EL34 Bias Help on New Set of Tubes

[OldGuySGFusionPlayer]

For anyone very knowledgeable on Tubes , Specs of Tubes and biasing, I sure could use some advise. I have been fixing my JCM 2000 TSL 100 for a while now and finally I can say that all the noise problems I have been having all were due to worn out tubes.

Here is my situation and I am very concerned. I received 4 Svetlana EL34 matched tubes today with an apparent mA rating of 54.

This is the first time I have ever gone this far in doing a complete Tube change. I have researched the hell out of how it is done and early in my findings I read that the Voltage taken from both pairs of EL34 is supposed to be 90mV.

So after putting in the new tubes I realized that I was dealing with a new much more powerful animal so I realized that I would have to use Milliamps to bias it. So I set both pair to what they have settled in at which is 52 milliamps.

My amp is SO Friggin loud now and the heat that is being dissipated seems to me dangerous. I am concerned that I am about to ruin my Amp.

I have to admit. I am just a Player with this amp. I have never had to service it ever. So I dont remember how loud it was and what the heat situation was mainly because the back steel guard has always been on it so I could never feel inside how hot it was.

The Amp sounds better than I can ever remember it. I dont have any crackling sounds or other noise now.

My questions are:

When measuring the mA from the 3 prong bias probes are 2 tubes being measured in series?

If so then per the tube spec sheet the readings i am seeing would make since. The spec sheet says each tube has a normal current rating of 1.6 amps.

The tubes dont appear to be overloading.
There is only so much you can adjust the bias with the two screws.
So I set it according to the number written on each tube box.

I would think that the amp would start making popping loud noises if it was in trouble.

The chassis is so hot I am very worried. But again I dont know if it was originally supposed to be like it is. It damn sure sounds great.

Any help would be much appreciated. Like I said on each EL34 box it says 54.

 

[jd267]

One prong is for two tubes other prong is for the other two . Center prong is ground

 

[StratoMarshall]

If you want to hear it sound better (just my opinion and do it with my TSL100), instead of biasing at 90mv, back it down to about 85. If your amp is un-modded, the slightly cooler bias will get rid of the fizzies and really create a warm tone with killer growl.

 

[Fusedbrain]

The spec sheet says each tube has a normal current rating of 1.6 amps

I believe this is the heater current draw for EL34's and is not related to the bias.

Now, when you measure from one of the outside prongs to the center prong you are reading the bias of 2 tubes on that side. You are actually measuring the voltage drop across a resistor, so make sure your meter is set to milli-VOLTS, not amps. The resistor is sized so that milliamps = millivolts under OHM's law.

If you are reading 52 millivolts, your a little cold, but if the amp sounds OK- no prob. If your reading 104 mv ( 52mv for each tube, then your biased a little too hot, and should adjust the bias until your reading 90mv.

If your meter is set to milliamps, I have no idea what your reading, so switch it to volts and report back.

One other thing to note: The matching number that tube vendors put on the tubes is only a relative indication of how much plate current those tubes draw ON THEIR TEST EQUIPMENT vs other tubes measured on the same equipment, and won't mean much when the tubes are installed in YOUR amp. The bias measurements you make on your amp are what matter.

Hope this helps and glad to hear your amp is rockin' again!!

 

[Fusedbrain]

And StatoMarshall is right, 90mv is on the high side. A little lower might be better. My JCM2000 DSL 100 has a similar biasing scheme and I run it around 80mv.

YMMV

 

[OldGuySGFusionPlayer]

Thanks for replying.
Yea the problem is that I cannot lower the Voltage for either pair below 100+ mV. Thats why I am kinda worried. I was able to bias my original bad EL34s to under 90 mV.

What do you do if you cant adjust even down to 90mV?

 

[Fusedbrain]

Oh Oh, I was kinda worried about that.

Even though I said that the number on the tube was only relative etc, that 54ma rating is really high for EL34's. Most of the EL34's I see are in the 28 - 36 range.

