Bias Rite readings on Mark IV

[Blueracer]

Ok first a dumb question. Which tubes are you measuring. If you are measuring the inner tubes with the switch in class A that could be your problem. Class A lifts the cathods of the inner two tubes which should send those tubes to zero current. If you are measuring the outer two tubes then yes this reading does seem low.

In terms of the master and output volume being low this is the more proper way to make the measurement. The idle current of the output tube is the current without an input. If anything you should set the output volume to zero to make this measurement.

The 475 and 400 volts for full and tweed power seems reasonable.

Have you tried your bias rite on other amplifiers successfully. If not it could be suspect.

 

[stokes]

The 35ma's you see on the box is what the tubes current draw was on the suppliers tester,and are only for their purposes.The low numbers you are getting dont surprise me,they seem extra low,but that is likely just because of those particular tubes.Like I have been saying forever,Mesa sets their bias way too cold.Any MKIV's I've worked on had 11 to 23ma's.If you measure the bias while playing you are going to get the wrong readings,you want to see idle current with zero signal,so volume on zero is best.

 

[Blueracer]

Ok I had to ask about which tubes you were testing. Do you have the original tubes you had in these positions. You might see what they draw. The comment stokes made about the difference in the characteristics of the KT77 is well taken. Also he is right about Mesa biasing things pretty cold. I imagine this is what you do if you fix the bias on your amplifiers.

Aside from the bias how does it sound.

 

[stokes]

"I imagine this is what you do if you fix the bias on your amplifiers."
A popular belief is that "fixed" bias means it is set and cant be changed.The term "fixed bias" means that a "fixed" amount of negative voltage is applied to the control grid,whether that be via a resistor or variable pot,and can and indeed is intended to be changed when changing tubes.What do the inner tubes measure?

 

[JOEY B.]

any ideas? the tubes are JJ KT77 marked 35 on the box (meaning will draw 35ma).

The plate voltages showed 475 for full and 400 for tweed which sounds reasonable.

Ask Bob what plate and bias voltages he sets the maxi-matcher to when testing the 6L6 and KT-77 tubes. Tell him that your amp, as per the schematic, has a plate voltage of 450-460 . The outer sockets have a -46v for the bias. The inner sockets have a -56v bias. This way he can select the closest plate and bias voltage on his tester to better your results in the MkIV.

P.S. When you get the current draw closer to the ideal range, that 475 plate voltage should ease down closer to 450.

http://maximatcher.com/spec.htm

 

[sbalderrama]

the most obvious answer would seem to be that those tubes are just the wrong specs for the amp, especially if a set of Mesa tubes check out better. Some tube vendors don't like the concept of fixed bias, but all it means is that instead of adjusting the bias for the tubes, you adjust the tubes to match the bias.

 

[disassembled]

Rabies,

I also remember you replaced a couple sets of KT77's because you were redplating them and would return them in a short amount of time. Maybe Bob was trying to allow you to keep the tubes running longer by giving you a colder set.


I wonder what the 6L6's are running at? Since your amp is out of warranty you might want to have a tech add two bias circuits (inner and outer) so you can dial in the tubes exactly as you want.


Greg

 

[Boogiebabies]

Something is amiss here. I don't see how a tube can be tested at say, 400 plate volts with -48 bias setting and run a lower bias number at 475 plate volts and -46 bias setting. Any higher plate voltage should cause the tube to draw more bias current. Every tube vendors number matching system means something different. Sadly, it seems a 35 in a KT-77 means let me sell (Or Hose) them to a Boogie owner who won't know. I mean, ****, what would you be able to get that tube up to even if you had a pot maxed out, 14ma? They should know what these tubes draw !!!
For instance, a KT-66 at -56 Volts may draw 35ma, but it will skyrocket at -45 to over 60ma.

I think you have been hosed with a set of tubes that is not fit even for an amp with a higher plate voltage and a full range bias adjustment. Most of these Boogie tube vendors rely on the adage that Boogies are supposed to be biased cold, or my # 35 would be perfect. Many years ago, I bought a quad of E34L's from a popular west coast vendor. I clearly explained the plate voltage and where I needed the tubes to be in natural plate current. I was sent a particular rating number (26) and was furious when they would not even reach 20ma with the bias pot maxed and 480 plate volts. Needless to say, I have never dealt with them again.
A week later, I had an order of old Svetlana's come on from Triode Electronics. I spoke to Ned and told him what I needed and what amp I wanted them to go in. He sent a #35 6L6 and a # 32 EL34. They read 35ma across the board in a simul-class amp at 483 plate volts and were all matched exactly to the ma. You have to find vendors who want to work with you and know what they are talking about. There are good ones, but the bad ones that send you B stock to make a dollar are BS and bad for the industry.

Stokes has your explanation dead on.

 

[JOEY B.]

I have seen a matched quad of winged =C= 6L6's that had 25mA written on the boxes give the following results.

Inner sockets of MkIIC+ simulclass -56v bias: 24.5-25.5mA - Bullseye!!!!

