Is this going to need a repair? (VH4) HELP! (Fixed :) )

[Ailean]

Hi all

Sue was playing her VH4 yesterday when it suddenly got noisy and then a smell of burning filled the room. It was a smell I associate with electrical components.

The amp continued to work, but we switched it off and checked it over. Fuses 3 and 4 had blown, three looked almost new but had a tiny break, 4 had gone discoloured and had very obviously gone. (Fuse 2 had gone the day before, but the amp seemed fine when it was replaced so I put it down to a bad fuse).

I took the amp out of it's head and did a visual inspection for any scorch marks but couldn't see any, the tubes (power and pre) all looked ok, so we powered it up again (used a 16ohm Hotplate on LOAD). No more smell, tubes all glowing evenly, no fault lights, all looked good. Switched to run and tubes 1 and 2 took on a faint blue glow, 3 and 4 didn't. Plugged the speakers back in and the amp appeared to be working.

We decided to check the bias while the amp was out of it's head, using a BiasRite I checked the current. Tubes 1 and 2 were showing around 37/38 mA, tube 4 was around 35/36, but tube 3 was around 18mA. All voltages around 475v.

I assumed it would be the tubes and we've ordered a new quad.

However... Where did that burning smell come from? It filled 2 rooms quite noticeably, I'm not sure a fuse could do that. Given that the amp still works and tube 3 is actually showing a current is it likely that it's not the tubes and actually a component that's died?

The amp is under a year old.

Thoughts?

 

[electroid]

It could have been the tubes getting really hot, screen resistors or maybe even the fuses.

 

[FourT6and2]

It could have been the tubes getting really hot, screen resistors or maybe even the fuses.

Hey! Welcome to the board! I've heard good things about your work.

 

[Peter Diezel]

Check the screen resistor of tube 3.
It should have 1kOm. It´s the white
big resistor located on the tube socket.

The age of the amp has nothing to do with Your tube fault.

Tubes last 2 seconds or 20 years. If all customers would
complain to the tube manufactures they could close their
business.

 

[Peter Diezel]

It could have been the tubes getting really hot, screen resistors or maybe even the fuses.

Welcome on board, Jeff

 

[electroid]

Hey! Welcome to the board! I've heard good things about your work.[/quote]

Thanks for the welcomes! I wish I could say that I'll be watching all the time but honestly, I'll just be checking in from time to time when I get a chance. Anyway, good to be here!

 

[Ailean]

Check the screen resistor of tube 3.
It should have 1kOm. It´s the white
big resistor located on the tube socket.

The age of the amp has nothing to do with Your tube fault.

Tubes last 2 seconds or 20 years. If all customers would
complain to the tube manufactures they could close their
business.

Hi Peter

Thanks for that. I've checked the screen resistor it's measuring 1KOhm within tolerances.

Yeah I know the age of the amp not related to the tube issue, I was referring more to the age of the components etc. I'm well aware of your constant struggle to get reliable tubes!

I was hoping these tubes would last a bit longer as they are NOS Phillips JAN spec, but you never can tell!

If we closed all the tube suppliers who'd make our tubes!!!

It could have been the tubes getting really hot, screen resistors or maybe even the fuses.

Hi and welcome to the board. It's not the tubes, they are 6-8 months old and this was not a 'hot' smell, it was a burning wire or insulation smell. My immeadiate thought was the fuses, but they are sealed and don't usually smell, I guess the seal could have broken as it blew (or be the reason it blew).

 

[mraajr]

If any of the tubes redplated it could definitely give off a burning smell. Probably just whatever oils are on the tube or whatever paint or whatever burning off. I had a set of Fender branded GTs and when they went it stunk to high heaven. It just had a painted on logo that fried when the tubes got hot.

 

[Sixtonoize]

It's not the tubes, they are 6-8 months old and this was not a 'hot' smell, it was a burning wire or insulation smell. My immeadiate thought was the fuses, but they are sealed and don't usually smell, I guess the seal could have broken as it blew (or be the reason it blew).

