OT: KNIGHT preamp

**spiros54** · October 21st, 2014, 04:23 AM

hi guys
I recently bought in a second hand store here in Sweden a set of Knight stuff, one main amp, one preamp and a tuner. All of them are MONO. Yesterday I was working with the preamp. When I bought it the tubes was missing, it is easy to take them off and somebody maybe wanted them! I try with 12AX7, 3 pc's but I get low voltages from the transformer, after rectified I get 135V, very low for those tubes. The filaments are in series, pin 4 and 5 go to the next tube 4 and 5 etc, the dial lamp is also in series. The filament voltage is DC and a part av HT through 2 pc's 450 Ω 10Watt each. I get 26Volt for all of them, 8Volt/tube and 2Volt for the dial lamp. Normal this tube need 12,6Volt. The main AC input voltage I have adjasted to 117V. The AC secondary voltage is 2x125V. Selenium rectifier of course. I can not find any information about it, maybe somebody from you know more about this American Knight stuff. There is not one type label on it. Help appreciated.
Best Regards
Spiros/Sweden

**bowtie427ss** · October 21st, 2014, 09:02 AM

Can you post a pic of the top side of the chassis showing the tube and x-former layout?

Maybe we can ID the model from that.

The Selenium rectifier is a time bomb seeking a place on the calendar, get it out of there. You can replace it with a pair of 1N4007 silicone diodes, you will need to add a series resistor to keep the B+ from running away. The dropping resistor will probably run hot, choose a good quality wire wound and keep it isolated where it won't cook other nearby components or wire insulation. You really need a schematic to determine voltage spec and to be sure you're getting proper voltage.

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Just for clarification, your mains voltage is 220-240 VAC?

And you are running through a variac to reduce voltage to 117 VAC?

Is it possible that the transformer has been replaced with one having the proper 220-240 VAC primary for your country?

**spiros54** · October 21st, 2014, 03:16 PM

The filter capacitors are rated at 150V, so the voltage seams pausible.
The main transformer looks like original American to me!
The last tube is wired as cathode follower. The others are wired like a typical 12AX7 amp stage.
Best Regards
Spiros/Sweden

**Old Guy** · October 21st, 2014, 04:27 PM

It was VERY common to wire 12ax7 to run at 6 volts, which would likely give you an 8 volt no-load reading. The original 12AX7 had 2 separate 6 volt heaters with a common connection meaning they could operate on 6 volts (one heater or both in parallel) or 12 volts(BOTH IN SERIES). I'd have to go back and look at a schematic, but am almost sure that's right... Not so sure about modern ones... don't use them.

I did, tube block diagram below. Center tap to one end or both ends tied together is 6 Volts.

I have to wonder if you are measuring the plate voltage right...the 150 volt caps if there are 2 could each be across one secondary of the HV...the HV might be double that 135.

Model number if you can, and as many closeups of the undersides as possible please.

**spiros54** · October 21st, 2014, 04:51 PM

The filament connection is between 4 and 5, it must be 12,6V/tube, I have 8V instead. The right voltage for the 3 tubes is 37,8V + the dial lamp, 6,3V?
The 135V HT and the 150V rated VDC in the filter capacitor talks about ECC88, but was those tubes that time?
No dubble Voltage, I have check it early! You can see the sort red cable betoween the (-) of the 2 filter capacitors and the CT (0V) from the main transformet also connected to the (-) of capacitors.

**bowtie427ss** · October 21st, 2014, 06:32 PM

I think it's an 83 YX 754. It was a kit.

**spiros54** · October 21st, 2014, 06:55 PM

Thanks
any information about tubes?

**mah** · October 21st, 2014, 08:07 PM

Tube listed here:

http://www.radiomuseum.org/r/allied_rad_knight_83yx754.html

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FYI: http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=192150

**spiros54** · October 22nd, 2014, 03:35 AM

Thanks
I have NOS 12AY7 and 12AU7
I have all ready began to rebuild, it will be 135V HT and different transformators for 230V input voltages.
Saparate shunt regulators for filament and HT. The main circuit will remain unchanged but with upgrade all the components. Not such silly super high-end but common good quality like Philips resistors and Wima capacitors.
American Mallorys, my favourite, in the power supply. It will be, if it is sounds lovely, my preamp in my new MONO system. Then I have to rebuild the power amp with EL37 and the tuner. Staying tuning!
Best Regards
Spiros/Sweden