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Need help biasing ST-70

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VoodooCLD

Posts: 4
Join date: 2011-01-08

Need help biasing ST-70

Post by VoodooCLD on Sat Jan 08, 2011 2:46 pm

Hello folks, i have recently stumbled onto this forum as I'm having some trouble with my st-70.
This is my first tube amp, however am pretty handy with a soldering iron and troubleshooting. However my lack of knowledge is leaving me stranded.
Anyway i got this st-70 and immediately replaced the quad cap (with an exact replacement) and all other capacitors with orange drops of the stock value.I also replaced the selenium rectifier with a 1n4007 diode. i then bought a matched quad of jj tesla's el34's that were all marked 26/5.0 on them. My problem is that i can't bias the tubes, they only bias up to about 1.44 before the potentiometers max out. Also there is a hum when i first start up the amp for about 5 to 10 seconds then goes away completely and doesn't come back.
Can anyone help me out as to were i should start, as i really wanna listen to this thing!
Thanks!
-Lucas

Bob Latino
Admin

Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Need help biasing ST-70

Post by Bob Latino on Sat Jan 08, 2011 3:57 pm

VoodooCLD wrote:Hello folks, i have recently stumbled onto this forum as I'm having some trouble with my st-70.
This is my first tube amp, however am pretty handy with a soldering iron and troubleshooting. However my lack of knowledge is leaving me stranded.
Anyway i got this st-70 and immediately replaced the quad cap (with an exact replacement) and all other capacitors with orange drops of the stock value.I also replaced the selenium rectifier with a 1n4007 diode. i then bought a matched quad of jj tesla's el34's that were all marked 26/5.0 on them. My problem is that i can't bias the tubes, they only bias up to about 1.44 before the potentiometers max out. Also there is a hum when i first start up the amp for about 5 to 10 seconds then goes away completely and doesn't come back.
Can anyone help me out as to were i should start, as i really wanna listen to this thing!
Thanks!
-Lucas

Hi Lucas,

When you replace the selenium rectifier with a diode there is more forward voltage in the bias system. The cure is to replace the two 10K resistors between lugs 1 and 2 and lugs 3 and 4 on the 7 lug terminal strip with two resistors of LOWER value. One thing you could do is to PARALLEL TWO 10K resistor with the TWO 10K resistors that are on there now effectively giving you a 5K resistance in both areas. The wattage value doesn't have to be that high. I am sure that two 1 watt resistors there would be fine.

As to the hum on startup, it is probably from the quad capacitor charging up. A quad cap in good shape usually won't do this. If you have an original Dynaco quad cap, I would replace it with the 80, 40, 30, 20 uF @ 550 volt quad cap that Dynakitpart sells.

Bob


Last edited by Bob Latino on Sat Jan 08, 2011 4:01 pm; edited 1 time in total

VoodooCLD

Posts: 4
Join date: 2011-01-08

Re: Need help biasing ST-70

Post by VoodooCLD on Sat Jan 08, 2011 3:59 pm

Awesome, i actually stumbled on another post were you had recommended that, and i have been trying to find some 1 watt resistors around town.
Thanks!

VoodooCLD

Posts: 4
Join date: 2011-01-08

Re: Need help biasing ST-70

Post by VoodooCLD on Wed Jan 12, 2011 1:50 pm

Ok, so i installed some 5.1k ohm at 2w resistors and fired it up. i was then able to bias the tubes to the correct 1.56. However after i listened to it for a while noting that it was really quiet (i thought it was my halfer dh-101, and wasn't meant to be played with the high input impedance of the dynaco) then i went to readjust the bias and it would only let me bias up to about .8 volts. is my rectifier tube bad? any other ideas?
Any help would be great
thanks!
-Lucas

Bob Latino
Admin

Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Need help biasing ST-70

Post by Bob Latino on Wed Jan 12, 2011 1:57 pm

Your rectifier *could* be bad? - OR maybe it works but does not give the proper DC voltage off pin #8. See if you get 420 - 440 VDC to chassis ground off pin 8 of the rectifier tube. Try a new GZ34 rectifier tube in there.. Also try replacing the two capacitors on the 7 lug terminal strip.

Bob

VoodooCLD

Posts: 4
Join date: 2011-01-08

Re: Need help biasing ST-70

Post by VoodooCLD on Wed Jan 12, 2011 2:51 pm

Well i have 413 volts at pin 8, and i found a short between the first resistor and one of the capacitors and fixed that. The tubes seem to bias correctly now, however i haven't ran it long enough to see if they again drop too low.
another thing i noticed was that the amp is not super loud. i've played 20 watt solid state amps at half volume that are equally as loud as this. Hooked up to the Dh-101 its really quite, then i plugged my ipod straight into the front rca's and at full volume its about half volume on a marantz 2220. Any reason this could be? does say a dynaco pas crank the input of the ipod up higher than the volume that leaves the ipod?
Again thanks!
-Lucas
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