P2P Redd 47 - a few questions

[alexc]

Thanks for the toroid info CJ - fills in a few gaps for me.

Cheers

 

[GabrieleP]

I wonder what the advantage of lowering the input transformer's ratio would be.

The lower the ratio, the lower the leakage inductance and the parasitic capacitance, which in turn increases the resonant frequency. Typically a 1:10 xfmr has a resonance just above 20k, which needs to be damped. Reducing the ratio (everything else being constant, of course) to 1:5 gives 6dB attenuation and the resonance goes up to 40k, making it much easier (even unnecessary) to damp.
[/quote]

Almost off the content of this excellent thread: can anyone tell me the reason why old "mic to grid" transformers installed in tube preamps had a 1:15 ratio or so, 200 ohms primary and 50k secondary? A technical one? Else?

Thanks a lot!

 

[simonsez]

Can i put a direct box transformer use for input iron? I have Jensen JT-DB-EPC .



Thanks

Simon

 

[abbey road d enfer]

Almost off the content of this excellent thread: can anyone tell me the reason why old "mic to grid" transformers installed in tube preamps had a 1:15 ratio or so, 200 ohms primary and 50k secondary? A technical one? Else?

Definitely practical reasons. In a tube preamp, there is no input noise current, so the higher the level the better; as a consequence, the higher the ratio, the better. The limiting factor is the parasitic capacitance, which puts a clear limit to the HF response. The tube input capacitance  can be minimized by using a cascode stage so in practice, the limiting factor is the parasitic capacitance of the xfmr's secondary.
I am not aware of any xfmr manufacturer capable of producing a secondary with more than 50kohm impedance having a good response up to 20kHz, within normal budgetary constraints.

 

[abbey road d enfer]

Can i put a direct box transformer use for input iron? I have Jensen JT-DB-EPC .



Thanks

Simon

It should work somewhat. It looks like it's been designed for about 2x the impedance of a mic xfmr. The HF response may not be as good as a dedicated mic xfmr. You'll have to find out.

 

[simonsez]

Thank' s Abbey....

 

[CJ]

<typically a 1:10 xfmr has a resonance just above 20k ...>

most power transformers will do that,

if you are not past the 80 k range with your peak, then your design work is not finished.

<can anyone tell me the reason why old "mic to grid" transformers installed in tube preamps had a 1:15 ratio or so, 200 ohms primary and 50k secondary...>

150-200 ohms is normal for a mic input, primary side,

but yeah, most people do not want to exceed 1:10 turns,
i think Lundahl has some 1:8 stuff, Jensen has 1:10,

more gain with a 1:15 but response is going to be a little off, noise should cancel if it is good,


<i am not aware of any xfmr manufacturer capable of producing a secondary with more than 50kohm impedance having a good response up to 20kHz, within normal budgetary constraints..>

Sowter makes a few things under the Pro Vintage section,

Yes you have to pay for anything higher,

1:30  V76 input, figure 180 K sec if you call the pri 200,

but with that core, you could call the pri 600 and so the sec becomes 540 k!

but you need tons of mu shielding for that ratio thus the 400 dollar price tag,

you can get a Peerless K-241-D on evilbay for big bucks,

1:12 ratio and real quiet with a great sound, i think my favorite input,

4665, K-241-D Transformers, Clean. Primary: 500/280/125/31 - 70K ohms...or 600/340/150/37.5 - 84K ohms. Max level +8dbm With a 10-30K

 

[earthsled]

Hi all,

Found this schematic the other day. I see a few differences: The Zobel after T1 is different (probably to accommodate the "B" model transformer), the small caps on used on the gain switch have different values, and there is increased filtering on the DC rail.

Please have a look and discuss!

 

[GabrieleP]

Thank you Abbey and CJ!

