The Steel Guitar Forum

All;
If you own a tube amp, have you ever had to change your power tubes? If you did, did you have to re-adjust the bias settings in your amp for the new tubes? I have acquired relatively inexpensive Mullard EL34 matched quad tubes for my Hiwatt. Currently I think I have the Teslas installed in the amp. Wondering if resetting the bias is absolutely necessary when I change the tubes. The guy who sold me the tubes seems to think it is necessary. In that case, I will have to look for a amp technician/repair person around here in Houston. Please share your wisdom. TIA

If it doesn't humm, and sounds OK, rebiasing isn;t really necessary. Mainly rebiasing is to get absolutely optimum performance.
Jim

I was about to say pretty much the same. But a bit more---proper bias can make the difference between an amp sounding ok but not great and an amp sounding real good. So you could be cheating yourself out of tone by running at incorrect bias. And you could possibly reduce tube life if not properly biased.
So the answer is: probably nothing blows up and nobody dies without biasing for the new tubes. But there are good reasons to tailor the amp (set the bias) to fit the tubes.

Although hum is a result of poor biasing, the main reason to bias a particular or new set of tubes is that they all will draw different amounts of current and run hotter or cooler depending on the bias. Each particular set of tubes should be biased for optimal performance and tube life. Too cool and the tone may be a bit to sterile, too hot and the tubes can burn up. Biasing is a very basic procedure for a tech. Dont mess with it yourself. The 400 plus volts can kill ya in a split second.

Brad Sarno

Buying matched tubes is also a good idea. I use a Bias King to adjust bias. I have not seen one set of 6L6 tubes yet that have had the same bias current! Every new set I get is a bit to a lot different. Eeven the "matched sets" I get are a few mills off from one another.
One thing to watch out for is red spots or red plates on the power tubes, that would be too high a bias.
Also few Fender amps actually bad a true bias adjust. The Blackface amps had a bias pot. 50's Tweed amps, early Browns and Blondes (60's), Silverface era, and most 1980's did not. Some has a bias balance adjustment.
The newer Hotrod series does, but the Blues Deluxe and Blues Deville amps did not have an adjustment. Most Musicman amps do not have a bias pot. Their bias voltage is usually fixed via a zener diode.
Best to check your amps schematic to be sure. Then biasing itself is another whole topic of great discussion. Many posts about the different procedures are available over on the Fender Forum.

I think proper biasing affects tone enough to consider it on that basis alone. I hate to think of all the tube amps that passed through my hands when I was younger and didn't realize this.

"I have acquired relatively inexpensive Mullard EL34 matched quad tubes for my Hiwatt."

Tell us more about where you got these "relatively inexpensive Mullard" tubes and how much they were.


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Artie McEwan

I got them through a very good friend of mine in CA who owns an electronic repair shop and I got them for $100 for these 4 EL Mullard tubes and I see the XF date codes on them and they are clearly Mullard tubes and they have been tested. I have seen some of the tubes go for over $300 in EBay. By the way I installed them last night in my Hiwatt and I didn't hear any hum or anything. And I compared these with the Teslas and there is definitely a distinct tonal difference between these tubes. Certainly Mullards seem to give a lot warmer sound than the Teslas. This friend tells me if I adjust the bias, the sound could even get better. So, I am wondering about that myself... Yes, Mullard tubes are getting to be rare and they so they can be very pricey.

How does the Hiwatt sound with the steel? I always thought these might be good sounding amps for steel (especially the 6550 versions).

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www.tyack.com

My Shobud does sounds pretty good even through a Hiwatt. It is quite different tone form a PV NV400 though. Hiwatt seems to have a lot more low end than the PV (my bass know is usually turned up all the way in my Hiwatt) regardless and you can certainly get the feel of the tube sound with the Hiwatt. It is just different sound altogether. Which one do I like better? I actually like them both, PV, to me is a very nice amp for the PSG, I must truely confess that. Hiwatt is different and would suit if I need a different type of a sound with my PSG. Tonight, I am going to play around more with my Hiwatt and PSG without any external effect processor gear. Would like to try to compare the raw sound. Will certainly let you know. So far I used a TC electronic G-Major processor with the Hiwatt. I was able to repro the Floydian sound, the tone from "One of these days" with my Hiwatt, a pair of EVMs, G-Major, Shobud and AnalogMan modded Tube Screamer. I had my Shobud screamin' nice and loud. I have been long time Gilmour fan, and I was pretty thrilled that I could repro his tone both with my Strat and with my PSG!!! I should say I am sort of addicted to this scraming tone with a lots of lows!

I never change my tubes. I don't want to risk losing the sound of my amp. If it ain't broke, don't fix it!

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Hiwatts sound great with steel, especially if you hook it up to a JBL. The overall sound character then falls somewhere between a Fender Twin and a Standel, depending on the Hiwatt model. Vox AC50 and a Selmer Treble'n'Bass heads are also similar sounding units and with a proper speaker work great for steel ( if you don't need to cause a hearing damage ).

