When all is well, all bias pots will be at about the same point, usually between 10 and 2 depending on tubes.
+9
chefothefuture
audiobill
buchela
peterh
BNR_1
Bob Latino
Roy Mottram
j beede
vtshopdog
13 posters
New Build Questions
audiobill- Posts : 425
Join date : 2014-03-13
Location : Albany, NY
- Post n°51
Re: New Build Questions
Are the f marks on the chassis indicating front? Those bias points are for the rear tubes,
When all is well, all bias pots will be at about the same point, usually between 10 and 2 depending on tubes.
When all is well, all bias pots will be at about the same point, usually between 10 and 2 depending on tubes.
corndog71- Posts : 840
Join date : 2013-03-19
Location : It can get windy here
- Post n°52
Re: New Build Questions
WireNut wrote:Bill, I will do as you suggest... but why would it take three days of continuous play to get to the point of blowing a fuse? What would that noise have been?
Thanks!
Could have been a short in the tube. Debris can and does get inside tubes from time to time and sometimes it gets zapped. Sometimes it'll kill the tube and sometimes it won't.
WireNut- Posts : 67
Join date : 2017-02-21
Location : Twin Cities
- Post n°53
Re: New Build Questions
Seems I found the issue:
Once the resistors are replaced, do I need to check anything else?
Once the resistors are replaced, do I need to check anything else?
audiobill- Posts : 425
Join date : 2014-03-13
Location : Albany, NY
- Post n°54
Re: New Build Questions
Not so fast- you need to find out what caused that. Either a tube or wiring problem, check all wiring as well as connections with a good light and magnifying glass from all directions. Be sure no shorts from switch pins to chassis. Good luck.
WireNut- Posts : 67
Join date : 2017-02-21
Location : Twin Cities
- Post n°55
Re: New Build Questions
I put on my jewelers visor and no wires were touching around there or to where the wires came from.
It could have been a bad resistor... but what can I check to see if there was another issue?
It could have been a bad resistor... but what can I check to see if there was another issue?
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
- Post n°56
Re: New Build Questions
WireNut wrote:Seems I found the issue:
Once the resistors are replaced, do I need to check anything else?
Dave,
Above is the photo of the wiring of your VTA ST-70. There are many differences between your amp and a standard VTA ST-70 amp. You added the bias meters, moved the inputs to the rear, and create your own (unnecessary) grounding buss bar. You also admitted to me that you had your bias meters incorrectly wired at first. When you go off the beaten path to create your own version of the VTA ST-70, you increase the risk of having playing and/or reliability problems. The VTA ST-70 is a VERY reliable amp if properly assembled. I still have the original fuse in my own VTA ST-70 which is now (2007 - 2018) 11 years old. This amp has thousands of hours on it and the amp is now on its third set of tubes.
Since those two resistors only blew on the RIGHT channel, there must be some right channel issue (wiring, bad solder connection, bad output tube ?) You had said in an Email to me that the amp played correctly for three days before this happened which usually means the wiring is correct. You did not get your tubes from me. > It could be just a tube that went bad prematurely ? I will ship you out a new triode/ultralinear switch and two 100 ohm 1 watt resistors later today. If you have any more issues like this with your amp, I am going to suggest you send the amp to one of our VTA amp technicians to straighten out the amp for you.
Bob
WireNut- Posts : 67
Join date : 2017-02-21
Location : Twin Cities
- Post n°57
Re: New Build Questions
I am going to put the tubes on my tube tester.
peterh- Posts : 1836
Join date : 2012-12-25
Location : gothenburg, sweden
- Post n°58
Re: New Build Questions
If this is the screen stoppers ( connected in series with the g2 ) then i suggest that both tubes are shot and should be replaced.
WireNut- Posts : 67
Join date : 2017-02-21
Location : Twin Cities
- Post n°59
Re: New Build Questions
Both tubes on that side tested good... am I the first person in the US that may have gotten a bad resistor?
peterh- Posts : 1836
Join date : 2012-12-25
Location : gothenburg, sweden
- Post n°60
Re: New Build Questions
A tubetester won't detect tubes that are prone to flash.
Junk them ! They will most likley flash again.
Junk them ! They will most likley flash again.
Roy Mottram- Admin
- Posts : 1837
Join date : 2008-11-30
- Post n°61
Re: New Build Questions
also, did it EVER work in that config, or was this the first time you powered it up with the triode-pentode switching in place?
First time up, yeah, wiring error somewhere. If it worked many many times before, probably a tube(s) went bad.
First time up, yeah, wiring error somewhere. If it worked many many times before, probably a tube(s) went bad.
WireNut- Posts : 67
Join date : 2017-02-21
Location : Twin Cities
- Post n°62
Re: New Build Questions
It was on for over 30 hours straight, and then a loud cracking noise.
All of the tubes test good for emission, grid, and shorts.
I am not about to toss a Philips 12au7 "For Medicine" tube unless it can be proven it is bad. They are going for $300 a pair now.
All of the tubes test good for emission, grid, and shorts.
I am not about to toss a Philips 12au7 "For Medicine" tube unless it can be proven it is bad. They are going for $300 a pair now.
peterh- Posts : 1836
Join date : 2012-12-25
Location : gothenburg, sweden
- Post n°63
Re: New Build Questions
It cannot be the pretubes ( 12au7), it must been the power tubes that has experiencedWireNut wrote:It was on for over 30 hours straight, and then a loud cracking noise.
All of the tubes test good for emission, grid, and shorts.
I am not about to toss a Philips 12au7 "For Medicine" tube unless it can be proven it is bad. They are going for $300 a pair now.
a flash-over. When the flash is over, a tubetester won't detect it, but the tube have a
much larger risk of another flash-over. Thats why you should replace them as a pair.
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
- Post n°64
Agree with peterh
Get some decent and fully-tested output tubes from Jim McShane.
WireNut- Posts : 67
Join date : 2017-02-21
Location : Twin Cities
- Post n°65
Re: New Build Questions
Does he have a test that can check for arcing?
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
- Post n°66
Re: New Build Questions
You'd have to ask Jim about that.WireNut wrote:Does he have a test that can check for arcing?
jimmcshane@mcshanedesign.net
He's very responsive to emails and very patient with tube newbies, plus he warrants his tubes after putting them through some serious torture tests. A major re-seller for New Sensor and Gold Lion, when he's not restoring H-K Citations. Good guy to know.
-d-
Dogstar- Posts : 361
Join date : 2014-06-23
- Post n°67
Re: New Build Questions
Can you post a photo of the amp from the outside?
WireNut- Posts : 67
Join date : 2017-02-21
Location : Twin Cities
- Post n°68
Re: New Build Questions
Dogstar, I have the replacement parts and will post a photo soon- I need to put in the parts and go through the protocol to fire it up first.
WireNut- Posts : 67
Join date : 2017-02-21
Location : Twin Cities
- Post n°69
Re: New Build Questions
Bill and I worked on getting the unit going by bringing it up with my variac- it is running now and has about four hours on it. I owe a HUGE thanks to Bill!!!
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