Mix n Match

[valvehead]

Hi everyone, only just joined the forum though i have read it for a while.
I have always loved tube amps and used to have a small repair shop in Dublin Ireland a long time ago. I now live in New Zealand and own a Audio visual design and installation company.
Anyway i am just getting into construction of my first ST-35. I have a couple of questions:
I have a pair of Beacon OPT 12watt 9k 20KC to 20KC number S.135.
Anyone tried something like these? To me the P-P seems a little high for EL84s?I cant find any info on them.
Yellow_Light_Colorz_PDT_17
Just scored a power tranny with 400 @ 450v 180ma secondaries that i plan to use with a SS rec. It does have a 5v winding -thoughts?


Aaron[/b]

[valvehead]

anyone out there?

[6L6/807]

Greetings Aaron,

I'm not familiar with the transformers you mentioned but 9k plate to plate is a suitable load for push/pull EL84/6BQ5 valves.

Characteristics and Typical Operation

Push Pull Class AB1 Amplifier [TS}
Plate Voltage ................................. 300 V
Grid No. 2 Voltage ............................ 300 V
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 130 Ω
Peak Grid No. 1 Voltage........................ 28.2 V
Plate Current (Zero Signal) ................... 72 mA
Plate Current (Maximum Signal) ................ 92 mA
Grid No. 2 Current (Zero Signal) .............. 8 mA
Grid No. 2 Current (Maximum Signal) ........... 22 mA
Load Resistance ............................... 8K Ω
Power Output (approx) ......................... 17 W
Total Harmonic Distortion ..................... 4 %

The power transformer is going to be a little too high in voltage, though, especially with solid state rectifiers. EL84s will be happier with something less than 350 volts on their anodes. Is that transformer 450-0-450 volts? I'm not clear on what the secondary looks like. If it is, you will end up with well over 500 volts. If it is 450 volts with centre tap (225-0-225) you will be in good shape.

Best to you,

Wade

[valvehead]

Hi Wade, Thanks for that. The secondary on the PT is 450-400-0-400-450 so most likley it will be to high unless i can bleed off a good amount of volts...
Pity its a lovley transformer and a perfect fit for the chassis. Can be hard to get decent iron here with shipping and the weak currency etc.

Perhaps ill use it with a couple of EL34s later.

Cheers

[Ty_Bower]

Use the power transformer as is. Build a choke input power supply filter. Use an IRCL or some other NTC device to help prevent the supply voltage from running too high before the power tubes warm up and start pulling current. Or build in a standby switch to let you warm up the heaters before you apply B+.

[Shannon Parks]

Just like a choke loaded input, you can build a resistor loaded input. Though I haven't seen it anywhere, I've used it with mismatched power transformers with good results. Just a variation of adding more resistance to the secondaries which I do see (and would be kinder to the rectifier). But plugging in a 5W cement resistor at prototyping time is fairly simple and painless.

[valvehead]

Thanks guys for the advice. I worked out the component values yesterday and it seems a choke input should work using the 400v taps. A 20H choke and a 30MF cap and using a 5u4 rec will give me around 360v B+ minus a little for voltage drop thru the OPT.
Now all i need is a choke...
Is there much advantage in using seperate filters for each channel? I had an amp ages ago that was built this way.
I found this book http://www.pmillett.com/Books/Schure_Rectifiers.pdf Excellent info on power supply design!
BTW i have used 10OHM resistors between secondary and rec on tube radios with success.
Cheers