M610 Tube Microphone & Instrument Preamplifier

could you hit me with the o/p tranny wiring info?

I have those same ones :green:

I was considering saving em for use with the redd.. and going to something else for the 610's


what did you use for the i/p tranny in that one?

I dig the p/s :green: nice and compact

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Winston, whats the diff between the Redd 47 and the 610?
And yes, those custom Reichenbachs sound great driving the Melcors. :thumb:

John I like the modular build.

Cool sections of circuits on the PCBs looks like it makes prototyping faster.

>>> you could try impedance coupling if you want to keep DC out of the traffo but don't want the voltage drop of a resistor.

>> How about power supply filter chokes... or is the distributed capacitance too high?

> I think that by the time you've put enough of them in series to get sufficient L for good bass response, the high end might suffer yes. ...what is a typical distrib. cap. of a filter L, anyone...

When I first saw parallel feed transformer, I wondered "Why???!"

Obviously inductance/$ is lower, and stray capacitance is higher.

One trick is that cap. With judicious R-L-C-L-R balancing you can get a 3-pole high-pass filter shape that is flatter than the Rs and Ls alone. Or get a couple-dB bump to compensate bass-sag in the cascade of R-C coupled stages before this point.

But another trick is leakage inductance. The winding passing DC will need a gapped core, so it will need a lot of turns, so it will have a lot of leakage inductance. As a loaded transformer, this hurts trable response. But in a choke, leakage inductance does not matter (it is either meaningless or good depending how you look at things). By doing the coupling in a second core that does not have DC, you can use an ungapped core, fewer turns, less leakage inductance. I suspect that at typical triode plate impedances, winding capacitance is not a real problem but typical gap-core leakage inductance is a problem; separating the DC from the coupling allows a better solution.

No, I did not write this. Mike at MQ did.
It applies to a much higher power than a line level amp, nevertheless, some interesting points:

"the beauty or benefit of parallel feed in the case of PP doesn't really require any "optimization" of a particular PP trans. Use a good one of course is the cardinal rule. In a sense the optimization is a byproduct itself of parafeeding.

See if I can explain what I have in mind. Some of the alleged benefits of parallel feed is in separating out the AC signal path and the DC supply path. This applies whether the unit is PP or SE. The benefit is that you get much higher PSRR (power supply rejection ratio) in either case.

another benefit (perhaps subsidiary) is that the primary copper circuit has less "work" to do... i.e., once you remove the dc current from the primary of the OT (either SE or PP) then the copper current density is made much better because you don't have the heating effect (and I squared R losses) that accrue from the DC being added vectorially to the AC signal current. Or, in plainer english, the primary copper circuit (say it were wound with a 31 gauge wire) will appear effectively as though it were a larger guage wire once you remove the heating effects of the dc supply current.

Here's an example I worked out;

say a 10 watt output into a 5K CT transformer... the ac volts will be (at full power) 223.6 vrms and the ac signal current will be 45 ma rms. And let us say that there is 60 mils of dc supply current (power supply in series with OT) through the primary winding.

the I sub tot (total current) which is also called the heating current is the vector sum of the ac current and dc current.

In our example the heating current will be 75 ma.

a #31 wire has an area of 79.21 circular mils.

therefore the copper current density (always expressed as circular mils per amp) will be 1056. which isn't bad at all.

But if you remove the dc current and it's heating effects and divide the 79.21 (area of conductor in circular mils) by just the ac signal current of 45 ma rms then your same wire guage will have a current density of 1760 circular mils per amp. This is an improvement of 166 percent.

In order to get the series fed unit the same copper current density as our parafed example you would need to go to a 29 guage wire which has an area of 127.7 circular mils. Now divide the 127.7 by our total heating current (in the series fed example) of 75 ma and you get a copper current density of 1702 circular mils per amp.

so the series fed unit would (due to the additional burden of carrying the heating current of the dc component) have to be wound with a wire 2 guages thicker to nearly equal the "performance" of the parafed unit.

and, from a practical point of view, this would be impossible. there is not enough room in the transformer to wind the same number of turns with a wire guage two sizes larger...

So.. the improvement in copper current density from excluding the dc current provides for us a more efficient transformer with lower copper losses. The copper circuit doesn't heat up as much and it's resistance doesn't increase as much as it would if it has to carry both the ac and dc currents mentioned above.

this same benefit also accrues to parafeeding single ended output transformers... your copper circuit becomes much more efficient in the absence of the heating effects of the dc current when it is excluded from the priamry winding.

So... after all that... from a practical point of view... use a good quality PP output trans, parafeed it, and then just count up your benefits...