Installed in a 60/100 MkIIC+: 42-43mA, same plate voltage as the simul C+ (480-ish), but the bias was -52v. This is the stock bias for 60/100 Boogies.

Installed in my 60/100 MkI (450 plate volts and -52v bias) : 35mA

So what Ed says is absolutely correct. Find a vendor that will take the time to work with you on the nitty gritty details of your Boogie. Or put in bias pots, or buy Boogie branded tubes. 8)

 

[AJ_Newkirk]

I hate to hear about your bad experience. I'll make a note not to buy tubes from that place.

Has anyone checked power tubes from Doug's Tubes to see if he properly matches them to a Boogie? I recently ordered some from Doug to replace the stock tubes in my Mark IV...haven't had much of a chance to play the amp since installing the new tubes, but it sure seems different right off the bat. MUCH brighter now than it was with the Boogie tubes in the amp. I need to A/B the new tube swith the Boogie tubes to see which ones I like better I guess.

 

[JOEY B.]

Check this out.

http://www.groovetubes.com/SAG-AX7-MPI_P1723.cfm

As for the Maxi-Matcher, I talked to a couple of different tube vendors that use this unit to rate their tubes. Both of them said that they rate their EL-34 style tubes at 400v on the plates with the -36v bias setting. So if they were to rate a set at -48v bias, it will get you closer to what the amp will see. The -60v setting would work better if you were looking to buy 6L6's for the inner sockets. It would be nice if the Maxi-Matcher had 450 and 500 plate voltages available.

 

[6L6C]

I hate to hear about your bad experience. I'll make a note not to buy tubes from that place.

Has anyone checked power tubes from Doug's Tubes to see if he properly matches them to a Boogie? I recently ordered some from Doug to replace the stock tubes in my Mark IV...haven't had much of a chance to play the amp since installing the new tubes, but it sure seems different right off the bat. MUCH brighter now than it was with the Boogie tubes in the amp. I need to A/B the new tube swith the Boogie tubes to see which ones I like better I guess.

This thread has me a bit curious myself I think I may have to check on mine (after I get a bias meter).
As for Doug got a whole set for my Mark IV last year. It always sounded great, but after a set from Doug felt like I was missing something the last 13 years what a difference all past tubes were straight from Boogie. (But never again)
That reminds me also around the same time last year I had a cap job done on the amp (by “stokes”) and he was showing me some things. At the time I still had all Boogie tubes in the amp (straight from Boogie BTW not eBay or something like that) was using 6L6/EL34 combination and the EL34’s were running cold as hell. It was pathetic according to the math around 2 watts, however the 6L6’s in the inner sockets were fine. I did not need to change the Boogie tubes they were pretty new but at the time was looking for change, and happy I was in that mood.

 

[stokes]

I think what I've learned from all this (and other than the fact that Bob ***** can eat sh!t) is that Mesa's are designed for preamp distortion and Marshalls (the older ones anyways) were designed for power amp distortion.

I'm a power amp distortion kind of guy but the Mark IV still sounds badass to my ears, even with the eurotubes in them...

I prefer power tube saturation myself,but in the case of Mesa amps,they create so much distortion in the preamp,if you bias the power tubes too hot,or what would sound good in a Fender amp,your lead channel will get sort of ratty sounding.I have adjustable bias in my amps,if you do it to your Mesa,just be aware that you have to go easy on the bias.

"you'd think that mesa would offer adjustable bias versions (options) for all their amps...

fixed is convenient but not flexible like diezel."

If they did that they would have a warehouse full of tubes they would never sell.

 

[stokes]

1. how important is this balanced phase inverter preamp tube (that should be changed with a re-tube of your power section according to GT)? I usually don't mess with the preamp tubes unless there's microphonics.

2. Ruby 6L6GCMSTR

Extremely popular with Mesa users. These are simply rebranded Mesa STR 440's. These are best for those playing alternative, aggressive rock, with a mix of metal. Great low end sound, very articulate.

http://www.dougstubes.com/tonekits.html

re-branded Mesa 6L6 tubes? does that mean ruby has re-tested Mesa already tested and burned in tubes? anybody tried these? matched pair: $35.

The balanced phase inverter is a lot of hype.At least as far as guitar amps go.The idea is that your PI will deliver a balanced or even signal to both power tubes.Look at the schematic for your MKIV one plate of the PI has a 91k load resistor the other an 82k,there goes the balance in your PI.Add to that the fact that all components in the amp are +/- 10 to 20% tolerance and that "balanced" PI aint balanced anymore.

 

[Blueracer]

Ok first a dumb question. Which tubes are you measuring. If you are measuring the inner tubes with the switch in class A that could be your problem. Class A lifts the cathods of the inner two tubes which should send those tubes to zero current. If you are measuring the outer two tubes then yes this reading does seem low.

In terms of the master and output volume being low this is the more proper way to make the measurement. The idle current of the output tube is the current without an input. If anything you should set the output volume to zero to make this measurement.

The 475 and 400 volts for full and tweed power seems reasonable.

Have you tried your bias rite on other amplifiers successfully. If not it could be suspect.

