TG1 - advice from any one who has built it...

The fairchild 1n751a, I have also tried the HS2051 and some unknown old stock with no manufacturer on the band. They all performed pretty much the same in terms of gain, they just had a different  compression curve.

The 500 series came with 8 matched 4.7V zeners again these performed pretty much the same in both units.

Trampo

I wanted to make more TGs racks for my studio so i copied very closely the Igor layout.

RE : IGORS PCB of the TG comp :- Painstakingly I went through the schematic ( Igor's ) and  Igors PCB and they both were/are  100% correct as a match for each other,  apart from the already disclosed info about the rack's HOLD problem.

There are a few tiny variations from IGOR's to the original 1969 schematic ;  around the 1UF caps in the side chain but i guess you guys already know that.

HFE of transistors would have to be on the lower side. 600 HFE would be high for original transistors no? I have used BC549C and 559C and I have matched all mine for around 450HFE.

Thing sings.

Interested to know if there was any measurable difference between zener diode brands!


Also, what is the word on the BCY34 substitute? has everyone just gone PNP whatever on this one and put in a bc214 bc213 etc?

Michael

So is this project going to be brought back from the dead? Be a real shame - don't suppose anyone knows where to source some boards do they?

does anyone know if there is a real world difference between BC549c and BC184C ,(  both NPN with similar specs/HFE  )  and BC559C and BC214 , both PNP with similar specs and HFE.

Unless i'm missing something these things are interchangeable.

The original TG did not use BC184s and BC214s.

Is there something magical about these transistors?


regards


Michael

This is Tanner guarding the different Zener diodes we've tried.
Best,
Bruno2000

I have been doing a few more comparisons between the two units and it turns out I made a mistake connecting the side chain from one unit to the other. After correcting this they do actually perform the same.

I have also realized that the 500 series unit also has its output transformer wired with 6db of gain to achieve unity gain.

Can anyone else confirm how they have their output circuits wired?

Did the diodes tested help with any gain issues?

Trampo

This is mostly to Trampo, but others can ride along as well.

With the 1N751 diodes and the component values listed, the closest I was able to get to unity gain was 16 dB down.  I could change some R values and raise the gain, but that messed up all of the other current readings, and the meter was wrong.

It's interesting that the 500 series has the transformer wired 1:2.  So far I have found one 5V1 diode type that can get within 6 dB of unity gain and has really nice compression curves, spaced just like they ought to be with each click of the switch.  After all of the diodes I have tried, I will probably try a few more, then call it a day and use a 1:2 output transformer myself with a 1:1 input.  Those output transistors can easily drive a 150r load.

Another observation was that there were a couple of diodes that did better with the gain situation (I think within 3 dB or so), but at extreme ends of the hold control (I'm using the 10k switch), they either caused a great deal of distortion, had a significant jump in decreased gain, or oscillated.  There were also some diodes that exhibited a second "knee" about 8-10 dB above the first knee, i.e. at -8 dB the ratio was 2:1, and at 0 dBm it changed to 4:1 and showed an obvious second "knee" on the graph.

I plan on sharing my test results with whomever is interested, but I've got to catch up on some Honey Dos before I get any more shop time. 

Best,
Bruno2000

Hey,


will you tell us which 5v1 zeners you found to perform the best in the TG circuit?

kind regards

Michael

Yes, I'll share all of my info, but first I need to run a couple more tests to verify operation.  I'm also going to try the 5k r/log switch plus 5k fixed configuration.
Below is a graph of input level vs. output level (not A-A frequency response like it says at the top) for each of the 23 positions of the 10k hold switch.  This graph was made with a 1:2 Carnhill output transformer.  I'm pretty happy with the curves, and although I can't do any critical listening in my shop (yet), so far I'm very happy with the sound.
Best,
Bruno2000

Not to bore you with more home movies, but here's a "zoomed in" graph showing the nice 2:1 ratio with the hold switch set at "0".  Note the cursors, input +3dBu, output +1.492dBu also input +10dBu, output +4.785dBu.
Best,
Bruno2000

Really interesting study. I have also tried a couple of different diodes, but not to the same detail. The second knee you discovered would probably explain why I seem to get nice compression up to a certain level and then above this level the meter goes off the scale, making it very hard to achieve even compression.

What is also conclusive is that 1n751s are not all the same. I had problems with a few 1n751 diodes with low gain, if i remember correctly the worst was the silver motorola 1n751s and by far the best were the fairchild 1n751s. Is this because we are using the diodes for a different purpose other than their intended use?

Now i am happy that the output transformer needs to be wired 1:2 I will also conduct some listening tests with a few diodes including the original HS2051s and report back.

So... can we conclude that the gain problem is down to the diodes?

Trampo

trampo said:

So... can we conclude that the gain problem is down to the diodes?
Trampo

Click to expand...

No, the problem is that the limiter is surrounded by transformers in the console it came from. You need the 1:2 boost on the output transformer.

Mark

So are you saying that with diodes that have up to 15dB of gain loss we should make that up with the appropriate transformer, or should we choose a diode with 6dB of loss and make it up with a 1:2 output transformer?

Trampo

trampo said:

So are you saying that with diodes that have up to 15dB of gain loss we should make that up with the appropriate transformer, or should we choose a diode with 6dB of loss and make it up with a 1:2 output transformer?

Trampo

Click to expand...

I used the original NOS diodes in my build and have no issues with gain makeup.

No idea how substitutions would effect that.

Mark

If you have one left over, would you be able to post an IV curve for it if you have a curve tracer?
I'd be very curious to see a comparison of IV curves amongst the different diodes people have used to see what effect we might be able to garner from that.

Sorry for the tease, but no follow-through.  I've been a little distracted recently.

The best diodes I have found so far are the Fairchild 1N5231B.  They are very quiet, they produce nice compression curves, and they sound punchy.  They do, however, lose 6dB of gain through the circuit, so I make that up with a 1:2 output transformer. 

The batch of diodes that I tested was from DigiKey, but Mouser has them also.  Out of the 100 I bought, I found 6 pairs that were +/- 1mV at all three current tests that I use (10mA, 1mA, 0.2mA).  When I get time, I am going to run some automated sweeps from 10mA to 0.05mA of the final contenders of my shootout, plus some of the ones that really suck.

Best,
Bruno2000

I have some of the original hs2051s, might be worth including these in the sweep test?
Just PM me your address and I will send them over.

Trampo

trampo said:

I have some of the original hs2051s, might be worth including these in the sweep test?
Just PM me your address and I will send them over.

Trampo

Click to expand...

Outstanding!  Thank you!
PM sent.
Best,
Bruno2000

Anyone have the BOM for the Fester PSU for the TG?

Thanks
Bonasimaster

Anyone have the specs on the transformer for the TG power supply?