Poor Man's EQP1A (+MID) Mark 2

[ruffrecords]

Hi,

judging by many posts here, other forums and some tests it seems like haromic spread of ECC83 used in normal way (high anode resistor, etc.) has harmonic spread that isn't as good as some others. For example octal tubes like 6SN7/6CG7 you wrote papers about, can't find much that would go as deep about ECC83, except for mostly writing without real tests.
I would really like to know what you think about it, is it NFB that makes higher (and lower in this case)  harmonics so much lower that even ECC83 sounds good like this? Or is output stage giving more sound that masks input tube's harmonic spread? My tests seem to confirm this, although with pentodes where difference is obvious.
Couldn't find almost any comment from you about it while searching around. Please comments if you don't mind, i can write in another thread for Euroboard Mic Pre if needed. Thanks for any reply.

About the time I tested the loads of 6CG7 tube based mu followers you have read about, I also tested some other types of tube arranged as mu followers. The interesting thing about testing double triodes in a mu follower configuration is that you automatically have a very high effective plate load resistance so you get to see the tubes intrinsic distortion pretty much untainted. This basically means the distortion you measure using a mu follower will be about as good as it can possibly be for that tube.

I tested a range of different tube types - ECC88, 12AU7, 6N1P, 6N23P and a number of 12AX7s. The most surprising result of all this was that the 12AX7 has a very low intrinsic distortion - lower even than the 6CG7/6SN7 - in fact it was the lowest of any tube types I measured.

The 12AX7 is a preamp tube - it runs at about 1mA typically and so it has no output drive capability - hence it is used mostly in early stages. In triodes, distortion is directly proportional to output signal level so a 12AX7 working in an input stage should produce incredibly low values of distortion. High signal levels only occur in output stages so that is where 99% of all the distortion usually occurs. Any NFB really serves to reduce the output stage distortion - any distortion contributed by the 12AX7 is negligible.

This applies directly to the Eurocard. The output SRPP stage has about 28dB of gain so even when the output is running at +20dBu, the 12AX7 output level is only -8dBu. It distortion is no more than one tenth of the SRPP stage and its operating level is also below one tenth of the SRPP stage so its overall contribution to distortion will be way below 1% of the total distortion.

Cheers

Ian

 

[Humner]

Hi Ian,

I'm beginning to design my panels. Just want to confirm the rotation of the switches and frequency selection. The schematic says with regards to the MID band frequencies that clockwise(going from 1 - 12) means going from 500hz - 16000hz. However the HI and LOW do not show the numbers for the switches - I'm assuming its the same and that frequency selection goes low to high in a clockwise fashion?

Rob

 

[ruffrecords]

Hi Ian,

I'm beginning to design my panels. Just want to confirm the rotation of the switches and frequency selection. The schematic says with regards to the MID band frequencies that clockwise(going from 1 - 12) means going from 500hz - 16000hz. However the HI and LOW do not show the numbers for the switches - I'm assuming its the same and that frequency selection goes low to high in a clockwise fashion?

Rob

That is correct. The component values silk screened onto the PCB assume frequency increases in a clockwise direction.

Cheers

Ian

 

[Humner]

Hi Ian,

Can you do a me a favour? I want to check the clearance/dimensions of the prescott inductors coming off your boards. Can you provide those dimensions and the diameter of the inductors?

Thanks

 

[ruffrecords]

Hi Ian,

Can you do a me a favour? I want to check the clearance/dimensions of the prescott inductors coming off your boards. Can you provide those dimensions and the diameter of the inductors?

Thanks

Just so I am clear, you mean the height of the inductors behind the PCB? as well as their diameter.

Cheers

Ian

 

[Humner]

Thats correct.

The prescott inductors don't appear to have the extra metal supports the carnhill variants have.  This is a good thing as the enclosure I'm using has some space restraints I'm tying to adhere too.

Thanks

 

[ruffrecords]

Thats correct.

The prescott inductors don't appear to have the extra metal supports the carnhill variants have.  This is a good thing as the enclosure I'm using has some space restraints I'm tying to adhere too.

