VTA ST-120 and Variac 5 amp fuse Help appreciated

Ok so here goes.
I've had my new pre wired VTA ST-120 for about a month running now. I changed to the TUNG-SOL KT120 output tubes with no problems.
I reset the bias. Its been fine for a little while now. Last night the sound went dead and I looked up at the amp and it appeared as thought the right front output tube was glowing hotter. By the time I reached for the power switch on the variac it went down and the amp went down as well.
The first thing I did was check the 5 amp fuse in the variac and it was blown. I then checked the fuse on the st-120 and it was good.
Today i ran up to home depot and got more 5 amp fuses for the variac. The first fuse i replaced blew right away. After that I changed back to the original
Sovtek 6550s that came with the amp wondering if one of the TUNG-SOLS went bad. I replaced the variac fuse and powered back up. The tubes lit up and it ran for about
a min. and blew the variac fuse again. Any diagnostic suggestions appreciated. At this point I dont know if the variac is defective or if there is a problem with the amp.

Fritz

Just for shits and giggles I pluged my Emotiva XPA-5 into the variac and its running fine without the variac blowing the 5 amp fuse.
Must be a problem with the ST-120.

ImFritz wrote:Just for shits and giggles I pluged my Emotiva XPA-5 into the variac and its running fine without the variac blowing the 5 amp fuse.
Must be a problem with the ST-120.

I wouldn't be so hasty to assUme it's the ST-120. Many other variables are at play:

1. The new variacs are a quality-control crap-shoot. I've been through several. Yeah, they honour their warranties so you get a new variac or your money back, but what far more expensive components got wrecked in the variac's meltdown?
2. I won't belabour the multiple threads on the nature of fuses, but I would study up on them. PeterW has some good posts in this regard, but to distill it: all slo-blow fuses are not created equal. Automotive-grade slo-blows at your local Ace Hardware have no business in tube hi-fi-gear, period.
3. Comparing the Emotiva with a VTA tube amp is apples and oranges stuff unless you know the steady-state, peak and start-up surge amperage demands.

Just my tuppence worth.

ImFritz wrote:Just for shits and giggles I pluged my Emotiva XPA-5 into the variac and its running fine without the variac blowing the 5 amp fuse.
Must be a problem with the ST-120.

I will spare the complete rant, but, fair warning of a mini-rant on Variable Autotransformers. I will throw in, for giggles, a bit on fuses:

a) "Variac" is a registered trademark of ISE, Inc. purchased from General Radio, issued and renewed since 1933. Any other uses of that word other than on equipment made by General Radio and/or ISE is a copyright infringement and theft of intellectual property.
b) A VA without effective current metering as a bench tool is dangerous at best, worse-then-useless otherwise. Keeping in mind their original purpose was to dim lights, not test electronics. As a bucker, however, that is within their label.
c) From the symptoms you describe, it could still be either - much of  how/why the fuse blows is dependent on the type of fuse, and where it is in the circuit.

1. If you had a current monitor on your VA, you would know whether it or the power-amp were at issue. Essentially, a VA should draw 0-current by itself.
2. It very much depends on the _TYPE_ of fuse in use. A standard or fast-blow fuse used on a piece of tube equipment is always at risk, unless sufficiently over-sized as to for the protection of the real-estate rather than the equipment.
3. Whereas I can see the use of a conventional slow-blow fuse (element wound on a fine ceramic rod) in a variac, I cannot see such a fuse being used on a piece of quality audio equipment. The overload-over-time tolerance of these devices is nuts.
4. An audio fuse should be a DUAL ELEMENT fuse chosen to be fairly close to the actual operating current of the item. Such a fuse can handle the starting surge, but will act more like a conventional fast fuse after the surge. These fuses hate short-cycling - so they are not appropriate in a VA - which by design gets turned on and off fairly frequently.  

If you have a DE or slow-blow fuse on your amp, and a fast fuse on the VA, it would be a close-run thing as to which one would blow first in the case of a fault - but favoring VA failure first.

