Mccurdy AU300 pre diyed help needed.

Hya folks I recently built a Mccurdy mic pre with Sowter 9045 input tran but no output tran just 2 resistors & two capacitors, just to try it, and it sounded good.

I since ordered a 8650 output tran 9: 1 I think,have added this to the circuit & now I have loads of distortion ?

Brian Sowter mentioned that his transformer doesn't like DC something? off set was it? & that I may have to put a balancing pot on the cathodes I think?

Cant get hold of Brian at the mo, can anyone give me any ideas as to could this be why I have distortion? and how about this pot?
Im still green & want to learn.

here's the schematic..
http://www.nashaudio.com/DIY/DIY/McCurdy/au300.jpg
Many thanks guys...Gary O.

It´s hard to read, but I think you can substitute what seems to be R17(is it a 500R?) with a pot, wiper to ground and the other pins one for each cathode. Now, you have to measure plate current and set the pot until you have the same value for both plates. This will make the DC force be canceled on the transformer windings.

Yeah is hard to see R17 looks like 820R 1W thats what iv used anyway so its the current not the voltage that has to match then,thanks Rafafredd gotta find a 1 watt pot now.

I have another schem Will post it tomorrow. Maybe someone can do it before me, because I´m closing my eyes...

Another common ballancing trick, along the same lines, is keep the 820R but use a small value pot, like 100R with wiper to the 820R cathode resistor and a leg to each cathode. This will allow you to balance the output.

Use a multimeter across pin 1 & pin6 of the output tube and adjust 500R 1watt pot until meter reads zero. Becareful the probes will both be sitting at about 220v.

circuit modded for output tube balance.


Also worth checking that the 230v still is that voltage as the transformer will make the output draw more current. And are you running the tube filiments from 36v into pins 1 & 2 or have you wired pin 2 to pin 15 & pin 1 to 3 for 12v operation?

Joe

www.jlmaudio.com

Hi Group
The value of R17 IS 820 ohms. The B+ is 295V @16ma.
About 12ma of it is for the 12AU7's.
The schematic is kinda hard to read, but thats what I've measured.

RonL
http://www.nashaudio.com

What are PINs 7 and 19 used for?

7, 19 are the gain strapping connections.
(see the chart at the top left corner of the schematic.)

Cheers
RonL

Hey guys thanks for responses,what if I do like JLMs schematic but have around 700R 1W and a 100R pot then could I use a half watt pot?, I dont have any 1 watters at the mo, I have the tubes wired up for 12V ish at the mo, is 295V the correct HT voltage? I was trying to run mine with 239V measured with the OP tran connected, it says 235V on schematic, i,ll get the soldering iron warmed up later after the snooker.

Thanks Gary O.

If anybody has pics of the DIY McCurdy AU300, I'd sure like to see some... I'll even post from my photbucket account if need be... my email is [email protected]

I'm lookin' to build a pair, so a list of parts and pointers in the right direction would be a bonus.