The Rude Tube

[alexc]

very nice - just going thru that ampeg schemo

I completely believe some eq is a great idea with 'the rude tube'.
Rude Tube II  eh?  Awesomely cool.  ;D

And something not too complicated. imho it's actually hard to beat a straight ahead fender/marshall/vox style of eq! Even in a 'studio unit'.

Working on my recalcitrant proto this morning. I've done a fair bit of pentode reading since I last put it down so I hope some good stuff filters thru.

Thanks for the 12DW7 tip - having a look at it now also.

Cheers

 

[alexc]

So back to basics - I have the removed the feedback eq and trying to find operating conditions.

I have so far, with 1vpp input at the grid1 and driving a 100K pot :

quiescent 6as6 pentode mode :

245Vdc +ve rail, 153Vdc plate voltage thru 100K plate resistor
245Vdc +ve rail, 73Vdc screen voltage thru 390K screen supply resistor
kathode +2.5vdc, CV -2.5Vdc, kathode resistance approx 1.7K on a 2K pot

This is giving output voltage gain of x13 which is very low.

Now varying kathode to 0V gives increasing output voltage with 0.7Vdc giving x40 and 0Vdc giving x80 -> as expected

The CV does nothing from -10Vdc -> -2.5Vdc with zero output voltage
This is where it cuts in giving quite good control of increasing output voltage for -2.5Vdc -> +10Vdc
where voltage gain is max

Not too bad so far - all that is nice and stable and fine.
Not too much distortion so far, but not yet fine checking.

When the gain of the pentode is high (kathode is closer to 0Vdc), the CV is exerting very smooth control over output gain just like a volume knob.

I noted in triode mode (screen to plate connected) where the operating points were :
  +245V supply, 100K plate, +2.5V kathode  (kathode resistance approx 1.7K on a 2K pot)
      -> gain was x11 (low!) rising to x19 (still low!)
      -> CV did nothing at all thru the whole -10V -> +10V with a steady voltage gain of 11 or so

I am now trying to find some reasonable values for plate and screen supply, as well as plate and screen resistances etc.

Trying to get the gain high and the CV smooth in pentode mode and see if I can get the CV working in triode mode.

Cheers

 

[alexc]

So increased load on the pentode with thru 500K and now a 1M pot now and thats improved the gains dramatically.

Seeing gain 65x with kathode of +1V and plate voltage 100Vdc, screen voltage 45Vdc
with 1vpp input, 1M pot output

The CV is now better too :
  -10.0Vdc to -4.3Vdc no effect with a zero output amplitude
  -4.3Vdc to -2.6Vdc is controlling increasing amplitude and the output is crazy distorted
  - 2.6Vdc to +3.5V dc controlling gain nicely and with little distortion
  +3.5Vdc near maximum level without distortion
  +3.5V+10Vdc in increases amplitude only a little to a maximum with a little distortion

So it is going in the right direction. Still my plate and screen operating points are a little wonky.
I'm trying to get around 120V for both or so.

Looks like my kathode bias will be around 1Vdc so for decent gain.

 

[alexc]

So I have the eq circuit working pretty well now, in addition to the moderate gain (x70) pentode with reasonably stable cv control (-3V to +4V)

I ended up with 3 tubes : 6n2p -> 6as6 -> 6n5p

input  traffo -> eq network with feedback -> grounded cath  > cath follower
      -> pentode with cv and output divider  -> white cath follower -> output traffo

So now just working on tidying up and then focus on dividing networks, gain staging and zobels
All that and hopefully with reasonable noise performance.

*Then* I can look at some of the funkier  'Bernbrue Rude Tube' stuff.

Great education project.

Cheers

 

[Kingston]

It might be good to investigate about the 12DW7 , since it is used in the ampeg amp and the "vulture solo" as well.

The only thing going on for 12DW7 is that one half is 12AX7 and the other 12AU7. In the ampeg circuit it's used as a cost/space saving trick. In the EQ section only 12AU7 halves are actually used. The 12AX7 halves are used elsewhere as plain gain stages. If you want to replicate that you actually only want a single 12AU7 tube.

Here's more about such filter sections http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html

 

[alexc]

Getting some good performance now with my eq+6as6 proto

My setup is now :

  600:600 input traffo -> eq network with feedback -> 6n5p grounded cathode+cathode follower
            -> pregain pot -> 6as6 with CV pot, cathode bias pot, pentode/triode switch
            -> voltage divider + postgain pot
            -> 6n5p white cathode follower -> 15K:600 output traffo

My setup is now stable and reasonably quiet so far with a noise floor allowing quite a bit of gain.
Decoupling B+ for the various stages with dropping resistor + electro cap was important.

Am getting unity gain from the eq -> gca+cf stage and it can swallow around 4Vpp before positive grid.
This is needed because it is driven from an orange86 module. So I have a pretty low gain tube but can handle a lot of signal input. I may change it again, possibly to a 12AY7 or a 6BC8.

I can still easily provide many times that input voltage and overdrive the cheapish/smallish 600:600 (altronics) traffo - has a good bit of 'hair' when pushed.

The eq curves I'm still evaluating, but it seems mostly as expected.

Now the 6as6 is giving around 90x gain and the CV control is actually pretty smooth now. The more gain the pentode is providing, the smoother the CV control. It does appear to be doing useful stuff from -3V to +9V or so. I am using 120K plate resistor, 220K+0.22uF screen with 220V B+. Am finding around 57Vdc on the screen and 120V on the plate in  pentode mode.