To figure this out of a certainty:

* you will need to determine the current draw of the 120 - and how that compares to what it should be.
* you will need to see if adding the VA increases that draw - and if it does, send it back.

https://www.ebay.com/p/Heathkit-Ip-5220-Variable-Isolated-0-140-Volts-AC-3-Amps-Power-Supply/1701509401   is presently on that auction site. I keep two (2) of them as they have excellent metering, and work very well. Most useful on tube equipment as it will show with some precision the onset of B+ as tube rectifiers warm.

Thank you for the response Peter. I will check into the heathkit. At this point Im fairly certain my rectifier tube is the culprit that is causing my problems and causing the fuse in the variac to blow. Ive ordered a new tube from vacuumtubevalues.com. When I first ordered the amp from Bob, I ordered a set of Tung-Sol kt120 from them while I was waiting for the amp to be built and shipped. After I recieved the amp I ran it with the Sovtek 6550WE that came with it for about the first week. The interesting thing is the very first time I powered up the amp it blew the variac fuse. It came with three fuses so I replaced it and it ran fine for about a month. I just figured I had a bad fuse. After about a week I decieded to install the kt120s. As I said they ran good for about three weeks. Then the problem started happening this weekend. At this point Im hoping that if the rectifier tube did go bad (I saw it sparking) it didnt take out one or more of the Tung-Sol KT120s. Is that a posibility?
Also what may have caused the rectifier tube to go bad in the first place? I feel like Im spinning my wheels and getting no where so to speak.
Thank you for your interest.
Fritz

I'm not a tech but shouldn't you be running a solid state rectifier with four KT 120's (or 6550's)?

Peter W. wrote:

ImFritz wrote:Just for shits and giggles I pluged my Emotiva XPA-5 into the variac and its running fine without the variac blowing the 5 amp fuse.
Must be a problem with the ST-120.

I will spare the complete rant, but, fair warning of a mini-rant on Variable Autotransformers. I will throw in, for giggles, a bit on fuses:

a) "Variac" is a registered trademark of ISE, Inc. purchased from General Radio, issued and renewed since 1933. Any other uses of that word other than on equipment made by General Radio and/or ISE is a copyright infringement and theft of intellectual property.
b) A VA without effective current metering as a bench tool is dangerous at best, worse-then-useless otherwise. Keeping in mind their original purpose was to dim lights, not test electronics. As a bucker, however, that is within their label.
c) From the symptoms you describe, it could still be either - much of  how/why the fuse blows is dependent on the type of fuse, and where it is in the circuit.

1. If you had a current monitor on your VA, you would know whether it or the power-amp were at issue. Essentially, a VA should draw 0-current by itself.
2. It very much depends on the _TYPE_ of fuse in use. A standard or fast-blow fuse used on a piece of tube equipment is always at risk, unless sufficiently over-sized as to for the protection of the real-estate rather than the equipment.
3. Whereas I can see the use of a conventional slow-blow fuse (element wound on a fine ceramic rod) in a variac, I cannot see such a fuse being used on a piece of quality audio equipment. The overload-over-time tolerance of these devices is nuts.
4. An audio fuse should be a DUAL ELEMENT fuse chosen to be fairly close to the actual operating current of the item. Such a fuse can handle the starting surge, but will act more like a conventional fast fuse after the surge. These fuses hate short-cycling - so they are not appropriate in a VA - which by design gets turned on and off fairly frequently.  

If you have a DE or slow-blow fuse on your amp, and a fast fuse on the VA, it would be a close-run thing as to which one would blow first in the case of a fault - but favoring VA failure first.

To figure this out of a certainty:

* you will need to determine the current draw of the 120 - and how that compares to what it should be.
* you will need to see if adding the VA increases that draw - and if it does, send it back.

https://www.ebay.com/p/Heathkit-Ip-5220-Variable-Isolated-0-140-Volts-AC-3-Amps-Power-Supply/1701509401   is presently on that auction site. I keep two (2) of them as they have excellent metering, and work very well. Most useful on tube equipment as it will show with some precision the onset of B+ as tube rectifiers warm.

Sorry to hijack this thread a little bit ImFritz, but so many times Peter has brought up his excellent point of view on fuses & power.
We have really high AC (We're really close to the Lawrence Livermore Labs). I'm constantly worried about the AC clocking in at 123.

