Pentorama , a tube booster/buffer (idea)

I was reading a really good site about David Gilmour’s rig , Gilmourish.com . It’s know that he collaborates with Pete Cornish for all his gear (since the 70’s). The last “pedalboard” made was in 2006 for the “in a island” tour . I read that Cornish put several tubes in the signal path. Some in buffer configuration . In my opinion it’s not really worth it , but yeah! Gilmour knows his stuff and his job, needs what he thinks it’s best for him , and most importantly he has the money for it !

For those you don’t know Cornish is a Famous electronic builder who works for a lot of big names . he also offers some work for the amateurs but the price hurts (to say the least).

Anyway , I had a stupid idea and drew this schematic with the same principle . Everybody knows the importance of a good buffer but what about a “hype” tube booster/buffer ?

pentorama2

it’s a booster/buffer made of a pentode and a triode. The idea is to use a tube like the ECF80/6BL8 or something like that . It’s a pentode/triode tube is the same glass enveloppe. Why not a simple 12AX7 ? because it’s boring , I like original desings. But a 12AX7 will work as good as a pentode ! The pentode brings something special , don’t know what but you can hear it.

the pentode is a standard gain stage. A bypass capactior isn’t represented because it will offer too much gain and the circuit will overdrive too much the input of the amp.

The triode is a buffer or “cathode follower” to give a nice low output impedance to drive long cable. A tube like 12AU7 will work better , it generates more current that a 12ax7 . The diode gives the proper bias , it can be replaced with a resistor or an another type of diode to make the proper bias point.  The issue would be C3 . Considering the low impedance and the  high voltage the cap will be big in size and value , and electrolytic caps have some cons concerning their place in the audio path. C5 is here to bypass  treble from C3 .

there’s also a switching system made of a transistor controlled relay. A better switching circuit could be designed.

You can notice that some values and components can be missing  because they will depend on the bias point chosen , they aren’t fixed values.

I think I won’t make that circuit, unless I’m asked for , in my opinion such circuit isn’t that good for a pratical point of view . It’s anachronic to use this kind of circuit today (in real situation of course ,  band , gig , gear on the road ect…) It requires a transformer with High voltage , High current (heaters) , tubes are fragile. All that needs serious space . Some tricks are still possible (like in some EHX tube pedals) but make a small box with it will be a challenge. aking the circuit with a PCB will reduce the size , though . Power transformer for that kind of circuit is cheap considering the low power required (few mA for the anodes and 1A max for the heaters)

Beside guitarists always ask for small stompboxes and a circuit like this one won’t correspond to these wishes. For myself, I don’t like the idea of having high voltage at my feet . Problems happen and if they don’t they will ,  for sure ! Dancing with High voltage isn’t a safe thing to do.

There are serveral smaller circuits that can make the same without all these issues . However, if you’re a die hard tube user and want nothing else between your guitar and amp , that’s the way to go. No need to be Gilmour to have fancy stuff !

After all , electronic can also be only doing posh and hype circuits  without thinking about pratical issues and efficiency , just pleasure !

but if anybody wants to make it and tweaks it , I’ll be really glad to heart about it !

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