The "Assembly Instructions" for the Dynaco Dynakit Stereo 70 list on page 10 tubes 5U4G and 5U4GB as suitable replacement for the GZ34
Thank you Alfred,
One must be careful in considering replacement tubes.
In particular tye GZ34 has a lower heater current than many others of a similar HT rating. It may be OK for that amplifier but not for all!
The attached pdf gives a brief overview, but other factors such as series resistance and capacitor loading must also be considered.