Peter, on your 2 Heathkit IP-5220’s where do you use them from day to day?
Do you have 1 on the bench for troubleshooting & 1 running your gear that needs protection?
I know... assumptions... at least I can make one of me, not so much you.

Thanks,
Bluewater

One of the IP-5220s is on permanently on the bench.
The other is in the "Travel Kit" for Kutztown and when I do traveling diagnostics. At 3A, it is not quite rugged enough for protecting an entire system, although more than enough to handle any one item.

We have 118 VAC at the wall-plate pretty consistently, so we have no need for additional protection.
I am building an EMI filter as we are transitioning from CFL to LED lamps, and LED dimmer systems are quite noisy.

For some years at our previous house, I did run through a true-sine-wave line conditioner (Furman), but not since we moved (11 years this month). I am of the persuasion that a proper line-conditioner is a better solution to high/low wallplate power as it adjusts to the actual incoming conditions.

Enjoy!

So PeterW, are you going back to a Furman to get rid of the LED noise? We're making the switch from incandescent to LEDs as budget permits (never much cared for the EPA's darling CFL bulbs) and so far, the Furman has kept all racket at bay. Granted, it has its very own line to its very own 20-amp breaker.
Like Bluewater we're at the hot end of the voltage spectrum with some pretty wild swings due to the nearby silver mines each with their multiple 2,000-horsepower hoists and compressor rooms; the Furman clamps all that at 121 VAC, high for antique stuff but OK for VTA gear.
(Sorry if this sounds like a paid ad for Furman.)

deepee99 wrote:So PeterW, are you going back to a Furman to get rid of the LED noise? We're making the switch from incandescent to LEDs as budget permits (never much cared for the EPA's darling CFL bulbs) and so far, the Furman has kept all racket at bay. Granted, it has its very own line to its very own 20-amp breaker.
Like Bluewater we're at the hot end of the voltage spectrum with some pretty wild swings due to the nearby silver mines each with their multiple 2,000-horsepower hoists and compressor rooms; the Furman clamps all that at 121 VAC, high for antique stuff but OK for VTA gear.
(Sorry if this sounds like a paid ad for Furman.)

Nope. Not Furman as of yet, anyway. We have a pretty steady 118 V at the wallplate.

I will be using one of the EMI filters from Surplus Shed (10A) as the core of a home-made filter that will include a GFCI and toggle-switch. When those parts arrive, I will document the entire process.

At this point Im hoping that if the rectifier tube did go bad (I saw it sparking) it didnt take out one or more of the Tung-Sol KT120s. Is that a posibility?
Also what may have caused the rectifier tube to go bad in the first place? I feel like Im spinning my wheels and getting no where so to speak.
Thank you for your interest.
Fritz

Dave_in_Va wrote:I'm not a tech but shouldn't you be running a solid state rectifier with four KT 120's (or 6550's)?

Im not a tech either but the 6550 is what Bob Latino is selling the amp with. And he also recomended the KT120s.

FWIW, My VTA ST-120 runs Tungsol 6550's and a Tungsol 5AR4 w/o incident. Yes Bob sells them w/6550's.
I have this going through a variable transformer (variac) putting out 117v +/- a few 10ths. Same tubes since new (as of last February). I bias every couple months at .50 and this barely ever changes. I listen w/ the attenuator set between 12-3 o'clock.

Pretty much the same for me except mine came with Sovtek 6550's and the Tung-sol 5ar4
and using the variac at 117v as well.

So I went to a web vendor who had 1 used Furman P-1800 AR for sale.
Just under $600. After I pulled the trigger, I received a call from them offering a new with warranty model for $55 more.  He said that they had just received a whole bunch of them in and they were blowing them out for really cheap trying to earn my business.

PM me for details.

Bluewater

aguaazul wrote:So I went to a web vendor who had 1 used Furman P-1800 AR for sale.
Just under $600. After I pulled the trigger, I received a call from them offering a new with warranty model for $55 more.  He said that they had just received a whole bunch of them in and they were blowing them out for really cheap trying to earn my business.

PM me for details.

