Blackstar HT-20 Biasing question?

Discussion - HT Venue amplifiers. Inspiration from Studio to Stage.
prsfreak01
Posts: 31
Joined: Mon Jun 21, 2010 12:00 am

Sat Jul 17, 2010 9:57 am

Hi Guys,

Any chance you can tell me if too much heat generated on my HT20 studio head a sign that the Tungsol EL34B power amp tube is not compatible with the amplifier power amp specification.

Do you think its a sign to remove it?. So like the tone and sound coming from my tungsol EL34B power amp tube and thinking twice removing it.

Any thoughts on this predicament. Thanks

toneseeker
Posts: 64
Joined: Wed Jun 02, 2010 12:00 am
Location: Australia
Contact:

Sat Jul 17, 2010 1:23 pm

prsfreak01 wrote:Hi Guys,

Any chance you can tell me if too much heat generated on my HT20 studio head a sign that the Tungsol EL34B power amp tube is not compatible with the amplifier power amp specification.

Do you think its a sign to remove it?. So like the tone and sound coming from my tungsol EL34B power amp tube and thinking twice removing it.

Any thoughts on this predicament. Thanks
Is it running hotter than before you changed the power tubes without biasing them,most tube amps run hot,that's where the tone is!,but if the plate voltage of the newly unbiased tungsol el34's you put in is wrong you are looking at blowing your amp up :o .... power transformers are not cheap if you cook those,my advice is don't put new el34 tubes into any amp unless it is biased correctly...oh yeh and there is no warranty when you change power tubes yourself and dont have them biased by a qualified engineer! Image

imagimotion
Posts: 1
Joined: Thu Dec 30, 2010 1:00 am

Thu Dec 30, 2010 7:58 pm

Just thought I would jump in here and add my two cents. Having built several tube amps and gone through the biasing
calculations for various tubes, I can tell you from experience that as long as you replace the power tubes with the
same TYPE of tube, you'll be fine if you don't rebias the amp. So if you have Mullard EL34 tubes and replace them with
another set of Mullard EL34 tubes, you'll be fine. The reason for this is that most tube amp manufacturers bias the power
tubes at 70 - 80% of the max plate voltage, which means the tube can still tolerate a lot more current before creating
an excessive load on the power transformer, which would cause saturation in the windings and destroy it. Not good.
However, since the manufacturing process of creating the tube for any given manufacturer is fairly consistent, you're
probably only going to see a variance of +-10% on the tube operating specs from each tube made. In fact, it's probably
much less than that.

So in the worst case scenario your tube would have a 10% increase in plate voltage, which would
mean it's biased hotter. With respect to tube life, it decreases obviously. But with respect to tone, it may actually
improve your sound. In fact, a lot of guys like to overbias their power tubes for this reason. The Hiwatts in particular
really sing when the KT66 is biased hot. If you get a tube that has a 10% decrease in plate voltage, then the tube is
under biased. The net effect is it will last longer than one biased hotter, but it also affects the tone. Cooler biased
power tubes tend to change the saturation curve when pushed to distortion. That means it may actually sound
cleaner than a hotter biased tube. Some guys like that as well. Again, it's a matter of taste. If you're playing metal
shred, you probably don't want your power tubes under biased.

When the manufacturer gives their bias specs for plate voltage, those values are the recommended settings for their
circuit design. If you bias your tubes to their recommendation, then you are guaranteeing that the power tubes are
running as the designer intended. Does that mean you have to keep them at that bias? Of course not. In fact, it's
probably in your best interest to play with the bias and see what works for you as a player. Often times you'll find that
the factory bias point is too low because manufacturers generally go conservative on the plate voltage to extend the
life of the tube. That, unfortunately, does not mean that the setting is optimized to get the best sound. It's usually the
optimal setting for best operating conditions for product longevity. Those are two distinctly different design goals.