Thanks

The inductors are 25mm in diameter. It is not easy to measure the inductor height on a populated board, which is all I have, but my best estimate is that it is not more the 20mm.

Cheers

ian

 

[Humner]

Thanks Ian,

Diameter was the main dimension I was after. The design of the enclosures I'm using is not normal - hard to explain it, but pictures to come when the inductors arrive.

Rob

 

[Humner]

I've bought my capacitors and have extras as many have to be bought in groups of 10 etc. I can provide capacitor kits for Ian's default values for anyone who needs them. I have extra to make up 6 more of Ian's boards. Majority are WIMA FKP2/MKS2, the 3.9NF is vishay film, 12NF is Panasonic Film, 68NF is EPCOS Film

I can provide each kit at $28 USD including worldwide shipping. All capacitors will be labeled before shipping.

Please PM with your interest.

*Correction! - the 3.9nf is vishay film! So all Film caps.

I've populated 2 of Ian's boards. Some of the WIMA are fat so they need to be populated on the other side.

 

[Humner]

Hi Ian,

i want to implement a IN/OUT switch - I found this diagram of yours for the original PMEQP1A

https://drive.google.com/folderview?id=0B_n67A1hN3qtfkNFTjdqVUtDeUM0YlNzWHZUQ1BKUHpPTkt4TnB3aV8xREtIVmo4Z0xPVlU&usp=drive_web

Are those values you used for the pad going to work well with the MK2? Is the 1k pot used more like a means for calibration?

Cheers

Rob

 

[p0ulp]

Starting this project too, I have question about de HT350 Board that I received this morning , I ve read the HT350 documentation what you wrote, and unfortunatly I m still not able determine the resistor value to power two GMT ...  those mix between current, dB , volt is still a jungle to me.

Just to be clear :

We are using the bridge version of the capacitor input filter.

what does that mean ?

What is the current consumption of one GMT ?

From what I understand  I don't need no transformer to supply the board as I do have 240AC on my french wall plug ? that seems to easy to be true ...


btw the link to sowter transformer documentation is not working anymore : http://www.sowter.co.uk/rectifier-transformer-calculation.htm

 

[ruffrecords]

Starting this project too, I have question about de HT350 Board that I received this morning , I ve read the HT350 documentation what you wrote, and unfortunatly I m still not able determine the resistor value to power two GMT ...  those mix between current, dB , volt is still a jungle to me.

Just to be clear :

We are using the bridge version of the capacitor input filter.

what does that mean ?

It refers to one of the schematics in the Sowter documentation. Basically it means the rectified dc voltage is 1.414 times the the ac voltage of the HT transformer secondary. So a 240VAC winding will give about 340V dc after rectification. We aim to drop 10 to 15V in each of the three following stages of ripple filtering which leaves the final HT voltage around 300V

What is the current consumption of one GMT ?

It depends on which tube you use and whether you configure it as a mu follower or as an SRPP. If you go to the DIY tab of my web site:

http://www.customtubeconsoles.com/diy

and click on the PMEQP1A folder and then on the PMTGMU folder you will find a .pdf file that covers the alternative configurations you can build using the PMTGMA PCB. It lists the current consumption for each type.

From what I understand  I don't need no transformer to supply the board as I do have 240AC on my french wall plug ? that seems to easy to be true ...

STOP!!! NEVER EVER CONNECT THE MAINS SUPPLY DIRECT TO AN ELECTRONIC CIRCUIT. ALWAYS USE AN ISOLATING TRANSFORMER

Sorry to be so blunt but it is fatally dangerous not to use an isolating transformer even when the voltage you require happens to be the same as the local mains voltage. You need a mains transformer that will provide secondaries of 240VAC at about 50mA and 6.3VAC at 1.2A. I am not aware of a transformer with exactly these specs but many people have used the Antek  AS-05T240-50VA. It is a bit of an overkill but it is very cheap.

