REDD47 p2p - FINISHED

[PeteC]

Its not the TX's - I detached them from the sub assembly chassis and still have same symptoms of tiny signal when I dont earth the sub assembly and full signal when I do.  Strange thing is the sub assembly isn't electrically connected to anything on the boards !

Must be a stupid ground thing I am missing.  Or is an oscillation problem.



Rgds
Pete

 

[CJ]

with the sub assy out of the box, try injecting a signal into the Sec of the Sowter and see if it gets amplified,

also, with the sub assy out of the box,  try a 1 uf cap from the case of the Sowter to chassis gnd, this will sort out if it is an AC or DC problem,

 

[PeteC]

OK thanks CJ,

interesting.......

I just rigged up the whole preamp on  just the breadboard - so completely out of the chassis and using clip leads for connecting everything up between the PSU, preamp boards, xlrs, gain controls and the grounds to mains earth etc.
http://i55.photobucket.com/albums/g159/pedrocortez/2FED5FA8-B17D-4A51-AAFA-3D12A2A97596-1703-000001FE4D26BCA5_zps0df5533d.jpg

Leaving the input fully open I get a squealing oscillation, then if I EITHER ground the sub assembly chassis by connecting to my star earth point, OR connect a 1uF poly cap between the chassis and the case of the input TX,  the oscillation goes away completely.  So I guess this is some kind of AC oscillation problem ?
http://i55.photobucket.com/albums/g159/pedrocortez/290ECB01-AC79-4298-A0AF-EBB6175984CF-1703-000001FE36CCBF47_zpscf02ed57.jpg
I have used the Gorbut/Guitarmaker layouts for the REDD - only difference being I rotated the valve sockets so that the heater wiring is as far away from the signal leads as possible.  Could this have introduced some stray oscillation problems I wonder ?  I was trying to improve the original layout because of the heater orientation.

With the secondary/grid grounded the preamp is completely silent.

I have built loads of valve gear and never had this kind of puzzle before !!

Any thoughts ?

I have taken voltage measurements across the circuit - the only strange thing I found was that I had 44mv of DC at the end of the 0.22uF coupling cap/grids of V2.  That shouldn't be there I guess.  If it was a dodgy coupling cap I wouldn't expect to get the same symptoms on both channels sub assemblies I have built - must be a layout issue somehow.

I have a simple sig gen and a cheap scope but have never really used them much - I can try scope the pre's if someone can guide me through it.

Anyway - knackered now so will leave it for tonight and try again tomorrow - I'm itching to get this thing in my rack but don't want to finish the build with a possible problem lurking in there that I cant find !

cheers
PeteC

 

[CJ]

i do not think the tube sockets are the problem,

hi gain amps can get a little unstable when laying on the bench like that,

did it oscillate with the gain turned to half?

maybe that input trans has some stability issues that require the screen to be grounded,

easy to check, wire in a different input trans if you have one,

there might be phase shift issues without the screens grounded on the input trans,

you could check this if you have a dual trace scope, inject  signal and check phase angle with and without the screens/can grounded,

you might try floating the I/O jacks also unless they already are floating,

sounds like if you bolt everything up and have a good star ground that the amp will be ok, try keeping grid wires as short as possible,

 

[PeteC]

Thanks CJ,

do you think its worth trying grid stoppers on the EF86 grid ?
Rgds
Pete

 

[PeteC]

Having read up a bit more on EF86 circuits it seems its not unusual to have the screen at higher voltage than the plate - and thats what I'm seeing when I ground the sub assembly chassis.

Can anyone with any type of REDD47 tell me whether a higher screen than plate is normal.

Looking at the original EMI schematic I can see that my voltages on E88CC V2 agree completely - so no probs there.
On the EF86 I see that the cathode on the schem is at 1.65 - and I have 1.75v - so about 1.75ma going through the cathode.
The schem shows the plate with 1.35ma across 100K.
There is no screen voltage shown on the EMI schem - but I am assuming that with 1.75ma flowing from cathode, and 1.35ma flowing through the plate - that the missing 0.3ma must be the screen current.

