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j beede

Peter W.

peterh

Rabindu

talsmaja

9 posters

Blown fuse st-70

talsmaja: Posts : 9
Join date : 2016-11-20

Post n°1

Blown fuse st-70

by talsmaja Sun Nov 20, 2016 4:03 am

Hi all,

I have a Bob Latino kit st-70 that I built in 2012 that has worked perfectly until just recently when it blew a fuse. I bought some replacement 3A slo-blo fuses and after replacing the fuse and turning the amp on the following happened: The tubes lit up and within seconds of turning the amp on I saw an arc in the rectifier tube and the fuse blew again. Does this sound like I need a new rectifier tube or new output tubes? Thanks!

Jack

Rabindu: Posts : 6
Join date : 2016-02-13
Age : 69
Location : Mississauga, Ontario, Canada

Post n°2

Re: Blown fuse st-70

by Rabindu Sun Nov 20, 2016 4:22 am

I had the same thing happen with my ST-120 some time ago. I replaced the original blown Chinese 5AR4 rectifier tube with a Mullard GZ34 and did the diode mod to help protect the rectifier. My AC coming in is consistently at 123 VAC and therefore got a Variac which I've set to 118 VAC and haven't had any problems since.

Mike

peterh: Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden

Post n°3

Re: Blown fuse st-70

by peterh Sun Nov 20, 2016 5:16 am

talsmaja wrote:Hi all,

I have a Bob Latino kit st-70 that I built in 2012 that has worked perfectly until just recently when it blew a fuse. I bought some replacement 3A slo-blo fuses and after replacing the fuse and turning the amp on the following happened: The tubes lit up and within seconds of turning the amp on I saw an arc in the rectifier tube and the fuse blew again. Does this sound like I need a new rectifier tube or new output tubes? Thanks!

Jack

It is the most probable cause.
But, it might also be a problem with something draining B+, a broken can cap, a el34 that flashes.
If you don't possess an external B+ powersupply to test with you could
rule out the above at the cost of possibly breaking another 5ar4 by this procuedure:

replace 5ar4 with a new one. Remove the el34's on one side ( keep note of
their positions ) power on. The results is one of :
-1 works fine.
-2 fuse blows

If (1) then reinstall the el34's in their original positions and power on again
if everything works you are done, the fault was the 5ar4.
If (2) , reinstall the el34's in their original positions, and remove the other
el34, noting their positions. AT this point the 5ar4 might have been destroyed
but do continue. If amp powers up and works with one channel the fault was in
one of the removed el34. If the fuse blews again you have a problem
with either faulty can cap or the new 5ar4 has been broken . Replace the can
cap and get a new 5ar4.

The above sequence should identify the problem in 90% of all cases. If
not i suggest you find a workshop .

Peter W.: Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA

Post n°4

Re: Blown fuse st-70

by Peter W. Sun Nov 20, 2016 9:20 am

talsmaja wrote:Hi all,

I have a Bob Latino kit st-70 that I built in 2012 that has worked perfectly until just recently when it blew a fuse. I bought some replacement 3A slo-blo fuses and after replacing the fuse and turning the amp on the following happened: The tubes lit up and within seconds of turning the amp on I saw an arc in the rectifier tube and the fuse blew again. Does this sound like I need a new rectifier tube or new output tubes? Thanks!

Jack

In rank order:

a) Bad rectifier tube - especially if it is of Chinese origin. How to test: Remove the rectifier tube and turn on the amp. If the rest of the tube light up properly, that is a good sign, but not proof that they are good. Next with a VOM test for any shorts at the pins of the rectifier. You should get moderate resistance - a few ohms but not a dead short - between pins 2 & 8, but no other shorts or resistance. If so, the rectifier is toast and must be replaced.

b) A short in one/more of the output tubes - and something that is related to the onset of B+. Mark each OPT to its location, then remove a pair. Repower the amp. Does the fuse blow? If no, install one-at-a-time the remaining tubes. If one of the tubes is bad, it will pop the fuse within a few seconds of installation. If YES, then remove that pair and install the other pair. If YES, it is the rectifier very most likely.

It is quite unlikely that you have a bad filter cap in a Latino kit after only a few years.

