Redd 47 Pre with Drive+EQ DIY

[alexc]

Hi All

Here's my next project - a tube preamp with a 'drive' section and EQ.

It's to go with the fairchild 660 style limiter I'm building - you can see it in my PMSix661 DIY thread.

It's based on the redd47 style of preamp feeding a pentode 'drive' section before an enhanced fender style EQ.
All that onto a push-pull line amp with a centre tap fed output transformer.

The transformers are a shielded utc ouncer for the mic in and a Haufe for the line in.

I'm arranging those to have seperate connectors and be relay switchable along with the preamp relay switchable HiZ input to grid. So the front panel has a rotary switch for +48V, Mic and Line inputs, over-ridden by the HiZ input.

The redd47 style preamp is Guavatone's (Charlie) really nice Orange 86. My fav mic/line preamp really.
It is feeding an Edcor 10K:600 output transformer in the usual parallel triode stage parallel feed way.

The secondary of that transformer then goes direct to an EQ network - a fender bass/mid/treble style affair with a few enhancements.

Like the usual 'bright' and 'deep' switches as well as a switchable 'shift' pot that moves the centre freqs of the high and low along some. And also a 'depth' and bypass switch. Basically some variation on that venerable old tone stack.

After the EQ, there will be  a grounded cathode triode gain make up stage,  feeding a high-gain pentode with some 'drive' control. That is passed onto some simple HPF, LPF switches for final band limiting as required.

All that then goes to a simple split load phase splitter and the finals + push pull output  transformer, again an Edcor 10K:600.
They are to be cathode biased, hopefully to combine with the 1st stage 'squish' to make the whole thing somewhat 'compressy'.

PSU is the same transformer I've used lots of times : 250VAC non centre tapped secondary at 100mA giving around 315V B+.

I will use the Orange86 on board PI filter + HV regulator to give around 35mA at 275V or so of regulated for everything but the finals, and around 50mA of 250V or so unregulated for the finals. Around 85mA all up of HV I think.

Actually, the on board regulation is perfectly happy doing the full 85mA so I'll prolly just do that. I make sure I adjust the pi-filter to make the regulation dissippation as low as possible.

There is also a winding for 24VAC-0 at 1A for regulated utilities like lights, relays, JLM VU Buffer rail as well as 48V phantom.
I'm using a JLM 3-rail pcb here. The -ve rail is unused. The 'doubling' mode will do the phantom.

The JLM ac/dc is pretty versatile - you can use it with centre tap ac for bipolar dc or non ct and disable the -ve rail with a simple link.

The heater winding is 9VAC at 2.5A  converted to unregulated DC for the required 2.4A of heater.
Using a big bridge rectifier and 25KuF of filtering here, along with a high power dropping resistor of around 0.5R 25W.

Tubes will be :

- Russian 6J32 (EF86 style) type pentode (tube 1) and 6N1P (6922 style) in the Orange 86 pre (tube 2)
- GE 6BZ7 (AV7 style) for the EQ makeup gain (tube 3a)
- Sylvania 6BC5 drive pentode stage (tube 4)
- GE 6BZ7 (AV7 style) split load phase splitter (tube 3b)
- GE 6S4A x2 (single strong triode) for the push pull finals (tubes 5,6)

Right then.

So you get the idea  - a single channel 2RU chassis with hardware bypassing, a buffered VU meter in addition to an interesting and overly complicated set of features    somewhat reminiscent of a guit-amp  8)

Just my style!

This is a relatively cheap affair, around 600us or so I would say, with 150 of that in delivery for the parts.
After all, next stop southwards of me is Antarctica.

Here's a pic of the rolling chassis, main parts and the current state of front panel design.
I'll finalise the design and finish the front panel pretty quickly. Only 1 VU meter hole to file!

I'll be knocking this together in the short term, because I figure the limiter I will partner it with will be done soon.
And I'm thinking there is a nice place in my music room for this combo.

My last word in preamp+eq DIY  and limiters, for that matter    :'(  but  ;D too

 

[alexc]

Bit more progress here.

Finished more of the modules - psu for phantom and + utility rails, heater rectifier and filter caps (40Kuf), VU meter buffer, line bypass relays, Orange86 mainboard, gain switch, pad and phase reverse pcbs.

