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Old 23rd March 2012, 02:54 PM   #1
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Default ST 70 Power on!

Hooray!! I finally got the nerve to turn it on. I should mention that this has been rebuilt with a Dynamull driver board and CV Power Supply board ( both built by Gregg) and all new tubes, including a matched quad of Winged C EL34s. I shorted the RCA outs and connected a set of 8 ohm speakers. I removed the rectifier tube and fired it up. Gregg's initial power up procedure says to adjust RV1 and RV2 pots to max -VDC from pin 5 to ground. I have 48.2VDC on the left and 48.6VDC on the right.

I power off and replace the rectifier tube. I connect my meter to pin 8 on the left channel and power on. After several seconds the voltage starts to climb, but it gets to 0.500VDC and keeps climbing. I power off and look for problems. I don't SEE anything, so I connect my lead to the right channel and try again, with the same results.

I power off and swap out the rectifier tube with the original, with the same results. I tried the original tubes, I disconnected the speakers,and removed the RCA jumper all with the same results.

The good part is no noise through the speakers and no chassis voltage.

I should mention the that the amp worked when I got it.

This is what I'm working with. A bit sloppy I know, but it's my first. It also doesn't show the ground wire from the IEC plug to the chassis.

DSC02184.jpg

I have emailed Gregg and will likely get an answer tomorrow. I'm sure it's something I've done or haven't done. For the life of me though, I can't figure out what.

Anyone else with ideas? I guess I'll go cut grass and have a think.

Last edited by richamor; 23rd March 2012 at 03:13 PM.
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Old 23rd March 2012, 06:43 PM   #2
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Do you have a 8 ohm load resistor that you can use in place of your speakers? So far so good, but it's good practice to do first power up with a load R, it can save your speakers in case of issues.

Your pin 5 to ground voltages above are actually negative, correct? If so, they sound normal. You usually need about -30 to -35V to bias EL-34's. If you adjust the trimmer pots, does the voltage get less negative?

What values are the cathode resistors? The voltage drop across these gives you the bias current using ohm's law. If they are 15ohms (IIRC that's what a typical ST-70 has roughly) The cathode R's should be between pin 8 and ground, so when you are measuring pin 8 to ground you are measuring the voltage drop across these R's. So if you want 33 ma bias current, then I=E/R so .033=.500V/15 ohms, etc.

Have the coupling caps been replaced or are they original?

So your issue is that the bias current keeps climbing on power up, correct? Even with the trimmer pots adjusted for full negative voltage....
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Old 23rd March 2012, 07:16 PM   #3
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15.6 ohms is the value of the stock bias resistors (not being picky, just informational).
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Old 23rd March 2012, 07:30 PM   #4
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Quote:
Originally Posted by CharleyW View Post
15.6 ohms is the value of the stock bias resistors (not being picky, just informational).
Cool, thanks! I knew it was around 15 ohms and a crazy value to make the bias calculation non-intuitive....
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Old 23rd March 2012, 10:19 PM   #5
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Quote:
Originally Posted by richamor View Post
I should mention that this has been rebuilt with a Dynamull driver board and CV Power Supply board ( both built by Gregg) and all new tubes, including a matched quad of Winged C EL34s..
er.. that would be "Dynamutt". Dynamull is a different circuit (from the Triode website).

I have the very same configuration as you and I just finished populating the boards, so I'm only a bit behind you in progress.

..Todd
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Old 23rd March 2012, 10:32 PM   #6
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Quote:
Originally Posted by taj View Post
er.. that would be "Dynamutt". Dynamull is a different circuit (from the Triode website).
DynaMull is a relatively recent Geek creation.

jeff
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Old 23rd March 2012, 10:38 PM   #7
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Quote:
Originally Posted by boywonder View Post
Have the coupling caps been replaced or are they original?
They were replaced. The photo shows new MKP caps on the new PCB, not ones recycled from the original board.

..TJ
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Old 23rd March 2012, 10:43 PM   #8
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Quote:
Originally Posted by vinylkid58 View Post
DynaMull is a relatively recent Geek creation.

jeff
Oops, I stand corrected. Is it based on the Dynaco designed circuit of the same name whose schematic has been available on Triode's website for eons? Or is this a name conflict.

Edit: Ah, I can see it's based on the original Dynaco circuit, though he's added a few tweaks.

..TJ

Last edited by taj; 23rd March 2012 at 10:49 PM.
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Old 24th March 2012, 03:36 AM   #9
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Quote:
Originally Posted by boywonder View Post
Do you have a 8 ohm load resistor that you can use in place of your speakers? So far so good, but it's good practice to do first power up with a load R, it can save your speakers in case of issues.
These are junk speakers anyway. I wanted to hear for problems as well.

Quote:
Originally Posted by boywonder View Post
Your pin 5 to ground voltages above are actually negative, correct? If so, they sound normal. You usually need about -30 to -35V to bias EL-34's. If you adjust the trimmer pots, does the voltage get less negative?
Yes. In fact I checked twice.


Quote:
Originally Posted by boywonder View Post
What values are the cathode resistors? The voltage drop across these gives you the bias current using ohm's law. If they are 15ohms (IIRC that's what a typical ST-70 has roughly) The cathode R's should be between pin 8 and ground, so when you are measuring pin 8 to ground you are measuring the voltage drop across these R's. So if you want 33 ma bias current, then I=E/R so .033=.500V/15 ohms, etc.
Those are the originals, and about the only thing I didn't change. They checked out good.

Quote:
Originally Posted by boywonder View Post
Have the coupling caps been replaced or are they original?
Those went away with the original driver board.

Quote:
Originally Posted by boywonder View Post
So your issue is that the bias current keeps climbing on power up, correct? Even with the trimmer pots adjusted for full negative voltage....
Exactly. And just so you all know, I'm no electronics tech. I can read prints somewhat, and know how to use a meter and soldering gun. I spent 20 years as a plant electrician, but this tube stuff is all new to me.
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Old 24th March 2012, 07:56 AM   #10
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Hi richamor,
I suspect your AC power switch wire will induce some hum once you get this running. It's only millimetres away from the input pentodes. Best to keep AC away from the sensitive parts of the amp. The power switch is typically kept on the back near the power supply for that reason.

..TJ

Last edited by taj; 24th March 2012 at 08:00 AM.
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