In order to achieve more gain I changed the LL1660 in place of ALT Q SE to line output 4.5 : 1 to ALT T SE to SE Interstage 2 : 4.5 but in place to have more gain I have less gain?
Presumed that you wired correctly, this will worsen bass response dramatically as primary inductance drops from 100H to 33H.
100H is already "lowish" for 26 tube with 8k Rp.
I don't think you will ever get nice square waves when you measure as high as 10-20 KHz. Output transformers simply do not like that. However I'm more interested in sin waves. How is the frequency response? Do you have anything left at 20 Hz relative to 1Khz?
I'll try to get to it today. When I had a quick look the other day and saw these traces, the sine waves looked OK. I will run it through the whole frequency range, and beyond, sine and square, before and after the output transformer, to get a bit more understanding. Should I try different loads on the output? How low could I go? My Parasound A21 has 33k, but a couple of valve power amps have the usual 100k inputs.
Looking at this, from Lundahl's data sheet -
"The source impedances used
in the tables indicates a
recommended upper limit, (3.5k ohms)
unless freq. response can be
compromised.
At lower source impedance
resonance peaking will occur. It
can be reduced using secondary
load resistors."
prompts me to ask how to implement the secondary loading. (The ra for the #26 appears to be 7300 ohms.)
Usually secondary loading for a line output is 1k - 10k with heavier loading (lower resistance value) gives lower bottom end and less loading provides less damping. You should try different values, measure and listen since different setup needs different loading and the Ra of the 26 varies quite a bit depending on tube condition. Looking at the very strong +- 80kHz ringing that you have on the 20kHz square wave output, I would suspect a low 1k - 2k2 loading is more appropriate, but you need to try several values. You could even find that less than 1k loading is the best for your setup.
Using a 26 tube (Rp ~8k) with this transformer (upper source impedance limit 3k5 as indicated by Lundahl) already confirms that you will not have acceptable low frequency bandwidth (at least for me it would not be acceptable as I prefer the bandwidth to be straight from 20 Hz to 20 kHz in order to have a neutral sound balance).
My advice is to measure frequency bandwidth first with sine waves; also see what happens when you change secondary loading (when primary inductance is not enough you will not get more bass when lowering the secondary load). Then decide if the outcome is acceptable for you.
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Please excuse my ignorance (I did say I had a lot of help with this build!), but am I correct in assuming that the 1-2k would be across the secondary?
Physically constructing something like this is much easier for me than understanding and calculating the circuit. I was always much better at metalwork than maths at school!
Physically constructing something like this is much easier for me than understanding and calculating the circuit. I was always much better at metalwork than maths at school!
Hi!
Lundahl offers his transformers with many different air gaps. Not all are listed in the data sheet but a few examples. The 3k source impedance is given as the recommended source impedance for the 18mA version. Awkwardbydesign has the 8mA version which has more than twice the primary inductance. This version has been used with the 26 many times and works well.
Don't expect a perfect 20kHz square wave from a transformer. The ringing can be dampened by loading and/or with zobel networks. In my experience this often results in better square wave response but not necessarily better sound. Try and use what you like best
Best regards
Thomas
Lundahl offers his transformers with many different air gaps. Not all are listed in the data sheet but a few examples. The 3k source impedance is given as the recommended source impedance for the 18mA version. Awkwardbydesign has the 8mA version which has more than twice the primary inductance. This version has been used with the 26 many times and works well.
Don't expect a perfect 20kHz square wave from a transformer. The ringing can be dampened by loading and/or with zobel networks. In my experience this often results in better square wave response but not necessarily better sound. Try and use what you like best
Best regards
Thomas
Hi!
I suggest awkward checks the op point and health of his tubes. How much current do they draw? Also checking frequency response as hard been suggested is a good idea.
In order to get an impression of the performance of the output transformers in this application, you can measure them separately by feeding the primary from the sig gen through a 7k resistor. This emulates the plate resistance of the tube. Of course this is not exactly as it will behave in the circuit since there will be no DC in the primary, but is close enough. Then you can hook up the tube again and compare the results to the measurements of the raw transformer.
BR
Thomas
I suggest awkward checks the op point and health of his tubes. How much current do they draw? Also checking frequency response as hard been suggested is a good idea.
In order to get an impression of the performance of the output transformers in this application, you can measure them separately by feeding the primary from the sig gen through a 7k resistor. This emulates the plate resistance of the tube. Of course this is not exactly as it will behave in the circuit since there will be no DC in the primary, but is close enough. Then you can hook up the tube again and compare the results to the measurements of the raw transformer.
BR
Thomas
Attached pics 1kHz square wave, one direct the audio generator the other direct to hte 26 tube & the other after the LL1660 configured Alt S 4 : 4.5 SE to SE interstage, as you can see the LL1160 pic reduces the gain
Attachments
![WP_20161206_001[1].jpg](/community/data/attachments/541/541826-fba81d2f95805bc3569d7fc1f8035cf9.jpg)
![WP_20161206_002[1].jpg](/community/data/attachments/541/541845-58e4ea9adc7536a11b87970575a854f6.jpg)
![WP_20161206_003[1].jpg](/community/data/attachments/541/541860-df4b9e5d7fd35a9477a5756b14685528.jpg)
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Left direct output 26
Center direct signal generator
Right LL1660 output feed with 681R