EHX Hot Tubes (1979 version) - supply voltage?

[aion]

The original '79 version of the EHX Hot Tubes runs on mains voltage and has a transformer and zener-transistor regulator to step down to ~9V:



(full schematic here)

The zener is a normal 9.1V type, so you'd think it would be pretty close to 9V, but I've never seen those two resistors before - the 220R to collector and 470R to base of the transistor. These are always jumpers in the examples I've seen of zener-transistor regulators.

Presumably these resistors would drop some voltage, but how much? How would I calculate +V?

Wouldn't matter much in a vanilla op-amp drive, but I know CMOS chips sound drastically different depending on the supply voltage, so matching the original supply voltage seems crucial to matching the tone of the vintage unit.

[antonis]

In case of Si npn current pass BJT and 9V1 Zener, Emitter voltage should be 8.5V..
(neglecting voltage drop across 470Ω due to Base current..)

220Ω serves as 2N3568 power dissipation reducer (it tapers V_CE) and 470Ω acts as 100μF capacitance multiplier..

[antonis]

Presumably these resistors would drop some voltage, but how much? How would I calculate +V?

+V should be equal or higher than V_Emitter + BJT V_CEsat + 220R*I_Collector + I_Zener + 470R*I_Collector/h_FE

After a rough estimation (mainly from 1k2 and 9.1 Zener voltage) +V should be about 27V (for 15mA working current)

[Mark Hammer]

The original ran just fine off a 9V battery.  I know because I had a clone at the time, that a tech helped me make, and ran it off a 9V battery.  The trouble was that the battery wouldn't last as long as many were accustomed to with many other overdrive-type pedals.

[mdcmdcmdc]

I'm unfortunately away from my pedal box for a week and change, but if no one else has one on hand I'm happy to take whatever readings you'd like off of an original when I'm back home.

[aion]

The original ran just fine off a 9V battery.  I know because I had a clone at the time, that a tech helped me make, and ran it off a 9V battery.  The trouble was that the battery wouldn't last as long as many were accustomed to with many other overdrive-type pedals.

To your recollection, did the tone change at all as the battery lost juice?

I'm unfortunately away from my pedal box for a week and change, but if no one else has one on hand I'm happy to take whatever readings you'd like off of an original when I'm back home.

I would very much appreciate it! I couldn't find any substantial info on this in digging around.

[idy]

Stab in the dark, but general impression is for this kind of circuit (variations on CMOS invertor overdrive are legion and fundamentally not that different) head room is not a thing. And so they all run fine on the everyday 9. The Klon and some boutique OD run op-amps at 27(?) because the opamps may behave differntly when these pedals re being used as clean boosters. And in everyday like 9, 8.6...whatever.

Have never seen a CMOS OD advertised as "...boosts your signal hundreds of db and overdrives the bejesus out of any living amp." But some op-amo OD pedals that's the pitch.

[Rob Strand]

+V should be equal or higher than VEmitter + BJT VCEsat + 220R*ICollector + IZener + 470R*ICollector/hFE

Actually if it's regulating, which is should be,  the output is Vzener - Vbe - 470R*ICollector/hFE.

You would need to come-up with an estimate of the circuit's current consumption to get ICollector.
At 9V the 4049 could pull a fair bit of current when operating as a linear amplifier.

Guess ICollector = 20mA to 50mA then for hFE = 80, ICollector/hFE = 0.25mA to 0.5mA which only amounts to 0.12V to 0.24V across the  470 ohm.   So that part doesn't introduce much error and antonis's "ignore drop" is justified.   Vbe is probably 0.7V at these currents so you might end-up with 8.3V or so.

[PRR]

> I've never seen ...- the 220R to collector ....These are always jumpers

Short the output. Current wants to rise to 13V/0Ω or infinity. There are a bazillion way to limit current. A dumb resistor is often the smartest trick. Here 220Ω holds dead-short current down below 100mA even for inputs to 22V.

Zeners hiss. And are so very low-Z that a cap hardly damps the hiss. 470Ω and 100uFd is a significant low-pass. Yes, it adds 5-10Ω to output impedance but for <100mA loads that's no big deal.

[PRR]

> I've never seen ...- the 220R to collector ....These are always jumpers

Short the output. Current wants to rise to 13V/0Ω or infinity. Unhappy customer. There are a bazillion way to limit current. A dumb resistor is often the smartest trick. Here 220Ω holds dead-short current down below 100mA even for inputs to 22V.

Zeners hiss. And are so very low-Z that a cap hardly damps the hiss. 470Ω and 100uFd is a significant low-pass. Yes, it adds 5-10Ω to output impedance but for <100mA loads that's no big deal.

[aion]

throughout

Stab in the dark, but general impression is for this kind of circuit (variations on CMOS invertor overdrive are legion and fundamentally not that different) head room is not a thing. And so they all run fine on the everyday 9. The Klon and some boutique OD run op-amps at 27(?) because the opamps may behave differntly when these pedals re being used as clean boosters. And in everyday like 9, 8.6...whatever.

For me it's less a question of "will it run OK?" and more one of "will it sound exactly like the original?". If a modern supply is 9.6-9.8V, and the vintage EHX regulated voltage is more like 8.5V or even less, then I'd want to test the change in voltage to see if there was any audible difference.

As I understand it, that's the main difference between the Red Llama and the Tube Sound Fuzz. The Red Llama uses a 1k resistor in series with the supply, and by most accounts they sound completely different. I've never built one personally (though I will in the next few weeks) but this is just what I've absorbed throughout the years.