Bluewater

Calling all cars! Calling all cars!!!

getting back to the original problem and question. I'm guessing but very likely the fuse on your variac is a 5 amp FAST-BLOW.
Very likely the fuse on the ST120 amp is a SLOW-BLOW.
Also, we all know Bob sells these amps with tube rectifiers, but most of us also know that a rectifier tube in this amp is basically waiting for a disaster,
running tubes at 50-55ma plus the driver board, you are already at the MAX rating of a GOOD tube, and most of us know that finding GOOD new rectifier tubes is unlikely.
Highly recommended to run a WZ68 solid state rectifier in an ST120 amp, AND use a variac set at 115-117 vac.

tubes4hifi wrote:getting back to the original problem and question.   I'm guessing but very likely the fuse on your variac is a 5 amp FAST-BLOW.
Very likely the fuse on the ST120 amp is a SLOW-BLOW.
Also, we all know Bob sells these amps with tube rectifiers, but most of us also know that a rectifier tube in this amp is basically waiting for a disaster,
running tubes at 50-55ma plus the driver board, you are already at the MAX rating of a GOOD tube, and most of us know that finding GOOD new rectifier tubes is unlikely.
Highly recommended to run a WZ68 solid state rectifier in an ST120 amp, AND use a variac set at 115-117 vac.

First of all thank you for getting back on topic! Also thank you for the response. You are correct with the assumption about the variac coming with the 5 amp FAST-BLOW fuse and the ST120 came with the 5 amp SLOW-BLOW.
Im going to order the WZ68 solid state rectifier asap!!!!

No secret here im a noobie.
Thank you again.

Fritz

ImFritz wrote:

tubes4hifi wrote:getting back to the original problem and question.   I'm guessing but very likely the fuse on your variac is a 5 amp FAST-BLOW.
Very likely the fuse on the ST120 amp is a SLOW-BLOW.
Also, we all know Bob sells these amps with tube rectifiers, but most of us also know that a rectifier tube in this amp is basically waiting for a disaster,
running tubes at 50-55ma plus the driver board, you are already at the MAX rating of a GOOD tube, and most of us know that finding GOOD new rectifier tubes is unlikely.
Highly recommended to run a WZ68 solid state rectifier in an ST120 amp, AND use a variac set at 115-117 vac.

First of all thank you for getting back on topic! Also thank you for the response. You are correct with the assumption about the variac coming with the 5 amp FAST-BLOW fuse and the ST120 came with the 5 amp SLOW-BLOW.
Im going to order the WZ68 solid state rectifier asap!!!!

No secret here im a noobie.
Thank you again.

Fritz

Fritz, if you're going to use a Weber, even the -68, get the time-delay relay (TDR) option on your ST-120. Also, to keep your Weber alive for more than 6 months, remove the cover from the base (it's just glued on -- you're not violating a vacuum) and drill a few cooling holes in the copper-looking cover near the top. Not so's your exposing the few parts inside, but to permit air to flow through and cool it.
As to the TDR, it will keep the high B+ voltage from slamming into your output tubes for 17 seconds or so, giving them a chance to warm up first. The Weber won't hold the B+ back near that long.

deepee99 wrote:

ImFritz wrote:

tubes4hifi wrote:getting back to the original problem and question.   I'm guessing but very likely the fuse on your variac is a 5 amp FAST-BLOW.
Very likely the fuse on the ST120 amp is a SLOW-BLOW.
Also, we all know Bob sells these amps with tube rectifiers, but most of us also know that a rectifier tube in this amp is basically waiting for a disaster,
running tubes at 50-55ma plus the driver board, you are already at the MAX rating of a GOOD tube, and most of us know that finding GOOD new rectifier tubes is unlikely.
Highly recommended to run a WZ68 solid state rectifier in an ST120 amp, AND use a variac set at 115-117 vac.

First of all thank you for getting back on topic! Also thank you for the response. You are correct with the assumption about the variac coming with the 5 amp FAST-BLOW fuse and the ST120 came with the 5 amp SLOW-BLOW.
Im going to order the WZ68 solid state rectifier asap!!!!

No secret here im a noobie.
Thank you again.