This is one of the few times that an inexpensive tube tester - a Heath TC3, or similar - can be useful to this hobby. Usually, they may be had in reasonable condition for less than $50 at a Hamfest or at show/swapmeets such as Kutztown. This would find the problem pretty quickly if it is tube-related.

j beede: Posts : 473
Join date : 2011-02-07
Location : California

Post n°5

Re: Blown fuse st-70

by j beede Sun Nov 20, 2016 11:11 am

You can reduce the odds of damaging otherwise good components by temporarily inserting a 60-100W incandescent bulb in series with your power cord before performing the steps described. If all is well the lamp will glow brightly on initial startup and then dim down after a few seconds. If you have a short somewhere (steady bright bulb) the lamp will provide some current/smoke limiting. If the lamp dims, take it out of the circuit and repeat the test.

Tubes4ever: Posts : 167
Join date : 2015-07-14
Location : Star, Idaho

Post n°6

Re: Blown fuse st-70

by Tubes4ever Sun Nov 20, 2016 2:51 pm

I had this exact same issue with my ST70 awhile back. The problem turned out to be the 5ar4 was defective. It worked well for the first 6 months of use. Something to keep in mind when testing the rectifier tube on a tube tester is that the tester doesn't apply high voltage to the tube. I tested my defective 5ar4 on a heathkit tc-3 tester and it tested good with no shorts or issues. I also tested it on my Hickok 800A with the same good results. I then put it back in the ST70 where it promptly arced again and blew the fuse. It was the Chinese tube that came with the kit.

talsmaja: Posts : 9
Join date : 2016-11-20

Post n°7

Re: Blown fuse st-70

by talsmaja Sun Nov 20, 2016 3:16 pm

Thanks for replies everyone. Looks like I'll try replacing the rectifier tube. I'm using the one that came with the kit 4 years ago. I'm assuming this is the Chinese tube you're referring to? Anyone have any recommendations for a better quality brand to replace it?

LeGrace: Posts : 390
Join date : 2016-08-07
Location : Ontario, Canada

Post n°8

Re: Blown fuse st-70

by LeGrace Sun Nov 20, 2016 3:40 pm

Genalex Gold Lion GZ34 is regarded as one of the better ones. (same as 5AR4) Getting 4 years borders on the surprising, were you using the amp all the time or just intermittently?

talsmaja: Posts : 9
Join date : 2016-11-20

Post n°9

Re: Blown fuse st-70

by talsmaja Sun Nov 20, 2016 4:10 pm

I used it pretty regularly, around every week over the past 4 years. I'll look into that tube. I've seen some posts about the Mullard GZ34 also. I found one on eBay for $33.75 (can't post link because my account here is new) but that seems like a lower price than I expected for NOS.

LeGrace: Posts : 390
Join date : 2016-08-07
Location : Ontario, Canada

Post n°10

Re: Blown fuse st-70

by LeGrace Sun Nov 20, 2016 4:17 pm

talsmaja wrote:I used it pretty regularly, around every week over the past 4 years. I'll look into that tube. I've seen some posts about the Mullard GZ34 also. I found one on eBay for $33.75 (can't post link because my account here is new) but that seems like a lower price than I expected for NOS.

Its not NOS that's why. The Mullard brand name was acquired and is still being used. NOS Mullard rectifiers are GZ37 and GZ33. They are a tad pricier, ahem. But the one you mention is also a decent choice.

talsmaja: Posts : 9
Join date : 2016-11-20

Post n°11

Re: Blown fuse st-70

by talsmaja Sun Nov 20, 2016 4:24 pm

Ahh yes, now I see they are re-issues.

LeGrace: Posts : 390
Join date : 2016-08-07
Location : Ontario, Canada

Post n°12

Re: Blown fuse st-70

by LeGrace Sun Nov 20, 2016 4:33 pm

Still amazed you got 4 years. I've replaced 2 of these in less then 1 year. Although I recently applied the diode mod and have to admit this seems to be helping.

