[cgrums]

Hey all,

I've spent the last couple of weeks finishing up a few small projects (case work and little things) and have been reading up on p2p amp building specifically with tubes. I've been reading both of Morgan Jones' books and feel I'm ready to tackle my first project.

As I've just gotten a pair of decent vintage TTs I'm interested in building a tube phono pre-amp. I've come across several designs on the web (not trying to design my own the first time out) and am looking for something reasonably simple for a first try. I'm thinking of building this:

with this power supply:

First off, comments as to quality of the pre-amp?

Second question: my plan is to use p2p wiring with tag boards to hold small components and an attempt at a star-ground. I understand basically how to lay out the components...but what about the small stuff? Should small caps and resistors just be air wired? Or is it better to have them on tag boards?

Third question: The transformer(s) for the power supply. The schematic has two transformers spec'd one for high voltage (625 vct) and one for (I'm assuming) the heater filaments for the tubes (6.3 vct).

Does the 625 v refer to the secondary output? Isn't this a little high? Most of the transformers I've seen have had a max of 425-450 v secondary output.

Does the designation vct stand for center tapped?

Are these physically two seperate transformers? Any recommendations as to decent quality transformers to use?

Please forgive the multitude of questions but I'd like to better understand the components themselves as opposed to just blindly following someone elses parts lists.

TIA!

 

[dsavitsk]

Quote:

Originally Posted by cgrums /img/forum/go_quote.gif First off, comments as to quality of the pre-amp?

Note that what you have is a pair of grounded cathode stages with RIAA equalization between them. It is a pretty standard design, but there are a few tips and tricks that can improve things. First, as a general principle, loading the plates with a CCS or a choke will operate the tubes in a more linear region. Second, if you can find a different way to bias them, such as a battery, you will get those two cathode bypass caps out of the signal path. But otherwise, it is fine.

One other thing, 6dj8's can be a little noisy/microphonic. Some people have no trouble with them, but others find them difficult in phonostage duty. I think there are better tubes to use, but you'll need to look around for different designs.

Quote:

Originally Posted by cgrums /img/forum/go_quote.gif Second question: my plan is to use p2p wiring with tag boards to hold small components and an attempt at a star-ground. I understand basically how to lay out the components...but what about the small stuff? Should small caps and resistors just be air wired? Or is it better to have them on tag boards?

For caps, because they can be microphonic, tag board is probably a little better, but this is minor. Just be sure things can't move and cause shorts.

Quote:

Originally Posted by cgrums /img/forum/go_quote.gif Third question: The transformer(s) for the power supply. The schematic has two transformers spec'd one for high voltage (625 vct) and one for (I'm assuming) the heater filaments for the tubes (6.3 vct). Does the 625 v refer to the secondary output? Isn't this a little high? Most of the transformers I've seen have had a max of 425-450 v secondary output.

It will give you 312.5VAC with the full wave rectifier -- maybe a little high for 6dj8's that are pretty happy with about 100V on the plate. He is using choke input for th PS which will drop quite a few volts, so it might be okay. You should use duncan amps PSU designer to design your own PS -- it is pretty easy.

Quote:

Originally Posted by cgrums /img/forum/go_quote.gif Does the designation vct stand for center tapped?

Yes

Quote:

Originally Posted by cgrums /img/forum/go_quote.gif Are these physically two seperate transformers? Any recommendations as to decent quality transformers to use?

You can find lots of transformers with multiple windings. Look at Hammond's website to find an appropriate model. Don't forget that you will need a 5V windng too for the rectifier tube, if you use tube rectification. Also, DC is essential for the filaments in a phono stage. You won't get 6.3VDC out of a 6.3VAC winding so you may want a separate filament transformer.

 

[platenspeler]

Agree with most of dsavitsk answers. I also think there are several alternatives out there, and I would check a few sites where the builder offers more information than the two circuit diagrams.

