Need help determining correct bias adjustment

I have a pair of modified Mk III's. How do I go about determining what the correct bias voltage is? Have not had any success locating a hi-fi shop in the Pittsburgh area that I can go to for service. Want to check/adjust bias but am hesitant as the amps are not in their original configuration.

Thanks for your help!

Thanks. here is the bottom of 1 of them.



Here is a link to the pictures of the Mk iii's on picasaweb

http://tinyurl.com/ljebpp7

Please let me know if you can see them or not so I can try something else if this does not work.

Looks stock to me...just a few newer board components. Still has the 11.2 ohm bias resistor and 10K pot.

I would just set the bias to about 1.35 vdc and see how it sounds....Factory suggested setting is 1.56 vdc.

Kevin

Thanks. If factory suggested is 1.56 then why do you recommend 1.35?

Nice amps....i have 3pr of the stock MK3s.... what is your B+ ......if like mine it high over 500 i run my MK3s on a Varic.....an drop the B+ to about 480-90 for the best sound...then bias the Amps...
In the 50-60s when these amps were made the wall ACV was lower...like 117....today most are 125 are higher....so this push up your B+....Not good on the parts....



Thanks for the site Bob.................

zx, no idea what you are referring to. How about some detailed instruction?

hopefully since you have a tube amp you should have a voltmeter (or DVM) and you can check the AC wall socket to see about how much AC voltage you have. If around 115-120v you are OK, but if closer to 125vac you may want to invest about $60 in a variac, which will allow you to adjust the voltage down and it's easier on the amp. You'll also need the voltmeter to set the correct bias on the tubes.

Did not mean to spek Voodoo to you....


...Some times less is more..if your MK3s amps sound good... that all that matters...by the way how do thay sound??
This old guy some times acks like.... i need too get these Dynaco amps to last for ever....mine have over 50years on them now.... an thay are rocking.. with some mods...an the right over all B+
When tube amps were made in the 50-60s the AC comeing in from the wall could as low as 110-15....so the MK3s B+ at that ACv 470-480 B+....
To day it higher...if you have 120-125 B+ can go as high 550 .... some of the parts are rated at lower 500-525...This can stress the parts....an as Bob has said you need to drope the bias.....
but with the Varac i can set the wall AC back to 110-115...are what ever lower AC......to me tube amps sound better at the lower B+...480 b+ is about the top for me ......other well say







...it all about the sound.



thanks for the site Bob..................

Do you have a recommendation in what to look for in a variac? Can a single variac support 2 mk iii's ?

Yes............
I run two MK3 all the time....that way i know there each geting the same V.... 5 amps Variac fine...
i got most of minne at fle-market yard sals for $10-20....but $40-50 for a new one...
I do repairs on tube amps for others....an if i sale any tube preamps are amps over 40 years old..
thay get a Variac....in the sale...
I see a lot of my altime fav sounding tube amps an pres....hhshott...have all there parts socalled upgreaded....to last longer...an sound better....
then soon after most of theses are up for sale....
i have found some of hhscott old parts have a sweetness...that make the sound... i an others have not made better with new-better parts............
But the high AC coming in wall today can kill the old parts ...so the Variac saves the day for most of all the old parts i have found....an yes this is just one-mans o-pine.....
Now i would not say that SS amps should be ran for a long time with a Variac....thay pull a lot of Amps...
tubes pull mil-Amps.....others can say there thinking





thanks for the site Bob...........................

what current rating should i be looking for in a variac that would cover a pair of mk-iii's up to their power rating with some buffer....

for two power amps I'd recommend a 5A variac, there are some here:

http://www.parts-express.com/Search.aspx?keyword=VARIAC%20TRANSFORMER&sitesearch=true

Bob,
as you control the transformers, why don't you create some models that has extra primaries ?

Having just a few ( 5V 10V 20V 115V ) and a patching are one could connect and combine
windings to suit.

This is what a lot of stuff had in europe in the 50-60, just look at a phillips amp and
see what possibilyties they had. Some even had special "voltage selectors" where
the user could easily turn a "plug" to the wanted voltage.

Ask your transformer builder for a pricequote. Maybe your kits could benefit for this ?
The total cost for an owner could be less then having to get a variac.