Tube boost + overdrive running off a 9 volt battery

[seedlings]

Used an Rk of 5000 ohms and haven't decided on Ck value - leaning toward a .68uF.  I left g1b at 470K.  Before the change, at full gain there was too much compressive sag at the onset of a big humbucker chord.  It is still just noticable, but OK.

CHAD

[esdiezy28]

Chad-  just a suggestion, but if your contemplating several options for Ck may I suggest adding a SPDT switch to allow you to flop between two selections. Like the 0.68uf (with Rk of 5k-ohms, it looks like its full boosting at frequencies above 46Hz) and for the treble boost capacitor 0.022uf (1447Hz). (This mod is on my valvecaster, toggle switch for two capacitors [Ck] and a pot for variable resistance.)

And I'm no expert, but if you have this breadboarded it should be simple to test, about the compression sag try adding some series resistance to the signal entering the each triode grid and see if that helps tame the compression. Not sure if that will help or make it worse, honestly. I'm with you, read so many pages I get confused as to what I remember. I put the RE- in REsearch. 

Best of luck, ToNy

[lightning]

I need some help with my valvecaster it has a low output with very mild gain.
I am using a 12ax7 and a question I have is how does a tube actually distort?
As in how would you build a circuit that really drives the tube.  and what is actually
causing that delicious breakup?

[zambo]

12ax7 will act funny in this circuit a lot of times. Try 12au7. works better at low voltage. if you really want to drive this thing you need more voltage one way or the other. Put a clean boost in front of it and crank it is the easiest way to get more out of it.

[seedlings]

I'm running a 12AX7 with an 18.5V notebook power supply and it sounds very nice.  Sounded nice with the original schematic, and customized with some different caps and such.

CHAD

[zambo]

12ax7 can be hit and miss in my experience. I have some that work good at low voltage and some may not work at all. 12au7 works consistently well at low voltage. YMMV. 

[zambo]

@ Lightning - check your voltages against some of the posted voltages in this forum and make sure you built it correctly. re flow questionable solder joints and look at resistor values. make sure you dont have polarized caps backwards etc. if all of that stuff is correct i would say its the tube. as far as distortion goes, thats a huge discussion but from what i understand it has a lot to do with the signal getting amplified so much it actualy causes the tube to malfunction and work incorrectly causing harmonics and the valve shuts off (signal clipping) momentarily. there are much smarter people on here that can explain it better i bet 

[stallik]

Finally got round to buiding my first Twincaster. Seems to work fine (everyone's asleep so can't try it at any kind of volume) but is it normal for gain 1 to crackle a bit? - It's a new pot

[zambo]

totaly normal. Its because of the way it works by changing the bias of the tube.

[Renegadrian]

I like your work! What's that switch for!?

[stallik]

Thanks Renegadrian, you got me started on all this a couple of years back. The switch is merely swapping C1 - I'm using 4.7 & 47nf in my valvecaster so though I'd put the same in this one. Will check out the sound later & adjust to taste.

[Renegadrian]

4.7n - you got a sort of treble booster this way, do you!?

[stallik]

More of a bass cut (I think). I'm using on old vintage valve amp (Roost) which has some very serious bass. I like having a good level of bass for my bypassed sound but this leads to some serious booming when the overdrive kicks in. The 4.7/47 was just trial and error to limit the bass frequencies and give me a switchable choice This worked well on the single valvecaster - I use that as a serious boost, amp on about 8 (preamp) valvecaster full up on everything (lovely vintage overdrive tones) then turn down the guitar vol for a clean sound. I wanted a little more distortion hence the twincaster. This seems to give me the same as the valvecaster when Gain 1 is fully down but increases distortion as it is turned up. Just what i wanted.
Only issue is that I seem to have a loose connection somewhere in the new unit. works fine for a while then looses output slowly & descends into a nasty crackle, almost like a cap discharging. Moving the wiring about seems to get it back but no matter how many i replace or resolder, the problem persists. May be down to the valve bases which in this case are really low quality Chinese items with loose terminals....

[seedlings]

18.5V 1940s Tung Sol black glass 12AU7.  Makes the old Ibanez TS9 and Fulltone Fulldrive sound very grainy.

CHAD

[stallik]

Er - HELP please (lack of knowledge & all that) I've successfully build a valvecaster but am having an issue with my first twincaster. I'm getting a kind of sagging when i hit a power chord. Originally, i was using a couble of physically small 1uf 63 caps at c3a & c3b. Using these the signal would completely cut out and would come back after a few seconds. I replaced them with 2 physically larger caps which were also marked 1uf 63. This reduced the effect dramatically but it's still there. I had thought that the 63 refered to volts so why the difference in size? both types are radial. C 1, 2 & 4 are all small ceramic disks
Am i looking in the right place or have I overlooked something?
Thanks

[tasos]

Probably wrong place....it might reduce the hum because the caps were of higher quality...

[zambo]

@ Stalik- what tubes are you using? Sometimes anything but a 12au7 will act like that. Not always though. Solder joints is my first guess. When you replaced the caps maybe you moved a wire or part with a cold solder? just guessing. Good luck.

[stallik]

@ Zambo 12AU7 (NOS JAN 5963) work fine in the single valvecaster. Just tried putting the old smaller radial caps back in & sure enough, the cutout returned. Put the larger ones in and it's far better again. i've put a couple of long posts into the vero so that I can quickly swap the caps over so not suspecting poor soldering (this time).  I used metalised film caps on the single valve unit so will try to get a pair of them tomorrow to check them out. looking at the gut shots on this forum, it appears that many are using 400v 1uf caps.
my basic theory gives me to understand that a cap filters frequency as a byproduct. It's main use is to store an electrical charge. I think the tiny units I was using simply did not store enough to cope with the demands of a bass rich powercord.
i think that's whats meant by the phrase 'smoothing capacitor' but hey ho, I really am out of my depth so that might all be rubbish!

if I'm right however, this might give rise to the possibility of a switchable sag circuit as it kind of sounds like a valve amp running out of steam...

[iccaros]

Er - HELP please (lack of knowledge & all that) I've successfully build a valvecaster but am having an issue with my first twincaster. I'm getting a kind of sagging when i hit a power chord. Originally, i was using a couble of physically small 1uf 63 caps at c3a & c3b. Using these the signal would completely cut out and would come back after a few seconds. I replaced them with 2 physically larger caps which were also marked 1uf 63. This reduced the effect dramatically but it's still there. I had thought that the 63 refered to volts so why the difference in size? both types are radial. C 1, 2 & 4 are all small ceramic disks
Am i looking in the right place or have I overlooked something?
Thanks

What is your power supply. with two 12AU7 the heaters are going to use up 300 - 600 MA depending on how you wired them. Then you have 2 -3 ma per triode which is not much and another 2ma for rest, so a .5 amp power supply may sag. The Cap differences could explain the same effect, as the CAPS charge and discharge as the power sags..
if you are voltage multiplying, you can be using more power.. as to increase voltage you eat up amperage.

[stallik]

@ iccaros -
My power supply is 1.2A so I should have plenty of headroom. nevertheless, I take on your point that it could be the PS. I will take a very carefull look tonight to ensure that I've not got some kind of short? that's dragging it down