I appologize if this is too technical...but some might find it interesting or informative.

And I may be a little biased myself but readjusting your bias control every time you change tubes is not really necessary assuming nothing is broken and that the amp was properly biased at some point in time.

On a car engine you have the idle control. Set it too high and you run the engine at full power but you'll drain your gas tank as your car sits in your driveway.

On tube amplifiers you have a bias control. Usually a single control for 2 or 4 tubes. Differences between the individual tubes cannot be compensated for by the single control. That is why there are matched sets sold. Does it make a big difference? Not really.

The difference between unmatched tubes is about the same as the difference between new replacement tubes and the old ones coming out. If the bias control was set properly and hasn't changed value you are "in the ball park".

I don't readjust my bias.. but I'm lazy.

If you set the bias properly when the tubes are new that should be fine for the life of your amp. As the tubes age and cathode emmission drops the bias current drops also. If you keep screwing with the bias to 'make it perfect" then the bias setting will not be correct for the new tubes you stick in next time.

If you leave the bias setting alone (assuming again that it doesn't change or get touched) then it should be good enough for the next new set you put in.

There are 2 extremes...too much and too little bias.

If you put a large ammount of negative bias voltage on the grids of the tubes the "idle" current drops to 0 milliamps. The tubes don't draw and power through the plate when you are not playing your instrument. But it acts like a light switch switch...turning on and off when the audio level exceeds the bias voltage. VERY DISTORTED! You would hear it. You would also notice that you had to really crank up the input level before any audio could be heard in the speaker.

Nothing bad would happen to your amp or the tubes...it would just sound terrible!

The opposite extreme is too low a bias voltage level.

If you removed the bias completely your tubes would turn a bright cherry red before the plates of the tubes warped or melted. If you have a 100 Watt amp with 4 tubes each tube is exceeding it's rated plate power disappation by generating tons of heat with not a sound coming out of the amp. Your tubes are 100% "on" and you now have a 100 watt heater attached to your instrument.

Proper bias typically lets the tubes "conduct" a small ammount of power when you are not playing. If the bias is set for 50ma (the total current draw of the 4 tubes combined) and four tubes have 500 volts on the plate then each tube is idling at about 6 watts (25 watts of heat total).

Oh...and if you are playing loud and long and you lean back and see your tubes are glowing a dull orange...that's not a bias problem....that's your amp working to make you sound loud.

If you are sitting there with the amp on and no sound coming out and there is ANY orange or red color on the plates of your tubes then you DO have a bias problem.

(sorry for the length of this post...I can be wordy at times)

Thanks for the info, Tom. Yes, I found some interesting online articles on biasing myself. Some of them even shows you how to do it and what you need to do it yourself. I wouldn't venture into doing that myself. A good friend of mine is a good analog engineer and I was thinking of taking his assistance to measure actually the current flow to make sure that there is nothing abnormal there. I am not sure if my Hiwatt has the pot to adjust the bias current. If not, I would have to change the resistor or diode depending on the circuitry to attain the optimum bias current. I am not going to rush into doing any of these yet as my friend is travelling at the moment till the beginning of Dec. So, my intent is to use my Mullards as is and let my amp do the rockin' like it does now. The year of my Hiwatt is 1979 I believe, but for the age, it still looks new and lets me rock nicely. Very well built amp I should say. when I played thru this amp the first time, I knew I found what I was looking for all these years!

Sounds like a very cool rig - which Hiwatt head are you using? I'd always thought they would sound cool with pedal steel, but figured that to get a nice warm tone they'd be so loud you'd make everyone's ears bleed...can you turn it up loud enough to get a little crunch from the amp or are you getting all that from the tubescreamer?

I have a THD Hot Plate power soak connected to this amp output which soaks the power from this amp. I have this amp bright vol turned up to about 65% and main volume turned upto about %80. I control the loudness with the Hot Plate. That way I get the real amp crunch tone without loosing my hearing. This THD Hot Plate has been very useful for this reason.

Oh, I forgot to mention that I have Hiwatt DR-103 one of those classic amps made in 1979.

Hello my friend... I found this to be of great use.. http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=10171&item=2573517386

Basil Henriques www.waikiki-islanders.com


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<SMALL>Steel players do it without fretting</SMALL>

http://www.waikiki-islanders.pwp.blueyonder.co.uk

Yes Basil...that is a cool device. It allows you to safely measure the current in each tube separately. No unsoldering etc.

Thanks for the info. I just wrote to this guy about his device. Here is the URL to that one: http://www.biasking.com/ Has anyone tried using this device? Looks cool indeed, but no other info on the website. Hopefully he'll write back with more info.

The Weber Bias-Rite works just fine for me.
I think it was about $25. You need a multimeter with this.
www.webervst.com