“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”