Fritz

Fritz, if you're going to use a Weber, even the -68, get the time-delay relay (TDR) option on your ST-120. Also, to keep your Weber alive for more than 6 months, remove the cover from the base (it's just glued on -- you're not violating a vacuum) and drill a few cooling holes in the copper-looking cover near the top. Not so's your exposing the few parts inside, but to permit air to flow through and cool it.
As to the TDR, it will keep the high B+ voltage from slamming into your output tubes for 17 seconds or so, giving them a chance to warm up first. The Weber won't hold the B+ back near that long.

deepee99
Thanks for the tip. Will do. I was just about to order it and saw your response. The WS1 Copper Cap looks considerably smaller that the WZ68. The web site doesn't give you the option for the TDR with the WZ68. Only the WS1 comes with that option. Am I correct assuming your talking about drilling hole for air flow in the WS1 with the TDR?
Thank you sir!

ImFritz wrote:

deepee99 wrote:

ImFritz wrote:

tubes4hifi wrote:getting back to the original problem and question.   I'm guessing but very likely the fuse on your variac is a 5 amp FAST-BLOW.
Very likely the fuse on the ST120 amp is a SLOW-BLOW.
Also, we all know Bob sells these amps with tube rectifiers, but most of us also know that a rectifier tube in this amp is basically waiting for a disaster,
running tubes at 50-55ma plus the driver board, you are already at the MAX rating of a GOOD tube, and most of us know that finding GOOD new rectifier tubes is unlikely.
Highly recommended to run a WZ68 solid state rectifier in an ST120 amp, AND use a variac set at 115-117 vac.

First of all thank you for getting back on topic! Also thank you for the response. You are correct with the assumption about the variac coming with the 5 amp FAST-BLOW fuse and the ST120 came with the 5 amp SLOW-BLOW.
Im going to order the WZ68 solid state rectifier asap!!!!

No secret here im a noobie.
Thank you again.

Fritz

Fritz, if you're going to use a Weber, even the -68, get the time-delay relay (TDR) option on your ST-120. Also, to keep your Weber alive for more than 6 months, remove the cover from the base (it's just glued on -- you're not violating a vacuum) and drill a few cooling holes in the copper-looking cover near the top. Not so's your exposing the few parts inside, but to permit air to flow through and cool it.
As to the TDR, it will keep the high B+ voltage from slamming into your output tubes for 17 seconds or so, giving them a chance to warm up first. The Weber won't hold the B+ back near that long.

deepee99
Thanks for the tip. Will do. I was just about to order it and saw your response. The WS1 Copper Cap looks considerably smaller that the WZ68. The web site doesn't give you the option for the TDR with the WZ68. Only the WS1 comes with that option. Am I correct assuming your talking about drilling hole for air flow in the WS1 with the TDR?
Thank you sir!

Nope, I was talking about the WZ68, Fritz. It's the one that needs the cooling holes. First, it sits in about the hottest place on the top deck of the amp. Second, its innards generate a lot of heat on their own. Just pull the cover sleeve off, slide same over a dowel for support, and drill a few 1/4-inch or so holes (4 or so) above where the innards would be, and glue it back together. I don't think there's any cooling needed for the WS1 as its just an array of diodes, with no resistors or thermistors or whatever they use to make WZ68 act more like a tube rectifier.
I probably have a couple of drilled-out WZ-68s around here; not going to go looking for them but if I bump into them I'll PM you. Troy Madden, who has gone onto other things, built my first Tubes4HiFi pre-amp gear (I built my own VTA stuff), came up with this elegant mod. Another option, this one developed by Holger at Erhart Audio, was to make holes for two rectifiers in parallel, which was a great idea for the M-125s given the paucity of decent rectifier tubes made these days. If you want to spare yourself all of that grief and stay with tube rectification, the only option is a genuine NOS Mullard GZ-33 or GZ-37 in that hole. They run a few hundred bucks nowadays but are bullet-proof. Plus they look b-a-a-a-a-d-ass.

Some niceties:

The WS1 has a very small voltage drop (1V Max), and no delay.
The WZ68 has a 17V Voltage drop - and some delay.

Tube rectifiers generally have a significant voltage drop - per the table below.

Tube Type DC mA DCV DROP
5AR4 / GZ-34 250 10
5V4-GA 175 25
GZ37 350 37
5U4-GA 250 44

Given the chronic high wallplate voltages experienced by many here, this information should be taken to heart. And as Bob has pointed out on more than one occasion, post-blight (modern) tubes are nowhere near as rugged as pre-blight devices.

