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Old 19th June 2004, 04:47 PM   #1
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Default VTL deluxe 300 amps - adjustment question


maybe anyone could help me with a hint regarding the adjustment of these monoblocks. The amps I have here need a complete new set (16x6550) of output valves. There are 8 black sockets (4mm) in front of the tubes of each amp - I suppose these are the bias measuring points - and there are 8 blue trimmers near the loading caps; I think they are for bias adjustment. I will set these for about 300 mV each, exactly as David Manley suggests. No problems so far.

What makes me worry is that there is one single additional black potentiometer near the pre/driver tubes, and there are also two red (smaller, not 4mm) sockets in that area. What has to be adjusted there?

thanks in advance for any help;

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Old 19th June 2004, 06:10 PM   #2
benny is offline benny  Australia
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it might be an AC ballance control for an isodyne phase splitter... just an idea. if it is so, nothing to stress too much about, it won't effect the safety of the tubes you just replaced, although if unballanced it will cause uneven wear on them... and again, if it is so, it is only worth trying to set this if you have propper test gear... if not, just leave as it's probably close enough to ballance as it is, or find someone with the gear.

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Old 22nd June 2004, 05:24 AM   #3
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I think you should leave the main bias adjust alone. Have you adjusted around 300mV to see what bias voltage you like most? It takes a lot of effort to fine tune a monster like vtl 300. One bias point is related to the other. You need to bias all of the points back and forth.

By the way, why not get a copy of Vtl book? Do you have one along with the machine? It gives a introduction to their products.
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Old 22nd June 2004, 11:00 AM   #4
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Default VTL deluxe 300

Hello contaxchen;

thanks - but I am not sure if this is a "bias master" control. YES, I do have both issues of "the book"; they are my own and did not come along with the amps, BUT: I can`t find anything about this matter in these books.
There is a passage in the books where D. Manley speaks about a master bias control, this is correct, but there is another passage where he speaks about a symmetry adjustment (like benny`s theory); seems to be symmetry but I hoped someone would read this who did this adjustment already.
I also sent a mail to VTL but I do not have a reply yet... seems I will have to try out what happens when I move the potentiometer...

thanks again, audiomatic
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Old 23rd June 2004, 02:17 AM   #5
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both issues of "the book";
This is great. Then read the schematcis to find out which theory is correct. In my VTL amp, I cannot reach the symmetric adjustment from the outside of the amp chassis.
The symmetric adjustment can only be and should be done by manufacturer, if you don't want to lose any optimal parameter.
My guess is that if you can reach that variable resistor outside the amp chassis, then that's a main bias.
Any way, it's helpful and fun to read the schematics.
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Old 23rd June 2004, 02:57 AM   #6
ben goh is offline ben goh  Singapore
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Default VTL Adjustment

Hi audiomatic,

To adjust the bias, get a good digital meter and a trimmer.

Place the black probe onto the black terminal of the speaker

post and the red probe onto any of the black slot near to the

6550. adjust for 250mv-280mv for healthy lifespan of tube.

Please do not adjust the additional black trimport as this is the

AC balance.

Hope this is of help to you.
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Old 23rd June 2004, 03:02 AM   #7
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Hello audiomatic;

I have a set of deluxe 150's which also have 6550's in them, just four instead of eight in each monoblock. if you are just replacing the 6550s and not the--- I'm guesing you also have 12ax7's and 12bn7's signal tubes you don't need to touch the other adjustments just bias the output tubes.
And IMO the Svetlanas 6550c's sound the best

If you need more info please feel free to e-mail me.
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Old 23rd June 2004, 07:23 PM   #8
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Default VTL 300 adjustment


thanks to everybody for your comments. After studying D. Manley`s book a second time carefully I also came to the conclusion that this MUST be a symmetry control. However, it is not pointed out clear.
Well; I just fit the first amp with 8 Svetlana 6550 C`s right now and the bias adjustment worked fine... of course it has to be done back and forth like the alignment of an FM Tuner...
These amps are real monsters. Mains inrush current when not driven with signal (idling current) is around 1,5 amps@230V! I recently serviced a pair of audio research classic 150`s - this is the only "high end" audio amp (means: no musicians amp) I remember to be a similar "monster".

Well, thanks again to all;

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