Unfortunately, it sounds like your out of range on your bias pots and it will require a resistor change in the bias circuit to get those tubes in spec. However, if you swap the resistor, chances are that the next set of tubes you buy would now be too cold. Any chance you could exchange the tubes for a colder set??

If your stuck with the tubes, I would probably suggest a trip to a qualified tech to sort this out.

This really kinda sucks and hopefully the tube vendor will help you out.

 

[RickyLee]

90mA/90mV means 45mA/mV on each EL34. That is too hot of a bias unless your wall voltage is quite low at 110VAC, which I am figuring it is not that low. Actually I should ask if you are in the U.S. or North America?

Your bias should be set according to what your plate voltage is.

 

[OldGuySGFusionPlayer]

I live in the USA and I really cant see myself sending back the tubes again. That would be the second time. I am out of money now.

So the questions are:
What could happen to me running this power? Will I damage my amp?
It sounds fantastic. Better than it ever did. Right now I am doing the revolving Preamp tubes with a Sovtec WB which I am putting back in V4 in a few minutes.

Yea modifying the Resisters just isnt an option. So can I ask for a specific Current rating for a Quad of EL34? I cant blame the Tube Store for this but if it is not common that an EL34 should be that high its kinda crappy they sent me these. Ill have to call them I guess to find out what the deal is on that.

Am I gonna burn my amp up that is the question?

 

[mickeydg5]

Bias drift board problem? Maybe.

Unless that tube set just runs really hot.

 

[OldGuySGFusionPlayer]

I just made my final decision on my preamp tube order and rebiased the EL34 set.

I set the voltage as low as i could which is 109mV. Bummer.

So what do you think can happen if I run my amp like this? It sounds very nice now. No inferior noises and very clean. No hum. The tubes seem overly hot. I am worried. It is close to 90mV you think?

I am just worn out and to find out that all my problems ended up being tubes and it would seem Im still in a jam is too much for an OldGuySGPlayer if you catch my drift, no pun intended : (

 

[Fusedbrain]

Hey did you get those from the TubeStore in Hamilton Ontario??

If so, get in touch with them and tell them what's going on. They are good guys. I'm really surprised that they would send you a set of tubes that hot for a TSL. Usually, when they have tubes that draw high plate current they discount them as they acknowledge that the bias circuit will need to be modified. For example, if you check the website www.thetubestore.com - Electro-Harmonix 6CA7EH (High Plate Current) Audio Tubes they have discounted EH6CA7's that draw high plate current. I actually have a set of these to swap into an amp that runs KT66's so they work perfectly with the bias range of that amp, but they would be way too hot for a normal EL34 amp.

Also, why would this be the second time sending the tubes back?? What happened the first time??

I know the amp sounds great, but if you run those tubes like that, they won't last very long at all and could cause some additional damage to the amp when they fail.

The other option if your truly stuck with those tubes might be to hold off on buying preamp tubes and get a proper set of EL34's. You maybe could save some money buy buying JJ's or Ruby's and you won't run the risk of damaging your amp. The JJ's or Ruby's will sound fine in your amp and are the least expensive tubes ( I think ) I would suggest ordering a set rated around 30 - 33ma

Sorry your so frustrated and I hope you get it all sorted out!!

 

[jd267]

Thanks for replying.
Yea the problem is that I cannot lower the Voltage for either pair below 100+ mV. Thats why I am kinda worried. I was able to bias my original bad EL34s to under 90 mV.

What do you do if you cant adjust even down to 90mV?

Need to swap resistor in bias supply .

 

[jd267]

Bias drift board problem? Maybe.

Unless that tube set just runs really hot.

I had a tsl once that would red plate tubes in seconds. Bridge rectifier I believe? It was used from GC so I returned it .

 

[OldGuySGFusionPlayer]

Man I am just worn down. I have been reading every thread and post I can on this Forum about the TSL 100 and Tubes, Tubes , Tubes. I have never ordered tubes like this. I am Praying extremely hard that I am done right this time. It will be my 3rd try with a new set of EL34 tubes. Maybe it will be the charm. What can you do. I am not mad or anything like that, just sad and physically drained. The one Great thing is I now know that all my woes have simply been a worn out 10 year old set of 8 tubes.

