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KT88 replacement

Former Rogues

Former Rogues

Sep 22, 2016
92
65
Oslo
www.facebook.com
#1
Hi folks,

I have some faulty power tubes in my Laney Nexus tube head. I figured I might as well replace all 8 of them in one go. Not sure if that is necessary or not. Coming from solid states for many years, or backline gear, this is my first tube change so I would appreciate advice on brands, if I need to change all 8 KT88 tubes and if I will need to bias the amp (I assume I will).

I am living in Norway, so I face two problems, 1) expense, and 2) difficulty sourcing tubes (see problem no. 1 for European or transatlantic shipping).

Anyways, suggestions on quality tubes, or advice from Nexus Tube users would really help out.

Thanks!
 
#4
A qualified tech can evaluate the existing tubes to identify the actual cause of the failure (it may not be the tubes by supporting circuitry) as well as the condition of the tubes themselves (including any information needed for matching).

I see many perfectly good tubes replaced at great cost and no real world benefit. In general, any time you replace a power tube, you run the risk of a defective tube failing, possibly damaging the amp itself.
 
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#5
Thanks agedhorse.

Techs are surprising hard to come by. I have reached out to some, and only one came back to me about how booked he was. So not much help there. I am NOT going to mess around in the amp and kill myself, so a tech needs to bias it for me. Just thought some advice on how many tubes I might need to buy so I don't face a big upcharge.

The Nexus has an LED indicator identifying the failing tubes, so if anyone is familiar if I can just change the dying tubes instead of all, that would be good info.

Do they need to be matched if I replace a pair?
 
#6
A (good) tech will identify failed tubes first, then verify no collateral damage, and then order the number of tubes needed.

Once the tubes are installed, then the tech will bias and verify proper current sharing.
 
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#7
Former Rogues said:
Hi folks,

I have some faulty power tubes in my Laney Nexus tube head. I figured I might as well replace all 8 of them in one go. Not sure if that is necessary or not. Coming from solid states for many years, or backline gear, this is my first tube change so I would appreciate advice on brands, if I need to change all 8 KT88 tubes and if I will need to bias the amp (I assume I will).

I am living in Norway, so I face two problems, 1) expense, and 2) difficulty sourcing tubes (see problem no. 1 for European or transatlantic shipping).

Anyways, suggestions on quality tubes, or advice from Nexus Tube users would really help out.

Thanks!
Click to expand...

I believe TAD does a good job testing and matching tubes.
Tube Amp Doctor Onlineshop
 
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#9
Former Rogues said:
That is what I assume, but I guess I just haven't found a good tech yet.
Click to expand...

I took a look at the owner's manual for your amp and it does not include a bias procedure. The manuals show TAD branded tubes.

I did find a bias schematic for the amp. It actually shows most of the circuitry for the output section. Each output tube is fused and has a blue and red indicator LED.

The amp does have internal trimmers for bias and bias balance, but it also has trimmers for heater hum balance, phase inverter balance, and also a bias control for the circuit that powers the LEDs. So with five trimmers, you really need to know what you are looking at.

The notes on the schematic say the red LEDs indicate a fault and the blue LEDs indicate signal. I suspect bias is set somewhere between where all of the red LEDs turn off and all of the blue LEDs turn on. Unfortunately I can't tell you the exact procedure, so my recommendation is find a tech who knows the amp or can get the tech manual from Laney.

Owner's Manual and Bias Schematic attached if you want them.
 

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#10
Former Rogues said:
Thanks agedhorse.

Techs are surprising hard to come by. I have reached out to some, and only one came back to me about how booked he was. So not much help there. I am NOT going to mess around in the amp and kill myself, so a tech needs to bias it for me. Just thought some advice on how many tubes I might need to buy so I don't face a big upcharge.

The Nexus has an LED indicator identifying the failing tubes, so if anyone is familiar if I can just change the dying tubes instead of all, that would be good info.

Do they need to be matched if I replace a pair?
Click to expand...

The tubes may not actually be failing, rather they may have drifted a little and just need to be rebiased. In the event one of the tubes fails, the respective fuse should open and this will cause the respective red LED to light. However, the red LED may also light if tube current in one of the tubes is too low....I can't say for sure.

Ideally all of the tubes would pull exactly the same amount of current, but in real world conditions this rarely occurs. As long as the current between all tubes can be balanced within about 15% they are considered reasonably matched.

This amp organizes the eight output tubes into two groups of four, and has a bias balance control so the total current pulled by the first set of four tubes can be matched against the total current of the second set of four tubes. If the tubes are not balanced within 15%, it may be possible to re-sort them in a different order to get a decent match.