Just because the tubes are relatively new doesn't mean that it can't be the tubes.
The fuses blew for a reason, and the most likely reason is the tubes. If there was a short in tube 3/4, they could blow due to redplating and take out the tube fault fuses.

Bad tubes is the most likely reason that the fuses went, so replace them first.
If the amp is still acting strangely, then explore other options.

 

[Peter Diezel]

Check the screen resistor of tube 3.
It should have 1kOm. It´s the white
big resistor located on the tube socket.

The age of the amp has nothing to do with Your tube fault.

Tubes last 2 seconds or 20 years. If all customers would
complain to the tube manufactures they could close their
business.

Hi Peter

Thanks for that. I've checked the screen resistor it's measuring 1KOhm within tolerances.

Yeah I know the age of the amp not related to the tube issue, I was referring more to the age of the components etc. I'm well aware of your constant struggle to get reliable tubes!

I was hoping these tubes would last a bit longer as they are NOS Phillips JAN spec, but you never can tell!

If we closed all the tube suppliers who'd make our tubes!!!

It could have been the tubes getting really hot, screen resistors or maybe even the fuses.

Hi and welcome to the board. It's not the tubes, they are 6-8 months old and this was not a 'hot' smell, it was a burning wire or insulation smell. My immeadiate thought was the fuses, but they are sealed and don't usually smell, I guess the seal could have broken as it blew (or be the reason it blew).

Sorry, Im´just a bit rude because 99% of our issues are tube related

 

[Ailean]

Sorry, Im´just a bit rude because 99% of our issues are tube related

You are forgiven, I'll forgive you just about anything except taking my amp away, that may cause irrational behaviour

If any of the tubes redplated it could definitely give off a burning smell. Probably just whatever oils are on the tube or whatever paint or whatever burning off. I had a set of Fender branded GTs and when they went it stunk to high heaven. It just had a painted on logo that fried when the tubes got hot.

That's a very good point, there is all that plastic at the bottom of the tube too.

It's not the tubes, they are 6-8 months old and this was not a 'hot' smell, it was a burning wire or insulation smell. My immeadiate thought was the fuses, but they are sealed and don't usually smell, I guess the seal could have broken as it blew (or be the reason it blew).

Just because the tubes are relatively new doesn't mean that it can't be the tubes.
The fuses blew for a reason, and the most likely reason is the tubes. If there was a short in tube 3/4, they could blow due to redplating and take out the tube fault fuses.

Bad tubes is the most likely reason that the fuses went, so replace them first.
If the amp is still acting strangely, then explore other options.

Yeah I know Tubes can blow quickly, my first set blew inside a month!

Sorry guys I think I paniced. Cool amp + burning smell = panic!

 

[steve_k]

Sorry, Im´just a bit rude because 99% of our issues are tube related

You are forgiven, I'll forgive you just about anything except taking my amp away, that may cause irrational behaviour

If any of the tubes redplated it could definitely give off a burning smell. Probably just whatever oils are on the tube or whatever paint or whatever burning off. I had a set of Fender branded GTs and when they went it stunk to high heaven. It just had a painted on logo that fried when the tubes got hot.

That's a very good point, there is all that plastic at the bottom of the tube too.

It's not the tubes, they are 6-8 months old and this was not a 'hot' smell, it was a burning wire or insulation smell. My immeadiate thought was the fuses, but they are sealed and don't usually smell, I guess the seal could have broken as it blew (or be the reason it blew).

Just because the tubes are relatively new doesn't mean that it can't be the tubes.
The fuses blew for a reason, and the most likely reason is the tubes. If there was a short in tube 3/4, they could blow due to redplating and take out the tube fault fuses.

Bad tubes is the most likely reason that the fuses went, so replace them first.
If the amp is still acting strangely, then explore other options.

Yeah I know Tubes can blow quickly, my first set blew inside a month!

Sorry guys I think I paniced. Cool amp + burning smell = panic!

Are you running 34's or 6550's? Just curious......

Steve

 

[Ailean]

Fixed!

New tubes, bias readings all good, no smells, cool tone. Happy, happy, happy.


Steve, 6L6's.