The stuff some of you guys as saying like the discount tube comment is just alarming. What can I do. There has to be someone really cool there that will do me right you know? I am a super nice guy.

I took those hot tubes out and yes I was told I shouldnt have been sent those tubes and advised not to use my amp with them. So I just got through putting my original tubes back in so I can play over the weekend.

If anyone here has any connection to the Tube Store maybe you can help me out. This time I specified in a comment box what Amp I have and even the Voltage and Current specs that apply to my amp.

The customer service I am getting couldnt be any better. I am remaining positive through all of this.

Thanks

 

[Fusedbrain]

OK so you talked to the TubeStore and explained the situation and they are working with you. That's cool!!!

My only connection to the TubeStore in Hamilton is that I've bought a TON of tubes from them over the years and their customer service is stellar. That's why I was confident that if you just talked to them and explained your situation, that they would help you out.

Just stay positive man, I'm sure this is all gonna work out for you.

 

[jd267]

If it does not work out just have a tech swap out the bias resistor to pot . Very simple man

 

[USAPatriot]

When I order power tubes I always send a note along telling them what amp it's going in and what bias range I'm looking for. The one time I didn't do that I got a set that would not bias below about 80 on one tube and 110 on the other. That obviously wouldn't work and I learned my lesson.
Tell them you need a set that will bias down to around 40 each. That will give you a little headroom to go a bit higher if needed. Biasing to mV rather than mA will always be more stable. -Rod-

 

[OldGuySGFusionPlayer]

I have learned a great deal about EL34 tubes and what My TSL 100 Head is supposed to have.

What is disturbing to me is that I have learned that the Bias Point and Headroom(when break up occurs) are distributed along a Bell Curve.

So they sold me 4 tubes that are significantly rated to the Right side of the Curve and Do NOT fall inside the Curve.

It must have been in another thread where I read someone sort of jest that with such a high bias point that would give you "Hell of a Headroom".

When I read that I searched and now understand the relationship that the Bias Point and the Curve has as it pertains to Breaking Up(Overdriving, Distortion)

So say I am sent 4 tubes with a Bias Point of say 25 then this will fall to the left of the curve out of the Bell (Optimal placement should be just to the right side of peak) and will indicate the opposite in Headroom.

This leads me to an interesting thought when it comes to selling tubes. I really hope these people didnt intentionally sell me tubes that obviously would not be wanted by a Player with a Marshall like mine. That goes for a DSL as well.

I guess I am going to find out this week.

I hate to even say this but this guy at another store suggested that I would want an EL34 Matched set in a Bias Range in the upper teens. He knew I didnt yet understand the Bell Curve and how Tubes are rated.

What do you think?

I also know that even though the people I have talked to act like they are not even Guitarists it turns out they really are. So they do have amps and they are familiar with say my amp the TSL 100. Then they really do know what Bias Point I need. Also since some of these places sell "Tube Sets" by Amp selection then the Bias Point MUST be a factor in selecting the tubes for said order.

Unfortunately for me I have to conclude that getting tubes for my amp is and will never be easy because of all this I have said.

Furthermore I know that many on this forum have lives and wont drive themselves crazy to figure all this out if they dont know it yet.

No one here has explained the Bell Curve and Headroom and that unless the Bias Point is most likely to the right of Center (40-43) I will not be able to comfortably reach 83mV to 87mV per pair.

What a friggin bummer. I have played my entire life and am too old for this man. Seriously.

 

[Slammintone]

What's the B+ measurement? I take that measurement which on my amps has been anywhere from 450-475vDC and then I bias my EL34s to about 60% max plate dissipation to start and usually that's where I finish. To do that you figure the EL34s max plate dissipation is 25watts x 60% = 15watts. Divide 15(watts) by 450(vdc) and you get 0.03333333 or 33ma. That's a good starting point. If the amp has a higher B+ I might start even cooler, say 50-55% and see that the tubes arent going to redplate. If I'm dong it wrong someone please feel free to correct me, but this has worked fine for me for years.