 

[soundcollage]

I've searched, but can't find specifics on the zener string.
    Would 3 100v 5w zeners be a reasonable substitution, or will a longer string of lesser values have a real world difference in noise?
    Would it be worthwhile to parallel a small value cap and large value resistor across either each zener, or the entire string?
    Would adding a high watt 5k(ish) resistor after the zeners approximate some of the sag the 0a2 is said to exhibit?
Thank you.

 

[abbey road d enfer]

Would 3 100v 5w zeners be a reasonable substitution, or will a longer string of lesser values have a real world difference in noise?

No. Anyway noise is clamped down by the decoupling cap.

Would it be worthwhile to parallel a small value cap and large value resistor across either each zener, or the entire string?

No. There would be no point in doing this. The Zeners are always in their conduction zone, so their voltage does not need to be determined by resistors, as is the case in rectifying diodes, where they are half of the time in non-conducting zone and their voltages may be all over the place when series-connected.

Would adding a high watt 5k(ish) resistor after the zeners approximate some of the sag the 0a2 is said to exhibit?

You may want to add a resistor in series with the zener string in order to simulate their internal resistance. The internal resistance of the OA2 is 80r typical, 240r maximum, so you would use a resistor of 160 to 480r in series with the zener string. However, since the circuitry operates in class A, I don't think the sag is really consequential.

 

[devonhaas]

Almost off the content of this excellent thread: can anyone tell me the reason why old "mic to grid" transformers installed in tube preamps had a 1:15 ratio or so, 200 ohms primary and 50k secondary? A technical one? Else?

Definitely practical reasons. In a tube preamp, there is no input noise current, so the higher the level the better; as a consequence, the higher the ratio, the better. The limiting factor is the parasitic capacitance, which puts a clear limit to the HF response. The tube input capacitance  can be minimized by using a cascode stage so in practice, the limiting factor is the parasitic capacitance of the xfmr's secondary.
I am not aware of any xfmr manufacturer capable of producing a secondary with more than 50kohm impedance having a good response up to 20kHz, within normal budgetary constraints.

If you go to Tab- Funkenwerk site, Oliver Archut will touch on why V72/76 level Radio standard Mic Pre had 1:30 ratios and down with  some using a choke input(3 chamber -on plate..)... you can do a search on Archut transformer etc... On a related issue,..I have just finished a tedious turret board- built Redd 47 , that 'photographs extremely well', but using this 460 volt transformer with a 6.3 volt secondary, fails to supply 6.3-7. volts  to 'Two' tubes', namely, the E88cc and the EF86.  I have 2.5 amps of current available, but have not been able to reconcile how to terminate- this' often 'overlooked topic.  I build Microphone systems with OB2's that sound better than Neumann.. but when it comes to providing  "AC" 6.3 voltage to -two 'differing tubes'  such as is found on the REDD 4 7,  I can only get 3.5 volts on both.  I have only done DC discreet choke fed shunt regulation for filament and have no experience in AC heater  design with diferent tubes. 1. First attempt - simple parallel- ( load to pin 4 and 5- both tubes..)no ground ref.. gave me a steadily changing '5-7volt' on the E88cc and '4- 5.1 volts' on the EF86(receiving current last..).  2. Second shot, I changed to the Guitar Amp approach where you short 4 to 5 and then use "9" on the E88cc as the other terminal.. with the same on the EF86- except using pin "7".3.  Third attempt, I -now have 3.5 volts --across load , with E88cc pins 4 and 5 tied together, other lead to "9" , with the EF86  simply going to pins 4 and 5-  however with a ground reference.There is no center tap, so I built out two 5 watt Ohmite resistors to signal ground and took my 7 volts from their.  Also, can I not 'clamp' this with some simple voltage clamp at 6 volts?  Across the load?..The transformer is "6K49VG"  Hammond(La2A) and is capable- more so than I (ha..) with a 460 volt secondary @50ma/[email protected] WITH CT for B+.  I have great B+ voltage at 294volts and I'm using an AMI Tab Funkenwerk BV8r at .6.5:1 for input and output- since they are around..  Input secondary load resistor is 63k. Output is 10k  over secondary.  Any help with this Multiple  Tube AC Heater dilemma would be appreciated!