1)improved power supply rejection ratio (keep the power supply out of the music business)

2)your primary's copper circuit will become more efficient and smile at you. "

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I'm not sure I follow the theory on current density. Yes, the resistance in series with the load could be less, but this is always small compared to plate resistance. The resistance in series with the supply is going to be about the same. The DC heat will be about the same. The AC heat is a non-issue, even considering the way the two currents sum, because we flow maximum AC signal much less than 1% of the time. The PSRR for a pentode (does anybody still use those?) is actually better if the winding returns to B+ instead of to cathode. A triode will gain 6dB-12dB if returned toward the cathode, true. I see the point about getting the power supply out of the audio business, but I'd view that added choke as "power supply", just a hi-Z supply instead of a low-Z supply, and possibly with more audio "character" than good caps. Efficiency difference will be very small, and if efficiency were any sort of goal, we'd be running class-B transistors. I don't disagree with much said there, just that I see the balance of factors differently.

> try an active CCS instead of the choke.

A choke lets the tube tell it how much current to pass.

A CSS tells the tube how much current it may pass.

So tube bias becomes critical. In a triode, the plate voltage change will tend to find a "working" bias current. This may not be the operating current and voltage you would pick, and will generally be lower voltage than you'd get with a choke. In a pentode, you have two high-impedance current sources that have to pass the same current: it won't be easy to make them play together.

Nevertheless, it is often done. Someone sells a board.

Another approach is a gyrator to emulate a many-many-henry choke with low DC loss and eating any DC current it is fed. A gyrator can be made with a unity-gain amplifier, so implementation may be simple. If unidirectional DC current is drawn, it can be a simple cathode/emitter-follower. The nice thing is that it will self-adjust to any current the tube wants to draw (within reason).

In Rod's Fig 7, R1 is similar to a choke's DC resistance. R2 is similar to shunt resistance across the coil. For high Q (in this case, for high gain without large DC drop) we want small R1 and large R2.

I forget what the hell we wanted a super-choke for. 12AY7 output stage with 30K load? Right away we have to face one problem with a CCS or a gyrator: they don't store or kick-back energy. With a 300V B+, a choke or transformer can swing up to 600V peak-peak, a resistor, CCS, or active gyrator only 300V peak-peak (and in reality much less). For the CSS or gyrator to "replace" the choke with the same max output we need more like a 600V B+ and double the power consumption. Sometimes we down't need max output, but in this case with a 7:1 output ratio we might.

Say the 12AY7 runs at 300V and 10mA. Use a BJT (if you can find one with the ratings). Gm of a BJT running 10mA is like 1/(3Ω), so try a 100Ω emitter resistor to the 12AY7 Plate to swamp BJT nonlinearity. Your load is 30K, so there aint much point in raising shunt resistance above 300K. Tie 300K from Base to +300V. Since this is a large-signal stage, you need to take the Collector up to +500V, +600V or so to cover the swing (no energy stored or kicked).

Using L=R1*R2*C1 {or more exactly" L=C1*R1*(R2-R1)} we need about 3uFd for 100H. This cap has about 1.6V across it, and "should" never go over 2.6V or under 0.6V, so it could be quite cheap. Leakage current will cause a voltage offset (drop) which I think is insignificant for good Japanese caps and these conditions.

> get the benefits of parafeed without having to buy an expensive choke or raise the HT to compensate for voltage drop across a resistor...

This is an old-old problem.

You are near the limit of what you can do with one-half of a twin-triode. 100mW is easy, 400mW is hard, output level is +20 to +26dBm and significantly less for low THD with no feedback. (Ah, I just spotted the feedback from the output plate.)

Silicon is IMHO not the answer.

The classic answer is to go push-pull. More than double the power at lower (but different flavor) THD, and a very much cheaper output transformer. SE 6L6 and P-P 6L6 working at 8 to 12 Watts are very nearly the same price: a 15W push-pull iron is much smaller and cheaper than a 9W SE iron, enough to add heater power in the power tranny, another 6L6, and a phase-splitter.

However if this is a "classic" circuit, going P-P is a radical change.

30K is an awkwardly high impedance for audio iron. 10K with a fatter (possibly dissimilar dual-triode) tube would mean more current but fewer turns, and less trouble with capacitance resonating with leakage inductance.

If this is a "classic design", I vote for not messing with it. If you are open to a new design on similar lines, add a 3rd tube as WCF so you can cap-couple a low-ratio no-DC transformer.

the schemo lists a UTC o-1 on the input and UTC PA-5946 (30K:600) min 55HY for the output.

anyone suggest some drop in replacements? Also no B+ listed. any guesses?