This is a Nice Piece of Kit!!

This THD Univalve is a real nice amplifier; extremely well made, very solid and full of all the facilities that one would expect from a modern day tube amp of this quality and price. All the THD products are good and well made. I love the quick release thumb screws for the top cover, the excellent quality, and the overall great concept (please see photos). All the THD products are designed by Andy Marshall, with "top-marks" to him for an excellent product.

Now, having started an article off with a tremendous appraisal like that, it’s a shame that the story ends on a slightly sad note! Please read on and all will be revealed.......

Like many another amplifier, this THD was brought in with the request to sort it out, as all was not well. It didn’t take much knowledge to realise that there was a completely burnt out 10k resistor (R16) and the same for the front mounted (5k) "attitude" potentiometer. Wow! These had both blown to smithereens! (This can be seen in the photo below, circled in red, where these two components used to be.) Being yet another amp utilising printed circuit boards (uh!!), and not being familiar with the circuit, I decided to e-mail THD for the schematic. In doing this, I explained to them in my mail that I had no intentions of displaying the said schematic on my web-site for the world to see, without their permission. I respect company policies like this.

OK. With the Americans being a few hours behind us here in the UK, it wasn’t long before I heard that familiar "ping" and found a solitary e-mail in my in box…….from THD. How kind of them. The schematic attachment was there sure enough but......nothing on the mail itself; no "With Compliments"; no "Hi there John"; no "Kiss my ass"; no nothing! Now, whilst this didn’t really matter as I had the schematic, I did find this a little less than polite.

Anyway, back to the amp. Now, looking at the circuit I could see that R16 and the attitude pot were in the cathode/grid circuit of an ECC83…….Hell, there are no high voltages around here. What on earth could have blown both these components up with such devastation? My first and only assumption was the ECC83 itself had suffered a very serious short; an extremely unlikely thing really, but I could think of anything else and I have had this problem with crap tubes on very rare occasions.

I changed the suspect tube and proceeded in cleaning all the burnt material off the board; replaced the 10k resistor and 5k pot. Turning the amp on, and with my meter connected here just in case there was going to be a high voltage from somewhere, she warmed up fine and all seemed OK........well, for a while at least! Everything was now working fine so, like all my repairs, I intended to leave it on test for a good few hours before handing it back to the customer.

After about an hour of this soak testing, all of a sudden there was a flash, crack, and a fizz of smoke coming from the circuit board! Power was removed rather promptly. I had actually spotted where this had come from and it was nowhere near the other previously blown/burnt components. In-fact this was from a 1meg resistor (R17) mounted further down the board, but in reality is the grid return resistor for the very ECC83 which I had assumed to be the troublesome bottle. However, now I could see the real problem and cause of all this. This 1meg resistor is lying hard down right across a piece of main HT track on the circuit board. This track is carrying around 300 volts, and basically as this 1meg is from grid to ground, it had simply arced across pushing the main HT through the 10k resistor and attitude pot. Its no wonder they both disintegrated! You can see this on the photo, circled in yellow.

I now ripped out the 1meg resistor, cleaned this part of the board, and mounted a new 1meg, but, a little up in the air this time to clear the board! Oh and by the way, the original ECC83 was fine too, so this went back in. And now she is all fine and perfect. After a further five hours of testing she went back to a very satisfied customer…!

OK, at the beginning of this article you may remember I said it was an unhappy ending, the reason for this is as follows:-

Though I wouldn’t have called this problem/fault "a design fault" I would at least call it a design/build "issue". This being the case, and trying to be helpful to THD, I sent them an e-mail outlining this problem that I had found. I thought that if they were aware of it they could (a) get their production line to mount this 1meg resistor "up in the air" to eliminate any further problems here and, (b) they would know the reason if they had any of their amps come in with the same fault/problem and/or could contact their service agents worldwide with the possibilities of this happening. But guess what………………they never responded! Below is a copy of the e-mail I sent them. At least all you good people out there are appreciative of my work and help!


PS. Have you noticed how all things seem to come back around!! In reality the THD amplifiers can quite easily be classed as "Bird-Cage style amplifiers", just like the good ‘ole UK Linear products of the sixties!! Check out the photo below!


The Linear Conchord - "Bird Cage" amp, 1960's style!



Email sent by John to the Service Team at THD:

Hi guys, my name is John (Chambers) and I run "Champ Electronics, The Valve (Tube) Amp Hospital" here in Nottingham, UK.

Yesterday I had one of your model UniValve amps on the bench for repair. On first investigation it was obvious that the 10k resistor (R16) was completely burnt-out as was the 5k "attitude" pot too!? To save removing the main board to source were these components were connected I e-mailed you guys for the schematic. Ed very kindly sent me same.

Now, looking at the place where these components are wired, being in the cathode/grid circuit of V2b the only first assumption, though very rare was a badly shorting ECC83, as I had about 295 volts sitting here!?

OK, after removing the board, replacing the 10k resistor and checking everything else out all seemed to be well. Just for good measure I put a new ECC83 in here. On power-up all was fine for about an hour but then there was an instant flash, fizzle and smoke!! I spotted the problem immediately and thought it best to let you know. I wouldn't call this a "design problem" but, more a "hick-up!"

The 1meg grid resistor (R17) is laid down hard on the board and the end of the resistor has been arcing to the 300 volt rail sitting right under it!! The guy who owns this says he has been having trouble with it for a while, during the course of this time the arcing has burnt itself into the board causing carbon tracking.

I have replaced the 1meg and lifted it up in the air, I also had to gouge-away the carbon build-up and she is now back to rights. (please see attached photos).

I thought I would let you guys know about this as, although it may be a rare fault, at least you would be aware of it should the problem cross your path again?

My web-site is very well respected over here in the UK and I get an immense amount of hits, e-mail and phone calls. It is my intention to display your amp on my site with an article about it too. Other than this small issue I am very impressed with your product and intend to voice this all on my site, should this be acceptable to your good selves?

Best regards, John.

PS. I have no intention of displaying the schematic on the site!







Circuit board at location of burnt-out resistor and pot



Typical circuit board ! -wires have to be removed to gain access.


Carbon tracking burnt into board  -  the initial cause of faults.



New 1 meg resistor fitted away from the board. Carbon build up removed. 






One of the quick-release screws for the top cover.