Help with ARC D79

[pacha]

I've purchased a new set of tubes for my ARC D79. Setting the bias on the outputs is simple and straight forward on this amp but here is my question.

Since I'm replacing all input and driver tubes is there anything else I need to bias or reset?

How about the regulator tube, also a 6550.

TIA,
Pacha

 

[battradio]

If you have a schematic it would help , the bias is the priamary thing you need to do . The voltage regulator most likely would have a control . I set the bias on a D75 back in the early 80's ,but thats been a long time ago .

 

[www.records]

Audio Research Database has the schematic for your unit. They are also very helpful for info if you give them a call.

http://www.arcdb.ws/index.html

 

[battradio]

It apears there is a bias control for each output tube and a control for the regulator tube .The AC ballace control would need to be adjusted , but you would need a distortion meter or a scope and signal generator to set it correctly

http://www.arcdb.ws/D79/ARC_D79_schematic.gif .

 

[pacha]

Thanks

Thank you for helpful replies.
I have acquired the schematic and see the regulator adj. for 325v and knew the outputs would be OK.

Can anyone point me to a procedure for setting the AC balance? I have access to a Nakamichi T100 and a scope for equipment.

Thanks again for the quick and helpful replies.
Pacha

 

[battradio]

Power the amp up on 100 watt dummy loads and let warm up about a half hour . With a sine wave in monitor the output with a scope , adjust amplitude of the of the generator till the output clips , if cliping is symetrical ( top and bottom half of wave form ) , no adjustment is needed , if unsymetrical back of aplitude and adjust AC ballance till both sides of wave form clip at the same time .

If a distortion meter is availible adjust for minimum distortion

 

[pacha]

I can do that....:yes:

Thanks for all the great help.

 

[truep]

I use a different method that you can use to set the low level AC balance or at any level you want. I think the low level (1-5 watts makes the most difference in sound).
Make sure your scope is calibrated. It must be dual trace with the ability to invert and add. Using AC input on both channels, connect Channel 1 to the grid of one output tube (or to one of the output tubed on "the top" pair on a PPP amp) and one to the other (or Bottom pair on PPP) Set the scope to sum, and invert one channel.. Use the resistive load on the output. Connect a volt meter (AC) across the resistor. use a signal generator and insert a sine wave signal to the level you want across the load (v squared divided by the load resistance will give the power output). Adjust the AC Balance to a point as close to 0 as you can get.

 

[pacha]

Thank you truep, I think I'll bring it close with the first method and fine tune with yours.