EHX Hot Tubes; too much noise

[Bernardduur]

Hello all!

Lately I am using a cloned EHX Hot tubes (old version, 4049) for my main bass OD and it is great in this position. The only downside I have on this unit is that is generates a lot of hiss (noise); is there a way to get rid of this?

Unit runs on 18V; I used multiple opamps but they don't really alter the signal

Schematic

Oh, btw, does someone knows if you should use a Buffered or an Unbuffered 4049? When I use an buffered 4049 (as stated on the schematic) I get oscillation on higher gain settings. The UB version does not have this issue

[Mark Hammer]

ALWAYS use an unbuffered version for these types of circuits.

As for hiss, that can be cured without too much sacrifice by sticking small-value caps in the feedback loop of one or two of the invertor sections.  Since you are using it for bass, you can afford to trim back more of the treble.  A 47pf-100pf cap in parallel with the 1M resistors in the invertor sections comprising pins 14/15 and 2/3 would help.  The 68-82pf range is probably ideal.

[Bernardduur]

Thanks man!

I encountered another problem; the unit is extremely bass heavy when you wire the tone section / bypass section like that. When I make em switchable (from the begin) I regain all my tone again.

[Mark Hammer]

Not sure what you mean.

[Bernardduur]

Sorry; I will try to make it more clear:

In the schematic, right after the second inverter the signal gets split into two lines; one goes through a .22 cap and is the tone-bypass line, one goes through the tone section.

If you leave it wired like that, you lose a lot of treble in the end signal. The signal will be extremely bassy and, to my ears, unusuable

(I encountered it as I didn't wire up the tone section and used the circuit main in the tone bypass mode; then, when I wired up the tone mode and made it switchable, I noticed the sound altered drasticly)

If you make those two signal lines switchable (so the signal goes through OR the tone bypass OR the tone section) you solve the problem. This can easily be done by using a DPDT switch instead of a SPDT switch.

Last thing; running the unit on 18V makes the sound bigger and serious cuts the hiss. Also, now the unit does not function as a radio receiver when unboxed

[Mark Hammer]

18v?  Not sure about that.  Certainly, the circuit can survive with more than 9v applied (I run mine off a 9v battery), but normally CMOS chips fry with more than 15v fed to them.  Are you feeding yours a V+ of 18v, and if so is that coming from 2 batteries or a wallwart/transformer?

[Bernardduur]

18V from my Dunlop DC Brick to the V+;

I checked the datasheet and it can handle up to 20V, so I am pretty close

Sound to die for btw; sound becomes bigger

[Bernardduur]

OK man; you were right (as always)

Chip burnt today up (it worked for 2 days). I will replace it with 15V

[Mark Hammer]

I'm reminded of how much everyone thought, for a while, that the "smart" thing to do was to follow Eddie Van Halen's example and use a variac between the AC socket and their amp....until their amps started frying.

I guess you didn't realize how prophetic your description "to die for" was, eh?

15v is certainly tolerable,though it might be smart to go just a shade below that by sticking a 1N4001 in series with V+ coming from the regulated supply to drop it by a half volt.  Certainly the op-amps themselves are comfortable with 18v.  The weak link is the CMOS chip.

On the other hand, you've inspired me to try out my 12v adaptor with my own EH-HT clone.

[Bernardduur]

OK; thanks man!

Chip hold out for quite long (I think I played it last night for 1 hour or so.......) just today it became hot and sounded a bit aweful

Will do as suggested! Thanks man!