What you supply: One Dynaco ST-70 with good transformers and good output tubes. You pay shipping both ways, or deliver it and pick it up.
What we do:- Replace the output tube sockets with ceramic sockets. - Install Tubes4Hifi’s excellent 6SN7 input board with the Russian paper-in-oil cap option. - Extensively rebuild the power supply. We use the original power transformer. We install UF4007 rectifiers, a motor-run input capacitor, the original Dynaco choke, massive filtering, another chole, another capacitor, and then for each channel, a diode and a final filter capacitor bypassed with a film capacitor. We've measured the ripple at the output of the filter at 7 mV (7 thousandths of a volt) on a 480 volt supply. - Supply used, excellent testing, excellent sounding 6SN7s. - Install switches to allow switching between ultralinear (stock) and triode operation. Triode operation gives you about 14 watts per channel. - Burn in the completed amp for a minimum of 6 hours. - Supply a schematic of the modified amp and biasing instructions. You’ll need a cheap meter, and it’s safe, as you’ll be measuring a fraction of a volt. What’s extra cost: - Output tubes, if needed. - Transformers, if needed (We haven’t needed any yet!) The Result: - Once you listen to the amp in triode mode, you’ll never listen in ultralinear mode again. The amp puts out about 14 watts per channel in triode mode. - We measured 7 mV of ripple on the 480 V B+ supply. That’s about -96 dB. With your ear at the speaker and no music playing you hear nothing.
Recommendations: - If you need output tubes, we recommend Russian reissues of American or European tubes. We’ve used those for several years and had no failures or bad-sounding tubes. Sylvania grey plate 6L6s are great in this amp. Notes: - All the mods we do are electrically reversible; that is, they can be undone and the amp restored to its original electrical state. The holes needed to mount the choke are permanent. - We don’t expect you read this ad, and just send your ST-70 and a pile of money to us without discussion. Feel free to email or call. You can also audition a pair of modified amps at our shop in Omaha, NE by appointment. You are welcome to bring your own music (LP, SACD, CD or whatever) or listen to ours. - The photo shown is for information only; it is from the Tubes4Hifi website. Yours will not have a rectifier tube. FAQ: 1. That's a lot of money to rebuild/modify an amp! Yes, it is. 2. What if the output transformer is bad? We don't know, we haven't run into a bad one yet. 3. How do you know it's worth it? Make an appointment and come in and listen.
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