http://www.antekinc.com/as-05t240-50va-240v-transformer/

You could also use the ML250V63V48V transformer by Don-Audio:

https://www.don-audio.com/Toroidal-Transformer-Mila-sec-250vct-63vct-48V

This is not as cheap as the ANtek but it is available in Europe so shipping will be less. Don-Audio are currently developing a small mains transformer for me for another project that is basically a pair of PMTGMU amps and power supply on one board. This transformer will also be suitable for use with the PMTGMA board. I expect the first prototype transformer at the end of this month.

btw the link to sowter transformer documentation is not working anymore : http://www.sowter.co.uk/rectifier-transformer-calculation.htm

It seems Sowter have revamped their web site. The link is now:

http://www.sowter.co.uk/rectifier-transformer-calculation.php

Cheers

Ian

 

[p0ulp]

Understood, I will be gone for the next weeks or so ... I wait until then for your custom transformer, I also have an order pending at don audio , so ..

thank you for the safety tips, I will look if there is any "meta safety don't kill yoursell before having made something cool"

happy sunday to everyone

 

[ruffrecords]

thank you for the safety tips, I will look if there is any "meta safety don't kill yoursell before having made something cool"

happy sunday to everyone

I don't think we have one on groupDIY but there is one here:

http://www.diyaudio.com/forums/tubes-valves/30172-safety-practices-general-ultra-high-voltage.html

Cheers

Ian

 

[p0ulp]

Ok still waiting for some component and looking for sourcing those inductors ... no answer from the member who uses to make them and looking for a cheaper alternative than buying them at audiomaintenance, if possible ...

Just curious , woulnd't be possible to make a separate board with Toko inductor  (as it is in G pultec 22mH , 47mH , 100 mH , 100mH ) instead of buying those particular one ? Those inductor are quite cheap in comparison, I don't think I have seen this "solution" in any thread so I think that it may be impossible or prone to add noise ?

 

[ruffrecords]

Ok still waiting for some component and looking for sourcing those inductors ... no answer from the member who uses to make them and looking for a cheaper alternative than buying them at audiomaintenance, if possible ...

Just curious , woulnd't be possible to make a separate board with Toko inductor  (as it is in G pultec 22mH , 47mH , 100 mH , 100mH ) instead of buying those particular one ? Those inductor are quite cheap in comparison, I don't think I have seen this "solution" in any thread so I think that it may be impossible or prone to add noise ?

Unfortunately this will not work because it is not a G-Pultec. The inductance values I use are quite different - they go up to a couple of Henries.

Cheers

Ian

 

[Whoops]

Another really nice project from Ian.

Thank you

 

[p0ulp]

Ok I received my transformer which is
primary 230
Sec 6,3V 1,6A / 250v 30mA

So if I understood well, if I go for a 6922 SRPP /Mu type of PMTGMU which current consumption is (11mA  and 5.3mA)  , this one should be ok right ? I think I ll try both configuration , I ve allready have used 3575 for the input and also BBC L81 10k/10k , and maybe go for a A262A7E OEP for the output .

Does it make sense ?

 

[ruffrecords]

Ok I received my transformer which is
primary 230
Sec 6,3V 1,6A / 250v 30mA

So if I understood well, if I go for a 6922 SRPP /Mu type of PMTGMU which current consumption is (11mA  and 5.3mA)  , this one should be ok right ? I think I ll try both configuration , I ve allready have used 3575 for the input and also BBC L81 10k/10k , and maybe go for a A262A7E OEP for the output .

Does it make sense ?

The mains transformer you have is fine as are the two input transformers you mentioned. I am a bit concerned about the OEP A262A7E for the output transformer. Looking at its spec sheet it has rather low winding inductance of just 0.5 Henries per winding and the level it can tolerate at 20Hz is only 0dBu none of which is surprising given its small size. Instead I would recommend using the Carnhill VTB2281. It has much higher winding inductances of over 12 Henries, is much bigger and can certainly tolerate any level the tube can throw at it. It is more expensive but at £18 instead of the £9 for the OEP A262A7E it won't break the bank.

Cheers

Ian

 

[p0ulp]

Ok I ll order them from audio maintenance as for the inductor , this project is a trap  ;D , anyway money doens't always matter , thank you again for the support Ian.