Amp hum...

[garey77]

…once standby is off, not before. So there's this VHT Classic 212 combo on consignment at a local guitar shop near to where I live. I went down to check it out, turned it on, let it sit for a minute before I switched the standby off, and there was a very pronounced hum while playing. You could hear each channel just fine, and all channels sounded great, save for the hum. I tried a different cable, tried a power conditioned power source and figured it was for sure coming from the amp. I'm thinking tranny hum. Any ideas how much it'd be for a replacement? It's an older combo, actually more like the CLX; el34's. They want 1k for it (which it in it's current state it's not worth).

 

[2tallmusic]

Honestly, I would try to run it with other tubes before thinking of any kind of repairs.

 

[moronmountain]

A pre amp tube can hum.

 

[garey77]

Interesting… I might go down there with a couple of preamp tubes and see if they'll let me mess with it. I'm sure they'll be happy (including the actual owner) to not have to try to sell a piece of gear in need of repair… especially if the amp actually needs a tranny replacement. The actual owner of the amp is not even remotely a musician. He wandered into a stupid awesome deal on the amp, and the owner of the shop is a buddy of his.

 

[xzyryabx]

a friend had a vht with the same problem...ended up being the rectifier tube (the last pre-amp tube I think it was)

 

[suhrimmetal]

Pre amp tube...almost for sure. Either its bad or the prongs are bent and not seeding in the socket correctly. Should be an easy fix with a little time and attention

 

[garey77]

So, a rectifier tube is considered a preamp tube? No biasing needed?

 

[moronmountain]

So, a rectifier tube is considered a preamp tube? No biasing needed?

It is not a pre amp tube, but it doesn't need to be biased either. Can you turn of the tube rectifier on a VHT like you can a Mesa? If you can, do that. Does the 212 Classic even have a rectifier tube? I thought only the CL amps did? Any tubes can hum, but the pre amp tubes will be the easiest way to tell at a shop if they will let you mess with that. Consider this though......... Fryette charges a flat $60 fee for fixing their own amps plus parts and shipping. Even if you got the amp, bought new tubes, and it still hummed, you wouldn't be out much coin to have them fix it. I sent my Ultralead to them less than 3 weeks ago, and it's going to be back on Wednesday for $118 shipped. It will be like brand new again.

 

[JTyson]

If your output tubes are poorly matched, it will induce hum
can you pull a pair out (like a Marshall) and see if you can at least isolate it?
If you pull both pairs out (1 pair at a time) at least you have eliminated that possibility without spending a dime

 

[tfridgen]

Make sure the preamp tube in the PI position is a 12au7 and not 12ax7. I recall when I bought my 212 Classic, the guy said it was time for a change of amps. When I fired it up it was noisy and sounded terrible. Looked at the back and there was a 12ax7 in the PI spot. Changed it out and the noise cleared up and woke up the amp. I was a little pissed that he told me it was working fine, but dude would have shit himself if he heard that amp after the change. Never should have sold that thing, damnit.

 

[garey77]

If your output tubes are poorly matched, it will induce hum
can you pull a pair out (like a Marshall) and see if you can at least isolate it?
If you pull both pairs out (1 pair at a time) at least you have eliminated that possibility without spending a dime

Is that safe? Also, how, while amp is on? Outtermost set first?

 

[JTyson]

If your output tubes are poorly matched, it will induce hum
can you pull a pair out (like a Marshall) and see if you can at least isolate it?
If you pull both pairs out (1 pair at a time) at least you have eliminated that possibility without spending a dime

Is that safe? Also, how, while amp is on? Outtermost set first?

Well, on a Marshall, if you want to drop the output, you can remove either the 2 inner or 2 outer tubes without a problem. I dont do it, but it can be done for at least diagnostic purposes. Not sure about this amp in particular, but I'm sure you could at least email Fryette about the ramifications. You would pull one or the other set and then power up to see if it makes a difference, then power back down and put those back in and pull the other set and power back up
I was just looking for a way to possibly resolve the question of what exactly is the problem without an investment. Your trying to figure out if you even want to purchase the amp because of the hum, and whoever has it is probably not gonna sink any coin into it to resolve the issue. If its a preamp tube, its probably only 1 of them (usually V1) I've had way more issue's with output tubes than pre's. You probably have a preamp tube laying around that you could use as a swapper to see if it makes a difference. If you happen to have a biasmaster, you could always check the output tubes that way.
My JMP came back from Mike with more hum than normal, and he told me before it was ever shipped back to me that my output tubes were poorly matched and that was causing the issue. It was a set that came in the amp when I got it. I popped in a fresh set and problem solved.
There are several possible causes of hum in an amp, but if its ok to pull the output tubes in pairs, you could at least explore the results without spending any coin on the amp. If you have eliminated tubes as the problem, the other possibilities are far more in depth than the act of pulling a set out and seeing if it makes any difference or swapping a pre tube in all positions starting with V1 and working your way across
If none of that makes any difference in the hum, and you suspect the issue is more serious, you just got yourself some leverage in dropping the price

 

[garey77]

If your output tubes are poorly matched, it will induce hum
can you pull a pair out (like a Marshall) and see if you can at least isolate it?
If you pull both pairs out (1 pair at a time) at least you have eliminated that possibility without spending a dime

Is that safe? Also, how, while amp is on? Outtermost set first?

Well, on a Marshall, if you want to drop the output, you can remove either the 2 inner or 2 outer tubes without a problem. I dont do it, but it can be done for at least diagnostic purposes. Not sure about this amp in particular, but I'm sure you could at least email Fryette about the ramifications. You would pull one or the other set and then power up to see if it makes a difference, then power back down and put those back in and pull the other set and power back up
I was just looking for a way to possibly resolve the question of what exactly is the problem without an investment. Your trying to figure out if you even want to purchase the amp because of the hum, and whoever has it is probably not gonna sink any coin into it to resolve the issue. If its a preamp tube, its probably only 1 of them (usually V1) I've had way more issue's with output tubes than pre's. You probably have a preamp tube laying around that you could use as a swapper to see if it makes a difference. If you happen to have a biasmaster, you could always check the output tubes that way.
My JMP came back from Mike with more hum than normal, and he told me before it was ever shipped back to me that my output tubes were poorly matched and that was causing the issue. It was a set that came in the amp when I got it. I popped in a fresh set and problem solved.
There are several possible causes of hum in an amp, but if its ok to pull the output tubes in pairs, you could at least explore the results without spending any coin on the amp. If you have eliminated tubes as the problem, the other possibilities are far more in depth than the act of pulling a set out and seeing if it makes any difference or swapping a pre tube in all positions starting with V1 and working your way across
If none of that makes any difference in the hum, and you suspect the issue is more serious, you just got yourself some leverage in dropping the price

That's what I was thinking. Thx!