Ampex 351 Conversion---> DC Heater Issue

[schmidlin]

In the process of doing the 351 strip-down/improve mod ala:
http://greg.electrical.com/351-photos/ea351.jpg

And it's the heaters: I get proper DC from the heater supply but ONLY when I leave it untied to ground.  Untied I get -1 and -14 VDC to chassis.  Tied I get nothing (I only bring it up about third on the variac: I know a dead short when I see it).

I suspected a non-isolated winding, and I'm getting 45K to ground from that tap when unhooked from the system.

Anyone know what causes this and is there a good fix??  I triple checked for an odd ground I may have missed in the strip-out: very confident there are none.  Cap is proper polarity (as marked)

Right now a have a very robust 60hz generator on the bench.

 

[PRR]

> Untied I get -1

-1V suggests there is a Full Wave Bridge involved, not the 2-diode scheme shown.

 

[schmidlin]

Thanks PRR, and I agree.

I am using the original diode(s)  Mostly original format (same tap), save the schemtic alterations.  Any suggestions what to do?

 

[bluesbaz]

I really simplified my wiring when I did mine which cleared up some the crazy ground loop problems. This project turned into a freakin huge headache but in the end I sussed out all of the craziness. You do want to make sure the flybacks on the output are in place when you test. Also, the volume pots had to be grounded with a wire to the back of the pot, simply bolting it in was not enough. Are thing quiet when you ground the input to the second card?

 

[nielsk]

Why are you tying it to ground? Do you have the proper schematic? The center tap on the AC heaters and the pin 9 of the tube on the DC heaters is what should be tied to ground. There is nothing wrong with the original design.
If you remove the bias and record head tubes (not needed in a preamp only) you can change the output tubes to DC heaters, but you must make the all the required wiring changes.
Why not restore them properly? I have a 351 transport with good heads that needs a loving home!

 

[bluesbaz]

Why are you tying it to ground? Do you have the proper schematic? The center tap on the AC heaters and the pin 9 of the tube on the DC heaters is what should be tied to ground. There is nothing wrong with the original design.
If you remove the bias and record head tubes (not needed in a preamp only) you can change the output tubes to DC heaters, but you must make the all the required wiring changes.
Why not restore them properly? I have a 351 transport with good heads that needs a loving home!

Tying the input to ground allows one to determine the source of the buzz. Im not suggesting a permanent change. v1 is originally dc and it should stay that way. the other two tubes which continue to be used in the electric audio schematic were dc and can stay that way. If you do remove the unused tubes you will need to accomodate for the removal of a path to ground that happens when unplugging v3. In other words make sure v1 pin nine is grounded. As for wiring this helped me

 

[schmidlin]

Thanks Gang.  Funny story- you're going to laugh really - I had the gain pot wired backwards...so it was maxing on 1kh input.  That n00b feeling stings a little. 

So I'm getting the feeling you want it wired as it was, and not a bad suggestion at all.  I have all the heaters running on the DC from pins 4 & 5 each, so grounding pin 1 should be no problem.  I was hoping too improve the circuit by having all positive DC on the heaters, as is: all negitive.

And I don't (can't, really) get fancy with measuring hum-out, but I am now getting ~4mv AC into a 600 ohm load.  Too much?

 

[bluesbaz]

I am now getting ~4mv AC into a 600 ohm load.  Too much?

How does it sound?
-s

 

[schmidlin]

Great question.  I am a tech and need to know if something is *worthy* of my audio engineer trying it out.  Sort of a symbiotic relationship.  I don't want to waste either of our's time. 

Seems I should drop it off and check the feedback.

 

[Bill Wilson]

This has nothing to do w heaters, C16 should be connected to the plate of the preceding stage & not to B plus.

 

[abbey road d enfer]

I have all the heaters running on the DC from pins 4 & 5 each, so grounding pin 1 should be no problem. 

Just make sure that the heaters section is really fully floating.

  And I don't (can't, really) get fancy with measuring hum-out, but I am now getting ~4mv AC into a 600 ohm load.  Too much?

It all depends on the actual overall gain and if the input is loaded or not. With all pots fully cranked up, the overall gain is about 70dB for the electronics, the input xfmr adding another 20dB or so.
4mV is -46dBu, with 90dB of gain, it would compute at -136dBu e.i.n., which is clearly much better than can be expected. So your measurement is either wrong (a multimeter is not a good audio meter) or taken in unsuitable conditions.

 

[jwhmca]

Tying the input to ground allows one to determine the source of the buzz. Im not suggesting a permanent change. v1 is originally dc and it should stay that way. the other two tubes which continue to be used in the electric audio schematic were dc and can stay that way. If you do remove the unused tubes you will need to accomodate for the removal of a path to ground that happens when unplugging v3. In other words make sure v1 pin nine is grounded. As for wiring this helped me

Old thread, I know... but dealing with the same ground hum issue and getting huge noise when I remove v3.

v1 gets its ground through v3? Pin 9 on v1, which is in the Record board?

 

[NadMk]

This is not an issue. V3 has pin 9 tied to ground, and is the only reference to ground this part of the circuit has. If you unplug it, DC heaters power rails will be floating, and will lead into hum.
If you want to unplug V3 you have to reference the circuit to ground at somewhere else.