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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all products

    Can ST-70 Be Bridged Mono


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    Can ST-70  Be Bridged Mono Empty Can ST-70 Be Bridged Mono

    Post by EddieGnz1 on Mon Mar 31, 2014 12:29 am

    1) Can a Vintage ST-70 manufactured in the 60's be wired as a mono amp for a total of roughly 70 watts into one channel ?...that is, without major mods ?

    2) Can the new VTA ST-120 kit be wired as mono for roughly 120 watts into one channel?

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    Post by tubes4hifi on Mon Mar 31, 2014 12:41 pm

    sure, the wiring setup is in the original Dynaco ST70 manual

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    Post by GP49 on Mon Mar 31, 2014 1:34 pm

    It works for theoretical specsmanship but it doesn't work well for fine sound.  Parallelling the two channels results in mixing of the feedback-correction signals (derived at the output transformer secondary) between the two channels.  Unless the two channels are exactly identical (they never are), this means that the correction offered by the feedback will be non-optimum for each channel...meaning that instead of reducing distortion, it may well increase it.

    Additionally, the Stereo 70's power supply will not sustain two 35-watt channels at once (if you doubt that, read the FTC-mandated specifications that were published in the early 1970s).  Driving both channels simultaneously with the same strong low bass signal (which is the only place you would need 70 watts) will result in power supply voltages going down, limiting the power output which is the opposite of what you are trying to achieve.

    The only PROPER way to parallel two channels is to invert the phase in one, and drive the loudspeaker across the positive speaker outputs.  This is called "bridging." It keeps the feedback signals separated but requires additional active circuitry: a phase inverter which must be so good that it will do no harm.

    As for the VTA ST-120 the power supply not be an issue if sufficient storage capacity is provided.  But if it takes its feedback signal from the secondary circuit of the output transformer, as in the Dynaco Stereo 70, I cannot see any difference in the behavior of its feedback circuits when the two channels are paralleled.
    Bob Latino
    Bob Latino

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    Can ST-70  Be Bridged Mono Empty Re: Can ST-70 Be Bridged Mono

    Post by Bob Latino on Mon Mar 31, 2014 2:32 pm

    I concur with 100% of what GP49 has said about "bridging" the two channels of the Dynaco (or VTA) ST-70 or the VTA ST-120 .. It does work but for the reasons outlined by GP49 these amps do not sound as good as they do as stereo amps ...

    If you have an ST-70 and want to use 70 watts on ONE speaker then BIAMP with the amp. Most speakers have a dual input with separate terminals for the woofer annd tweeter sections .. These are usually connected together with two metal straps. Remove the two straps from each speaker. Just run the left (or right) channel of your ST-70 into the woofer section of the speaker and the other channel into the tweeter section. Your speaker will now receive 70 watts from two channels. Do the samething with another ST-70 on the other stereo speaker. Hopefully your preamp will have two left and two right RCA outputs. If it doesn't just use a "Y" connector with two male outs (into the two ST-70 RCA input jacks) and one female in (to accept the interconnect from your preamp) to make the connections. I have done this a number of times over the years without issue ...


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    Post by stromberg67 on Sat Apr 05, 2014 8:33 pm

    I agree about the cathode feedback. I have four Bogen MO-100 amps, same series run, that can be bridged for increased output power, and they have a jack on each for cathode feedback that must be connected to each of the bridged amps. It works well for one of the intended purposes. These amps are rated at 100 watts RMS, and have 115 volt output taps to allow the powering of equipment of various sine wave frequencies. The are rated at minus 2 dB at 100 Hz and 40 Hz, and sound quite good with my AR 2AX's, for which only two are used.

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    Post by dronepunk on Mon Dec 09, 2019 7:19 pm

    I am looking for a single MO100A myself....

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