The one thing to keep in mind here is the power handling capacity of the power transformer. Most manufacturers will
overspec the power transformer to allow for variances in the plate voltage requirements of various tube manufacturers.
As long as there is additional power handling headroom in the transformer, you can swap in different brands of tubes
and not worry about blowing up your power transformer because of the added current draw. It would be nice if someone
from Blackstar could chime in on the rating specs of the power transformer used in the HT-20. If you know that, you can
pretty much determine whether or not there will be any issues with swapping out your power tubes with different brands.
In general, I have found that as long as you use the same tube type, like an EL-34, the amp will still run fine without changing
the bias, even if you use a different manufacturer's tube. And the reason for this is that the tube type itself has an established
spec for power consumption that is fairly consistent across manufacturers. Again, it's the 10% rule. There will be variances in
the real world application, but it's generally within the operating guidelines of the amp itself, even if there is a slight increase in
current draw.

The area that ALWAYS requires rebiasing is when you change tube types. So you can't just throw in 6L6 tubes where EL-34 tubes
were and expect your amp to run properly without rebiasing. At best the amp would just sound like crap, and at worst you'd
exceed the max operating conditions of the tube and red plate it causing the tube to melt and/or frying your power transformer
in the process. Unless you have a provision on your amp to switch the bias setting for different tube types, don't EVER swap out
different tube types without rebiasing.

Last note. For Blackstar to say that rebiasing your amp after swapping out power tubes voids the warranty is just retarded. So
I guess what they're saying is you can replace the power tubes as long as you don't rebias it, but if you somehow damage the amp
by doing so they won't cover it because it was supposed to have been rebiased. Huh? This sounds like grounds for a lawsuit
if you ask me.

michael_dba
Posts: 49
Joined: Fri Feb 25, 2011 1:00 am
Location: California

Mon Apr 04, 2011 2:16 am

@imagimotion

Excellent info. Image

tlhamon
Posts: 1
Joined: Sun Apr 10, 2011 12:00 am

Sun Apr 10, 2011 11:12 pm

Imagimotion is pretty spot on. Most cathode-biased amps can handle a tube swap as long as it is the same type of tube (he may have been a little too specific in that you need to replace a Mullard with a Mullard). As long as you put a different set of matched EL-34's in, you will probably be OK.

Although, even a cathode biased amp is "self biasing" to some extent, if the new tubes fall at an extreme, it could create an issue. If possible, buy the new tubes from someone that has a tube tester and can tell you what the specs are (David Allen of Allen Amps will sell you a set of tubes with the correct values) and if they will work with Blackstar's numbers.

Having said all of that, Egnater's Tweaker says "pop them in, no biasing required!" I'm sure Bruce knows what he's talking about.

classicampguy
Posts: 37
Joined: Sat May 07, 2011 12:00 am

Wed May 25, 2011 8:56 pm

toneseeker wrote:hi,i have worked on many amps,mainly marshall jcm's dsl etc with el34 and all have had to be rebiased,also all fender with 6L6 or 6V6 have to be biased,the only ones i have had that didn't need rebiasing were el84 amps,and blackstar would have told me if the ht20 was self biasing,they told me what to set the bias at if i was to change power tubes,so as far as i reckon and this is from all amps i have owned that have had el34 tubes,THEY ALL HAD TO BE BIASED!

NOTE:THIS IS FROM BLACKSTAR,IF IT WAS SELFBIASING THEY WOULD HAVE TOLD ME:
here is a reply i received from blackstar customer support on the bias of the HT20:
question: does HT20 amps need to be biased if changing power amp tubes....eg EL34


Thanks for your enquiry regarding your HT Studio 20. The bias settings for the el34’s should be 11.5v across a R221 resistor but please be aware that performing any sort of technical maintenance on your amp will void the warranty. We also recommend that any operation such as this is carried out by a qualified engineer. I hope everything with your HT Studio 20 is going well!
Thanks for getting in touch.
Customer Service

