SE Tube Line Amp DIY

[alexc]

Hi All

Now I'm in the home stretch of the last of my current build, the 'Redd47 + Drive EQ' I'm starting up my next couple of DIY projects.

After completing and using pretty much a representative set of builds covering the whole spectrum of tube, discrete and even chip tech, across amps, preamps, sumamps, eqs and compressors, my feeling is ..

'tube is king'    8) 8)

My bias, no doubt - as a guitar player first and foremost, and then 'homey recordist' second.

Tubes just .. do it for me in a way that other techs don't.

Of course, the classic neve, api and even modern vca configurations are wonderful too, but .. yep.

Tubes. Again.

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In light of the great result I have been getting with tube stages after after my discrete stuff, I'm doing this next  ...

A dual tube line amp build - simple and basically a THD 'color' box with respectably drive levels.

This one as always will have a few twists ...

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First up, it's a wooden/aluminium chassis - like the tube monitor amps I previously did.

It's 'tubes on top' point-point wired, which is totally the only way to go .. after you've caught the 'bug'

Using my familiar Edcor traffo with EZ40 (90mA) tube rectifier + choke + pi filtered PSU.

It has a B+ of 275Vdc or so.
And 2.5A of dc (unreg) heater.
As well as a 24V utility rail.

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Signal path front end has  either or both (sum)  of :

Line In  -> pikatron traffo  -> 6BZ7 triode pair as 'constant current drive draw amp'   
ie. grounded cathode amp direct coupled to cathode follower

HiZ In  -> 6BD6 pentode as grounded cathode amp

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Signal path back end has :

EL95 moderate power pentode (5W max diss) -> single ended Telefunken 12K:4ohm transformer

Tele SE traffo loaded in variable resistance (4,8,12ohms) -> paralleled 5K:10K utc traffo -> bal XLR


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Here's a pic of the basic layout - once again, this is a 'use up spare and cheap parts' build.

I like ad-hoc projects with the 'what's on hand' parts constraint    8) 8)

I plan to use this as a stereo color box after notable DIYs that I've done which could use some extra .. beef  .. to be used as a balanced insert on a 'stem' to DAW.

Cheers

 

[ruffrecords]

Cool project. Couple of questions.

I am not clear what you mean by the 'constant current drive amp'. Could you post s rough schematic. It's probably something I am already familiar with but not by that name. Second I am not clear what is going on with the two output trafos? Is the intention to switch between them.

And of course I agree, tubes rule!

Cheers

Ian

 

[alexc]

Thanks Ian

Firstly a correction - should be 'ccda' - constant current draw amplifier.

It's a fancy name for a common config - grounded cathode amp direct coupled to cathode follower with a little component selection to provide some possible benefit.

Here is a description - comes from a discussion of the 'apex 460' mic circuit.

It's basically the front end from a fender bassman, early version. I just like it. 

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The two traffos at the line amp is the single ended one provides some THD with a voice coil output Z, loaded with some 4R, 8R 12R resistor option - across all that is the 5K:10K step up.

Because the step up is much higher Z should not load down the SE secondary - it may be a bust, only one way to find out.

I've done this before and it seems to work out OK.
If I have to, I can buffer the step up with another tube.

So - the SE traffo is driven reasonably hard by the 5W pentode for THD, and that makes it's way to a fairly standard traffo suitable for line amp duties.

We'll see how it goes 

Cheers

 

[ruffrecords]

OK, got it. The CCDA I have seen before on the web - was it Glass Audio??

The line transformer is driven from the SE output with various loads to set distortion. Neat idea. Look forward to hearing some samples.

Cheers

Ian

 

[alexc]

I have the chassis now. Here's an update pic of the general layout.