Transformer Coupled Linestage

[hpn5350]

Hi Guys,

I’ve been fooling around with this preamp for a while and would appreciate your experienced opinions before I start drilling holes in my chassis.

I started with the transformer coupled linestage schematic on the JE labs website, but quickly became hooked by the “Ultrapath” topology.

The main problem is that the B+ from the JE labs power supply is way too high and I am unsure of the best way to bring it down. I added an RC stage to the end of the supply, but wonder if it would be better to add more chokes, or try a VR regulator tube? (Note: I use a Mac, and am unable to do simulations in PSUD2)

Of course, I would appreciate any other suggestions you may have!

Thanks,
Matt

Here are some pictures of the linestage:









Voltage at the first 500uf capacitor is 291.5VDC, at the second 500uf 290.1VDC, at the 43uf Solens 276.3VDC and at the Ultrapath cap 271VDC. I end up with 251.5VDC on the plates once I subtract the bias voltage (Design centre max for the 76 is 250V).

[zigzagflux]

Connect that unused 6.3V filament winding in series with the 120V winding. In the correct polarity, it will lower the secondary output.

This would lower your 300V winding to somewhere around 95% (285V) or so. Percentage change in 5V filament winding would be similar, if you can handle it.

[rdf]

Just add a bleeder resistor to ground after the 12H choke. Should help with regulation as well.

[JoshK]

Does the 76 have low enough rp to have any reasonable bandwidth with tx output?

[kevinkr]

Depending on operating point the Rp of a 76 varies between 9.5 - 12K typically. Depending on the quality of the opt (whether or not there is enough primarly L) the bass could start to roll off early in this particular application. I would use a 15K - 20K plate to line transformer with several hundred H primary Z, better still in this case might be parafeed with a ccs..

I use a 26 (8K Rp) with Ha-133 15K:600 plate to line transformers into 70K input impedance and I have good performance to both extremes. With these transformers I do get down to slightly below 30Hz and to something in excess of 25KHz (IIRC) on the top end with better than +/-1dB - way better as I recall.

I was suspicious of the whole transformer coupled gig for quite a while after I built this one, it was just in comparison to a number of other pre-amps I owned or had access to that this one finally won out.

Incidentally an excellent choice for dht based line stages is the 12A/112A, its Rp is low enough for good performance with a 10K transformer. The 6J5/6SN7 is another excellent choice..

[hpn5350]

Connect that unused 6.3V filament winding in series with the 120V winding. In the correct polarity, it will lower the secondary output.

Like this ?

Just add a bleeder resistor to ground after the 12H choke. Should help with regulation as well.

How do I calculate the value of this resistor? (I'm just a hobbyist with one term of engineering under his belt )

The 6J5/6SN7 is another excellent choice..

I was under the impression that the 6SN7 and 76 were very simmilar - would it be better to use a 6SN7 with this transformer? I did most of my listening through a pair of Senheisser HD600's and while I felt the high end was slightly rolled off, the bass seemed fine.

Here's a copy of the data sheet for the transformer if it helps!



Thanks,
Matt

[zigzagflux]

Exactly.
It will either be correct or not. If not, change the polarity of the 6.3V winding. You can do some simple tests beforehand to be sure you got it right the first time.

The method you choose really depends on your end goal. I don't think you mentioned what voltage you were shootin' for?

[Sheldon]

Originally posted by hpn5350
How do I calculate the value of this resistor? (I'm just a hobbyist with one term of engineering under his belt )

Download this baby: http://www.duncanamps.com/psud2/index.html

It's free, it works (very accurately if you measure your transformer primary and secondary resistance, and unloaded primary and secondary voltage). Best of all, you can plug in a bunch of different values and begin to get a good feel for the relative effect of various parameters.

Oh, and it's real easy to use. Just ask here if you have any questions.

Sheldon

[kevinkr]

Actually the 76 and 6J5/6SN7 family are not all that similar.

Depending on operating point the 76 has an Rp of 9.5K - 12K typical, a gm of 1150 - 1450 umhos, and a mu of 13.8.

The 6J5/6SN7 family depending on operating point will have an Rp of about 6.5K - 8K, a gm of 2600 - 3000 umhos, and a mu of 20.

I believe the early 6J5G is actually a lineal descendent of the 76 - all of which can trace back to the 27, the first idht introduced in the late 1920's.

Other obvious differences are better (perhaps unstated) cathode construction and materials, better gettering, and octal bases allowing the use of a universal socket design for lots of different tube types.

IMHO The 76 is not the best match in terms of Rp that you could make to the 10K James transformer. I think the 6J5G or 6SN7 would be a better choice. Other types with somewhat to considerably lower Rp would be even better.

5687, 5842/417A are other types I would consider for this application as long as Ip is kept to slightly below 20mA to avoid saturation issues in the transformer.

Note you will get better low frequency extension with tubes with an Rp below 10K when using the 10K tap - you will also probably get less distortion as well. Kind of the point with my harping on the Rp issue..

Edit: kept adding thoughts LOL

[hpn5350]

Download this baby: http://www.duncanamps.com/psud2/index.html

I wish I could, as it appears to be a very useful program. However, I use a Mac.

You will get better low frequency extension with tubes with an Rp below 10K when using the 10K tap

Ah, now I can understand why the 76's Rp of 9.5K isn't optimal with my James transformers.

The 5842/417A seems to be the best option. As I was "google-ing" for more information, I came across the Euridice linestage which appears to be quite popular. However, I have a feeling that it is running the 417A over 20ma. Are there any specific operating points you would recommend? (I am not sure how to adopt the 417A into my present linestage/PS)

Would something like this be reasonable:



Out of curiosity, why is the value of the ultrapath cap so low?

Thanks for your help,
Matt

P.S. Anyone wanna trade a pair of 417A's for some 76's