starved plate design

[sweetspence]

heres a question and it might have been answered somewhere before but here it is. if you have gear that runs on a starved plate design is there a way to up the plate voltage and truly have a tube in the signal path? what would be the problems with this modification? any thoughts are greatly appreciated

[J M Fahey]

Yes, if you can get from, say, 160V up, basically you can substitute the original circuit values with "real" ones.
Starved circuits can't push enough electrons with such low voltages, so they often use no cathode resistor, and a few even apply *positive* bias to the grids, go figure.
With *real* +B voltage you can go to datasheets and pick values from there, such as classic 100K plate and 1K5 cathode for a 12AX7.
That said, 12AT7 can work with as low as 100/120V and 12AU7 with as low as 60V.

[sweetspence]

so if i were to do something like this how complicated would it be in my berhinger tube composer where the tubes are just used as coloration in the circuit?

[J M Fahey]

Well, what I told you was the general idea, but then things must be analyzed step by step, considering the actual circuit and intended result.
For example, in a circuit where the starved tube can provide, say, 1Vpp , or 300mV RMS, the "real world" tube, may put out 50 or 60V RMS and absolutely nuxe the next stage.
And so on.
So;
1) we need that Behringer schematic , to see what it's actually doing now, and what can we achieve
2) that said, I'm quite certain that the Behringer gets 95% of its sound from some DSP circuitry and the tubes may not even be connected.
And if they are, do very little.

So, if we don't know what do those tubes do for "warming" (which by the way is Marketing Dept babble), don't know what mod to suggest.

I can talk as example from circuits I *do*know and have had on my bench.
a) Hartke Bass amps: they split preamp signal in 2 paths, one goes through an *flat* SS preamp and is called "Transistor"; the other half goes through a tube, is *equalized* and has more internal gain, amd is labelled "Tube".
They sound different because they are *cheating*; otherwise both would sound practically the same. How's that?
b) The Eden Bass amps: the use a permanently connected triode connected as a unity gain cathode follower, fed from the +/-70V (or thereabouts) rails, the same from the power amp.
That cathode follower does very little, if anything, to the sound.

[mexicanyella]

I don't mean to derail this topic, but I am very interested to read JMF's description of what the tube actually does in the Hartke and the Eden. Back in the '90s it seemed like these low-voltage tube "enhanced" products were popping up everywhere, and at the time I had a Real Tube RT-922 rack preamp powered by an ADA Microtube 100 power amp (in my defense, I bought both because they were used, cheap, compact and lightweight, not because they had preamp tubes in them). But I read somewhere that the RT-922 had something like 16 volts on the two 12AX7's plates.

After using that thing for awhile I decided I'd like it better if the gain was a little lower overall and came on more gradually as you twisted the gain knob, so I swapped both channels' tubes for 12AT7s. I thought I could tell a difference at the time; it didn't get quite as snarly/buzzy with the gain maxed and it was easier to find a medium gain sound I liked. So I guess those tubes were doing something in there, or I was really experiencing the tube placebo effect. Eventually I developed a taste for lower-gain sounds and didn't like the RT-922's sound for the band I'd joined. I wish I'd tried some even lower-gain tubes--12AU7s, maybe--before getting rid of it, but I got into small combo amps and got rid of the rack box and its contents. And I never did try other tubes in the ADA Microtube power amp, nor did I ever really look into what those tubes were actually doing in there. Anybody know?

Regarding the Hartke tube, at the time our bass player had a Hartke 3500 head with the dual preamp path thing in it, and I remember us jacking around with it and generally agreeing we liked the clean SS pre as well or better than the "tube" pre. Never gave it much thought beyond that. That's why it's interesting for me to read what's actually in there now. I wonder how much of an Ampeg SVT's signature sound comes from being a tube amp and how much just comes from its tone stack design and the 8 x 10" cabinet...?

[teemuk]

In Behringer it's probably their usual (patented) scheme:

The signal is phase splitted and fed into two common cathode stages that practically operate in push-pull.  A differential opamp stage sums the two opposite-phase ouput signals of the common cathode stages. A "warmth" potentiometer controls the amplitude of the input signal to one half of this push-pull circuit, effectively fading between true push-pull operation (odd order harmonics) and "single-ended" operation where the circuit half in question receives no input signal at all (even order harmonics).

In some designs the "warmth" control is also ganged to a post-tube stage EQ that enhances higher frequencies; sort of giving a further boost for the harmonics the tube stage creates when distorting. I'm not sure if they have that one in the Composer products but it is found from the Ultragain series.

The circuit operates very subletly for a good purpose; the primary application is not some über high-gain thing but spicing up pretty much clean signals. If you push it hard the tubes will most definitely distort and Behringer's circuit is actually a pretty good mix of both generic SE tube sound and PP tube circuit (depending on warmth setting).

I think it's a clever circuit, way more clever than stuff commonly found from "hybrids", and anyone who claims that tubes do nothing in Behringer products simply doesn't know what he's talking about.

They also do the same thing with FETs  instead of tubes (and sans warmth control) in certain guitar amps. The device choice really makes no drastic difference, it's more about the overall circuit architecture.

Behringer has DSP-based products but most of their mic preamps, compressors, enhancers and alike are purely analog. They usually pretty clearly state if the unit in question is of digital nature.

----

Speaking of mods... If you want to further bring out the "tube characteristics" of a circuit like this then you most certainly do not want to increase B+ voltages but actually decrease them. Too high B+ is simply giving too much headroom and you end up gaining no overdrive from the tube stages (effecticely they do nothing then) and with bad luck you'll gain plenty of it from the following solid-state ones, which might not sound all too great. IMO, the "starved plate" has quite undeservedly become sort of a swear word and it's ironic that people who bad mouth "starved plate" circuits can be the same ones who sing praises to stuff like "power scaling", which's main idea is to ...yes... starve plates.

[mexicanyella]

Thanks for that explanation; now I want to go find one of those Composers and try it out for myself!

[sweetspence]

I got this thing for free so i looked it up and it got some good reviews most people only complained about the tubes in it (others just the brand) but over all i figured it wouldnt be a terrible thing to have around. will let you know how it sounds when hooked up. Many thanks

[mozwell]

Found these in my travels, hope these help,   the tubes work from 48VDC

[joecool85]

Found these in my travels, hope these help,   the tubes work from 48VDC

Are these those preamps that work on XLR "Phantom Power" ?

[sweetspence]

 mozwell, you are the man!!  i almost gave up on trying to mess with this thing. Many thanks