Speaking of noise issues, what are your opinions about DC heater supplies? Is it possible to change 6.3V AC into DC using a voltage doubler, rectification, smoothing, and regulation?

Although theoretically possible, it may not be. Regulating a voltage implies wasting energy, and your transformer may not have enough power to start with.
Voltage doubler is a nasty thing because it is single-wave rectification, meaning you need huge caps to maintain a low enough ripple, which means the peak current will be very high, and may create too much stress to the transformer, and the voltage loss created by the DCR of the windings may be too high.
DC heaters voltage is a nice thing if it's built-in from the start or you take it as a redesign, with properly dimensioned transformer, full-wave rectification, reasonably large caps and quiet regulators (some are less than others).
I've seen too many improperly DC heaters that were noisier than a clean AC. Clean AC is 50/60Hz with very small harmonic content. Dirty DC is full of upper harmonics that cunningly go direct from the filament to the grid.

 

[Gearsix]

I am becoming to realize this preamplifier and I'd like to know if someone can help me to find the schematic for a definitive 290V dc power supply without the tubes
I know that tubes contribute to the sound, but I prefer a traditional power supply, but I have seen in a photo that the Revolution version of Redd 47 is without the tubes ...
My project is to realize 4 modules so I need a "powerful" power supply
Thank you in advance for the help

 

[ruffrecords]

Almost off the content of this excellent thread: can anyone tell me the reason why old "mic to grid" transformers installed in tube preamps had a 1:15 ratio or so, 200 ohms primary and 50k secondary? A technical one? Else?

Thanks a lot!

Any or all of the following:

1. Noise. Tube mic pres have noise problems so the bigger the input signal the better.
2. Frequency response was only expected to be flat to 15KHz so you could get a higher ratio than if response needed to reach 20KHz
3. Low output ribbon mics were common so more gain was needed.
4. Many mics were 50 ohms nominal impedance so their output was lower and more gain was needed.

Cheers

Ian

 

[PeteC]

Hi all,

I have digested most of this excellent thread on P2P Redd47 - as I'm most of the way through building a 2-channel version.

Just one question on the power supply.... 

I have a Power TX that will provide the necessary voltage and 150ma of current - and I know I just need just 85ma for the two channels really - so lots of current headroom etc.

As I'm running 2 channels in the same box - do I need to duplicate any of the filtering and regulation stage ( OA2's or Zener string )  or can I just take the regulated supply to each channel as its HT without worrying about it ?

Do I need an extra filtering stage for each channel after the Regulator  ?
BTW I'm using UF5408 diodes in the supply, not 1N4007's.

Sorry,  that turned out to be more than one question !!

I realise I might have to mess with the dropping resistors in the power supply as I'm going to be pulling 60ma for 2 channels rather than just one channel at 30ma. So maybe I might need to reduce one or more of the 1K resistors to 500ohms by paralleling another 1K across.

Any thoughts ?
Cheers
Pete C

 

[CJ]

you could split the B+ line off into another RC filter just in case there are cross talk issues

you have a single ended output stage pulling some hefty current which means that a good bass signal could move the B+ node around a bit,

so cheap insurance to add a few more parts,

 

[PeteC]

Many thanks CJ,

how about the single Reg , the 2x OA2's ( or 300v zener string )  - Can I get away with just adding an R-C filter after the Regulator ?

Cheers
Pete C

 

[CJ]

yes that should be enough isolation,

there are a few words about the pwr supply in here that you might want to check out>

http://www.groupdiy.com/index.php?topic=7292.0

 

[PeteC]

Many thanks CJ - I will check out the info

Rgds
Pete C

 

[CJ]

maybe use a Choke-Cap instead of the RC if you have too much voltage drop across the RC resistor,