With 0.3ma screen current flowing through the 390K screen resistor , there must be a voltage drop of 117v ( 0.3ma x 390K screen resistor )
With B+ to the first stage at 205v the screen should be at 205-117 = 88 volts .  I.E -HIGHER THAN THE PLATE VOLTAGE - which is shown as 73v on the EMI schematic.

So , based on this it seems to me that my circuit is working properly when the sub-assembly is grounded - in this condition I see similar voltage to the ones on the EMI schem - as long as I have calculated the screen voltage correctly - based on my workings above.

Would be great if someone could confirm if their REDD47 voltages at plates and screen of the ef86 for me  :-\

still cant explain why the sub-chassis needs to be earthed for the circuit to work.  Still puzzled by it..

Rgds
Pete

 

 

[CJ]

i found that lowering the screen to about 100 volts on those ef86 tubes will make them last longer and make them less prone to developing noise and microphonics for which they are so famous for,

you need to increase the screen resistor quite a bit to do this,

 

[PeteC]

Hi again

screwed around with screen resistors tonight and landed on 470K for screens which brought down the screen voltage to 88volts and plate up to to 83volts. 

still cant fathom the chassis earthing thingy - but have recorded a couple of tracks with Martin acoustic and fender bass ( DI through the 47 ) and sounds lurrrvely......

so gonna stop worrying about the sub-assembly earthing thing and get on with case metalwork. (  I think it must be something to do with screening the fields around the EF86. ????)

Amusing to track tonight with the preamp laid out on a breadboard with lots of clip leads - Dont try this at home kids......

cheers
Pete

 

[PeteC]

OK,

been advised that shunting of the meter will probably be giving me wrong screen grid reading.
With say 1 meg resistance of meter across 390K I am probably getting c 280K with 108v reading - so real current of probably nearer to 38ma.  in circuit with the 390K this is a voltage drop of 150volts from B+ to screen so screen really sitting at lower voltage than plate - I think !!

Anyway

cos I like to have things just so,  I injected a 1KHz, 50ma test signal and measure gain from input to output

Hope I have done this right ....first time I have done this so tell me if I'm doing this wrong.

AC Voltage readings at output .....
Reading          Gain        dB
1)  3.5v          70            37
2)  6.7v          135          43
3)  11.8v        235          47

So looks like I am in the right ballpark vs the 34, 40, 46dB gain settings on the original unit.  Hurrah !

Cheers
Pete

 

[PeteC]

Some progress between work assignments this week....

just the metalwork for the faceplate to do , then final hook-up tasks.

http://i55.photobucket.com/albums/g159/pedrocortez/69A6B2DE-8EEE-435D-B17E-9AC845458E95-680-000000C6C6437CE7_zpsd94c8241.jpg

Just heard from Tim C that my capsule is ready for the G7 - so really looking forward to the new input chain !

Rgds
Pete

 

[MartyMart]

Great work Pete
I'm watching the progress you are making = Impressed !!

Have only done P2P a couple of times, a 7 watt valve guitar amp and a small 1.8watt amp also.

Great fun

Marty.

 

[PeteC]

cheers Marty, much appreciated

slowly but surely getting there - I keep adding features - pad, input impedance, phase etc and finding better ways of doing things.
For example the Sowter TX have flying leads, and terminating them is not obvious.  So I have made phenolic circles to slip over the mounting screw so I can terminate the leads properly and give me an earth point for shielding input wiring etc.  Seems to have worked out OK.  Forgive the rough phenolic - this was the first one I cut and couldn't be bothered to do a better one - the others are smoother !
http://i55.photobucket.com/albums/g159/pedrocortez/55ED7400-823E-4EF9-ABE8-661B606C78B7-417-0000003682B0C049_zpsb28fb0d6.jpg

Cheers all,
PeteC

 

[PeteC]

front panel progress
http://i55.photobucket.com/albums/g159/pedrocortez/8F98384F-D67F-44D6-8C14-801ADB4688F2-376-000000ADE6F025EB_zps1c72e164.jpg

apologies for the shaky iphoto shot.

For each channel - Combo input, bass lift, phantom, z, -20dB pad, Polarity switching
Gain and Output attenuation selectors. No fine gain trim - not worth it for me.

