Self designed tube pedal/preamp problem

[omt]

To B Tremblay : i have built this design with 2 stages, it gives clean sound to mild breakup with a good boost, i know this one works





(sorry about the sloppy drawings)

[George Giblet]

The first thing I have to say is don't run the tubes with the heaters at 20V.  If you look at the mctube there's a low value resistor on the heaters, that's there to drop the heater voltage back down to 12.6V.

The second thing is I think RG got it.  The fact you have two stages working and four stager is not working points to oscillation.

If you can get two separate two-stagers working you are almost there.  If you connect them together and it oscillates try RG's supply isolator suggestion.

Also try:

- Keeping the signals later stages away from the input stage.
- Use shielded cable for the input stage
- Try adding a small cap across the plate resistor on one or more stages.  Too large a value will lose highs, too small a value won't stop it oscillating.  If the cap has to be so large that it affects the sounds you should really be trying to reduce the problem by good layout techniques.

[Steben]

I still don't know what tubes are used. On the sopht amp site, 12u7's are used, right? They are designed within the 12-24v range of supply, so I thought they would work nicely as pedal tubes?

[omt]

I have used 12ax7 But i have 12au7 and at7 lying around, i just havent got araund to try them yet.
I messured the frequensy of the noise  and got 100htz and 200htz. il try soldering sockets on it and try out capacitors il also try to run it on batteries so that im shure its not my powersuply (i run the 2stage one with batteries)

Thanks for the help you guys, i wold have a hard time figuring this stuff out by myself

[slajeune]

Hi omt,

ok with such a low plate voltage (12V), 4 gain stages is simply too much.  Go see the following link:

http://doorstopelec.typepad.com/bore/schematics/index.html

Scroll down to Goldtooth Softy.  He describes that with 4 gain stages, it would oscillate.  But with three, it seemed fine.  Even though he is using a 12U7, I still think that the problem is the same with a 12AX7.  So, please try removing the last stage (i.e. only use 3 gain stages).  This might get you closer to what you want.  Once the oscillation stops, you can start biasing the tubes to get the sound that you want.

Let us know how it turns out.

Cheers,
Stephane.

[Ge_Whiz]

I have used 12ax7 But i have 12au7 and at7 lying around, i just havent got araund to try them yet.
I messured the frequensy of the noise  and got 100htz and 200htz.

Sounds like 50Hz mains hum causing the noise - you need to smooth the power supply more. Also, lower mu valves such as the 12AU7 or 12AT7 might help a lot. But don't try them until you get the heater voltage down to the right level.

[fikri]

Check the fillament supply, use shielded cable for the input, you may want to take another reference project and collect them as much as possible.

[omt]

I have found my enemy, its the powersupply, i tried it with batteries and then no noise . even though i added a large cap on the powersupply i still messure 100Hz anyone know why i didn't have my guitar at the moment so i couldn't test if i got any sound, but it was heating up and there was no noise on the amp. IL try tomorrow with guitar connected-

[sonic66]

I built the last design posted by omt, and besides hum , from power supply which i haven't filtered out , it appears  that it could  work ok, but i get some problems that are probably fixable.

I left out the input cap  and the other two i've used are .01 and .02 mf , i tired a 500k A  pot for gain and it had a good range but sounded like there was a some oscilation or something else in there when full.

Also i had a vey muffled , loaded down sound which cleared up when i rolled the volume on the guitar back abit  (which is ok, infact good for marshall lead 12  which is painfully thin and brite,  i'd like a variable loading pedal be made to combat this).

How can i sop the loading if thats what it is?

Besides that i get alot of crackles and cut outs, the wiring seems ok , do't know what's causing that

voltages were 12.65 on the plate  with 500k pot and 13.45 with 100k pot ( gives a clean range only)  and 14.56 on th heaters

[sonic66]

Its working better for me now

i found a regulated powersupply, the awful hums gone, changed a shonky pot, and it works fine.

Innitally i used a 1M instead of a 2M2 grid to ground and the gain may have been more.
This could be  the resistor helping  to bias the tube maybe? Or just the the voltage from the different is lower,
though i'm not sure what makes more difference. also i got diferent voltages at each plate 10.6
or something v about 8.5 on the second one? hmm.... sounds more stable with the 2.2M.

still haven't added a input cap and seems that with the guitar vol up full it sounds weirdly muddy and loaded,
but cleans up back at about 7 ,  like you can hear it swithchin in like a crossover point  ?

So it seems to be a good simple build , that even a noob like me can do,

thanks  to omt

[George Giblet]

>like you can hear it swithchin in like a crossover point  ?
An input cap will stop that (may have to lower the 2M2 after adding).

[sonic66]

I've now tried  several different caps values, they cure the muddness, but made it sound distorted at all levels of gain
like a 'low battery',

So instead I added a 100k input resistor and it seems to work nicely,  the muddy 'loaded' sound has gone'
( though the strange 'crossover point' can still be heard like static kicking in and out but.)

Finally adding a 1M to gound after the output cap but before the volume pot seems to help somehow....? its not really noticable though.


Can some one explain. Does the gain control sets the bias for the second half of the tube?  does it also raise / lower the signal from the first half?
Is the overdrive or clipping derived from both halfs of the tube or just  the the second tube? Does the the it clips asymetrically?

Thanks