I made a few myself (see my homepage), and there are many other sources of people showing their solution with pictures etc. This might help you in building your own amp. Some comments:

1. The first stage has not much gain. A 12ax7 would give much more headroom. You can use an SRPP stage there but it is not really necessary for the distortion.
2. The tube rectifier used by RJMaudio, 6CA4, seems to run on 6.3V ac so you do not need another winding on the power transformer. The AC is usable for heaters, but you have to be very careful when building the phono amp or you'll get humm.
3. I would prefer to use DC for the heaters, especially in phono applications. Like said by dsavitsk the power supply for the heaters will be very different if you go for DC. You'll need 9V secondaries and a voltage regulator etc. There are several circuits available on the web. For one of my projects I had my transformers and chokes made for me. As the power consumption of phono amps is not high it did not even cost that much. Amplimo/Plitron offer several standard transformers although most of them have 6.3V secondaries for the heaters.
4. C1 would be hard to find I think (0.0295 uF) with a tolerance of 1%. As it is easier to find resistors with 1% or 0.1% tolerance in several values I wonder why the value was taken this way.
5. The gain of 35dB is on the low side. And if the amplifier you connect to the output has an input impedance of 10kohms instead of the 100K assumed, the gain drops another 5 dB (and a lot of amps do not have 100K inputs)
6. Also I do think that the tube based power supply offers rather high B+ voltage for the tubes chosen.

When amp and psu are in separate boxes one can build very small:

.

The power supply is in an equal box. I used open air connections with soldering strips on both sides to solder the components on.

Remember that tube rectification with lots of chokes makes the power supply very large and heavy.

So apart from the quality of the design, the layout, groundplan, enclosure and component choice is very important for phono amps. Aluminum enclosures for me work very well. And you'll either need lot's of space in the amp (transformers far away from inputs and tubes) or choose for separate power supply and amp setup.

Success with yor choices, please show us the pictures when your project is finished.

regards,

Maarten

 

[cgrums]

Very nice work there Maarten!

I've been thinking more about using a 12at7 / 12ax7 but have yet to turn up anything (I must not be looking in the right places

)

I'm going to be using your website for reference for sure. Thanks again.

 

[cgrums]

Well, I just ordered a few EZ80s, ECC83s and a 5751...I think I'm going to give a stabe at building that scratch phono stage from...scratch

 

[cgrums]

Here's another quick question: I'm planning out my shopping list for the Scratch build based on this schematic from Maartens website:

Right now I'm looking at transformers. I understand I'm ultimately going to need 275 V for B+ voltage, 12.6 V for the filaments of the 5751 and ECC83s and 6.3 V AC for the filament of the EZ81 rectifier.

I'm thinking of ultimately using two transformers: one for the B+ and rectifier filament and the other for the 5751/ECC83 filaments.

For the B+ transformer I'm considering the Hammond 272JX with ratings of

- 236 VA
- Secondary voltage: 300-0-300, 250 mA
- Filament: 6.3 V @ 8 A ct

For the 5751/ECC83 filaments I'm considering the Hammond: 167N14

- 14 V ct

Am I on the right track?

Edit: to answer my own question: not quite.

Checking the datasheet for the EZ80 showed that it only needs 250 VAC to get 275 VDC and hammond makes a nice 250-0-260 with a 6.3 VAC filament winding....this looks a bit closer.

Any reasons for not using the 167N14 for the 5751/12AX7 filaments?

 

[cgrums]

Now that I've got the transformer question sorted I have a choke question:

The schematic specifies 20H @ 25 mA...I'm having a difficult time finding this particular value. I can readily find 20H @ 20 mA...is the power supply going to suffer for not being able to pass the extra 5 mA through the chokes?

 

[platenspeler]

Quote:

Originally Posted by cgrums /img/forum/go_quote.gif Now that I've got the transformer question sorted I have a choke question: The schematic specifies 20H @ 25 mA...I'm having a difficult time finding this particular value. I can readily find 20H @ 20 mA...is the power supply going to suffer for not being able to pass the extra 5 mA through the chokes?

Hi,

I have not measured the power concumption myself, but accoding to simulations the power comsumption is much less than 20 mA, so you'll be fine with 20 mA.

By the way: I published the circuit on my site, but it's Triodedick's design, see http://www.triodedick.com

The transformer + chokes are available from AE-Europe, see http://www.ae-europe.nl/trafosets.htm , for 70 euros.

regards,

Maarten

PS. I sent you a personal message ...

 

[cgrums]

Quote:

Originally Posted by platenspeler /img/forum/go_quote.gif PS. I sent you a personal message ...

Thanks Maarten, I've sent you a message as well.

 

[cgrums]

I've taken the plunge and ordered all the parts for this build. Tubes should be here today and the rest of the ridiculously expensive caps and such will be here next week...this should be fun.