valvecaster/twincaster woes

[runmikeyrun]

I built the twincaster... it is having some issues.  It sounds ok on minimum gain, decent breakup if a bit ratty.  But anything much higher than min gain and it gets thin with tons of sag, finally splattering out.  If i play bass through it gets really rattly and splattery with lots of lows and brittle highs but zero mids.

When i tested just the first stage as the valvecaster it sounded good. 

i used the standard twincaster circuit, except for the following mods: I changed all the plate resistors to 100k to lower gain, and increased coupling caps to .47mfd since i'd like to use it on bass. 

Any ideas?  Lower coupling cap values?  I'd really like a warm tone with good mids like i had on the valvecaster just with more gain.

[kurtlives]

The Valvecaster is known for being a bit dark, not much high end content but not flabby.

Hmm...
Can you bypass the second "stage", is the first stage fine?

Have any voltages?

[robizz]

have you tried to feed it at 12 volts?

what circuit did you used?

[frequencycentral]

Most Valvecaster build issues are power supply related. As you are not mentioning any hum it sounds like your power supply is regulated/filtered okay. It's worth bearing in mind that each 12AU7 heater requires 150ma, so your Twincaster is looking for at least 300ma just for the heaters. I would be using 500ma just to be on the safe side, though the plates have very little additional current requriement. But you mention sag, so it's worth stating how you are powering it.

As Chris says, voltages would be good to see. I have posted voltages on that huge Valvecaster thread a few times, I can post them here again if you wish. My voltages are based on the original circuit, so would differ from yours a little as you used 100K for all the plates. it wouldn't be too much hassle for me to breadboard your version and get voltages though.

It would also be useful if you linked/posted the schematic you built from. It occurs to me that one of the original Twincaster schematics posted had an error - there was a missing 1M grid resistor between the two Valvecaster stages.

I never quite got the twincaster idea anyway - especially if you're using 100K plate resistors. I've built a couple of Valvecasters, I've also built a Pepper Shredder http://www.tube-town.net/diy/lov/lov02-peppershredder-eng.html which is a similar idea to a Twincaster, but more refined IMO, and it doesn't suffer from the high end losses that the Valvecaster does, and the gain control works better too.

[runmikeyrun]

Thanks for the ideas guys.  Rick i'll check and compare voltages with yours later, i have to run out to work and then practice later.  Checking the 2nd gain stage separately is a good idea too, i know the first one is okay. 

As for hum, i have zero hum at min gain.  When i crank it up, it gets a bit louder and then really amplifies in the last 1/8 of a turn of the gain pot.  it's exaggerated with both stages in series.  It also gets somewhat more pronounced when i increase the plate resistors and increasing gain.  It could be a lead dress issue since everything is just loose on my bench wired PTP at this stage.  I'll check for DC in my coupling stages too.  I've tried 3 different supplies rated at 300, 400, and 500ma.  All are unregulated putting out about 17vdc, but i do have a heatsunk 12v regulator taming things to 12.2vdc.  I designed a filter system around the regulator based on the 7812 datasheet and I have 100 uF of filtering across the power/gnd as well.  I tried an additional 100uf across the power rail with no change.  I kind of ran out of time to experiment last night when my wife got home 

If the 2nd stage checks out ok by itself then worst case scenario I will run both stages in parallel with one being clean and one being higher gain.  Then i can use a pot to blend between the two.  It would work out well on my bass anyways, which will be the primary application to warm things up a little before my SS preamp/poweramp setup. 

Thanks for your suggestions. I'll give them a shot when i get a chance and then report back.

[runmikeyrun]

Ok, i powered up the circuit with a 500ma 15v adaptor (putting out about 19v) and the sag got much better and hum disappeared.  When i pull one tube there is no sag whatsoever so i think it's a supply issue  I think i'm going to be in the market for a regulated 12v 1A adaptor. 

Voltages are as follows:

(original valvecaster voltages per Rick)

Pin 1: 3.02
Pin 2: -0.518
Pin 3: 0v
Pin 4: 0v
Pin 5: 11.97
Pin 6: 8.21
Pin 7: -1.285
Pin 8: 0v

My readings:

V1
Pin 1: 9v
Pin 2: 0v
Pin 3: 1.1
Pin 4: 0v
Pin 5: 12.2
Pin 6: 3.7
Pin 7: -0.42
Pin 8: 0v

V2
Pin 1: 10.0
Pin 2: 0v
Pin 3: 1.0v
Pin 4: 0v
Pin 5: 12.2
Pin 6: 4.0
Pin 7: -0.3
Pin 8: 0v

they're pretty close now, with the exception of pins 1 + 6 being about flipped between mine and yours.  Checking the voltages revealed a short between V1 1+2 which made a big difference in the tonal quality.  Still sounds very ratty on bass though, maybe just stick w/ the valvecaster for that.  I think once i get the power supply crap sorted out it'll be killer on guitar.

Thanks guys!

[kurtlives]

Swap the triodes plates around so you get readings like Ricks.

I think that will help, try it!

[runmikeyrun]

You mean switch the leads going to the pins?

[kurtlives]

Ya, so then you have the lower plate voltage on the first triode.

[runmikeyrun]

right on, should this help with stability at higher gain?  I'm getting oscillation and feedback.

[esdiezy28]

For those of you who have sucessfully build a valvecaster or a twincaster I have a question. Do you have to mount it inside a metal enclosure box? I have a tube shield for the socket but I think my problem is with wiring up the power. My Valvecaster build just hummed louder as I turned up the volume pot. And this whole having to wire in a DC jack is a bit of a problem for me, I almost always work with 9V battery applications, but at 150mA for the heaters alone on a 12AU7 I thought a DC jack would be a good choice.

[runmikeyrun]

your battery life will be measured in minutes.  Use a good quality power supply.  I tried 4 on mine, all with varying degrees of hum.  It seemed the higher the power supply was rated (in amps) the lower the hum.  I also used a 7812 regulator mounted to a heatsink and then used the datasheet for a reference on filtering. 

I'm finding that 500ma for a twincaster isn't enough, i'm going with an amp to be safe, that's what my (2) 12ax7 budda pedal came with.  I think i'm going to go for 15v regulated so i have a little more headroom and still not overheat the regulator. 

[kurtlives]

your battery life will be measured in minutes.  Use a good quality power supply.  I tried 4 on mine, all with varying degrees of hum.  It seemed the higher the power supply was rated (in amps) the lower the hum.  I also used a 7812 regulator mounted to a heatsink and then used the datasheet for a reference on filtering. 

I'm finding that 500ma for a twincaster isn't enough, i'm going with an amp to be safe, that's what my (2) 12ax7 budda pedal came with.  I think i'm going to go for 15v regulated so i have a little more headroom and still not overheat the regulator. 

Don't worry about the regulator, it can take the heat (no pun intended  ).

If you go with 15V DC make sure you keep the filaments at 12V DC.

[runmikeyrun]

I don't want to get crazy re-doing my whole vero layout so i'll keep the 12v regulator.  As such, i can't swap pins 1 and 6 because the coupling cap to the next stage is on that piece of vero.  If it were PTP it'd be ok.  I didn't want to start messing with plate resistor values and such as i just want to get it done so i can start on my bass valvecaster.

I got it to stabilize and lower the gain some by inserting 56k resistors in line with the 50k gain pots.  On max gain i still get good crunch but not sag or oscillation.  So i'm pretty happy with it at this point.

Thanks for the help guys!  I'll make sure to post some pics when i get it done, i have a really neat enclosure for it.

[kurtlives]

Glad it's working.

Can't wait to see the pics!