I just typed a long reply and Vista just f%&# me over. f%&# that.

I was doing some calculations using http://www.webervst.com/tubes/calcbias.htm

and determined that my amp in classA with the new KT77's supposedly puts out 6watts for both tubes.

In order to get 25watts @ 475V, catchode current must be 52ma.

So how the hell is my amp that sick loud @ 6watts???

I doubt that the outer tubes are running in class A mode. Your own measurements on these tubes would bear this out. At a plate voltage of 460 volts and assuming a 70% of max plate dissipation the biasing for class AB would set the plate current at 38 ma for EL34/KT77 tubes or 45 ma for 6L6 tubes. None of your measurements are even close to this with the exception of the 68.1 ma reading. Note that if the plate voltage was still 460 volts for the 68.1 ma bias point this would translate into an idle power of 31 watts which exceeds the maximum rating for the tube. So if the outer tubes are really operating in AB mode you are getting more than the 6 watts output that you found on the Weber site.

 

[Boogiebabies]

you'd think that mesa would offer adjustable bias versions (options) for all their amps...

fixed is convenient but not flexible like diezel.

My first VH-4 had no bias pot. It was fixed resistors. My second had a pot due to public demand.

It seems that you are in a bad place with all these power tube readings.
It's a gradual learning curve and it takes a bit of time to really find what you are looking for in terms of power tube tone. When I started, it took me three days to decide on a bias reading because I would always think I would be happier at 50,60,70 or 75% MPD because of the different tone I got at different readings. When I finally settled on a bias % I would put the amp back in the case and put the chassis bolts and backplates on only to be unhappy the next day and pull it all back apart. It's the same with Boogie's. I got to the point where I vowed that if I got them between 30 and 60% I would work with what I have. Tone chasing is an obsessive and expensive habit. It's like crack to amp owners. Sit back, relax and let the tone find you. It's not about buying the $ 250 Phillip's 6L6GC's, it's realizing that you may prefer the Sovtek 5881 over them and living with it.

 

[JOEY B.]

Their 6L6's look much more "matched" than their KT-77 tubes (It's almost like the 68 mA KT-77 was close to shorting out itself). But looking at your email, they may have you by the nutsack on the 6 month / 1 mA claim.

JJ E34L outer sox (new)

pent/classA/full

08.6/08.0

It's definately not right for the amp, BUT matched by their standards.

 

[AJ_Newkirk]

Their 6L6's look much more "matched" than their KT-77 tubes (It's almost like the 68 mA KT-77 was close to shorting out itself). But looking at your email, they may have you by the nutsack on the 6 month / 1 mA claim.

JJ E34L outer sox (new)

pent/classA/full

08.6/08.0

It's definately not right for the amp, BUT matched by their standards.

Would that set of tubes bias correctly in any amp tho? I think any "standard" should include an acceptable range of use and function.

Either that or slap a green sticker on the box that says

"Eco-Friendly night lamps. One will be as dim as the other."

I am glad rabies brought this issue to the board. I am now planning on buying a Bias Rite (or building a similiar tester) and checking my tubes. If they don't put the amp in the right range I hope Doug's Tubes will work with me to get a better suited set the next time.

 

[stokes]

"So has anybody noticed "little sustain and a harsh, scratchy tone" with their cold-biased Mesa amp?"

You wont likely notice little sustain from a Mesa,all the sustain is generated in the preamp.Harshness you will get,but it may not seem so bad till you find warmer tubes. As cold as Mesa sets the bias,the tubes are still not in the zone where you get crossover distortion issues,it takes a lot colder tube to get to that point.That is exactly why the "crossover method" of biasing sucks.

"so why does mesa use the term "fixed bias" instead of cathode bias?"

Because as I keep saying,fixed bias doesnt mean non-adjustable,it is absolutely meant to be checked and changed with every power tube change.The term "fixed" means a fixed amount of proper negative voltage is applied to the control grid.If this neg. voltage is not "proper" then you must "fix" it by adjusting the fixed resistor or replacing it with a bias pot.If Mesa didnt buy huge lots of tubes,check them for current draw and cull the ones that dont fit their bias point,you would be adjusting the bias just like we did in the old days,there is absolutely nothing special about the bias circuit in any Mesa amp,it is no different than amps made way back in the early days.Mesa amps are not cathode biased.Cathode bias is meant to be left alone,the cathode resistor that sets the bias point keeps the current draw in check,to a large degree.It can be adjusted but in general doesnt need to be.I do like to use an adjustable resistor on all my cathode biased amps,and reset it from time to time,as the tubes drift with age,but admittedly,I am kind of obsessed,and have been for many years,with having the tubes biased to my liking.

 

[HalfDeaf]

rabies wrote:

yeah I think I get really bored with playing guitar sometimes and start obsessing about the amp tech stuff.

This is precisely when I have learned to put the guitar down, turn off the amp, and walk away. The madness has eased somewhat by trading my mkiv for a single channel VHT, but I still am confronted by the urge to remove the back panel and start swapping tubes. Must resist.....