Waterloo - Valve Overdrive

sinner · Post by **sinner** » 11 Oct 2014, 22:23

Here's the story

I'm a big VOX early AC series freak. I like that early AC30/4 and AC15 pentode channel a lot

Recently - two months ago or so I have purchased Merlin's second edition preamp book. It contains great section dedicated to pentode based preamps

It seems that Mr Blencowe doesn't like the idea of using penthode to serve as an first gain stage (pron to microphonic, noisy) and suggested to use it as a second stage preceding by triode. Merlin also suggests that the Vox's pentode channel was poorly designed. The gain factor of 200 combined with pentode drawbacks listed above was a bad idea resulting design changes in later issues ac30/6 (shift to triode channel). Vox basically took the idea from Mullard datasheet without any changes.

Why overdrive? Well. I was thinking to 'design' valve overdrive since quite a while, I like valves, I like them more than any other construction approach. My recent drive is to read old component (valves, transistors) datasheets and application notes, I like the one made by GEC the best. It's great way to see from where big names, classic companies ware taking inspirations. I like component oddities too, like this ECL80 hybrid valve. You having pentode AND triode in one package with separated cathodes (usually common). This seems to be ideal for what I could use in overdrive pedal with Merlin's suggestions. One glass = less heat, less power consumption, less space.

What I needed is just an output stage. I like that new fashion to use high voltage mosfets in SS FXloop designs (metro and clones). Since they're operating on full HT the headroom is preserved, small package, relatively low heat dissipation. Seems ideal for output stage where clipping 'job' is already done and all we need is to preserve headroom (or nor squash it to much) and set the output level.

The design itself contains half the Merlins book. Actually only the output (ss) stage is not from Merlin. There is no component values right now, but they will be in rev.2 - simply I don't have much time to update it right now, but I will somewhere next week.

- C1 can be omitted - it's just a RFI kill cap
- C4/C5/R5/R6 are the parts of simplified/fixed bridget T mid control. I've chosen to have it in footswitch form
- C6 bright cap - Probably will make it fixed in my version
- shape pot, c8/c9 is Merlin's morph control. Blending between triode/pentode operation - cool mod, should change the character big time. Whole penthode section will have value numbers reducing the stage gain to less than half vox-ish gain factor
- Tonestack - It's BMP (tilt) type with R12 addition to make it work more evenly
- and finally SS output stage.

PSU is in works. I was going to do it with Fender Reverb transformer, but I really like that DC/DC converters from GTFO tread, I just have to gain more how-to about them

Anyway, it's early stage design, that have potential IMO. I'm just hoping it doesn't have deadly HT potential. I'm also hoping for community input and criticism

Thanks

DrNomis · Post by **DrNomis** » 12 Oct 2014, 15:48

Looks like an interesting design Sinner, I like working, and building, with Valve technology too, I find it fascinating and also like the sound of it too, I'm currently working on re-building an amp head as a Soldano SLO50, so far I've done all the heater wiring, I've been thinking of building a Matchless Hotbox, please keep updating us on your progress.....

ppluis0 · Post by **ppluis0** » 12 Oct 2014, 23:57

Hi Sinner,

Perhaps it's a typo, but the ECL80 has only one cathode common to both sections... so: what tube are you talking about ?

The ECF80 (that you mentioned in your post regarding subminiatures) has effectively separate cathodes for each section and looks best suited for your design.

Keep us posted about the evolution of this project...

Cheers,
Jose

sinner · Post by **sinner** » 13 Oct 2014, 00:04

Yep. Typo on my side. It's ECF80

I'll keep you posted lads

iq01221 · Post by **iq01221** » 13 Oct 2014, 04:55

Wow!!!
Nice proyect!!

I'd put a 1K resistor (grid stopper) in grid1 of the pentode section, soldered just in the socket, to avoid noises.

sinner · Post by **sinner** » 13 Oct 2014, 08:12

Good, ideas... keep them coming.

Does anybody know the math to calculate the output level? I suspect it sould have 6 to 25db output, but I never heard of any rules nor regulations about it

DrNomis · Post by **DrNomis** » 13 Oct 2014, 09:24

sinner wrote:Good, ideas... keep them coming.

Does anybody know the math to calculate the output level? I suspect it sould have 6 to 25db output, but I never heard of any rules nor regulations about it

Do you mean the output power of an amp?, Check this thread out(it's on another forum):

http://el34world.com/Forum/index.php?topic=12997.0

sinner · Post by **sinner** » 13 Oct 2014, 09:44

No man. How to calculate the final/output stage in guitar pedal.

DrNomis · Post by **DrNomis** » 13 Oct 2014, 09:58

sinner wrote:No man. How to calculate the final/output stage in guitar pedal.

Oh....

My misunderstanding, what kind of output stage were you thinking going with?, a common-cathode buffer?.....

sinner · Post by **sinner** » 13 Oct 2014, 10:01

What a fuck Simon? Check the schematic in my entry post

Honestly mate...

DrNomis · Post by **DrNomis** » 13 Oct 2014, 10:32

sinner wrote:What a fuck Simon? Check the schematic in my entry post

Honestly mate...

Silly me, I forgot to look at the schematic, my bad.....

You could probably try these resistor values in the output stage:

1M, or 2M2 for R14/15.

220k for R16.

100k for R17.

R 14,15, and 16 sets the input impedance as well as sets the biasing of the output stage, you might need to tweak one of them to get the source of the Mosfet to sit at roughly half the supply voltage.....hope that helps.....

ppluis0 · Post by **ppluis0** » 13 Oct 2014, 23:23

Hi Sinner,

The last stage is a source follower and has unity gain, if my memory works fine...

The output level you will have at the output jack will be the same that be present at the tone potetiometer, that can be a lot after the amplification of the pentode stage.

I don't handle simulation softwares so, in my case, the next action will be to breadboard the tube stages and measure what levels can be found.

Cheers,
Jose

teemuk · Post by **teemuk** » 22 Oct 2014, 14:46

- shape pot, c8/c9 is Merlin's morph control. Blending between triode/pentode operation - cool mod, should change the character big time.

"Big time" is probably an overestimation but it will indeed change how the stage overdrives (triode vs. pentode).

...But do note that gain of pentode and triode modes is very, very different so built as is you're basically just building yourself another volume control. What I would recommend is using a dual, ganged potentiometer, one half wired as is, the other half wired as an attenuator to level out the gain difference between pentode and triode modes. That way you actually have a useful circuit for changing the characteristics of overdrive without it greatly affecting the overall volume.

---

Your schematic doesn't show the power supply but I trust you fit adequate "coupling" between stages so that power supply voltage modulation due to current draw won't reflect to other stages and at worst cause severe instability.

The last stage is a source follower and has unity gain, if my memory works fine...

It does. Source's AC voltage would be closely following that of the gate, but is gate-to-source voltage drop less than that.