So to answer your question, you don't need to buy the tubes in matched pairs because the tubes are grouped in fours in this particular amp. If one tube goes bad, you either need to find another tube that is reasonably close enough to your remaining good tubes, or you need to buy one or two matched quartets.

Ideally all eight tubes would be perfectly matched, but it's okay if you have two quartets that have similar operating characteristics. Some techs have a collection of old tubes that still test good, that can be matched up with your remaining good tubes. The best decision will depend upon how much you want to spend and how trashed your tubes are.

If you find a good tech, IMHO it's generally better for them to buy the tubes in case something goes wrong. The tubes should carry a warranty and the tech should also warrant his/her work. Usually tube warranties range from 90 to 180 days. I believe TAD tubes carry a 180 day warranty.

If you wind up buying a new tubes, ask the tech to give you back any good tubes, and if possible group tubes that are still reasonably matched together.
 
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#11
Wasnex, thank for the extensive info.

I did find the manual (bought the amp used) and the bias manual, although I really can't read the schematic.

It is good to know that I may be able to save tubes, and that it might be a bias drift issue. The red LEDs are lit on 2 tubes, but if it is a bias drift...even better.

Basically, the amp has a subtle speaker pop going on, and I have swapped preamp tubes, but the pop continues. So my assumption is the power section.

I will continue searching for amp techs who can help sort me out.
 
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#13
mesaplayer83 said:
I can't add much to this discussion other than I had my Mesa 400 fitted with a matched sextet of JJ KT88s from Eurotubes that was spec'd for my amp a few years ago, and they have been 100% trouble free...
Click to expand...

I am not sure how much mileage this amp had on it before I bought it, so I am sure when I fix the problem I should have no problems for some years.
 
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#14
Listen to these guys- agedhorse and Wasnex. They always give sound advise. I remember having an old tube V4 that had different brand output tubes. It ran perfectly for many years. The tech matched their VALUES, so that says there is much more to it than just plopping in a matched set of tubes. Maybe the popping is a bad resistor or something in the circuit that is taking out the few tubes mentioned. It sounds like you are trying to side step a tech visit, but you may be making a worse mess by not.
 
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#15
Former Rogues said:
I am not sure how much mileage this amp had on it before I bought it, so I am sure when I fix the problem I should have no problems for some years.
Click to expand...

I forgot to mention a couple of things: my Mesa 400 was made in 1985, so it was well used when I got it, but appeared to be fully functional... I cleaned it up thoroughly, then handed it over to a known and trusted tech to look it over, add an adjustable bias circuit, then set it up for the new KT88s... It was money well spent - I've had a few years of trouble free service from it, and expect many more...
 
#16
Former Rogues said:
Wasnex, thank for the extensive info.

I did find the manual (bought the amp used) and the bias manual, although I really can't read the schematic.

It is good to know that I may be able to save tubes, and that it might be a bias drift issue. The red LEDs are lit on 2 tubes, but if it is a bias drift...even better.

Basically, the amp has a subtle speaker pop going on, and I have swapped preamp tubes, but the pop continues. So my assumption is the power section.

I will continue searching for amp techs who can help sort me out.
Click to expand...

The popping could be a tube issue, or it could be unrelated.

If the popping is one of the tubes, it will most likely cause the LEDs associated with the tube to flicker.

If the fuses are accessible for the two tubes that have the red LEDs displayed. You can pull the fuse and inspect to see if the fuses are blown. I believe the circuit is setup so the red LEDs light when the fuses blow. If the fuses are blown, it's likely the tubes are bad.

The good news is it sounds like you have up to six tubes that the amp considers in range. So you may get by without needing to buy a complete new set of 8 tubes. However, you may consider buying a complete new set and keeping the remaining good tubes as spares.
 
#18
Rockbobmel said:
Can't he also pull 4 tubes (2 bad and 2 remaining good ones)- to run the amp at 200 watts?
Click to expand...
Without knowing the specifics of plate matching and how the bias circuit is set up to measure plate current, it could result in improper operation.

If the bias circuit sums plate currents, pulling 4 tubes could cause the circuit to overbias the tubes by 50%.
 
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#20
agedhorse said:
Without knowing the specifics of plate matching and how the bias circuit is set up to measure plate current, it could result in improper operation.

If the bias circuit sums plate currents, pulling 4 tubes could cause the circuit to overbias the tubes by 50%.
Click to expand...

See the schematic in post #9. Each tube is individually fused and has it's own circuit to monitor cathode current and display the tube's operating status.
 
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