VHT - Valvulator

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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mnemonic
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Post by mnemonic »

I quite like mine also, I bought it on a whim and wasn’t sure it made any difference until I took it out of the signal chain.

After the noise fiasco I was concerned with reliability so I built a clone using one of those cheap high voltage boost converters off eBay, so I could just run it off a 12v dc wall wart. 12v dc for the heaters, and for the boost converter to convert to the required ~350v dc for the tube plates.

It worked (albeit noisy, maybe the power supply or maybe the boost converter to blame), for about 10 minutes until one of the filter caps on the boost converter blew up.

Never got around to fixing it since I fixed the real Valvulator. Might give the boost converter a try again, otherwise maybe a small 9 volt or 12 volt ac transformer wired backwards and some diodes and filter caps.
6B272C11-B11E-470F-853D-0694F4B49EDD.jpeg
I got a bunch of smoke pouring out of that cap, smelled terrible.

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shakti
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Post by shakti »

Well, here I am half a year later trying to fix my Valvulator. No one can accuse me of rushing things...

As I had a problem with slight ticking noise, DC across the output and an overall ever so slightly weak/wonky sound, I/we figured the two 10uF output caps might be faulty. After I finally got those ordered I went and replaced them yesterday. I didn’t take any measurements first. This is my first time working on a PCB, I’ve built loads of amps on turret boards but I am a novice at this sort of thing.

I might have a soldering problem here, but continuity checks out fine and I hope I didn’t burn the caps with too much heat, but in any case it didn’t get any better after the change, in fact it got worse. I now have a very weak and dull output, and still get popping when switching it in and out.

I then started taking measurements, and get some very strange readings. The plate voltages on the 12AX7 are all very low, and what’s even stranger is that they are dropping as I take measurements (with the tube pulled out), and continue to drop. However, out of the rectifier before R13M I get the appropriate 450ish VDC.
I noticed that someone else had a problem with R9M and indeed I can’t get a good reading on it in the circuit. Perhaps it has suffered the same problem? However, I hate shotgun repairs and prefer to try to understand what’s happening. Could a burnt R9M cause these symptoms, i.e. gradually worse behaviour with noise at first, then lower and lower output and DC across the output jacks?

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shakti
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Post by shakti »

I’d like to answer my own question: can a faulty R9M cause these symptoms?


YES, it can!! I am happy to report complete success. It probably wasn’t the output caps at all, but rather a burnt R9M 100k 0.5W resistor, just like in mnemonic’s case. Many thanks to mnemonic for putting me on the track. Like on his case, I couldn’t get a reading on it in circuit, nor after I yanked it out. There were no outside signs of damage/burns/cracks, but it may appear that this resistor is a little underspeced and prone to wear and burn over time. I replaced it with a 1W resistor to be on the safe side, and all was back to normal.

I still dearly love how this buffer sounds and feels. It feels so much like plugging into an amp, with a lively, bouncy feel with shimmering harmonics and the slightest hint of compression, at least it appears like that to me. Very subtle, but I like it better than any solid state buffer I’ve tried. It also sounds really interesting when running a fuzz before it. With some fuzzes I clearly prefer running straight into the amp without the buffer in between (fuzzface for instance), but with a Castledine Supa Mk I, which is an amazing Tonebender variety, the buffer tightens up everything in an interesting way. I keep the Valvulator in a true bypass loop which also runs out of the «motherboard» so I can patch in my second pedalboard without any signal loss but with the ability to bypass the auxiliary loop and just use it as a plain buffer on the motherboard, Best of all worlds.

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