Diezel VH-2 Bias Weirdness

[Hexjibber]

Hi,

New forum user here, wondering if anyone may be able to help with some biasing strangeness that seems to be happening with my VH-2.

My set up is amp -> Suhr Reactive Load IR. I noticed that one tube was red plating so did the usual checks such as swapping tube positions around but the issue remained with that position (2nd tube from the left looking at the front of the amp). Decided at that point to buy a new matched quad (from Watford Valves) to see if that solved the issue which it seemed to initially but before long I had the same tube red plating again.

At that point I took it to my local tech to have a look but despite a few days of trying, he couldn't replicate the issue. Obviously this would seem to indicate an issue with my set up at home but I have several other amps that I use and they are all operating as expected. I took the amp home and as soon as I plugged it in, the same tube is red plating. I measured the bias using the measuring points shown on the amp PCB and sure enough that tube was reading ~60mV, where all the others were reading at ~35mV as per spec. Just out of interest I put one of the older tubes in that position and it read around ~35mV which I think was just dumb luck that the old tube was in that range. The amp seems to be working correctly from a sound point of view but even though now the tubes are all within 1-3mV of each other, that problem position tube and the one to the left of it now have a noticeable blue glow and the problem position has slight red plating, where the other pair doesn't. I feel like there is some issue here that I'm missing but haven't managed to nail it down yet.

The only thing I have tried to do is measure the resistance of a few components around the tubes and the other thing somewhat out of the ordinary was that the 1k 5W resistor near the problem tube socket read at around 900 Ohm whereas the other three were all pretty much at 1k. I realise there is component tolerance to account for etc but could this resistor be causing a bias issue?

Any help or advice would be greatly appreciated, thanks!

Graham

 

[Leo Diezel]

Replace the 1K resistor and it should be ok!

 

[Hexjibber]

Ok awesome I'll give that a try, thanks for the response!

 

[Hexjibber]

Update on this, I replaced the 1k resistor but I am still seeing some noticeable red plating on the problem tube position when using channel 2 and only when I am actually playing, is this normal? Not quite sure why just this position would be affected if it was a bias issue so would seem to point to something else. I am reluctant to shell out for a 3rd set of tubes if they are not the culprit. The bias readings are all between 30-35mV.

Any advice on how best to troubleshoot this would be a great help, thanks!

 

[belensky]

I'm interested to find solution as well, had same issue with Herbert, in the end tube got brighter and brighter till I replaced them all.

 

[Hexjibber]

I'm interested to find solution as well, had same issue with Herbert, in the end tube got brighter and brighter till I replaced them all.

I'm really hoping that spending another £130 on a new quad isn't the only answer. I've read some other forums posts on one hot tube and most of the prevailing advice was that if it's done it once, it will do it again. What I can't make sense of is that the bias readings are all within normal, so how is this tube getting so much more current than the others? I'll be the first to admit I'm no amp expert but it's not that complicated. I checked the sockets as that was a possible issue that came up a lot, they all look fine. I was hoping replacing that slightly out of spec resistor was the answer but sadly it wasn't so now I'm stuck in no man's land!

What I don't understand is that my local tech couldn't replicate the issue which means one of two things, they either didn't try it on channel 2 at volume, or there is an issue with my set up. Again, I use a Suhr Load IR with this and several other heads, all the others are fine so that would appear to rule that problem out. I have one more thing to try which is put it through my cab the next chance I can get to the studio and see if it happens then. The only issue is whether it confirms the problem or not, I'll still be in the dark as to what I can do about it.

Very frustrating as I really want to use this amp but at the moment it's just causing me stress.

 

[cardinal]

Sorry I can't help but I'm curious: are the pots and jacks in the VH2 attached with flying leads or mounted directly to the circuit board?

 

[Leo Diezel]

I'm really hoping that spending another £130 on a new quad isn't the only answer. I've read some other forums posts on one hot tube and most of the prevailing advice was that if it's done it once, it will do it again. What I can't make sense of is that the bias readings are all within normal, so how is this tube getting so much more current than the others? I'll be the first to admit I'm no amp expert but it's not that complicated. I checked the sockets as that was a possible issue that came up a lot, they all look fine. I was hoping replacing that slightly out of spec resistor was the answer but sadly it wasn't so now I'm stuck in no man's land!

What I don't understand is that my local tech couldn't replicate the issue which means one of two things, they either didn't try it on channel 2 at volume, or there is an issue with my set up. Again, I use a Suhr Load IR with this and several other heads, all the others are fine so that would appear to rule that problem out. I have one more thing to try which is put it through my cab the next chance I can get to the studio and see if it happens then. The only issue is whether it confirms the problem or not, I'll still be in the dark as to what I can do about it.

Very frustrating as I really want to use this amp but at the moment it's just causing me stress.

Does it red plate on idle? Meaning with master at zero when you read the bias or only when you play?