Still to go is EQ+Drive module - you can see it start to take shape. It is a small 600:600 input traffo, pentode 7pin socket, dual triode 9 pin socket and a pair of 9 pin sockets for the single triode drivers in push-pull mode, feeding the 15K:600 output traffo, centre fed with B+.

I changed the tube line up - redd47 is ef86+6N1P and the eq+drive is 6BJ6+12AY7+pair 12B4A.
That makes for some 2.5A of dc unregulated heater and 80mA of regulated hv B+.

Also will use around 100mA of +12Vdc as well as +48V for phantom.

The  input traffos are now a utc O8 with O17 shield for the mic step up, utc a44 with a35 shield for line input step down. Preamp output is Edcore 15K:600 as is the eq+drive output.

I'm just making up a little veroboard to allow switching between the mic, phantom and line. I think I will make it a little more elaborate and have some impedance switching for each of the mic/line.

In addition to all that, I'll be spending more time deciding the operating point of the ef86 in the orange. I plan to run it with a higher bias than stock and higher current. I found previous ones to be a little too lowish for the line inputs.

------------

So about to get the chassis powered up and diverting a little to make up a tube-characterisation rig for the tube limiter that will partner up with this guy.

Cheers

 

[alexc]

A bit more progress yesterday.

I have the psu all going fine and finished the Orange86 (redd47 style) pre.

Also did the mounting for the eq+drive board. It's got the 12B4As in the there which are taller than usual.
So it was a tight fit heightwise in a 2RU. But I have it sorted now.

So the first thing to play about with is the biasing and gain arrangement of the redd47 ef86 pentode and so far 6N5P driver tube.

Now what I'm going for here is a moderately clean path - not fizzy at all and just some harmonic complexification . 

So, I'm biasing the pentode to run low sensitivity and gain, higher current and a larger input bias.
Cathode at around 3V, plate at 150V thru 100K, screen at 140V thru 560K.

A bit different - previous one was more usual with warmer bias, lower everything etc.

My main signal sources are active and noiseless guitar/bass pickups which can often do 2Vpp typical and peak at 4Vpp. This is a direct to grid affair.

Also line inputs, which among my analog stuff, levels are generally pretty hot, maybe 8Vpp or so.
This is via a step down which I will switch between divide by 5.2, 2.6 or 1.7
I'm just going to play around there and see how I like. I just want to avoid grid limit here.
There is a 3-setting pad here too.

The mic is going to be around 250:15K or x 7.75 turns. Which is quite a bit. My main mic is the diy mod apex 460 and I have it with quite capable output. So no prob at all driving this guy. My other main one, an mxl sdc is also not short of drive.

So - first stage clean, clear, not too much gain, with the 20dB or whatever on the feedback gain rotary switch. The output shouldn't clip too early, having at 280V B+ but with a gain of x40 or so will do on the highest gain thru scenario.

That gets divided before the driver - there is the stock attenuation plus line driver local feedback.
I'll play with that for quite a large reduction. Currently I have the driver biased for 20mA total and a fairly warm bias of 4V or so. I'd like to raise that to more like 7V. So I'm looking at different tubes.

The AU7 is the obvious candidate, but I've done that before so something else here ie. some russian types.

All that  will go to the edcor pretty standardly. Typical levels here would be 10Vpp after stepdown.

Next is the little Altronics 600:600 traffo - this is a surprisingly wide bandwidth traffo which is bulkier than it looks. I really like these. Basically like a smaller edcor - wide, clean, can take a strong level without freaking out and just a little rawness when pushed.

That is matching to a simple tone stack type of thing, low Z input with a bunch of bells and whistles on the basic theme. Instead of a post-eq 'volume' control, it has a 'blend' control. So, how hard the EQ is driven is done on the preceding redd47  'gain' rotary switch.

After that is the 6BJ6 pentode make up stage - also set biased reasonably cold at 4V or so. That will do  a fairly high gain with an attenuator + simple 'drive' pot control after it. Twists here are the pentode/triode morph pot, maybe some cathode partial bypass.

It will feed  a cathode follower AY7 stage with a simple 'post' pot after it, folllowed by a split-load phase inverter feeding a simple cathode biased, B4A triode push-pull into 10K or 15K to 600 stepdown.

This B4A are obviously overkill for such a duty but again, I enjoy doing things just for the hell of trying something different. They  probably will do 20mA or so a piece, gain will be lowish. Have to see how they go with the edcor - I'm sure it will be fine. I've used these tubes before and I do think they sound good.

