TUBE BIAS, socket to me!

oldhvymec1,917 posts12-28-2020 6:36pmhttps://www.pinoydvd.com/index.php?topic=139769.0

OP check this thread out pretty good, pics in all...

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thanks, finally had a look today.

Preamp Connected???


"amp should be connected to your preamp but volume is turned fully down ( NO signal applied to the amp )."  


 jomar

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Re: Cayin A88t - Biasing

I owned previously a A88T amp below are the biasing procedures. Please take note that the amp has HIGH voltages thus extra care should be taken.

1. Open bottom cover to have an access on the Test Point ( TP ) and Variable Trim pots (VR) .
2. Speakers should be connected and amp should be connected to your preamp but volume is turned fully down ( NO signal applied to the amp ).
3. Turn-ON power amp for about 10 minutes to warm-up tubes.
4. Attached positive ( Red ) probe of DVM (Digital Volt Meter) to Test Point . Negative probe of DVM to GROUND.
4. Adjust the VR which corresponds to the tube being biased to get 0.4Volts (400mV) reading
    *** VR1 for V5 , VR2 for V6, VR3 for V7 and VR4 for V8
5. Verify reading after 30 mins to finetune .
6. Power OFF amp and put back bottom cover

lewm

now I see what you were referring to re 'surprised'. but this was a quote found and pasted by another poster, not me.  

"Like me, after living with the MX110Z for a while, you may wonder why all modern preamplifiers don’t include FM tuners and equalization controls—they’re great fun! Also, tell me, is there anything even remotely comparable to the vacuum tubed McIntosh MX110Z currently available today at any price? No, which is a pity."

Price: I don't know when that was written, I got lucky, the mx110z has increased in price over the last year.

A tube tuner/preamp with tuner and phono only would be cool, no need for so many inputs as the integrated amps have them. Gotta have remote volume and remote balance on the integrated so that's available to all sources. 

In my long experience, tube amplifiers that require periodic bias adjustment typically provide both external test points (female jacks where you stick the pos and neg meter probes) and externally accessible potentiometers with which to do the bias adjustment.   Some owners of ARC amplifiers have already mentioned this. Some brands (like Atma-sphere on at least some of the model line) even provide a built in external meter, so you don't need to own a DVM, etc, in order to adjust bias. Another large set of tube amplifiers have an autobias circuit built in, which automatically senses a drift in grid bias voltage and compensates for it, until the tubes themselves are worn out.  Your Cayin amplifiers are unusual in that they apparently do require manual adjustment of bias, but you seem to have to open up the chassis to get at the adjustments.  That may reflect their "made in China" provenance, where perhaps the liability laws that provide legal recourse for persons who are injured by electrical shock whilst biasing are less stringent.  (But perhaps the US distributor could be made liable in the US.) So just take the warnings seriously and proceed carefully.  I always keep one hand in my pocket when working with power on, to remind myself never to grip the amplifier with two hands, which could send a shock across your heart muscle.  It doesn't take much to be lethal.  I don't think you need a non-conductive screw drive blade so long as the handle is non-conductive.  But it couldn't hurt.

Maybe Ralph can chime in, but I have no idea what is being adjusted to 0.4V in your amplifiers.  Surely that does not represent grid bias voltage (between grid and ground or grid and cathode) for an output tube.  Perhaps you are adjusting the voltage across a fixed value resistor which in turn affects bias current.  Also, you need a fairly good quality meter to be accurate in that under 1VDC range.

Now your talking electrocution precautions? Come on.... it can’t be worth it.

i guess as a physics lab hobby, its OK- I’d rather listen to music than play in the lab with tubes. 
I had physics lab in college 40 years ago. No desire to go back there.

@elliottbnewcombjr Before the owning my VTL 450's I had VTL 300 Deluxe mono blocks and they were not Auto Biasing so I feel your pain!