 

[Hexjibber]

Sorry I can't help but I'm curious: are the pots and jacks in the VH2 attached with flying leads or mounted directly to the circuit board?

They're attached with flying leads

 

[Hexjibber]

Does it red plate on idle? Meaning with master at zero when you read the bias or only when you play?

Hi Leo, it only red plates when playing. If I stop playing for a short while, the tube presumably cools down again as the red plating slowly fades

 

[cardinal]

Hi Leo, it only red plates when playing, if I stop playing for a short while the tube presumably cools down again as the red plating slowly fades

That stinks. I have a '69 Superlead that did the same thing. Never did figure out what was wrong.

 

[Leo Diezel]

Hi Leo, it only red plates when playing. If I stop playing for a short while, the tube presumably cools down again as the red plating slowly fades

Ok at least that may explain why your technician can't replicate it.

He should check that all the nearby components are ok, there are a couple of resistors for each tube and replace the faulty one, if any.
If you swap tube position and it follow the tube it's tube, otherwise may be a component in the amp or even the socket that may need a bit of cleaning, but all things your tech should be well able to do.

Thank you!

 

[Hexjibber]

Ok at least that may explain why your technician can't replicate it.

He should check that all the nearby components are ok, there are a couple of resistors for each tube and replace the faulty one, if any.
If you swap tube position and it follow the tube it's tube, otherwise may be a component in the amp or even the socket that may need a bit of cleaning, but all things your tech should be well able to do.

Thank you!

Thanks Leo,

I have checked all the resistors around the faulty tube and all check out ok. I have also tried moving the tube and it is that position that is the problem as red plating occurs with any tube. I have also retensioned the pins in the troublesome socket but this unfortunately made no difference. Is there some way of verifying the socket is faulty before I desolder it and replace it? It would seem odd for such a new amp to have this problem but I suppose it does happen.

The only variables I have left to consider it using it through a real cab which I will try. If it red plates then also I will just have to insist that my tech takes another look.

If you have any suggestions in the meantime I’d appreciate it. I’m comfortable working on amps and am well aware of the potential hazards concerning capacitors etc.

Thanks again,
Graham

 

[Leo Diezel]

Thanks Leo,

I have checked all the resistors around the faulty tube and all check out ok. I have also tried moving the tube and it is that position that is the problem as red plating occurs with any tube. I have also retensioned the pins in the troublesome socket but this unfortunately made no difference. Is there some way of verifying the socket is faulty before I desolder it and replace it? It would seem odd for such a new amp to have this problem but I suppose it does happen.

The only variables I have left to consider it using it through a real cab which I will try. If it red plates then also I will just have to insist that my tech takes another look.

If you have any suggestions in the meantime I’d appreciate it. I’m comfortable working on amps and am well aware of the potential hazards concerning capacitors etc.

Thanks again,
Graham

Does it happen also with master volume low?

You need to check:

2K2 grid resistor
220K from 2K2 resistor to BIAS pot
1R cathode resistors

I also don't think the socket is the issue but to correctly identify the problem it needs to be checked with an oscilloscope and all the points measured while running, all in all it's extremely easy as components are just few, but it's not something on the user side, a tech is definetly needed.

Unfortunatly when a tube decide to break it may brings with it some components.

Thank you,
Leo

 

[Hexjibber]

Does it happen also with master volume low?

You need to check:

2K2 grid resistor
220K from 2K2 resistor to BIAS pot
1R cathode resistors

I also don't think the socket is the issue but to correctly identify the problem it needs to be checked with an oscilloscope and all the points measured while running, all in all it's extremely easy as components are just few, but it's not something on the user side, a tech is definetly needed.

Unfortunatly when a tube decide to break it may brings with it some components.

Thank you,
Leo

Hi Leo,

I tried reducing the volume, when I turn the master down (to approx. 8 o'clock) the red plating tube actually got brighter, channel 2 level was at around 10 o'clock.

I have checked those resistors, the 2k2 and 1R resistors checked out ok but the 220k was really hard to get a reading from though for some reason in both instances, the one near the bias pot and the one near the middle tubes.

Just to try something different I put the 4th tube from the new matched quad back in that was reading high at ~60mV, the reading is still high no matter what position it's in so it red plates but the tube in the troublesome position is now ok.The readings are as follows;

New set
V1: ~30mV
V2: ~31mV
V3: ~30mV
V4: ~58mV

After that I re-tried the tubes that were in the amp when I bought it (they have matched values written on them) and the readings are all over the place;

Old set
V1: ~28mV
V2: ~40mV
V3: ~30mV
V4: ~41mV

Obviously these readings would all seem to point the tubes being the real issue, could I just have been unlucky with my new matched set? I've also no idea how the original tubes are all so different when they are clearly marked as a matched quad also.

It's just making it all the more confusing! I realise that buying a new set of tubes could perhaps confirm the issue being with the amp or the tubes but that's an expensive experiment!

