Leak stereo 20 modification

[Gudmund]

Originally Posted by Eli Duttman

Neutralize the alkali with vinegar (acetic acid). Sodium acetate is harmless and can be safely flushed down the drain. The electrolyte is a paste that may be partially or completely dried out. Dry or not, NaOH is dangerous, until neutralized by a weak acid. Wear thick rubber gloves and safety glasses until neutralization is completed.



Keep C13 32 μF., to avoid overheating the power trafo. Increasing C12 to 100 μF. makes considerable sense, as this is the reservoir position. Don't bother with a bypass here, as modern industrial parts by Panasonic, Nichicon, etc. are MUCH better than OEM.

Increase C10 and C11 to 47 μF. Increased capacitance in those decoupling positions can't hurt and is likely to help. A 4.7 μF. Solen bypassing C10 and C11 might be of be benefit. Those parts supply the small signal circuitry.

Don't forget the cathode resistor bypass parts, C2, C7, and C8. As those parts are very much in the signal path, top quality is in order. Use Nichicon KZ series 'lytics. Increase C7 and C8 to 120 μF.

Shall use the weak acid you suggested for neutralization! - remembered some about the alkali / acid relations after you mentioned this.

- Really think that we have achieved something in this thread. - shall try to find the Nichicon KZ capasitors.

- Did you notice any difference in performance together with the mentioned changes in ST20?
Do you have further suggestions for ST20 improvements?

- Have been wondering why the power transformer went that hot at normal drive by use of 47uF as C13!! - was difficult to imagine that H.J.LEAK LTD. company should have used such undersized powertransformer for public sale. - most of the heat might be the result of glow for GZ34 and approx. 4 A?? - what is your experience?

Gudmund

[Gudmund]

Originally Posted by Eli Duttman

Increase C10 and C11 to 47 μF. Increased capacitance in those decoupling positions can't hurt and is likely to help. A 4.7 μF. Solen bypassing C10 and C11 might be of be benefit. Those parts supply the small signal circuitry.

Don't forget the cathode resistor bypass parts, C2, C7, and C8. As those parts are very much in the signal path, top quality is in order. Use Nichicon KZ series 'lytics. Increase C7 and C8 to 120 μF.

- Could only find Nichicon KZ 100uF/100V radial and 220uF/100V. What do you suggest we do? - is 220uF too much for C8 and C7? - C2 may have to be close to the stated value of 50uF// 2K2 - working with the FB signal?

[Eli Duttman]

Make C2 100 μF./25 WVDC. Make C7 and C8 220 μF./50 WVDC.

How deep do the speakers you plan to use with this amp go? We really should provide some protection against O/P trafo core saturation. That's done by rolling infrasonic noise off at the unit's I/Ps. Installing 68 nF. caps. instead of wires between the RCA jacks and the voltage amplifier triodes' grid circuitry sets F₃ for the unit at 23.4 Hz., when combined with the 100 KOhm value for R1 previously discussed. A well matched pair of Soviet surplus paper in oil (PIO) parts would be superb here. BTW, the lowest note a double bass can produce is 31 Hz.

[Gudmund]

Originally Posted by Eli Duttman

Make C2 100 μF./25 WVDC. Make C7 and C8 220 μF./50 WVDC.

How deep do the speakers you plan to use with this amp go? We really should provide some protection against O/P trafo core saturation. That's done by rolling infrasonic noise off at the unit's I/Ps. Installing 68 nF. caps. instead of wires between the RCA jacks and the voltage amplifier triodes' grid circuitry sets F₃ for the unit at 23.4 Hz., when combined with the 100 KOhm value for R1 previously discussed. A well matched pair of Soviet surplus paper in oil (PIO) parts would be superb here. BTW, the lowest note a double bass can produce is 31 Hz.

- Have already ordered both values Nichicon muse KZ 100uF and 220uF at 100V.

- by normal sound level AR11 is used. - know that we only have approx.12W from the ST20, but sufficient for AR11 in spite the low power. - like the low-low bass sound.

When you say 68nF from the RCA jack - you mean CR filter = 0,68nF followed by 100K? 23,4Hz should do it!!

The PIO from Soviet - the green ones with glass-surface?

Thanks
Gudmund

[KJ2005]

Originally Posted by Gudmund

- by normal sound level AR11 is used. - know that we only have approx.12W from the ST20, but sufficient for AR11 in spite the low power. - like the low-low bass sound.


Thanks
Gudmund

Hi Gudmund,

My stereo 20 drives occationally AR3a - The used Siemens EL84 gets very hot and burns black and grey inside and I will not recommend the stereo 20 for such heavy driven speakers. It is much easyer for the ST20 with JBL L96 if you like vintage sound.

Best regards
KJ

[KJ2005]

Who knows?

In the circuit we have 0,25uF as C4 and C6. This value is not standard today. I have used 0,33uF together with 1M for R13 and R12. Does the changed value have any influence on performance ??

Best regards
KJ