• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

ST70 iron recommendations please

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Spectral distribution of distortion can be as important as the amount of distortion. The 12AT7 is well suited for service in PP amps. The 12AT7's distortion is skewed towards 2nd order. When combined with the cancellation of internally generated even order harmonics inherent in a PP O/P stage, the net distortion spectrum is an ear PLEASING "waterfall" of 2nd > 3rd > 4th.
 
Got it. The 12AT7 is just right for the task.

The draw to the ECC99 was the low Rp. Couldn't find the specs for grid capacitance in triode connection from the Mullard EL34 datasheet, but I assume it's fairly high. Then again, a Zout of 13K from the 12AT7 doesn't seem to bad.

Guess we'll start building and see what we end up with.
 
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My friend is working on an updated of this ST-70 (stock), and he has 5AR4 for rectifier, can he can raise the capacitance of the first cap after the 5AR4 to 40-50mf?


Dude,

Let's look at the PSU 1st.

SS rectifying the PA-060's B+ winding is going to yield a high rail voltage. The extra Volts present an opportunity. Original ST70 power trafos are somewhat undersized. Ease the strain on the trafo by CLC filtering the B+ using 10 muF. in the 1st position followed by a Hammond 193M (10 H./300 mA.) choke. The smallish 1st cap. yields a large conduction angle and a cooler trafo. The large inductance takes a BITE out of the ripple and isolates the trafo from the remainder of the filter. BTW, a 'lytic is fine for the 10 muF. part, as it is not in the signal path. The B+ rail can be soft started by placing a CL150 NTC inrush current limiter between the diodes and the filter. Series connected pairs of UF4007s will rectify the B+. A RRSF will be used to keep SS diode switching noise out of the power trafo.

With all of the above, B+ still rates to be on the high side. Combination bias using a shared RC network in each channel under the EL34 cathodes consumes the remaining extra Volts. Combination bias allows the use of a single bias set pot. per channel and stabilizes the tubes against runaway.

The 5 VAC rectifier filament winding can be voltage multiplied to yield a negative rail that can power the LTP current sink and provide EL34 bias. The OEM bias tap gets tied off. The 2nd schematic here will do nicely. As drawn, a positive rail is created. Stand everything on its head for a negative rail. Use "noiseless" Schottky diodes in the multiplier.
 
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