THD Electronics – customer service commendation

Was trawling through some old e-mail today doing a clear out and came across this old exchange between myself and Andy Marshall of THD Electronics. I’m always a fan of companies who give personal time to each and every customer and this only draws me closer to them as a loyalist.  I still have my UniValve and although it’s not super versatile live, at home it gives some of the greatest tones I’ve ever heard – in fact this little baby is positively dripping with tone – on some occasions I’ve even laughed out loud that there is TOO MUCH TONE coming out of this little head!

Anyway – this is what I wrote.. and for the record I believe the tech screwed me making up some crap to overcharge me.

Dear THD guys,

I am the proud owner of a THD Univalve that I absolutely adore – it is positively the best amp I have ever owned – for this I thank you. 🙂

I would like to relate a recent experience to you and ask for your advice please: recently, I had some technical difficulty with the amp – I turned on the power, let it warm up a while and then flipped up the standby. There was a crackling noise and I saw a purple incandescence inside the power tube – like a quick flickering flame. After this, there was no sound emitted from the speaker. I changed the power tube but still the amp did not work.

I brought the head to a local tube amp repairman. I had described the symptoms to him and he immediately told me what his suspicions of the problem were. At the workshop, he replaced one of the fuses, turned it on and we heard the same crackling noise. He told me he had to do some work on the PCB. I collected the amp the next day in working order and he told me something to do with ‘screens and humidity’ (Singapore is very humid except my amp resides in an air-conditioned room) and having to raise up components from the PCB so they weren’t so close in order to cause arcing (the root of the crackling he said). BTW, repairs weren’t expensive – approx USD $30-$40.

Not being a tech head, I didn’t understand this, but he mentioned something about the attenuation and more recently has posted this on a local forum:

-You must also have a good technical background to use a product as a tube amp attenuator.

-Improper connection, or using it outside the boundary (very easy to happen for the untrained and unfamiliar), could cost you irreversible damage.

-Tube amps are meant to be ‘impedance’ matched properly for the output stage. Improper matching could result in shorted out tubes or worst transformer meltdown.

-Either case, I personally frown upon putting an amp on a “treadmill” situation. This I surmise from some of the THD univalve units I have seen so far with built-in attenuator.

-With the case of the SL sales that is happening on a regular basis, I do suspect the resale values of such amps will deteriorate.

I have not heard of similar problems in running the amp with attenuation *all the time* which is essentially what I want to do as a bedroom player. I certainly don’t want to suffer transformer meltdown.

If the info is relevant, then my tubes are: stock China 12ax7, Mullard NOS ECC83, Mesa Boogie EL34.

Can you comment please?

Thanks in advance and kind regards


Andy Marshall personally replied:

Thank you for your email and for purchasing a UniValve.

Let me start off by stating that I am very sorry that you have had any problems with your UniValve. I am glad that the repairs were inexpensive.

The situation that you are describing is almost always caused by a short in a power tube itself and is almost always rectified by replacing the power tube and HT fuse. As you changed the power tube and still had the problem, clearly your situation was somewhat different.

As for your tech’s assertion that some components needed to be raised off the board, this would only make sense if the board were not completely coated with a moisture-proof completely insulating solder mask layer. Furthermore, the components in the screen circuit are either raised at least 1 cm off the board or are mounted point-to- point at the tube socket, implying that your tech must have been mistaken in his assessment of the cause.

Now, as for the humidity in Singapore being an issue, it is not so much the humidity itself, but more likely a matter of small crawling or flying critters making their way into the amplifier through the vent-holes or the jacks themselves. I have seen countless cases over the years of exactly what you are describing (in other amplifiers, not THD amps) being caused by spiders, ants and other uninvited guests strolling across the terminal strips, the tube sockets or the circuit boards, providing a nice, juicy path for arcing and leaving a nasty burnt trail where they were cremated when the amp was turned on. I see this in many amps from the tropics as well as amps that have been stored in dark basements or attics. Often there are little legs and wings left over, but sometimes there is just a burnt smudge, which could be misinterpreted as inadequate relief for voltage and humidity. Our relief spacing on the boards, sockets and terminal strips are adequate for 95% humidity at up to an altitude of 2000 meters, so unless a little visitor was involved, I would have to guess that you had a “freak” accident with the amplifier.

We have over 2000 UniValves in the field, with roughly 1/4 of them sold in tropical areas, and this is the first we have ever heard of this kind of problem being anything other than a bad tube, so I would have to go back to my assumption that your experience, though unfortunate, is an isolated incident.

As for your tech’s prejudice against attenuators, that is another matter altogether, and clearly had nothing to do with the problem you had with your amplifier. The UniValve was designed from the ground- up to be run 24/7 with it’s built-in attenuator and doing so will not shorten the tube’s or the amplifier’s life at all compared to running at the same settings into a speaker. As for our external attenuators, they are constant-impedance devices and always show the amplifier the same impedance, assuming that the amplifier’s output impedance, the Hot Plate impedance and the speaker impedance all match. For these reasons, I fear that your tech is expressing more of a prejudice than anything else.

I hope this helps.

Please do not hesitate to contact me if you should have any further questions.

Sincerely yours,

– Andy


One Response to “THD Electronics – customer service commendation”

  1. JMGuitars Says:

    THD is always great! and I love how Andy signs off each amp himself and takes the time to identify and advise on user problems.

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