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Author Topic: Foreplay III Voltage readings  (Read 1326 times)

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Offline mpdd

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Foreplay III Voltage readings
« on: January 13, 2016, 10:53:58 AM »
I have a Foreplay I built approximately one year ago I haven't been using due to extremely high gain. I ordered a Kaiju so I want to get the Foreplay going. I checked voltages and there are some descepancies from the stated voltages in the manual.
The transformer tabs read as follows:
Tab 4 & 5 manual voltage 3.3v.      measured voltage 30.30v
Tab 6.      manual voltage 165.5.   measured voltage.   5.84v
Tab 7.      manual voltage.    0.     measured voltage. 227.0v

Terminal tab voltages all match the manual except tab 6. Tab 6 should be 216 but measures 5.84.

Is this an issue with the transformer or wiring? I've traced each wire several times to confirm correct termination.

Offline Paul Birkeland

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Re: Foreplay III Voltage readings
« Reply #1 on: January 13, 2016, 11:06:14 AM »
I'm going to guess that these are the tabs on the power transformer?  If so, these are AC voltages, and not really necessary to measure unless there are other problems in the circuit that call for verifying that the power transformer works.

(In the Kaiju, the transformer voltages are measured pre-construction, but not at the end)
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Thoburn

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Re: Foreplay III Voltage readings
« Reply #2 on: January 14, 2016, 03:33:52 PM »
I have a Foreplay I built approximately one year ago I haven't been using due to extremely high gain. I ordered a Kaiju so I want to get the Foreplay going. I checked voltages and there are some descepancies from the stated voltages in the manual.
The transformer tabs read as follows:
Tab 4 & 5 manual voltage 3.3v.      measured voltage 30.30v
Tab 6.      manual voltage 165.5.   measured voltage.   5.84v
Tab 7.      manual voltage.    0.     measured voltage. 227.0v

Terminal tab voltages all match the manual except tab 6. Tab 6 should be 216 but measures 5.84.

Is this an issue with the transformer or wiring? I've traced each wire several times to confirm correct termination.

I think the question is, "why would this foreplay have so much gain that even on the lowest point of the volume control it plays loud". It might be the amp, but other preamps don't exhibit the problem. So, it might be something else. Any ideas?
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Offline Paul Birkeland

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Re: Foreplay III Voltage readings
« Reply #3 on: January 14, 2016, 03:54:23 PM »
You are starting to uncover why we don't have a 12AU7 preamp anymore.  If we presume a C4S is used as loading, a single half of a 12AU7 has a voltage gain of roughly 25dB or so depending on the operating point. 

If you have the standard 2V of signal coming from your source, then a wide open Foreplay would try to make that 35-45V, which is unreasonable.  (Our amplifiers, and most amplifiers in existence for that matter, don't want to see more than 5V at their input for full power)

To combat this, there is preattenuation in the Foreplay III circuit in the form of that fan of resistors by the selector switch.  Each resistor pads an input, and with sufficient padding everything works nicely.  If you have a Foreplay III that's just too dang loud on ever setting, try measuring each of those resistors and let us know what they are.  I, for one, would likely just set them all around 300K-400K, to be somewhere in the neighborhood of unity voltage gain.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline mpdd

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Re: Foreplay III Voltage readings
« Reply #4 on: January 15, 2016, 08:59:58 PM »
I have the 33k resistors installed per plan. I'll try some 400k and see how that sounds. I'll post results.