The HT-5 preamplifier is based on the HT-Dual pedal. As is the case in the
HT-Dual, we use solid-state circuits to provide "pre-gain" before the ECC83
valve. The diodes are part of this "pre-gain circuit" and are used to limit
the signal level being driven into the valve and hence maintain the correct
operating conditions for the valve in this application.
Most importantly, the valve is the last amplification (and clipping) device
in the signal chain and the overall gain structure is designed such that the
compression and harmonics you hear are generated solely from the valve
itself