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Specifications These charts show the Frequency Response of both units. The lower unit has a bit more roll off on the high end as can be seen. This first chart is for the Mic input. Notice that the vertical divisions are only 6 dB, so even a small variation can be seen. The vertical position of the curves is meaningless. I arranged the curves to make this chart easy to see and understand. In other words, the cathode follower/term off outputs are NOT 15 dB lower in level than the Altec/term on. Microphone Inputs Line Inputs Instrument Inputs The high freq rolloff is intentional in the design and typical of guitar preamps. I saw no point in providing a freq response chart for the overdrive mode. It appears that the CF output's transformer has a resonance at about 12Hz when unterminated. One unit is worse than the other indicating to me that there is variation in the output transformers. The only explanation I could find to explain the other differences between the units came down to all the old resistors which were not replaced. These vintage resistors have 10% tolerance and might account for the differences. The power supply voltages of the 2 units are very close with less than .5% difference on both the B+ and the filament power. Distortion figures: Max Gain: (with termination on) Line input: Instrument input: 40.8 dB (CF output only)
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More Ampex 600 series info: Most of the Ampex 600 and 601 tape decks/preamps (with the mic transformer) work pretty well with dynamic mics like the classic shure sm-58 and also vintage ribbon mics. In fact, I have never heard a 58 sound as good as it does through the 9er-ized 601. Unmodified 601s still use the 4th amp stage which limits (IMHO) the 601 for general purpose recording. It does provide overdrive capability and is a worthwhile circuit. This stage adds a pot/voltage divider(can be used as an attenuator for overdriving) to lower the signal followed by an amplification stage to bring it back up then the cathode follower. This colors the sound and is less transparent than the Altec 15095 (or other transformer) tapped off after the mic preamp section. If you Google for Ampex 601 you'll see people talking about using it for a special effect - distorted vocal or bass guitar preamp. You'll also find mention of how noisy they are. They can be noisy, but they can also be fairly quiet when they are setup well. The 601 tape decks were only rated at 40 - 15k frequency response. The preamps are a diamond in the rough however, since their circuitry is based on classic tube preamp design incorporating a pentode first stage, followed by a dual triode. In the 601, it has been compromised by size and cost restrictions, as well as the compromises due to the tape electronics. These are some of the reasons the unmodified 350 / 351 sells for 4 times as much as the 601. The 601 is a great candidate for mods and upgrades, though. The belt driven decks like the 601 aren't prized for their mechanical performance, so converting them isn't too frowned upon. |
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