[Rob Strand]

You would need to build one up to experiment.

There's no details on the transformer, if the input voltage isn't high enough and the CMOS gate currents are high the regulator might not even be regulating with that 220 ohms in there. (220*20mA = 4.4V loss)

When you add resistors to the supply like the Red Lama it helps prevent the 4049 operating at high current, perhaps even evening the supply current out with device to device variations, buy maybe not the sound.

Unfortunately I can't remember the currents those things pull in linear mode with 5V and 9V etc.  (The number of gates in linear mode is also a factor in the total supply current.)

The non-transformer version is quoting 18mA but I don't know what the circuit differences are.

See nano on this page,
http://mirosol.kapsi.fi/category/electro-harmonix/

[Rob Strand]

For me it's less a question of "will it run OK?" and more one of "will it sound exactly like the original?". If a modern supply is 9.6-9.8V, and the vintage EHX regulated voltage is more like 8.5V or even less, then I'd want to test the change in voltage to see if there was any audible difference.

Some power supplies are right on 9.0V.   The issue is of you have to cope with 9.0V to 10.0V DC input range and you want 8.5V out you can only do that with a low drop-out regulator.   The EHX circuit isn't going to work.  It probably needs 18V in.

A different angle, which would need experimenting with, is to simply add a low-valued series resistor to drop the supply volt 0.5V to 1.0V, then do a test to see if it works OK at 9V and 10V.

(Batteries are out since you want to go down to 7V or so to get good life out of the battery, which is already less than 8.5v)

I dug up this old thread,
https://www.diystompboxes.com/smfforum/index.php?topic=120074.msg1123269#msg1123269

The thing still missing in all of this is the actual DC supply voltage on a real unit.

Based on that thread we can estimate the current vs voltage

Vdd[V]    Idd[mA] (per gate)
15            7.5
10            3.4

estimates   
10           3.4
9.5          3.078
9            2.772
8.5         2.482
8            2.208
7.5         1.95
5            0.9

keep in mind they are only estimates, not measurements.

That ends up with a total 14mA of supply current without LEDs etc, a bit lower than the 18mA of the nano.


Here's some estimates for what happens when we choose a supply resistor to drop a nominal 9V down to 8.5V, then with that resistor value we increase the supply to 10V.   What that's saying is with such a low value resistor the 1V DC input variation translates to 0.9V DC variation on the gate supply side.  In other words a small resistor value doesn't help even things out at all.


Choose RDC for 8.5V with 9V in       

RDC =         34.65
       
VDCin        Vdd
9.00        8.5
9.11        8.6
9.22        8.7
9.32        8.8
9.43        8.9
9.54        9
9.65        9.1
9.76        9.2
9.87        9.3
9.97        9.4

I suppose the issue could be ignored altogether if the unit sounded OK from 8.5V to 9.4V!

[CheapPedalCollector]

Mine supplies 9.05 volts or so, and the transistor is a 2n5088 instead of the 2n3568. It's a revision B board, reverse graphics.

[Mark Hammer]

I think it bears noting that a lot of pedals at the time the Hot Tubes was released did NOT have external power jacks.  I imagine a big part of this was because people didn't have pedalboards and the wallwarts available were often of dubious regulation.  So the pedal either ran fine off a battery or two, or else it had an onboard transformer if current consumption was enough to drain a battery too quickly.  I could be wrong, but my sense is that there was nothing magic about the supply used.  It provided enough current at a useful voltage and that ticked all the boxes.

Decent pedal.  Yields a nice grunt that is suitable for Keith Richards rhythm tone.

[antonis]

Actually if it's regulating, which is should be,  the output is Vzener - Vbe - 470R*ICollector/hFE.

Trouble in mind - I was thinking voltage drop and current consumption simultaneously..

[Rob Strand]

Trouble in mind - I was thinking voltage drop and current consumption simultaneously..

No worries, it's easy to do.


Can anyone see a transistor or zener on the nano?
http://mirosol.kapsi.fi/category/electro-harmonix/

Maybe they just have series resistor.  They even run that thing on batteries.
All the "issues' seem to be left to ride.

[antonis]

As fas as I can see, no transistor at all..

[Rob Strand]

As fas as I can see, no transistor at all..

I can't see any transistors or zeners either.

The best guess is R22 is in series with the power but it's hard to tell and I can't read the value.

The diode D1 looks like a parallel protection diode more than a series protection diode.
(From the tracks on C18 I'm not even 100% sure of that.)

[amptramp]

Back in my old days designing remote sensing amplifiers for infrared scanners, we occasionally did some strange things.  We tried CMOS linear amplifiers running in a bath of liquid nitrogen at 77K.  Since this is a quarter of room temperature, we would expect to get half the noise, so that worked in our favour, although the series resistance for the input protection kept the design out of the low noise region.  We characterized the current drain of a single CMOS inverter and found that as we got to an input approaching half the supply voltage, we ended up with an 8 mA current drain.  This dropped off to leakage current as the input approached either rail.  I forget what voltage we were running but it was probably 12 VDC.

Every once in a while we would get extreme pulses at the output indicating that maybe we were getting tectonic quakes in the silicon die at liquid nitrogen temperatures.  At room temperature and various supply voltages, you might get a different current drain at the midpoint but the value of current drain for a single CMOS inverter would be about the same as for an op amp or larger.  The other point is the current varied widely with the input so power regulation would be necessary to avoid one stage coupling into another.