The planar v72 from scratch

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solkatten

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The Planar V72SE emerged from a discussion in the tread:


Many thanks to Winston Oboggie and abbey roads d efer for sharing great ideas and solid advice (y)

I have been fooling around with the planar tube 6s17k-v, a low noise/microphonics and high gain triode, as front end on a mic preamp.
The idea in this design is to use a cascade (jfet + triode) to avoid miller capacitance which tend to degenerate the performance of high gain triodes. The cascade has a gain of 50-55dB. With a varible gain between +20 to +50 dB plus a -20 input PAD. I think you got a practical range for the preamp.

The preamp share some of its topology and got its feedback derived in the same way as the classical v72a. The initial idea was to have a parafeed choke loaded CF. A more elegant solution emerged by simply letting the dcr of the SE transfomer elevate the cathode. This way we avoid to many poles and parasitic elements of the cap, choke and transformer solution...

Its a clean and simple design: gain controll, -20dB input PAD and pole reverse.

The preamp is going to be a single channel unit, built with chassi, transfomers and chokes I already have at hand... ...so some compromises is necessary... As always Im going to build on eyelet boards. The power transformer has a 6,3V winding for the heather of the triode strapped E81L tube and 9v winding which is rectified and filter and feed to a Rod Coleman regulator (a gyrator ccs combo) for the 6s17k-v. (Which has a shared cathode and filament connection.) The HT 180VAC is going to be C-L-C-L filtered to obtain good smoothing without dropping to much voltage. The PT has no 48V winding.

The build is gonna be from scratch, so I´ll post pictures along the way...

The schematics...
 

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solkatten

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The Lundahl line up + Screening can...

The IPT is going to be Lundahl LL1538 1:5, it´s gonna sit in an screening enclosure to ensure an extra quite front end. Its pretty much flat between 10Hz-100Khz and does not need termination.

The OPT is also a Lundahl (we are both from Sweden(y)) LL1671SE 30ma very fine and capable trafo. Each primary winding can be terminated with a 10K resistor for flattest response.


 

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solkatten

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Building a DIY tube socker for the 6s17k-v tube. Just solder 1mm solid wire to the tube, then mount it on a terminal👍
 

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solkatten

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The 6s17k-v needs a really quit heather supply because of shared cathode/heather connection. Both the cathode voltage and the heather supply is gonna be floating on top of the jfet... Rod sells a convenient compact pcb that can be used if your really in to quit supplys or, like I, playing around with esoteric tubes... :)
 
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solkatten

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Templates for the eyelet boards...

After a bit of contemplating I will start this project by building one channel, but it makes much more sense to do 2 channels in the end. The only thing that a dont have right now is another LL1671...
 

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Winston OBoogie

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Nice job. I really like your way of building/prototyping, not too much hassle to alter things as need be.

Have you ever tried those Coleman regs on regular old indirectly heated tubes? Wondering if there's a benefit to be had there too?

👍
 

solkatten

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Nice job. I really like your way of building/prototyping, not too much hassle to alter things as need be.

Have you ever tried those Coleman regs on regular old indirectly heated tubes? Wondering if there's a benefit to be had there too?

👍
Its quit allright... but I dont know if its necassary... I mostly use ac-heather lately, its simple, the problems are predictable and curable... I actually had bigger problems with rectifier noise in dc-heathers.... Maybe the front would benefit....

Its really easy to build and modify the eyelet boards....
 
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solkatten

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The tools for the craft:

Eyelet board prototyping EU source:

Eyelet TR-10 :: Terminals :: Interconnects :: Electronic Parts :: Banzai Music GmbH
Eyelet Tool :: Accessories :: Prototyping :: Electronic Parts :: Banzai Music GmbH
Board FR4-RED :: PC Boards :: Prototyping :: Electronic Parts :: Banzai Music GmbH

or you could buy from Canada:

 
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Winston OBoogie

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Thanks for the info :)

With a cascoded 2SK170 being fed from a nice 1:5 like the Lundahl, you're then in a new Arena as far as input noise is concerned.
What once was a very low gentle hum from your input tube's AC heater might now stick out like a sore thumb.

Anyway, not an issue in your case as your 6SK17 has that nice reg :)
 

solkatten

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Thanks for the info :)

With a cascoded 2SK170 being fed from a nice 1:5 like the Lundahl, you're then in a new Arena as far as input noise is concerned.
What once was a very low gentle hum from your input tube's AC heater might now stick out like a sore thumb.

Anyway, not an issue in your case as your 6SK17 has that nice reg :)
Yep u are right it totally depends on the circumstances. I have built tube pres with relatively low gain and now feedback. A triode strapped 6j32p is pretty quit with ac heathers in that application...
 

solkatten

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Thanks for the info :)

With a cascoded 2SK170 being fed from a nice 1:5 like the Lundahl, you're then in a new Arena as far as input noise is concerned.
What once was a very low gentle hum from your input tube's AC heater might now stick out like a sore thumb.

Anyway, not an issue in your case as your 6SK17 has that nice reg :)
I really like the the flow of this thing. The transformer do the first 15dB, then a really quit jfet take over, finished by a really quit triode with high headroom on top...
 
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Tubetec

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I have an EL84 based line driver amp I made,nothing fancy just a Sowter 9705 in the anode 10k:600
One day for test purposes , I rewired the cathode to the positive end of the heater supply, at the operating point , the heater supply (6.3v)coincided with the value of cathode voltage I required , so simple single ended supply instead of resistor balanced to ground ,
I found I needed at least a 30 db quieter heater supply for the DHT style wiring to compete with seperate heater and cathode RC to ground in the usual way .

I would think with the super high gain of the 6s17k-v a quiet heater/cathode supply matters even more, alot more than a power tube like EL84 with a gain of 20 or so .
 

solkatten

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I have an EL84 based line driver amp I made,nothing fancy just a Sowter 9705 in the anode 10k:600
One day for test purposes , I rewired the cathode to the positive end of the heater supply, at the operating point , the heater supply (6.3v)coincided with the value of cathode voltage I required , so simple single ended supply instead of resistor balanced to ground ,
I found I needed at least a 30 db quieter heater supply for the DHT style wiring to compete with seperate heater and cathode RC to ground in the usual way .

I would think with the super high gain of the 6s17k-v a quiet heater/cathode supply matters even more, alot more than a power tube like EL84 with a gain of 20 or so .
Yep the filament bias calls for clean dc👍
 

Tubetec

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I couldnt find a translated spec sheet on the 6s17k , what grid resistance will it tollerate max?
 

Tubetec

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I was wondering how the 6s17k-v might fare in a condenser microphone as a cathode follower , here high grid impedence would be required . It has the gain of a pentode with the distortion of a triode , as a cathode follower youd expect very low distortion and low output impedence .
 

solkatten

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Maybe CF but you have to elevate the cathode and you need a super clean heather supply. Your right the low allowed grid resistance dont work well with 1giga ohm resistor in the condenser mic. But it work grid biased in a active ribbon mic sitting between a ribbon trafo and OPT. I think that is what I should do if I were to put the tube inside a mic.... (y) And probably the ribbon starts to roll off highs before the miller c becomes a problem :)
 
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