Possibly even some feedback if I get the hankering to tinker some more 

Plan is to have a 'balance' pot on the cathodes, for  .. balancing .. and also to do some unbalancing! A bit of unbalance alters the even-odd harmonic set mix so I'm going to try for an audible difference here. Maybe it's possible. Certainly it is visible ont he RTA.
The chunky and not easily intimidated edcor laughs from ahigh at a little unbalance

And finally, there will be an attenuator after the output traffo. I have to make one which can take a fair bit of push. Like a strong line amp'd tube limiters of the past, these guys are essentially little power amps
So probably an attenuator network with a pot inside it that can handle a couple of watts.

------

So there it is - hoping for a relatively clean pre but with lots of character and eq variation available.
It sounds like a lot but I do think I can get there.

I recently redid the 2x ax7 intrument pre with ay7+au7 instead - this is  paired with an la2a in a 3ru.
Lots of stages biased clean interspersed with traffos and it came out very nice. Very different from the original fizzy version of 7yrs ago!

 

[alexc]

Damn it's cold here!  At 4C right now, I have migrated operations to the fireside!

Have the redd47  biased up nicely. So now to the eq+drive board. This one has plenty of space so should be fun

 

[Junction]

Nice project 
Wondering why you have chosen a fender style eq? Have you used a Pultec EQ before? .... mine just works on anything I put through it.
Anything with 10+% alcohol content works well at this time of year.
Cheers
Michael

 

[alexc]

Thanks - yep I have a bunch of diy pultec. I like to mix it up.

Plus this fender style tone stack will have a fair bit of twists and turns. I'm looking for much less variable freqs than a pultec and I don't need the peaking hi band. And I don't need seperate boost and cut controls. Apart from that, it's pretty similar 

You're right about the alcoheel - I like the Jagermeister but not during the day!

Here's a pultec I did - in a channel with an api pre and an 1176 limiter. Not bad at all.
But - sounds a lot better with a tube limiter after it.   Like just about *everything*

 

[alexc]

and another .. there's another two as well So well served on the diy pultec already!

Apart from that, I mostly like fender style tone stacks. Prolly long years of guitar amps and pres.

Many bands of fully adjustable surgical stuff makes me feel like it's a medical appointment.
A sure inspiration killer. Best suited for someone paid by the hour.

I'm finding adapting tube amp eqs to studio racks is my favourite diy thing to do. 

But kind of dull with the usual super clean maxi feedback pre and super clean everything.
And also tired of the fizzy mesa type of thing. Like snorting alkaseltza and doing shots.
Gets tiresome quickly.

Hence the eq+drive - not fizzy distortion, as I have done a few times, but a midrangy hifi roar.
It may be my ears changing, in my later middle years as I am experiencing, but it's all about mid range thickery for me now.

Just like my acoustic, now 20+yrs old, doesn't do treble like new  - the trebly stuff is for the young.
But the mid range is 'where it's at' for me and 'roars' in a controlled, middle aged sort of way.

I'm trying to find that  8)

 

[Junction]

... ah ok, preaching to the converted! Some awesome handiwork there. I have preferred not to go down the "put it all in one box approach" possibly for the reasons you prefer a tube comp over the 1176 you've stuffed in there. But rather one or a pair of tube products into a case and also the 500 route, that way its kinda more elegant to mix and match pres with eq's with dynamics. Got a few on the go at the moment but not much to show off yet. Keep up the good work and keen to hear how you get on with this one.
Cheers

 

[Dr Gris]

I guess I'm not the only one reading your threads as a silent bystander.
But now I have to break the silence and say I'm impressed by your dedication and good work.

Best regards
//M

 

[alexc]

Hey thanks for the kind words.  8)

I've certainly taken away from this experience what I've wanted. There's plenty of ways to go and my choice has always been about discovering what's what with a wide range of the classics.

That means build quality and crafting is always the major casualty. Especially compared with the top end of this community where there is so much that totally carries you away with the awesomeness in the design and the building.

I wish I was starting again with it now - I'd take it slower and with more care.

But life can be short - there's no time to waste

---

So I have the line amp done - it is doing 15mA per phase at a bias of around -27V. I have a 1K balance pot on a cathode bias. So it needs around 50Vpp max input to stay relatively clean. The gain after stepdown won't be massive but thats good. I'll know soon enough.

