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Hi Greg,
Did you have a chance to look at my post right above rambi10's one.
I still have not touched them since I posted this thread and would like to know if my questions make sense.
Thanks
Alex

Idoia wrote:Hi Greg,
Did you have a chance to look at my post right above rambi10's one.
I still have not touched them since I posted this thread and would like to know if my questions make sense.
Thanks
Alex

Sorry Alex. I have been in the recording part of the studio all summer (that is why I haven't posted much on the forum). Your questions are very specific and I would need a preamp, the schematic and a bunch of time to answer some of them. I'll try when I can to get some info off to ya, but I don't know when that will be.

Idoia wrote:Maybe one question that could really help me, on your schematics, both 9 pin of V5 and V4 are connected together with the GN/Y of the main transformer. Am I reading well ?

Least but not last, on the same part of the board, pin 5 of V5 is connected to pin 4 of V4 but on the original board, both pin 5 of V5 and V4 are connected to eachother, which one is it ?
Alex

The schematic shows pin 9 connected to gn/y. This does not need to happen. I should draw a new schematic but probably wont.
The pin 5 of V5 to pin 4 of V4 .... is from the old Ampex schematic. I don't know why they wrote it that way but it doesn't matter. As long as there is roughly 12 volts AC between pins 4 and 5 on both tubes, they will work.

Idoia wrote:Hi greg,
I have put another transformer before the main one so that the current is transform into 110 before. The only thing is that the transformer should at least provide 100W unless the ampex won't power up.
I have done two of this units and have for both of them the same problem:
Looking at your schematics, on the amplification board, the middle one, I got all voltages right.
On the record one however, I get 13.5V between R25 and R28 and 15.4V between 4 and 5 of V1.

I don't understand, R 25 and 28 are on the repro/middle board. 13.5 V is only a little high. Was that with tubes in and being powered? With the tubes in, the voltage will drop a bit.
The voltage across V1 will also drop in value if a tube is installed.

I triple checked all resistors and cap values and got them matching your schematics. Is there a slightly chance there are still an error on the schematcis ? For ex on the photos you gave us, I can see that you kept the original R60 which is on the original schematics 3ohms. On the modification schematics you put 1.5 ohms. I might be wrong but at this level this makes some difference, no ?
thanks.
Alex

I don't know of any more faults in the schematic. R60 should be 1.5 ohms, 1W though.

Hi

The output attenuator does just that- lowers the level of the entire output. This is not simply for the VU meter (though the readings on the VU meter will be affected by the output attenuator since the VU meter monitors the output of the 351).

The Daven attenuator is no longer available. You'll have to look in vintage parts stores or eBay to find one. It'll be expensive also- probably $70 or more. A company called STATE ELECTRONICS will make reproductions of the Davens, but only in very large quantities at a very high price ($100 or more).

You can build the 351 without the output attenuator, but then you won't have the option of overdriving it- unless you have some sort of pad on the device following it.

As far as your other questions go: Since I don't have the schematic near me at this moment, I would say just look at the schematic and compare it to the wiring and traces on the 351 circuit boards. This is a complicated project (as far as wiring goes) and it takes time to logically figure out where everything goes. Just go slow and you'll discover the solution eventually.

Good luck.

rambi10 wrote:Hi greg

Hi

I have got few questions about the wiring of the output scheme.
In fact, what is the output attenuator with the three pots just before the XLR output?
I don't understand the principal of this attenuator pot, I just think that it's for the VU METER position and to adjust the output level on the VU meter. Is it possible to wire without this pot? that's to say yellow and green wires directly on the lugs near output transformer and after on XLR output, VU meter and headphones jack?
If I do that, can I always adjust the VU meter?

You can simply connect the o/p trans to the XLR, and you will have a normal preamp. The output attenuator is just there to trim down the output when you are driving the tubes harder with the i/p gain.

After, who did you find the DAVEN output attenuator GP2T because on the NEWARK site, there isn't this kind of pots.

Some hifi resale place. They were bought before I worked here.

To finish, there is two GN wires for V4 and V5 filaments, I must wire on GY and V on the repro board?
But GN/Y where is it wired?
R 25 and R28 are wire on GY and V tracks on the repro board?

Sorry for the confusion. The 12VAC is only used for the meter lights (one G + GN/Y = 6 VAC for lamps). V5, V4, and R25, 28 are in parallel with V1 (12VDC supply). However you do that. The color coding gets confused when you start rewiring so our 'green' may be something else for you. The color code of the transformers are true. Try to keep the color to board connection true if you can, otherwise make note of your changes on the boards. We would install jumpers in some places to keep it consistent.

hey greg,

I have seen these kind of stepped attenuators on ebay and am wondering if they would they work as output attenuators for the 351?

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... 0032487174

punk wrote:
I have seen these kind of stepped attenuators on ebay and am wondering if they would they work as output attenuators for the 351?

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... 0032487174

No. Those won't work. Those are meant to replace normal potentiometers. What you need is a balanced "T" pad attenuator- not a "ladder" type.

hey greg,

i wanted to know if you have any schematics for the power supplies you built for EA 351's? i'm just getting started on the mod and i'm trying to gather as much info before i start.

thanks