Dynaco ST 70 Tube Amplifier Modified Clean Works Great Modifications

Description

Modified Dynaco ST70 – includes new F4 glass circuit board configured for 6GH8A tubes to replace 7199 drivers. 6GH8As are quieter and readily available at modest cost. Circuit replicates original Dynaco design/component values and is composed of 1% metal film resistors and precision WIMA metalized film poly capacitors.. Improved power supply includes new 1N4007 silicon diode to replace selenium rectifier (safety issue), increased bias/filter capacitors and resistors to compensate for lower resistance of new diode and allows for more bias adjust range. CL60 NTC varistor to reduce inrush current surge and improve tube/transformer life. 1N4007 safety diodes installed to protect circuits/tubes if rectifier tube fails (shorts). New power cord and RCA input connectors. Stereo/mono switch disabled to eliminate switch contacts and resistors that can degrade sound. Quad cap tested for value and low ESR/ripple current leakage. Input choke tested for value and leakage. Output transformers tested for value and impedance matching. All tube sockets cleaned and pin contacts tightened. Chassis shows some mild pitting and patina fitting age of unit. Transformers run cool and it sounds great with solid bass and smooth high end with great separation. Enjoy as is or is a great starting point for additional modifications.
Tube compliment:
2 ea NOS Raytheon 6GH8A drivers
4 ea NEW Electro Harmonix 6CA7EH big bottle matched quad from VIVA Tubes (EL34 output tubes can be subbed but will require bias adjustment after break in) 1 ea 5AR4 rectifier (Japanese origin – stronger than Russian variants) tested 95%
Output tubes will be removed and boxed in original sleeves marked for location in amp.
Will ship via FedEx Ground. Unit will be professionally packaged at FedEx Store with expanding foam in a heavy duty shipping box. Full insurance will be placed on package and signature will be required