So now to the cathodyne or split load inverter.

It needs a clean swing of 50Vpp more or less. Gain is around unity, so it will need to be driven with the same 50Vpp or so. I am starting up with a 6N2P. I have limits on heater current here - this is a russian ax7 type needing 325mA heater.

I'm still deciding whether I will feed it from a grounded cathode gain stage or a cathode follower.

So I'll stop here and tonight figure out what I want.

 

[bernbrue]

You're right about the alcoheel - I like the Jagermeister but not during the day!

Hi Alexc,
the "Jägermeister" factory is about 200 meters away from my house. The Mast family, owner of Jägermeister, are the VIPs here in our small town. They do a lot of social projects and support educational activities directly here in Wolfenbüttel. So after working on your phantastic projects take a break from time to time and enjoy yourself drinking one or two Jägermeister. It has to be served ice cold. Prost!!
Bernd

 

[alexc]

hey Bernd - that is very cool indeed

And so I did do just that, last night.

Cheers to the Mast family and their wonderful contribution to civilised life !

Regards
Alex

 

[alexc]

OK - I have my design for the eq+drive amp. I changed phase inverter choice again - back to the good old au7.
Fits my limited heater budget and gives me the low gain, high bias I am looking for.
My next choice, ay7 I have used up my stock already

Running around 6mA  each of the au7 gain stage and split load stage with a combined gain of around x8.

I'll be doing a dc coupling between the AU7 stages, with a step network to drop the relatively high 150V plate of the first stage down to the relatively low 70V on the second stage grid. No cathode heater referencing to higher dc is required as the cathode-heater difference voltage is well below max.

So  - should have that up and running in a couple of days.

I'll test theose operating points and then get to doing the eq which also is not to complicated.
Low, mid, high with a 'blend' pot, some 'shift' switch, a 'bright' switch.

That's plenty.

 

[alexc]

Done the au7 today - now up to the pentode makeup stage.

Here's a (really) rough draw (updated 1_1) schematic - put a face on the thing

The screen supply resistance should be more like 150K. The coupling caps need to be adjusted too.
I always go too large and sub down invariably.

I may also need to increase grid stoppers here and there to keep things under control.
Forgot to add the divider after the pentode make up stage and before the 'post' pot.
And may need to have more attenuation between some parts...

But that's the basic idea.

It's similar to the output amp of my fender champ 25se - a 25W hybrid ss front end, tube back end, little 12" combo that I use whenever I whenever I play 'on the outside'

It's EQ is similar also, but it uses opamps and a diode clipper circuit for drive instead of the pentode.

This combo really does sound good when you don't feel like messing around. Instant usable  gigging sound. For the last 20years or so The Champ 25SE also interfaces to a recording setup pretty well,  having a few patch points available. Noise is surprisingly low. Perfect to direct record that gig.

I used a dc couple from the au7 gain stage to split loader - I like to throw them in from time to time.

 

[alexc]

Here's the champ 25 se schematic - there's a huge amount of really interesting circuitry.
Combining tube classic stuff with modern opamp techniquies.

Really very educational ... if you're into this sort of thing

 

[alexc]

Updated the schematic.

I'm going to move the pentode 'drive' pot off the grid and into the cathode and see which way I like it.
Also want to try the same thing in the cathode of the au7 first stage.

Anyway - just wiring up the pentode now.

 

[buildafriend]

oh dude thanks so much for the awesome pics. you just helped me out! Alex C saves my a** again.

 

[alexc]

OK - this guy is now my 'foreground' project

I've cleared my decks and it's time to nail this guy together.

I have my monster limiter pmSix61 in need of a front end, so now is the time!

When I left it, I had finshed  and tested the psu and Charlie's fine 'Orange86' module, which is in the Redd47 style.

I was in the process of building/testing the 'EQ+drive' module.

So - here is the current progress pic - no front panel yet.

Finalising my controls and will start metal bashing that soon.

Back is all done tho' 

 

[bieckmusic]

Hi Alexc
Great job, looks sweet!  8)

Do you mind my asking, what circuit you used for the led output vu?  Or was that your own design.  Looks nice!!

thanks,
Greg

 

[alexc]

Thanks!

The moving coil vu meter driver is from JLM  http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=40

If you mean the led vu driver from my other thread, it was from a long ago project, the  'pico compressor'.

Not often around anymore - I had a few I sold off a while ago.

Cheers