The cathode bias pot is working really well as is the pentode/triode switch, which more than halves the gain in triode mode.

So now am buttoning up a few things before changing the the screen bias network to something resembling Bernbrue's Rude Tube setup ie. switched cap/resistor combinations.

Then, I'll start listening to all this and start doing some tuning of the gain staging.

I still have a pair of EL95+Telefunken SE amps to add the monster box 

Anyway, it's getting better all the time  (can't get no worse!)

Cheers

 

[bernbrue]

Hi Alexc,

Now the 6as6 is giving around 90x gain and the CV control is actually pretty smooth now. The more gain the pentode is providing, the smoother the CV control. It does appear to be doing useful stuff from -3V to +9V or so. I am using 120K plate resistor, 220K+0.22uF screen with 220V B+. Am finding around 57Vdc on the screen and 120V on the plate in  pentode mode.

I´ve got almost the same loadline and voltages. With these settings the 6AS6 works like a charm. I would love to see a drawing of  your variable power supply (+-15V ?) and how you realized the neg/pos voltage with a pot.

The cathode bias pot is working really well as is the pentode/triode switch, which more than halves the gain in triode mode.

Same thing here. In triode mode from 0 - 0,2 mA there is a quite thin, slightly distorted sound (don´t change that, you´ll love this setting in pentode mode). Turning the bias pot cw from around 0,25 mA to 1mA very clean and rich sound, though almost half the gain in comparision to pentode mode.

Anyway, it's getting better all the time  (can't get no worse!)

I like this attitude!

I thought a while about the cathode switching. It might be better to put the Lorlin switch between cathode bypass caps and ground. That means that you have to put the inductor before (infront) of the bypass caps. (cathode -> inductor-> all the bypass caps -> Lorlin switch to ground.)

regards
Bernd

 

[bernbrue]

apropo triode pentode switching,
You might try a triode/pentode blending as shown in the schematic attached. The advantage is that the "Squish mode" is already implied in this arrangement. Keep us informed what it does.
regards
Bernd

 

[alexc]

Thanks Bernd

Glad to hear that I am converging on some similar operating points.

My CV voltage is simply a standard pair of LM317/337 regulator circuits set to +/- 10V connected to the outside legs of a 100K lin pot. The pot centre feeds CV input directly.
It is actually a centre tapped pot but it is not neccessary to use this. Works fine as is.

Haven't had any problems with loading the regs (very tiny current) or with added noise.

That pentode/triode morph looks very interesting. I'll definately check that out shortly.
I've been wanting something like this for my current SE amp (parallel pair 6550 8))

I will investigate your suggestion regarding the screen bias arrangement with respect to the ordering of components. Should be doing so in a couple of days. I have to sort out the EQ curves which are giving me a little grief now - have to try a few things there also.

All fun and games on this project - uses up my spare parts and gives me a platform for investigation while I do my more straight-forward projects (is there any such beast?)

Cheers and thanks for the suggestions!

 

[Niketouille]

Hi,
Is that possible to use the rude tube as a mic pre ? I was thinking using the original input stage of the redd and padding the input to use it with line level. Did I miss something ?

 

[drask]

Hi,
Yes it's possible. We're include a switch for pad on XLR input.

 

[drask]

Audio Demo of Rude Tube : http://www.mediafire.com/?hfjqtqbfitogq7n
regards

 

[bernbrue]

Rude Tube II ??
regards
Bernd

 

[drask]

Interesting things Bernbrue.
Mono version so?

I've almost finish the last prototype
Untill here all is fine !

 

[baadc0de]

it would look... well, impressive!

 

[bernbrue]

it would look... well, impressive!

Really nice frontpanel. Unfortunately it won´t work ...................................................... 


















There is no power switch  ;D ;D ;D ;D ;D
regards
Bernd

 

[baadc0de]

Just a quirk of mine, I have most of my gear remoted as far as power switches go

I've refined it a bit, it's very very busy but has all the features and more. Character means Triode/Pentode. the switch between clean/dirty pots changes between the clean/dirty processing. The FP is made for DIN72 meters, I assume they would meter between 0 and 1 mA?

 

[bernbrue]

Nice! Although the Squish mode is already implied in the Morph potentiometer (blending between Triode/Pentode and Squish) we could provide an extra Squish mode for the first valve stage (EF86 pentode). The meters are 1mA. Have you got a good source for these square meters (actually like the phoenix meters)? I tend to build the prototype completely unbalanced into an old guitar top case (mono only). With all the features a 3HE 19 inch case for a stereo version would be better. Since it´s only a pleasure & leasure project, it´ll take a few months until a working proto is ready to publish.
regards
Bernd

 

[bernbrue]

Hah, funny! You named it "Audio Destruction Unit". It can destruct audio, but it can also be very subtle.. The other day,  while doing MS mastering, I´ve sent the mid signal through the Rude Tube in Pentode mode with very low Drive setting. The units adds a very pleasant "into the face" character. Amazing!
regards
Bernd

 

[baadc0de]

I guess I was just too enamored with this:
http://www.youtube.com/watch?v=-D9UwfcivsY&feature=related

I suppose it can be used to gently punchify something, I'll have to build it to find out.. I guess my front panel is still missing one more vital thing.. a DI jack or pair thereof!