Help Please!! No Bass

[Qpiter]

I build the Leak TL/50 from scratch, point to point, following the schematic. Some of the voltages are a bit
high: EF86 165 (should be 150); 12ax7 315v (should be 300) Kt88 plates 478- cathode to plate voltage 425v.
across cathode 45v ;resistor ohms 493. The treble is wonderful but the bass is weak, very weak. I upgrade the coupling cap from .25 to .47uf, along with the screen grid cap on 12ax7. But still very little bass. Now I am a
novice. the only tool I have is a multimeter. Please, no complex theory. Any help to improve the bass will be appreciated. schematic link:http://www.44bx.com/leak/Leak/Circuits/tl50_ecc83.JPG

 

[Eli Duttman]

What did you use as the O/P transformer?

 

[Qpiter]

Eli, it's Edcor cxpp100 watts; 5000ohms ; taps 16,8 & 4 ohms. 40% screen.

 

[Eli Duttman]

5 Kohms is "tall" for KT88s. Something in the 3 to 3.5 K range is common with KT88s. I suspect the 5K "iron" could be OK using full pentode mode "finals", an above 500 V. B+ rail, and regulated g2 B+.

You can't rely on Leak's phase compensation configuration. Phase compensation optimization is an empirical process that must be repeated for each topology/"iron" combo.

 

[azazello]

Whit 5 kohm OT bass must be bigger than with 3 kohm.

 

[Depanatoru]

With a multimeter and a computer as generator it should be possible to indentify in which stage the bass is lost . Probably some kind of mistake , moderate Raa mismatch is not enough for "very weak" bass ... whatever it would mean

 

[GoatGuy]

There aren't many places where could selectively squash the bass, actually except C₂, C₃, C₁₁ and C₁₂. What actual values were used?

⋅-=≡ GoatGuy ✓ ≡=-⋅

 

[azazello]

Some cap, maybe, in the audiopaths isnt the correct nominal. Check caps, connected to the ground.

 

[20to20]

I build the Leak TL/50 from scratch, point to point, following the schematic. I upgrade the coupling cap from .25 to .47uf, along with the screen grid cap on 12ax7. But still very little bass.

The Gain control does not provide loudness. Does your preamp? Are you used to SS audio equipment with loudness on/off, or have loudness type volume controls? Without loudness it always sounds thin from a stand alone amp.

 

[Qpiter]

I disconnected the NFB and it's much better. GoatGuy the caps are OK, except for C3.If anyone knows the value please let me know.I am using 1uf. 20to20 I using amazon kindle fire as the signal source. Can it drive the amp to full power? Thank you, appreciating the help from everyone.

 

[azazello]

Maybe records. CD....that You listen are with small bass?

 

[Qpiter]

I found the problem which was the Kindle Fire I used as source; it was the input. Apparently, it was not able to drive the amp to full power. I have a small preamp(solid state) DJpre 11
which I stuck between the Kindle Fire and the Leak. Oh my God, the bass almost lift the roof off.

 

[wg_ski]

Your source might not only be low, but may be designed to play through small speakers which can’t reproduce bass- it may have filtering. Your preamp, just from the brand/model, I suspect may have some bass boost.

My latest experience with big tubes on high voltage through beefy iron almost qualifies as the kind of “iron fist bass” you get from 30 or 40 year old Crown amps. It was an eye-opener to say the least, for something that couldn’t have a damping factor over 10.

The experiment you did disconnecting the NFB would result in a bass boost - the gain and output will peak at the speaker’s resonant frequency. Old console stereos were intentionally designed to take advantage of that - to get “bass” out of two miserable watts.