What A Great Idea!

Back around November 2005 I was having a cup of tea with the guys at Hot Rox UK, down at their unit here in Nottingham. (They have built up a thriving business matching and supplying thermionic tubes (valves) across the world as well as supplying many other musically related products). We were all standing in the testing room and I was watching them testing and matching up tubes. I was admiring the routine that they had obviously gotten into after many years of doing so. I noticed that most of the testing is done on one machine, with 2 or 3 other machines having just one tube in each and basically being used just to warm up the heaters. The technician, with glove on hand (the tubes are now hot!), would pull a “warmed up” tube from one of the ancilliary testers and place it in the actual one doing the tests. This reduces the testing time as the tubes heater is now already on (hot) and only takes a matter of seconds to reach its working potential as apposed to having to wait for a “cold heater” to warm up every time....all very sensible. It was at this point that I said “why don’t I make you a heater warmer?” After I had explained my thoughts on what I could do, the management replied “what a good idea, can you do that for us?” Next came my usual reply; "no problem, leave it with me". And so………..

Firstly, I designed and made the appropriate power transformer. This unit has 3 secondary windings: 1 x 6.3 volt @ 25 amps (for up to ten output tubes in any combination), 1 x 6.3 volt @ 10 amps (for any combination up to 20 of the smaller B9A base tubes like EL84’s, EZ81’s EF86’s and all the ECC8?/12A?7 range of tubes), and lastly a 5 volt @ 30 amps for up to ten rectifiers, also in any combination. That makes a total of about 370.5 watts, so I simply made it a straight 380 v/a. You would be hard-pressed to buy a transformer like this “off the shelf!”.


Now, having done the transformer and also knowing the physical size too, I started to design/workout the chassis size. I wanted to use twenty octal bases and twenty B9A bases too. Knowing that KT88’s would be physically the biggest tube likely to be used in this warmer, I spaced the bases sufficiently to suit these. Anything smaller would of course then be fine. It was also important that enough space is needed for someone to get their hand around the tubes whilst they are hot and not get burnt on the adjacent tube! It was these two issues that decided the overall size of the chassis. The twenty B9A bases fitted nice and snug in two banks of ten each side of the centrally mounted power transformer.

Next job was to go down to my friendly sheet metal works for a piece of 14 gauge mild steel, and then to a friend of mine to have it bent and welded into shape. The chassis itself ended up being 25 x 14 x 3 inches, which is quite large.

Then came the tedious task of marking it out and lastly, the drilling/hole cutting. As there were forty main tube holes to be cut, I decided to do just two of them (one of each base size) and then returned back to my metalwork man for him to cut the other thirty eight on a pillar drill. (I didn’t relish the thought of cutting that many holes by hand with my punch cutter - too much like hard work….phew!) All the smaller (grommet and bolt) holes where of course no problem.

Next came a drive across town to my very reliable powder coaters, M and R Refinishing. (These guys do a great job. Check out their website ). The colour that the Hot Rox team had chosen is one I have used before and it just looks superb. Its title coding is “Antique Copper”.



Finally, after receiving the chassis back from the spray shop, it was time for the assembly. Whilst the actual wiring is extremely simple, the building of this unit was quite tedious and fiddly. All the nylock nuts and bolts are nickel plated and slip out of one’s fingers. There are eighty six altogether! The tube bases are all glazed ceramic with silver plated pins and gold plated mounts.



The wiring is extremely simple, but as we’re working with high currant here I wired all the octal bases with 16 gauge silver coated copper wire in a buss fashion to eliminate any voltage drop down the chain. The same thing was done with the B9A bases wiring using a lesser size wire of 20 gauge.

As simple as this unit is, and the simple task it performs of heating forty tubes (valves) for pre-testing, it is however an invaluable piece of equipment for this kind of work.



All the major tube manufacturers use this kind of thing, though on a much larger scale. They have banks and banks of tubes - in the hundreds - being warmed and (or) tested all at the same time. I doubt one could buy this kind of thing commercially and even if you could I’d bet the price would be……..well!?

When all the guys at Hot Rox UK saw this warmer for the first time they were well impressed to say the least. It is now in use on a daily basis fulfilling its job. All the staff say how they wouldn’t want to be without it now! I enjoyed designing and making it for them.

Cheers, John.

Footnote: There is a cover for the power transformer but at the time of the photos I was waiting for it to be sprayed.






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