Nor do I understand why Weber does not recommend other-than the WS1 for audio use. I have used the WZ68 with excellent results.

As an aside, in my checkered (as I was in Saudi at the time, chequered) past, I ran a 5U4 in a stock 70 as wallplate over there was ~122 VAC from SECo. Sure, the amp de-rated a bit, but I had no B+ or transformer worries.

deepee99 wrote:

ImFritz wrote:

deepee99 wrote:

ImFritz wrote:

tubes4hifi wrote:getting back to the original problem and question.   I'm guessing but very likely the fuse on your variac is a 5 amp FAST-BLOW.
Very likely the fuse on the ST120 amp is a SLOW-BLOW.
Also, we all know Bob sells these amps with tube rectifiers, but most of us also know that a rectifier tube in this amp is basically waiting for a disaster,
running tubes at 50-55ma plus the driver board, you are already at the MAX rating of a GOOD tube, and most of us know that finding GOOD new rectifier tubes is unlikely.
Highly recommended to run a WZ68 solid state rectifier in an ST120 amp, AND use a variac set at 115-117 vac.

First of all thank you for getting back on topic! Also thank you for the response. You are correct with the assumption about the variac coming with the 5 amp FAST-BLOW fuse and the ST120 came with the 5 amp SLOW-BLOW.
Im going to order the WZ68 solid state rectifier asap!!!!

No secret here im a noobie.
Thank you again.

Fritz

Fritz, if you're going to use a Weber, even the -68, get the time-delay relay (TDR) option on your ST-120. Also, to keep your Weber alive for more than 6 months, remove the cover from the base (it's just glued on -- you're not violating a vacuum) and drill a few cooling holes in the copper-looking cover near the top. Not so's your exposing the few parts inside, but to permit air to flow through and cool it.
As to the TDR, it will keep the high B+ voltage from slamming into your output tubes for 17 seconds or so, giving them a chance to warm up first. The Weber won't hold the B+ back near that long.

deepee99
Thanks for the tip. Will do. I was just about to order it and saw your response. The WS1 Copper Cap looks considerably smaller that the WZ68. The web site doesn't give you the option for the TDR with the WZ68. Only the WS1 comes with that option. Am I correct assuming your talking about drilling hole for air flow in the WS1 with the TDR?
Thank you sir!

Nope, I was talking about the WZ68, Fritz. It's the one that needs the cooling holes. First, it sits in about the hottest place on the top deck of the amp. Second, its innards generate a lot of heat on their own. Just pull the cover sleeve off, slide same over a dowel for support, and drill a few 1/4-inch or so holes (4 or so) above where the innards would be, and glue it back together. I don't think there's any cooling needed for the WS1 as its just an array of diodes, with no resistors or thermistors or whatever they use to make WZ68 act more like a tube rectifier.
I probably have a couple of drilled-out WZ-68s around here; not going to go looking for them but if I bump into them I'll PM you. Troy Madden, who has gone onto other things, built my first Tubes4HiFi pre-amp gear (I built my own VTA stuff), came up with this elegant mod. Another option, this one developed by Holger at Erhart Audio, was to make holes for two rectifiers in parallel, which was a great idea for the M-125s given the paucity of decent rectifier tubes made these days. If you want to spare yourself all of that grief and stay with tube rectification, the only option is a genuine NOS Mullard GZ-33 or GZ-37 in that hole. They run a few hundred bucks nowadays but are bullet-proof. Plus they look b-a-a-a-a-d-ass.

Thanks deepee99 and peter for the feedback.
Ill go with the WZ68 and drill it for sure. When my new VTA ST-120 with the Tung-Sol 5ARA crapped out I ordered the Gold Lion Genalex U77/GZ34 and got in 3 days.
As of right now its sounding sweet. I'm running from puter (Tidal) to the Schitt BIFROST multibit straight to the the ST-120 with attenuator to my B&Ws.
I've got to say wow! The resolution and imaging are incredible compared to the Emo xmc-1 and the Emo xpa5. Frankly i'm just glad it didn't take anything else out with it.
Again thanks for the replies.