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No bass from ECC88 PP OT headphone amp

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#22
PRR said:
Loads, but did you approximate the source? A cheap transformer may do OK with 50 ohm source, but a pair of triodes is maybe 10k source. 1000mH 1H from 10k is 1,600Hz low-cut, which sure would be "no bass".

Can you get a 240VCT to 24V(CT) 50/60Hz power transformer? 40VA 24V transformers are often used with heating thermostats and sell for $25.
https://www.amazon.com/Control-Transformer-Primary-Secondary-Furnace/dp/B07RDBS6NK
The 240V side will (probably) be over 10k down to below 100Hz. If that is a lot better, then your "unknown" irons are just wrong.
Click to expand...
rongon said:
5V on a nominal 6.3V heater is almost 20% below normal. That will result in reduced transconductance from the tube (it won't be lit up as hot), which will result in higher internal plate resistance, which will interact with the far too low inductance of your OPT's primary to make the bass response even more deficient.

From what I can see,

1. The OPT primary inductance is far too low. For a 6DJ8 you want the primary inductance to be at least 30H. Is your OPT's primary inductance really only 1H? That's way too low.

2. The rp of a 6DJ8 at these operating points is probably no lower than 2.5k ohms. You might want to try a different twin-triode with lower rp, such as 6N6P or 5687. The downside is that they have lower mu (less gain) than 6DJ8 (aka ECC88, 6922, E88CC) and require double (or more) the heater current (800mA for 6N6P vs 300mA for 6DJ8 or 365mA for 6922/E88CC).

3. The heater needs to be the right voltage, and with enough current capacity, or the operating characteristics of the tube you use will be degraded (gm goes down, rp goes up).
Click to expand...

1058mH on the primary says my LCR meter.

Dug up a 220:18 30VA transformer of a kind I have a few off, the primary has an impressive 145H of inductance but it's not got a very good high frequency response, it already starts falling off at 5KHz and it's pretty much gone on the scope at 10KHz, so the inverse issue... Tried a 220:30 30VA transformer that came out to 25H primary inductance and that was even worse. I wouldn't take these inductance values as the absolute truth though, my LCR meter is only ment to go to 20H.

The PSU has semi destroyed itself from bringing the amp when I went away for Christmas. Transformer came loose and broke the mount for the caps and they were all laying in a pile when I opened it up. Don't think anything took permanent damage, just some bent metal and broken solder joints. Glad I checked before flipping it on 😅

Think I'll start building another OTL amp instead(6C8G and 6AS7G, already have it working well in LTSpice), or go back to fixing my 8136 project that's been sitting since the PSU arced over again a few months back.

Not sure what to do with this build or my ECC88s now though. Don't belive they would do any good as OTL.
 
#23
bananasplit_00 said:
Dug up a 220:18 30VA transformer of a kind I have a few off, the primary has an impressive 145H of inductance but it's not got a very good high frequency response, it already starts falling off at 5KHz
Click to expand...
Thanks for doing the experiment. Yes, 'power' transformers do NOT want extended highs. The one in my picture, dual-bobbin, might be 5kHz (with load). Another kind with separate bobbins on separate legs may drop by 1kHz. A for-audio transformer will have more intimate windings so the highs couple better.

While I too doubt the precision of the "145H" reading, modern control transformers for 24/7 operation may have high inductance to reduce idle losses and reactance power-factor.
 
#24
I used a 1W6 toroidal mains transformer - but this rolls off at 8KHz. I had lots of NFB which resulted in a flat response but that's not what you have. You could boost the HF to compensate I guess. Headphones should be driven from about 100R not low impedance but that will affect the HF response not the bass. I also used these before:
https://uk.farnell.com/oep-oxford-e...2a3e/transformer-1-1-6-45-6-45-150/dp/1172344

https://uk.farnell.com/multicomp/mcfm16-12/transformer-toroidal-2-x-12v-1/dp/9531319?st=totodal transformer

What frequency does the LCR bridge use. Because of the primary capacitance it can give a very different reading of L. 50Hz is a good figure.
 
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#25
baudouin0 said:
I used a 1W6 toroidal mains transformer - but this rolls off at 8KHz. I had lots of NFB which resulted in a flat response but that's not what you have. You could boost the HF to compensate I guess. Headphones should be driven from about 100R not low impedance but that will affect the HF response not the bass. I also used these before:
https://uk.farnell.com/oep-oxford-e...2a3e/transformer-1-1-6-45-6-45-150/dp/1172344

https://uk.farnell.com/multicomp/mcfm16-12/transformer-toroidal-2-x-12v-1/dp/9531319?st=totodal transformer

What frequency does the LCR bridge use. Because of the primary capacitance it can give a very different reading of L. 50Hz is a good figure.
Click to expand...
The frequency of my LCR meter is not specified anywhere I can find. It's not a very fancy tool
 
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