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 Post subject: HH Scott 299 High Voltage on Pin 8 of 5AR4 Rectifier
PostPosted: Mar Wed 18, 2020 4:24 am 
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Joined: Jun Sat 22, 2019 5:51 am
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I'm restoring a HH Scott 299 stereo amplifier. It uses 12AX7 preamp tubes and 7189 output tubes. I have a problem with higher voltage than expected at Pin 8 and Pin 2 of the 5AR4 rectifier tube. The schematic shows that the voltage should be 380V at Pin 8; however, my readings are Variac line voltage : 115VAC. Pin 8: 450DCV Pin 2: 464DCV. Is there a mod to lower the voltage to 380DCV at Pin 8 & Pin 2?


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 Post subject: Re: HH Scott 299 High Voltage on Pin 8 of 5AR4 Rectifier
PostPosted: Mar Wed 18, 2020 6:59 am 
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Check to see if the power transfomer has been replaced , that would increase the B+ . Try a 5R4 in place of a 5AR4 , double check the 20 ohm 10 watt resistor connectected between the first cap and the 5AR4 . If all else fails increase the 20 ohm resistor to a higher value and wattage Start with 50 ohms at 20 watts .

Are all the tube in the sockets and lighting . Check to see if the 7189's have is B+ at the plates , check the voltage across cathode resistors on the 7189's .

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 Post subject: Re: HH Scott 299 High Voltage on Pin 8 of 5AR4 Rectifier
PostPosted: Mar Wed 18, 2020 1:48 pm 
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You did not mention if the Scott is working or not. If not, I might be inclined to look for something that is not drawing a load. No load on the B+ = higher than expected voltage.

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 Post subject: Re: HH Scott 299 High Voltage on Pin 8 of 5AR4 Rectifier
PostPosted: Mar Sat 21, 2020 7:04 pm 
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battradio@ wrote:
Are all the tube in the sockets and lighting. Check to see if the 7189's have is B+ at the plates , check the voltage across cathode resistors on the 7189's .

I agree. If the 7189 tubes are not drawing current, that will make the B+ soar.

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 Post subject: Re: HH Scott 299 High Voltage on Pin 8 of 5AR4 Rectifier
PostPosted: Mar Sat 21, 2020 8:39 pm 
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Location: 18424 PA
If the tubes are drawing correctly, 7189A would be the tube to use. But when you see the price of those, you will use one of the other suggestions, resistor increase, different rectifier etc. If your filaments are also running high, a bucking transformer would be the best bet.


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 Post subject: Re: HH Scott 299 High Voltage on Pin 8 of 5AR4 Rectifier
PostPosted: Mar Mon 23, 2020 2:11 am 
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Joined: Oct Wed 09, 2019 2:19 pm
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Location: Rochester, NY
The published schematics for the 299 show the A and the B versions were the only ones using the 7189. Later versions used the 7591. The 299A has a B+ of 365 VDC. The 299B has a B+ of 430 VDC. Both pin 2 and pin 8 should have the same voltage on them as they are connected together. The 299B shows a voltage of 440 VDC at pin 8/1 with an 80 ohm resistor reducing it to the B+ of 430 VDC.

If OldDave can shed some light on which version he has and weather or not the amp has all tubes on and functioning we may be able to help more. If it is the B version the 80 ohm resistor should be checked but the B+ value is not that far off. If it is the A version something is not right.


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