Can you tell when your amp was made?

[smokinjo]

I am looking at buying a used ht club 40 but would like to know when it was made. I should be able to tell by the serial number but I do not know how to decipher it. Any help would be appreciated.

[ht40guy]

HI, good choice! I bought a HT40 new in Dec 2010, it has the serial 201009AD1518. Im assuming the first digits refer to Sep 2010...if not Id be interested in what they are as well. As for the AD1518, Im guessing thats just a manufacturing serial number. If you can get to the speaker, you may find a label stuck onto the speaker with various information. The Celestion site has a chart to identify the age of the speaker, mine has (amongst other numbers and letters) 02GV. The chart says this speaker was manufactured on the 2nd July 2010. Various other components like transformers and pots have these date codes too, but all they show is when that component was made. Hope this helps,

Glenn

[DrGit]

I am a happy Blackstar HT 60 Soloist owner...i bought it from the original owner after a months use. Its a 2010 model that i have had at least 3 years now. I came here in hopes of finding out how to bias it with a multimeter. I just replaced the preamp tubes with reissue Tung Sols, and replaced the speaker with a Redcoat Governour. What a difference already. I highly recommend both. Anyone's help would be awesome. Cheers Pete

[DrGit]

2010

[thephantum]

I believe the main difference between the Soloist and Stage versions is that the Soloist has test points and the Stage does not (but I could be wrong about that). When you look at the PCB near the adjust pots, there should be two test points: TP1 and TP7. Put the red probe on TP7 and the black on TP1 to measure the bias.

Keep in mind that most amp manufacturers (BS included) set bias very cold from the factory. That video says 50mV, but you really don't know if that's correct unless you read the plate voltage and then do the math.

*** DISCLAIMER - All of the following measurements need to be taken with the amp powered on. There are lethal current levels flowing through these amplifiers. If you touch the wrong thing, best case is you'll get a good bite of live electricity....worst case, you'll be electrocuted. ***

So, to bias it correctly, there's actually two measurements you'll need to take: the first is Pin 3 of a tube socket to ground, which is the Plate Voltage.

From there you need to divide the desired plate dissipation of the tube by the plate voltage. The maximum plate dissipation of an EL34 tube is 25 watts. A general rule of thumb is set bias to 70% max dissipation, which won't work the tube too hard, but it will saturate nicely at volume.

So the math would be: (25 x 0.7) / Plate voltage = bias in volts. Then multiply that by 1000 to give you the mV reading. So for example, if you measure 500v at pin 3 and want to set it at 70%:

((25 x 0.7)) /500) *1000 = 35mV

Now, that 35mV is for one tube....so you need to double it. The bias in this example should therefore be 70mV to run an EL34 tube at 70% dissipation at 500V.

[DrGit]

Thanks for taking the time to explain to me the correct way to bias. Not sure if you happened to see the Blackstar Stage 60 video on youtube i was talking about, but the guy doing it is also a newbie at this like myself, here's the URL http://www.bing.com/videos/search?q=you ... BFA9BD5BC4 ....and what he did was put the red multimeter lead on the D27 resister and ground the black on a screwhole, sets his multi to mV and adjust to 70, which he said w/b 35 each tube

[thephantum]

I saw the video, but here's the problem with saying just set bias to "xyz" mV:

As I stated before, an EL34 tube has a max plate dissipation of 25 watts. Let's assume you want to set that tube at 70% of max dissipation which is 17.5 watts. If you have an amp that has a plate voltage of 500 volts, that works out to 35mV per tube or 70mV bias for both tubes. But let's say your plate voltage is closer to 350 volts. That's going to work out to 50mV per tube which is a 100mV bias for both. If you set that amp to 70mV, the tube is going to be running at around 42% of max dissipation, which is WAY cold. The amp will most likely sound like crap.

Granted, that's an extreme example. However you can take two of the same exact amps, and there will be variations in the plate voltage between them. In addition, different tubes of the same brand will have slight differences and cause even more variations. Different brands can have even larger differences, causing bigger variations.

Referring back to the video, 70mV would be the setting for a pair of EL34's running at 70% in an amp with a plate voltage of exactly 500 volts. But what if your specific amp is running at 490 volts? In that case 70mV is just a little cooler then your target. Yes, the difference is slight but a little too hot or a little too cold will impact how the amp sounds.

The point here is that setting 70mV on an HT-60 might get it in the ballpark, but it's always best to measure and do the math to set the bias properly for the specific tubes in your specific amp.

[DrGit]

cool, i see your point...i have a pair of matched EL34's Mullards i was thinking of dropping in this..i think i'll save the Matched NOS Winged C's for another time. I also have found this on the web
http://firebrush.co.uk/blackstar-solois ... de-part-2/

it shows your mentioned TP1 and TP7 and the bias adj screw, which is cooler then the HT Stage 60, as its closer together