Probably gonna use white ink pen for legends - cheap and funky !

Just want to get it finished now...

Cheers
Pete

 

[PeteC]

Very happy to report that the dual channel REDD47 build is complete and passing lovely audio.

Here is a low res shot - will put up some better pics soon.


Controls are combined DI/Mic inputs with rumble filter and Phantom, low-Z/High-Z, -20dB input pad, and polarity switching.
On output I have 0, -10dB and -20db attenuation. So plenty of flexibility.

And its all Mullard !!

Next up ....finish the G7 mic and do a couple of DIY ribbons !

Thanks to all who offered advice along the way on this build ! Great community !

Rgds
PeteC

http://i55.photobucket.com/albums/g159/pedrocortez/77B5D6FB-5155-4770-94E7-7A9E9710E580-1107-00000104AC6D495D_zpsadf0d53a.jpg

 

[My3gger]

Nice, congratulations for this build. Do you also get this huge bass and slightly agressive highs? And of course lots of details... I'm asking because you have complete Sowter iron and i have Cinemag.

 

[ruffrecords]

Well done Peter, that's a really nice looking build. It's always good to see some point to point.

Cheers

Ian

 

[baadc0de]

Nice, congratulations for this build. Do you also get this huge bass and slightly agressive highs? And of course lots of details... I'm asking because you have complete Sowter iron and i have Cinemag.

I have Sowter here and it sounds that way to me too. It is probably the EF86 pentode "sound" though. Very accurate description.

 

[PeteC]

Cheers all,

Early days so its hard to say for sure ref huge bass and "forward" highs- but yeah that sounds like a fair description.

Did some tracking with mic'd acoustic and DI'd jazz bass last night and the bass sound has really knocked me out.  Havent tracked any mic'd guitars or vocals yet which was primarily what I built this for.

Have limited test equipment so wondered about how best to test noise floor etc ?

Also I have a switchable rumble filter of 1uF and 3.3K0hms across the input TX primary and it doesn't seem to be doing much.

cheers
Pete

 

[alexc]

Testing the noise floor is best done with a good quality soundcard and a 'real time analyzer' application.

For a basic noise floor test of a mic input, you terminate the mic-in xlr with some reasonable value resistor or even short circuit the hot-cold phases together. Perhaps someone more expert in this can elaborate.

You then connect the output of the preamp to the soundcard input - good soundcards have balanced ins, outs.
It's very much worthwhile to get a good soundcard with balanced I/O if you are a serious DIYer

The 'RTA' app then shows you the spectrum (graph of amplitude vs freq) of the output of the preamp in 'real time'.
You can vary the gain of the preamp to see the effect on the noise floor.

RTA apps often give you a 'headline' figure for the incoming signal 'energy level' - often referred to as an 'SPL' (sound pressure level). This is the aggregated energy level over the audio bandwidth ie. 20Hz-20KHz or even up to 44Khz or 48KHz.

The SPL can be relative to the soundcard's 'full scale deflection' or FSD which is the max input amplitude before clipping, usually expressed in 'dB' or 'dB below FSD' or it can be referenced to some absolute voltage or power  ie. 'dBu or dBV and so on.

The next thing to do is to use the soundcard's output to send a tone (eg 1Khz) of a certain amplitude (eg -10dB or -10dBu) to the preamp and return the preamp;s output to the soundcard. The RTA application then shows the spectrum around and including that test tone - you can figure the gain and distortion of the preamp by measuring the spectrum 'peaks' and so on.

Good RTAs also let you 'send' a frequency sweep to the preamps input and record the preamps output and display a 'freq response' graph at various signal levels.

A good RTA to get is 'TrueRTA'. For the committed diy person on a budget, it's the best. It will grow with you.
edit  - it's a PC thing. For apple folk there's probably something on an iphone or whatever

Another one people use is 'Rightmark' - free and good for somethings. But not as real-time usable or awesome as TrueRTA.

PS - what a fabulous build - anyone would be proud to offer that beauty up!

Cheers

 

[PeteC]

Many thanks for the comments and advice Alex.

I will look into the RTA options for Mac.

Rgds
Pete