exciting for sure.

So, following Sodacose's advice on specifications for the OPT and following Kevin's advice on brands, I came up with the James JS-6123HS OPT since they come in three different built-in impedance (2.5k/3.5k/5k) and they are fairly priced in comparison with the electra print especially and even the Hammond.

Am I ready to order?

James Audio JS-6123HS Output Transformers - factory matched pair

on ebay.ca:

Black JS 6123HS Jame Audio Tube Output Transformer FOR Western Electric 300B | eBay
Those James will work and they'll let you try more than one primary impedance (great for experimenting). I haven't personally used James, but I have heard another James model in a very nice push-pull amp (SY's Red Light District).

Just to muddy the waters, here are the Transcendar 15W options:

Transcendar Transformers - 15 Watt SE

There's a 5k with UL and multiple secondary taps for $100 each. I have used Transcendar and think they're the bee's knees. I've read plenty of positive opinions about James too, though.

If you use either of these options, you can rest assured that the output transformer is not holding back the amplifier's performance. On the other hand, that may already be the case with your current output transformers. On the third hand, you'll have an extra set of output transformers to build a new amp around.
The amp has 15 wpc and I was told I should aim a bit higher than the amplifier's output power, so I was looking into 20 watts, should I go for 15?

Only the 15 watts 5K has the 40% UL tap

The 20 watts only comes in 3K or 5K both with UL 40% tap

I can't find if the James have a UL tap but it doesn't seem so.

I'm not using the tap as it is now. The 2nd grids are currently on B+.

Bottom line its either
Transcendar 15W, 3.5 K without tap
Transcendar 15W 5K with tap
Transcendar 20W 3K or 5K with tap

James 20 W 3.5K without tap
I don't see mention of the speakers being used. When you go to a Single Ended amp at only 15 watts, your speakers need to be fairly efficent or you will run out of steam. You likely will not have a lot more volume at 20w compared to 15. When you said you can hear it distort, that says to me that you're way into clipping and that why I think your speaker / amp combo is likely more at fault than the actual amount of power. That being said, welcome to the club! FWIW, I've used James, Audio Note, Electraprint, Magnequest and Bartolucci SE output transformers and I've settled on the Magnequest. Expensive and can be a wait for them to be built, but I've found their products to be well worth it.
Efficient speakers are next step :) I'm short on cash so I'm looking at vintage efficient speakers. I'll try a pair of Acoustic Research Ar 2ax soon... and I'm planning on investing in ASW genius 100 eventually... my girlfriend will kill me though...

I'm using mordaunt short MS35ti, 89db/watt at 6 ohm... the 6ohm I think is draining the amp in orchestral fortes, which is most of what I listen too... but I get good volumes when the music is not too demanding, like chamber music, which sounds great
The question of wattage rating in outputs is an interesting one. You have to also pay attention to the conditions that the transformers are rated under. The 15W transformer from one company may handle full power a lot better than the 15W transformer from another. Comparing these specs across manufacturers is tough and that's why opinions from the forums can be really helpful.

Are you running your 6 ohm speakers on the 4 ohm or the 8 ohm tap in the amp as is?
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