Last edited by LeGrace on Sun Nov 20, 2016 4:37 pm; edited 1 time in total

peterh: Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden

Post n°13

Re: Blown fuse st-70

by peterh Sun Nov 20, 2016 4:34 pm

talsmaja wrote:Thanks for replies everyone. Looks like I'll try replacing the rectifier tube. I'm using the one that came with the kit 4 years ago. I'm assuming this is the Chinese tube you're referring to? Anyone have any recommendations for a better quality brand to replace it?

5ar4 is "consumables". They live a hard life in st70 ( still within spec)
Buy 2 when you buy!

talsmaja: Posts : 9
Join date : 2016-11-20

Post n°14

Re: Blown fuse st-70

by talsmaja Sun Nov 20, 2016 6:51 pm

Ordered a Genalex Gold Lion GZ34. I'll update this post with the results when it arrives. Thanks everyone!

talsmaja: Posts : 9
Join date : 2016-11-20

Post n°15

Re: Blown fuse st-70

by talsmaja Sat Nov 26, 2016 4:01 am

Well, I just got the Genalex Gold Lion GZ34. Pulled the old 5AR4 and plugged this one in and the system has been playing fine for the last 2 hours. About 60 seconds after I first turned it on the front left output tube flashed and the speakers popped but everything seemed fine after that. So I'm just reporting back that it looks like the rectifier tube was the culprit. Also: the bad rectifier tube was a Ruby tube that came with the kit, are these the Chinese tubes people refer to?

GP49: Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon

Post n°16

Re: Blown fuse st-70

by GP49 Sat Nov 26, 2016 6:39 am

You should be looking at new output tubes now, not later

peterh: Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden

Post n°17

Re: Blown fuse st-70

by peterh Sat Nov 26, 2016 7:08 am

GP49 wrote:You should be looking at new output tubes now, not later

So very true.

And get a GZ34 as spare at the same time

Bob Latino: Admin; Posts : 3274
Join date : 2008-11-26
Location : Massachusetts

Post n°18

Re: Blown fuse st-70

by Bob Latino Sat Nov 26, 2016 10:35 am

talsmaja wrote:Hi all,

I have a Bob Latino kit st-70 that I built in 2012 that has worked perfectly until just recently when it blew a fuse. I bought some replacement 3A slo-blo fuses and after replacing the fuse and turning the amp on the following happened: The tubes lit up and within seconds of turning the amp on I saw an arc in the rectifier tube and the fuse blew again. Does this sound like I need a new rectifier tube or new output tubes? Thanks!

Jack

Hi Jack,

After 4 years with the amp ... Exactly what GP49 has said .. You should be getting a new quad of output tubes .. Pushing a set of output tubes beyond 2500 - 3000 hours runs the risk of one of the four output tubes going bad. What usually happens is that a bad output tube will cause the rectifier to "flash over" near the bottom of the tube. Sometimes when the rectifier flashes over, it may still work again but the flash over is more often than not a symptom of a problem someplace else in the amp. All vacuum tubes are "consumables" .. They have a lifespan and they do eventually wear out. Tubes should be replaced periodically ... Output tubes after 3000 hours ... Driver tubes maybe 5000 hours. Keep the old tubes and mark them as "spares" because in time you will forget which amp that tube came from and when the tube was pulled.

People should also check their line voltages. Line voltages above about 122 VAC (USA and Canada) raise the amp's internal AC and DC voltages and will shorten tube life. If you have high line voltage problems where you live, then run the amp off a variac set to about 117 - 118 VAC.

Bob

talsmaja: Posts : 9
Join date : 2016-11-20

Post n°19

Re: Blown fuse st-70

by talsmaja Sat Nov 26, 2016 1:21 pm

Thanks all, is there any consensus on the best output tubes for a VTA ST-70? I saw some older threads from 2009 talking about the new production Gold Lion KT66s. Are those still considered really good?

peterh: Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden

Post n°20

Re: Blown fuse st-70

by peterh Sat Nov 26, 2016 2:02 pm

talsmaja wrote:Thanks all, is there any consensus on the best output tubes for a VTA ST-70? I saw some older threads from 2009 talking about the new production Gold Lion KT66s. Are those still considered really good?

Any of EL34 E34L kt66 kt77 6ca7 is fair game.
I do not think there is a "best" tube, they all work. More important is choice of speakers and
source material.

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Post n°21

Re: Blown fuse st-70

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