High Voltage Readings

OK...I purchased a VTA st70 board awhile ago that uses 3 12AT7 tubes. Everything is new except for the iron. Just finished putting together. My line voltage is 124. On 5AR4 pin 8 I get 490v. I have a 6.8K resistor between sections 3 & 4 and 1 & 4 have been connected to each other on quad cap. My B+ is good @ 405v but all other voltages are 35-40v high. Is this normal?

Other problem is that there is a noticeable hum or buzz from only one channel. Noise goes away when input jack is in place. Why does this occur and only one channel? Thanks in advance for any info!

Hey Bob,

Is this a common problem? I had to use 5u4 with my 2 MK3 amps. I was not concerned about the power trans being over taxed but can the st70 tranny handle the extra amperage?

Also what about the noise issue?

Can someone please answer the question regarding the low hum/buzz noise comimg from only ONE speaker with the VTA board installed when there is no input jack in place?

Re; > "Can someone please answer the question regarding the low hum/buzz noise comimg from only ONE speaker with the VTA board installed when there is no input jack in place?"

This is NORMAL... > Without the input jacks terminated with two RCA interconnects plugged in which are either ..

A. Terminated on the other end by being plugged into a preamp
B. Shorted between the center pin and outer shield on the other end ...

the amp will pick up some hum/buzz from the amp's circuitry. Sometimes one channel will pick up more noise than the other channel ...

Bob

What amazes me is how other people run their vta amps with 5AR4 rectifiers, unless their line voltage is considerably lower than mine. I have even seen amps on ebay with no resistor on the quad cap. When they rebuild an amp do they even bother to take voltage readings? I understand it will work, but how long will the tubes and other components last?

rudes wrote:What amazes me is how other people run their vta amps with 5AR4 rectifiers, unless their line voltage is considerably lower than mine. I have even seen amps on ebay with no resistor on the quad cap. When they rebuild an amp do they even bother to take voltage readings? I understand it will work, but how long will the tubes and other components last?

We have 230V here and my voltages are pretty much "correct" using the official schematic.
As regards to hum mine vta is dead silent even when turned on :-) I used a "star grounding" and awoided grounding at the tubesockets, instead i installed solder lugs connected to the "star ground"

Running a different power trans. If running PA521 trans was designed for 240v operation therefore 230v will result with lower voltage. I also used "star grounding" and is dead quiet WITH inputs connected.

rudes wrote:Running a different power trans. If running PA521 trans was designed for 240v operation therefore 230v will result with lower voltage. I also used "star grounding" and is dead quiet WITH inputs connected.

If your amp has tendensys to hum it might have to do with the environment.
I have grounded all equip ( the vta has a IEC-well), the preamp a fixed grounded cable, in addition
the shelf has steelparts that are grounded.

But the fact that an open input picks up hum but otherwize quiet, is no reason to start rebuilding.
Just listen to the Music !

I agree...not rebuilding. Was just curious as to why...thanks!

an open input is an antenna to hum and/or hum pickup, that's why so many suppliers sell overpriced shorted RCA jacks for unused inputs on preamps.
Fortunately on the ST70 that input is only about 1" long of wire. Also dependent on the environment - stay away from florescent lights!