NOW I DONT AGREE WITH THE WARRANTY PART AS IF YOU ARE A PROFESSIONAL GUITARIST ON TOUR AND YOUR POWER TUBES STUFF UP,WHO IS GOING TO REPLACE THEM IN A HURRY.MOST GUY'S WOULD CARRY A BACKUP AND NORMALLY WOULD KNOW HOW TO REPLACE AND BIAS THEMSELVES.
I DONT THINK BLACKSTAR WILL HAVE A SERVICE VAN DRIVING AROUND WITH A QUALIFIED ENGINEER!!!
Image
blackstar did tell you in the handbook, where it says that the power amp is cathode biased. while you can change the bias internally, it's not necessary

clif
Posts: 1
Joined: Mon Jun 13, 2011 12:00 am

Mon Jun 13, 2011 6:29 pm

The reply with "11.5v across R221" worked perfectly for me. Ohms law shows this is about 52mv which is about the rumoured spec. I pulled the chassis out and located R221 - a large white resister rated at 7w / 220 ohms and clearly marked on the board as R221. Placed my probes on either side and adjusted the labeled bias adjustment micro pot till it read 11.5v. I let it warm up for a few minutes and measured / adjusted again as the bias will move up a bit when warm. Thanks to this forum I was able to save time / money and took about 10 minutes from start to finish, not to mention taking control of my tone! BTW, I was VERY carefull so as not to hurt /kill either myself or the amp. Best advice I found was don't touch the circuit board when doing this as lethal charges are still in the caps. Not reccommending anyone else do this: don't take chances - again lethal voltages.

I was bit un-happy with Blackstar's responses for the TP's. Called the uk and was told they, by policy, do not give these out. Same with an email reply. But, it's probably a liability issue. I remember getting my fender super 112 late 80's and it came with the schematic!

soundsickness
Posts: 34
Joined: Tue Jul 05, 2011 12:00 am

Fri Jul 22, 2011 12:43 am

The question of whether to re-bias or not to when swapping power tubes it's been asked and answered many times here.

I'd love to find the final answer: most people say that it should be re-biased in order to change power tubes, some say is not necessary (and according to the Tube Store is not necessary, of course they just want to sell tubes).

I just change the stock Sovtek preamp tubes for a pair of JJ ECC83S and it was a big improvement tonewise: I'm not longer need to turn the treble to 4 o'clock, the mids at 8 am and bass at 9 o'clock. Now I can finally tweak around.

After hearing how the tone improved I really want to swap the TAD EL34's for Tung Sol ones but after all the discussion about biasing I guess I'll wait to take it to my amp tech.

theDogger
Posts: 4
Joined: Wed Dec 18, 2013 10:26 pm

Thu Jun 30, 2016 4:00 pm

clif wrote:The reply with "11.5v across R221" worked perfectly for me. Ohms law shows this is about 52mv which is about the rumoured spec. I pulled the chassis out and located R221 - a large white resister rated at 7w / 220 ohms and clearly marked on the board as R221. Placed my probes on either side and adjusted the labeled bias adjustment micro pot till it read 11.5v. I let it warm up for a few minutes and measured / adjusted again as the bias will move up a bit when warm. Thanks to this forum I was able to save time / money and took about 10 minutes from start to finish, not to mention taking control of my tone! BTW, I was VERY carefull so as not to hurt /kill either myself or the amp. Best advice I found was don't touch the circuit board when doing this as lethal charges are still in the caps. Not reccommending anyone else do this: don't take chances - again lethal voltages.

I was bit un-happy with Blackstar's responses for the TP's. Called the uk and was told they, by policy, do not give these out. Same with an email reply. But, it's probably a liability issue. I remember getting my fender super 112 late 80's and it came with the schematic!
Has anyone taken the PlateVoltage of the HT20 Head? Searched the forum but could not find it. I am looking at buy one and ended up here because I wanted to make sure that I was able to adjust the bias. I fell that these new amps that control the bias are crap.

