Bias tutorial

for the DIY ST35, the Dynakit and every other PP EL84

Bias tutorial

Postby coolhandjjl » Sun Dec 30, 2012 3:39 pm

Before I make the jump to a tube amp and build an ST35, is these a bias tutorial somewhere to see if I am capable? All I have is a digital volt meter from RadShack. I've built crossovers, speaker cabs, done work on electric guitars like replacing pots and pickups. I'm sure I can build the kit, but I have no knowledge of setting bias.
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Re: Bias tutorial

Postby paart » Sun Dec 30, 2012 10:19 pm

If you feel confident that you can build a kit, and own a digital voltmeter, you should have no problem setting the bias. You didn't state which ST-35 kit you are planning to build, but I assume that it's the diytube ST-35, since there are no bias adjustments in the Dynaclone ST-35.
The manual for the diytube ST-35, which can be downloaded on this site, gives explicit, step-by-step instructions for setting the output tube bias. This involves first measuring resistance from the output tube cathodes to ground, then measuring voltage for the final settings. It's well explained and quite easy.
I'd suggest that you download the manual and read it thoroughly before starting assembly. Good luck!
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Re: Bias tutorial

Postby coolhandjjl » Mon Dec 31, 2012 9:58 am

I downloaded a manual, I believe it is for the diytube one. Is that the same one as the triode electronics one?

I was actually considering the DynaParts version as I preferred the form factor of the chassis better

Can the Gillespie performance mod be done to that one? I thought I saw some photos in a PDF that depicted it squished in there.

My voltmeter is digital. If I understand it correctly, this is done from underneath with some alligator clips, and I adjust some trim pots to get desired readouts on my meter?

I play electric bass, my preamp has three tubes, 12au7 & 12ax7. The poweramp I use for that is ss, so I never got into bias adjusting. I'm assuming it is just the four large tubes on the ST35 that reguire the bias adjust? Is the process a set and forget one until tubes are replaced, or the if the unit begins to perform oddly?
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Re: Bias tutorial

Postby rmyauck » Mon Dec 31, 2012 11:33 am

Yes the diytube ST-35 is Triode's kit.

http://www.diytube.com/phpBB2/viewtopic.php?f=4&t=5739

You can download the EFB mod from the Tronola site mentioned in the thread referred to. You can set the EFB mod up with 1 to 4 pots just depending what you want. You can check the bias from time to time (more often at first) till you feel confident in the Output Tubes. You may be fine checking it every 6 months in the end. You don't need well matched tubes if you set up the EFB mod with 4 pots. If you only use the one pot you could always have a number of extra output tubes to swap around to get them all biased similar from time to time just like the movie theaters did back in the day.

Anyway build the amp first, then when you feel confident add the well recommended for all the reasons mentioned EFB mod. It doesn't cost much to add either ($10-20 depending on the # of pots). JJEL84's and the Russian EV tubes can stand the high voltages of the ST-35 better than other tubes also. The EFB mod will let you roll in other production EL84/6BQ5's.

Lot's of help here in building the amp so keep asking!

http://www.diytube.com/phpBB2/viewtopic.php?f=1&t=4374&start=150

Randy
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Re: Bias tutorial

Postby paart » Mon Dec 31, 2012 12:02 pm

There are advantages and disadvantages of each. The original style chassis is smaller, if you consider that an advantage in your situation. The diytube ST-35 uses 12AX7 and 12AU7 tubes rather than the 7247/12DW7 which is only currently produced by one or possibly two manufacturers, and that's a limiting factor, in my opinion. The diytube amp also has some power supply enhancements, which I consider significant.
Dave Gillespie's EFB mod can be added to either unit with about equal effort. Like the above poster, I'd recommend that you build the amp first and add this improvement as a separate project.
Yes, you are correct, the bias is adjusted using either alligator clips or straight probes on your VOM. Again, is pretty straightforward, simply adjusting the "pots" to get the correct reading on your meter. Resistance adjustments are made with the amplifier off, but voltage checks are done with the amplifier "powered up".
If you would like to see how I added the EFB bias mod to an original ST-35 chassis, both in "stock" and considerably modified forms, you may wish to check out my links:
http://www.tronola.com/html/efb_in_a_dynaco_st-35.html
and
http://www.tronola.com/html/ps_mods_for_st-35.html

---ART---
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Re: Bias tutorial

Postby coolhandjjl » Mon Dec 31, 2012 1:04 pm

Thanks for all the help. The suggestions of getting the kit built first seem sound. Get it built and start enjoying it will be my first goal. Then I can entertain mods after I get a bit of experience under my belt.

Is there a Stickey discussing recommended soldering stations/guns and type of solder?
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Re: Bias tutorial

Postby paart » Mon Dec 31, 2012 2:35 pm

Some of this depends upon your preferences and budget, I guess. My wife bought me a Weller WTCPT station for Christmas nearly 10 years ago, and I'm very pleased with it! For me, it actually makes soldering fun! There are much cheaper units at places like eBay, but I'm unfamiliar with them.
Assuming that you are not in Europe, use only lead based, rosin core solder. If you are in Europe, try to get some anyway! :)) As you probably know, NEVER use acid core solder for electronics.
Eutectic (63% Tin/ 37% Lead) will be the easiest to use, but again, the most expensive.
Kester solder is a popular brand. Personally, however, I find the fumes from their rosin core solder to be very obnoxious! I have always liked "Multicore" brand, but I guess they are out of business.
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