 

[Leo Diezel]

Hi Leo,

I tried reducing the volume, when I turn the master down (to approx. 8 o'clock) the red plating tube actually got brighter, channel 2 level was at around 10 o'clock.

I have checked those resistors, the 2k2 and 1R resistors checked out ok but the 220k was really hard to get a reading from though for some reason in both instances, the one near the bias pot and the one near the middle tubes.

Just to try something different I put the 4th tube from the new matched quad back in that was reading high at ~60mV, the reading is still high no matter what position it's in so it red plates but the tube in the troublesome position is now ok.The readings are as follows;

New set
V1: ~30mV
V2: ~31mV
V3: ~30mV
V4: ~58mV

After that I re-tried the tubes that were in the amp when I bought it (they have matched values written on them) and the readings are all over the place;

Old set
V1: ~28mV
V2: ~40mV
V3: ~30mV
V4: ~41mV

Obviously these readings would all seem to point the tubes being the real issue, could I just have been unlucky with my new matched set? I've also no idea how the original tubes are all so different when they are clearly marked as a matched quad also.

It's just making it all the more confusing! I realise that buying a new set of tubes could perhaps confirm the issue being with the amp or the tubes but that's an expensive experiment!

Yes I this seems very odd, 28mV to 41mV is quite a big difference for a matched pair, this may seems obvious but are you measuring it withouth any jack connected (other then speaker) and Master set at 0?

Thank you

 

[Hexjibber]

Yes I this seems very odd, 28mV to 41mV is quite a big difference for a matched pair, this may seems obvious but are you measuring it withouth any jack connected (other then speaker) and Master set at 0?

Thank you

Yes I have been checking the readings with the master at 0 and no other jacks connected.

To be honest at this point I feel like my only option is to buy more tubes and hope for the best, or at least rule them out. Both sets I have seem defective in some way so I can’t really reliably try and figure out what’s happening. At least if I put new tubes in they measure correctly I’ll know it was the tubes, or if the bias runs away on one again, I’ll know there is an issue with the amp and can take it to my tech. Expensive option but seems like the best way forward from here.

 

[Hexjibber]

Update on this; I received a new matched quad of KT77s today and have just put them in and given the amp a good workout. So far there's no red plating in the troublesome tube position but I am a little perplexed by the bias readings;

Starting from closest to the OT
1: 28.2
2: 30.9
3: 35.2 (the socket I was having trouble with)
4: 28.1

Is it normal for a matched quad to be out from each other by this much? As position 3 was measuring the highest, I set the bias to that one, is that OK? Just wondering if the amp is running a little cold overall as a result.

The good news is that the original issue seems to be gone currently and I suspect may have been due to the 1k resistor on that socket being 100ohm out from the others, it's now been replaced. This would seem to explain why tubes in this socket essentially were receiving higher current than the others and burnt out quickly, causing them to measure in the 60s once they'd had enough. What do you think, a reasonable explanation?

Would appreciate any insight anyone is able to give if possible.

Thanks,
Graham

 

[Leo Diezel]

Update on this; I received a new matched quad of KT77s today and have just put them in and given the amp a good workout. So far there's no red plating in the troublesome tube position but I am a little perplexed by the bias readings;

Starting from closest to the OT
1: 28.2
2: 30.9
3: 35.2 (the socket I was having trouble with)
4: 28.1

Is it normal for a matched quad to be out from each other by this much? As position 3 was measuring the highest, I set the bias to that one, is that OK? Just wondering if the amp is running a little cold overall as a result.

The good news is that the original issue seems to be gone currently and I suspect may have been due to the 1k resistor on that socket being 100ohm out from the others, it's now been replaced. This would seem to explain why tubes in this socket essentially were receiving higher current than the others and burnt out quickly, causing them to measure in the 60s once they'd had enough. What do you think, a reasonable explanation?

Would appreciate any insight anyone is able to give if possible.

Thanks,
Graham

Yes the 1K resistor was definitely damaged and It's a common issue when a tube break short, so that explain it!
As for the higher bias it may be due to the new 1K or 1R being slightly different in value, if you swap tubes that position is still the highest?

 

[Hexjibber]

Yes the 1K resistor was definitely damaged and It's a common issue when a tube break short, so that explain it!
As for the higher bias it may be due to the new 1K or 1R being slightly different in value, if you swap tubes that position is still the highest?

Hi Leo,

I swapped around the middle two tubes and the higher reading stayed with the tube so I think that one must just be a little out from the others. From what I've read 5-10mV between tubes is generally considered ok? Obviously the closer the better in terms of hum but generally speaking not an issue if they are out by that much.

I've just been playing the amp for a solid hour and have seen no noticeable red plating and the bias readings have stayed consistent so very pleased to have resolved the problem. Amp is sounding fantastic and is going to get it's first real outing at show on Saturday so very exciting!

Thanks for your assistance Leo, it is much appreciated!

Cheers,
Graham