Most of them keep it so cold that it stay in crossover distortion :o I prefer to set the bias to around 75-80% dissipation using a Bias Probe.

So if anyone is about to attempt this you measure the DC plate voltage from pin #3 of any power tube to ground, that will give you the plate voltage. You can then plug it into this formula

Image

Image

Tube Plate Dissipation/Amp Plate Voltage = milliamps X (70%-85%) = milliamps

So if you had 480 plate voltes @ 70%
Example: 25 watts divided by 480 = .0520 milliamps X .7 = .036 or 36 milliamps

Use a Bias Probe and MultiMeter set to Milliamps and set appropriately to what you have calculated.

Would like to see what the plate voltage so I can figure out what BlackStar is setting the bias to...Cold or Hot?

theDogger

(Images and info. is from Eurotubes.com wanted make sure to give them credit. I have been a customers of theirs for years and they are people to work with always will to help!)

theDogger
Posts: 4
Joined: Wed Dec 18, 2013 10:26 pm

Thu Jun 30, 2016 4:04 pm

Well Google is my friend...found this..this morning
by thephantum » Tue Apr 14, 2015 2:41 pm

I have seen this video before (as well as this method posted in multiple places) and IT IS NOT CORRECT. It will set the bias way to cool. I know this from personal experience as I own both an HT-20 and an HT-5R and have swapped tubes/re-biased multiple times in both.

Using the HT-20 as an example, it is a straight up, fixed bias amplifier. There is no "one size fits all" bias setting. Every amp is a little different and every tube is a little different. You need to measure the plate voltage, do the math to calculate desired bias current, measure and set the bias/balance, then rinse and repeat. The reason for this is as you adjust bias, the plate voltage will change slightly. So each time you adjust, you have to measure and adjust again until you get it dialed in.

Like most manufacturers, Blackstar sets the bias of the HT-20 amps cool from the factory. They ARE NOT setting them to 11.5v across R221, but they aren't measuring either. They are assuming a plate voltage of 300V and setting them to 60% of maximum plate dissipation, which is exactly 50mV at each tube. This is a very conservative setting. If plate voltage went as high as 350V it would be closer to the target...but it doesn't. It tends to live between 305V and 325V, depending on the tubes used.

These are from my own experience biasing my own HT-20H (and debunking the method detailed in that video):
- From the factory bias was set at exactly 50mV (out to 2 decimal points...exactly 50.00mV) per tube when measured across a 1ohm resistor at pin 8 with a plate voltage of 314V at pin 3. That's 63% of max plate dissipation which is cool. That setting, when measured at R221, was 11.7V...so setting it to 11.5 is going even colder. :shock:

- With the stock (Ruby) tubes, rebiasing it to 70% of max dissipation was 56mV at each tube, with plate voltages of 308V. This measured 13.35V at R221.

- With winged C's, biasing it to the same 70% yielded 54.7mV at each tube with 320 plate voltage and 12.25V at R221.
You can clearly see that different tubes have different plate voltages and bias settings. You can also see that, when setting the bias correctly on the HT-20 and then taking the measurement detailed in that video, it's not even close.

The HT-5 is even more sensitive to bias adjustment. As I said earlier in this thread, part of the reason the amp sounds so good is that they run the power tube on the edge of it's specs. There is a very narrow sweet spot where the amp absolutely shines (80-85% max plate dissipation). Go too high (hot) and the tube will redplate. Go too low (cool) and it will sound like a fart machine.

You are better off biasing the amp correctly. The tubes will live longer and the amp will sound better. You don't have to have a probe....you can use the OT shunt method and measure/set that way. A probe is just safer since you don't have to poke around at live components in the amp.
Answers my question and what I thought that the tubes are really cold. Warming them up should let this amp really come alive.

theDogger

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