ST70 voltage readings a bit off

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ST70 voltage readings a bit off

Postby barbosas » Tue Sep 22, 2015 9:46 am

Hi,

I've just finished assembling an ST70 using several parts I sourced over the last months:

PT: Dynakitfriends (http://www.dynakitfriends.com/blog/files/trafo-schema.jpg)
OT: Hammond 1650n
Choke: Hammond 1650r
Driver and Cap board from triodeelectronics

I'm getting the following voltage readings with amp idle and all tubes on:
Image

It's my first tube amp and I've noticed the PT and valves getting a bit hot specially the GZ34 (don't know if hotter then what it should be), the VAC readings at pin 4/6 of the GZ34 also don't seem OK, they should be around 375VAC right? (I find it strange they are off by 50V up and 50V down)., also the 5.8VAC on the EL34 should be more like 6.3, don't know if this can be an issue or not, some other readings are a bit on the high side but not by much.

Can you guys help me out figuring if there's any issue I should deal with? although the amp plays apparently fine I don't want to over-stress the components, and I can swear I saw the GZ34 gave a blue spark.

Thank you in advance
barbosas
 
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Re: ST70 voltage readings a bit off

Postby 20to20 » Tue Sep 22, 2015 10:23 am

First guess is tranny leads misidentified between the bias tap ground CT and the HV tap. Pull the rectifier and get the two AC HV points in the socket identified as being nearly equal. Go from there.
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Re: ST70 voltage readings a bit off

Postby barbosas » Tue Sep 22, 2015 4:20 pm

Hi guys,

I opened it and revised some connections and I found a B+ from one of the OT that was partially broken near the solder joint, I re-soldered it, rebiased at 1.1V and I got almost all the voltage readings to levels within specs (+- 5% :))

I measured from the red/yellow (in my case black) to each red hv lead (pins 4 and 6) and i get 360VAC from both, (i guess this confirms that the hv/ct leads are not swaped, right?) http://www.dynakitfriends.com/blog/files/trafo-schema.jpg

From each red hv lead to ground I still get 320VAC/420VAC :(

Any other advice? can the choke (which is connect on the cap board to OT's B+, and to pin 8 of the GZ34) have any effect on this?

I decided to use the amp for an hour to see how it holds, and it played nicely for an hour, no GZ34 sparks or anything.

What I've noticed is that the tubes and the PT where running very hot, at the point that you cannot keep your hand on top of the PT for more than 5seconds, is this normal?

Thanks for your help
barbosas
 
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Re: ST70 voltage readings a bit off

Postby 20to20 » Tue Sep 22, 2015 8:31 pm

barbosas wrote:From each red hv lead to ground I still get 320VAC/420VAC :(

Thanks for your help


You have the 55v bias line grounded and you have the HV CT ground line tied to the bias connection point. The color coding of those lines could be reversed. It moves the grounding point 55v away from each leg (+) on one and (-) on the other side.
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Re: ST70 voltage readings a bit off

Postby barbosas » Thu Sep 24, 2015 2:36 am

Thank you very much for your help guys, that was indeed it, the bias and ground lead colors where swaped.

It sounds beautifully now.
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