117P7-based high voltage clean boost pedal

[Jeema]

Okay, here is a bit of a zany project I did: it's a high-voltage tube-based clean boost pedal, utilizing a very obsolete 117P7 beam power tetrode.

So first things first: why this crappy tube.

Well, first, since the filament voltage is so high, my thinking was that I could make something using only a single inexpensive 120V:120V transformer - no charge pumps or filament taps needed.

Second, although it's generally a very maligned tube by audiophiles (due to it's microphonic nature amongst other things), it is very low power for a power tube, so it's decently well suited for use in other applications.

Third, it has nice 2nd harmonic distortion characteristics (according to the datasheet).

Fourth, they can be purchased for cheap since nobody wants them.

And lastly, it's highly obsolete ...therefore automatically boutique.

Long story short, after a few false starts, I eventually made something that works, and it sounds pretty good to my ears.  It is definitely not what I would call a transparent boost - it adds kind of a nice high end chiminess, but without being too overly clean, if that makes sense.  I'm not sure if this can be attributed mainly due to the circuit design or the tube's characteristics or both.

One problem I ran into was the fact that the filaments in these 117 volt tubes draw a lot more current when cold compared to when they're hot.  Hot, they draw about 0.1A.  On initial startup, though, they draw more like 0.6A.  That meant I needed to add in some inrush current limiters on the filament tap to keep my power resistors and transformer from getting toasty.  The filament power resistors still get pretty hot during operation, but they're within spec of their dissipation capabilities even at startup, and the internal temperature of the pedal doesn't seem to get too unreasonable (I measured it in operation).

Schematic is attached.  Sound samples and pictures to follow.

I did manage to (barely) fit everything in/on a 1591 plastic enclosure, including the transformer.

Questions/comments welcome...

[amptramp]

The 117P7 is a beam power amplifier with a power diode that allows you to make single-tube devices like amplifiers or its most common use (when it was popular) of a capacity switch that worked by damping an oscillator so its grid bias dropped and more current would flow.  Miller made a coil (type 695) designed for just this purpose.  They were also used in optical switches, cheap phonographs and as output stages for 3-way battery portable radios.  The only difference between it and the more common 117L7 / 117M7 tubes is the pinout.

The heater current is 90 mA and when operated at 105 volts on the plate and screen, it puts 850 mW into a 4000 ohm load.  It is a decent tube if you can avoid the noise from the 117 VAC filament.  Keep the filament leads twisted tightly or run them on DC.  The power diode would enable you to make an interesting diode distortion.

I am not sure you need 44K across the filter capacitor - seems rather low.  The diode cathodes usually would have another capacitor to the centre tap of the transformer.  It looks like this would give you "beam power tube sound" which may be useful if you are going into a transistor amp.

[PRR]

I'm surprised it has any "chewy flavor" since the tube normally needs 5V drive to slam it, and your modes don't seem to get that down below 1V?

The 36VA transformer ought to be plenty. I am wondering why you took nominal 240V to feed a 117V heater. Seems to me you could just take 120V. Yes, it leaves half the secondary unused but it still should not suffer.

I'd suspect residual hum from a *single* stage DC filter to a low line level output. And in 1949 a 270uFd cap would have cost more than a 117P5!! Dozens-uFd would be more appropriate, and a 2-stage filter to get hum down.

Wonder why you blew the big 30 cents for 1N4004 when there is a perfectly good vacuum rectifier in there. Hey, Mesa Boogie says vacuum rects are cool!

The 500K output pot seems unnecessarily high, and susceptible to loading effects.

[Jeema]

I am not sure you need 44K across the filter capacitor - seems rather low.

I wasn't sure what best practices are for HV capacitor discharge resistors, so I just settled on a value that would ensure discharge within about 1 minute after power was turned off, which may possibly be overkill.  It does drop the B+ voltage about 5 volts being in there.  At first I didn't have any discharge resistor, but I thought maybe that was a bit dangerous...

I'm surprised it has any "chewy flavor" since the tube normally needs 5V drive to slam it, and your modes don't seem to get that down below 1V?

Yea, could just be my imagination. You're right that there shouldn't be much of any distortion - it's only about 30x voltage gain by my calculations.

I am wondering why you took nominal 240V to feed a 117V heater. Seems to me you could just take 120V. Yes, it leaves half the secondary unused but it still should not suffer.

Maybe 120VCT is not the correct way to describe the transformer.  I meant that it's 120V from end-to-end, so 60V at the center tap - and that would be if it were pulling 0.3A.  Since I'm only pulling a little over 0.1A, it's about 140V from end to end.

Wonder why you blew the big 30 cents for 1N4004 when there is a perfectly good vacuum rectifier in there. Hey, Mesa Boogie says vacuum rects are cool!

No good reason I suppose! ...except that I went through several iterations and kept changing things. The transformer could for sure handle a half-wave rectifier.

The hum and noise situation isn't too bad right now.  I did some measurements and the noise floor is about 55dB below the signal.  It varies with the tube, though.  One RCA tube I have is about 4dB noisier than the one I am using now.

I hadn't even thought of using a two part capacitor filter, that might be a good idea.

Interestingly when I was originally breadboarding it, I tried to run the filament straight off the wall.  That did work okay when the amp's ground was lifted, but caused bad noise when the amp was grounded.  All my experiments using DC on the filament seemed to cause bad noise as well.  It seems like the filament voltage needs to be AC with no DC offset relative to the circuit ground for best results., but maybe I interpreted things wrong...

[anotherjim]

Low B+ bleed resistors may increase ripple voltage. It should be ok x10 bigger. The scariest HT bleeds off quickly. A big reservoir cap is storing a lot of energy which could make for a big bang if you get an accidental short. Pauls suggestion of smaller (20uF ish) caps in a C-R-C arrangement is better.

[PRR]

> it's 120V from end-to-end

Sorry; I mis-read that.

> I tried to run the filament {heater} straight off the wall.

Please don't.

You are trusting a 2-mil spray of white oxide between you and fatal connection. This was even legal back in the day. But never a good idea.

And as you note having heater and audio on separate sides induces buzz.

[amptramp]

Another recommendation for two electrolytics, one at the output of the diodes as well as the one you have there.  With a second electrolytic, you could get better elimination of hum.  The C-R-C filter has been used in radios for ages and it worked well enough because the high Rp of the tube (17,000 ohms) usually allowed an output transformer to float up and down on the hum without injecting it into the signal since both ends of the primary were going up and down with power supply ripple.  In this case, you need good control of the voltage because you are taking the voltage to ground and the C-R-C filter will allow you to get that.

[Jeema]

Finally got around to recording some samples.  This was done using a dual-humbucker Washburn through a solid state Fender practice amp straight into the board - nothing fancy. The first part of each sample is the bypass signal, followed by the boost engaged and set to approximately unity gain for comparison:

https://soundcloud.com/user-567435059/sets/boobtooster