Hey my first post..have mercy on me. Have a pair of Mk3's that I recently inherited. Looks like they have been modded somewhat. Keep going thru rectifier tubes way too fast. I have pretty hot mains voltage of 125VAC and I know that Bob suggests about 40ma per tube (6550's). So I will be getting the 20a Circuit Specialties Variac. Here's my question. What is the correct way to use it? Use it as the main "on" "off" control..plug the Mk3's into it, leave the Mk3's on/off switches on all the time and just turn on the amps with the Variac, or turn everything on separately? Also start the Variac at about 50 wait a bit then take to 118VAC? Any thoughts would be helpful.
4 posters
Using AVariac
Bob Latino- Admin
- Posts : 3260
Join date : 2008-11-26
Location : Massachusetts
- Post n°2
Re: Using AVariac
Daveinthedesert wrote:Hey my first post..have mercy on me. Have a pair of Mk3's that I recently inherited. Looks like they have been modded somewhat. Keep going thru rectifier tubes way too fast. I have pretty hot mains voltage of 125VAC and I know that Bob suggests about 40ma per tube (6550's). So I will be getting the 20a Circuit Specialties Variac. Here's my question. What is the correct way to use it? Use it as the main "on" "off" control..plug the Mk3's into it, leave the Mk3's on/off switches on all the time and just turn on the amps with the Variac, or turn everything on separately? Also start the Variac at about 50 wait a bit then take to 118VAC? Any thoughts would be helpful.
Hi Dave,
With 125 VAC coming in, set the variac for an output of 118 VAC. Starting a variac out at 50 VAC and bringing it up slowly is only for old gear that hasn't see any voltage in maybe 30+ years. This will help in reforming (maybe) old electrolytic caps that are on the ragged edge of failure. On new gear just leave the variac set at an output of about 118 VAC. 6550's in any of the VTA amps should be biased at 50 milliamps per output tube. They will work fine at a bias setting of 40 ma but IMHO, they should be run at a little higher idle current.
Bob
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- Post n°3
Re: Using AVariac
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Last edited by PeterCapo on Fri Dec 04, 2020 12:14 am; edited 1 time in total
peterh- Posts : 1823
Join date : 2012-12-25
Location : gothenburg, sweden
- Post n°4
Re: Using AVariac
If there is problems with rectifiers i recommend to verify that the amp is builtDaveinthedesert wrote:Hey my first post..have mercy on me. Have a pair of Mk3's that I recently inherited. Looks like they have been modded somewhat. Keep going thru rectifier tubes way too fast. I have pretty hot mains voltage of 125VAC and I know that Bob suggests about 40ma per tube (6550's). So I will be getting the 20a Circuit Specialties Variac. Here's my question. What is the correct way to use it? Use it as the main "on" "off" control..plug the Mk3's into it, leave the Mk3's on/off switches on all the time and just turn on the amps with the Variac, or turn everything on separately? Also start the Variac at about 50 wait a bit then take to 118VAC? Any thoughts would be helpful.
according to the dynaco manual. In particular, the main B+cap should use 30uF
closest to the rectifier.Much larger then this will risk the rectifier.
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- Post n°5
Re: Using AVariac
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Last edited by PeterCapo on Fri Dec 04, 2020 12:14 am; edited 1 time in total
wildiowa- Posts : 237
Join date : 2012-03-19
- Post n°6
Re: Using AVariac
My line AC is usually always 125 and often hits 126. After several "incidents" I put my amp on a Variac, set at 118, and no problems since. Use a better meter than the ones that are included on the Variac. I fortunately have my amps near one of those wall outlets they used to wire with a wall switch, so all I have to do is hit the switch and the amp is on. Try and rig up a master switch for the Variac maybe even a regular wall switch in an electrical box of some kind you can probably do something to dress it up a bit if it looks too funky.
Daveinthedesert- Posts : 8
Join date : 2011-02-15
- Post n°7
Re: Using AVariac
Thanks to everyone for all the valuable information. Luckily, for me our good friend Bob Latino referred me to a Dynaco tech that lives right here in Tucson
Last edited by Daveinthedesert on Mon Nov 20, 2017 9:43 am; edited 1 time in total (Reason for editing : Added information)
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