What would you copy - 350 or 351 REPRO AMP?

[glimmie]

Given the choice which classic repro amp would you copy. A 350 0r 351. I am leaning towards the 350 as it's all octal tube based. I plan on using MC phono transformers to match my modern SS heads.

Of course any other suggestions for a tube repro amp are also welcome.

I'll post pics of my R to R project in a day or two.

[ecir40]

Hi,

I don't have any experience with either or but do you have a link to the schematics.

Why not look at the Eros.
http://www.bottlehead.com/et/adobespc/eros/eros.htm

[glimmie]

Here is where I got my 350 schematic/manual. It's compressed into a .rar file which is type of zip file. I used a freeware rar decoder program I found online.

http://www.analogrules.com/350index.html

I looked at the Eros but I need to build a custom chassis to fit in my console. Now if Doc would make schematics available (including his famed Repro) I would surely consider those as well. But I fully understand he is running a business and values his intelllectual property.

[mep]

Well, I will throw in my $.02 worth here for what it is worth.  I own a pair of 350s and I think they are great.  I have never heard a pair of 351s.  There were obvious cost savings that went into the 351s when compared to the 350s.  The 350s had dual outboard power supplies in order to keep hum and noise out of the main preamp.  The 351 has the power transformer built into the main chassis. The 350s were wired point to point and the 351s were built on a circuit board which I understand didn't age well.  As far as which one sounds better, I haven't a clue.

I am curious as to where you will get all of your parts for either design.  If you do build either one, it sure would be interesting to here it against the original preamp.  Good luck!

[glimmie]

Parts are easy. Due to the current interest in tube DIY for both HiFi and guitar there are many sources for tubes and transformers. NOS and current Russian and Chinese manufactured. Also due to state of the art switching power supplies, new well engineered high voltage capacitors are highly available. Plus there is the never ending supply of "botique" capacitors and resistors.

www.tubesandmore.com
www.tubestore.com
www.angela.com
www.triodestore.com

Just to list a few!

[Ben]

I think *improve* rather than copy is the keyword here.
I would like quality parts and design for a pre-amp. A amp
anybody can put together even me. One got-ya may be output
drive of the pre-amp -- it may expect 470k input impedance.

One other place for odd parts is Surplus Sales of Nebraska
http://surplussales.com/index.html

[steveidosound]

The 350 being all Octal tube might be harder to find some of the types.
Also, at least at the input and output ends these have transformers to run in a studio environment.
Finding transformers that work/sound the same might be a bit hard. I suspect some of the "magic" in the sound is how those transformers interface with the tubes and the whole thing with the rest of the world.
I guess you are  building them as tape repro. preamps?  Not including the record head drive and bias oscillator sections? How much can you re-design it before you have a "new thing" that may not sound the same?
Most people just want them as mic preamps today.

[Ben]

I keep forgeting these are broadcast rather than consumer products, so of course
you have a better design with 600? ohm transformers for output and a specific input
transformer for the type of amp you have designed. I am guessing that most octal
tubes can be still found as NOS with the common ones as new production tubes.
What studio quality transformers I know of, cost a mint but they are quality products.
Lundahl transformers seem to be the best off the self transformers I know of.
http://www.kandkaudio.com/

[stellavox]

Either set(s) of electronics sound very good - don't really have a favorite.  If you are going through them I shggest checking the values of all the resistors to ensure that are still in tolerance AND replacing any that aren't with the same type if you can find them.  I don't remember ever finding any bad electrolytics but if it were mine, I'd replace them all.  Replacing the coupling caps with your favorite botique type will also help marginally.  As I remember, the (unbalanced) headphone output comes before the output transformer so you may want to take the output to your preamp/amp from here.  I also remember finding a ground loop which added some hum but don't remember which model it was (my notes were borrowed and not returned)

Charles

Regarding head matching, before going nuts with matching transformers, just try running the "new" head straight in to the Ampex and see how it sounds - how flat the HF response is and can you correct any deviations with the existing controls.  May find that it works just fine.   

[stellavox]

Andy,

Since you also asked for suggestions on other tape repro preamps I'd certainly consider trying one of the MANY tube preamps produced in the early days of Stereo.  Nearly all of them had (direct) tape head inputs.  Some, like the Mac's and Marantz's had variable HF EQ adjustments for the tape inputs right on the back panel - making that adjustment a piece of cake.  You'd probably only have to play with the input loading resistor to minimize LF head bounce.  PLUS some of these old puppies have VERY GOOD tone control sections.

Charles