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    Looking for a Bob Latino build manual and specs.

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    polarflux

    Posts: 36
    Join date: 2010-02-12

    Looking for a Bob Latino build manual and specs.

    Post by polarflux on Mon Feb 15, 2010 3:15 am

    Sorry, I should have clarified. I picked up one an ST 70 built by you and was hoping to obtain the complete manual so I can set the bias correctly and work on it, if need be.

    Thanks

    Phil


    Last edited by polarflux on Mon Feb 15, 2010 8:26 pm; edited 1 time in total (Reason for editing : Clarification)

    Bob Latino
    Admin

    Posts: 1975
    Join date: 2008-11-26
    Location: Massachusetts

    Re: Looking for a Bob Latino build manual and specs.

    Post by Bob Latino on Mon Feb 15, 2010 8:16 am

    polarflux wrote:I picked up an ST 70 and am looking for the build manual so I can set the bias correctly, etc.

    Thanks

    Phil

    Hi Phil,

    Do you mean you bought an ST-70 and want to install a VTA driver board like on one of my kits ? The initial startup procedure after the installation of the VTA driver board and the setting of the bias is below. Note - The specs of the amp as listed below are based on the upgraded power transformer and quad cap that I use.

    Bob

    1. Make sure that the 3 amp SLO-BLO fuse is in the fuse holder.

    2. Place the three 12AT7 tubes in the sockets on the top of the driver board. DO NOT PLUG IN THE RECTIFIER TUBE OR THE EL34/KT88 OUTPUT TUBES at this time. Plug in the amp to an AC outlet and turn on the amp. Wait about 20-30 seconds to see if the three 12AT7 tubes light up. If they do and they stay on for a minute then turn off the amp and proceed to step 3. If the 12AT7 tubes don’t light up or if the fuse blows check the wiring below and measure the AC voltage where both sets of twisted wires connect to the driver board. Check to see that you get about 5.9 - 6.5 volts AC. If not, check the wiring back to the power transformer for an error.

    3. Get your multitester and set the range for 0 to 2 volts DC. CONNECT THE SPEAKERS. Place the GZ34 rectifier tube and just the LEFT EL34/KT88 output tubes in their socket. Turn on the amp. Allow the amp to warm up for a minute and check to see that the EL34 and GZ34 tubes light up properly. Measure the bias on the LEFT output tubes by placing the BLACK NEGATIVE probe anywhere on the chassis and the RED POSITIVE probe in the hole marked “Biaset 1.56 v”. With a small screwdriver adjust the FRONT LEFT bias adjuster on the driver board closest to the front left tube. Turn the adjuster CLOCKWISE TO REDUCE BIAS and COUNTERCLOCKWISE TO INCREASE BIAS. Set the bias for .400 volts. Now place the positive probe in pin 4 (which is on the opposite side of the power take off socket) and use the BACK LEFT bias adjuster to measure bias on the LEFT REAR output tube. Set it the same as the front tube. Turn off the amp.

    4. Plug in the two RIGHT channel EL34/KT88 tubes, turn on the amp and repeat the bias procedure as outlined above on the RIGHT channel tubes. Now go back to the left channel tubes and notice that the bias is now a little LOW. ADJUSTING BIAS ON ONE TUBE HAS A SLIGHT AFFECT ON THE OTHERS. Go back and forth between all output tubes until all tubes have the proper bias.
    Specifications VTA ST-70 amp
    Power output ................... 35 watts RMS per channel from 17 Hz to 30 KHz at less than 1% THD with both channels driven in PENTODE - 20 watts RMS in TRIODE.

    IMD.................................. < .45% (19 Khz and 20 Khz) at rated power

    Frequency response ....... 10 Hz to 30 KHz +/- .1 dB at 1 watt

    Power bandwidth ............ 17 Hz to 30 KHz +/- .3 dB at 35 watts

    Feedback .........................13 dB

    Input impedance .............. 270,000 ohms

    Hum and noise ................ > 90 dB below 35 watt output

    The ST-70 does not invert phase

    Dimensions - 13.0" wide X 9.5" deep X 5.5" high


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