Well Ladies & Gentlemen, I think you’re all going to enjoy this true (as all my articles are) and interesting story…..It’s gonna “blow your socks off"!

Back in 2006, a gentleman bass-player named Chris had got it into his head that he wanted a valve (tube) amp to use with his band, and rightly so too. He had heard about and read on the internet that valve amps are warmer sounding, and so after further web searching…to find out which one and why....he came across an American company called Red Iron Amps. After spending much time reading about them on their website and a few forums too, he decided that this was the one for him. He then placed an order with them for a 100 watt bass head at an all-in price (including shipping to the UK) of around £800, which seemed to be a pretty fair price.

When the made-to-order amp eventually arrived, which took about three months, (although some of this time taken-up with to-and-fro e-mails though), on removing it from its packaging there was some damage to the case evident. Chris however decided to still try it and did indeed do so. He was greeted with a puff of smoke and the amp went straight off! Now, as Chris was living in London at the time, the sensible thing for him to do was to take it into a local electronics repair shop. After receiving it back a few days later with an invoice for £88.50, which seemed quite reasonable, it worked OK and all was well for a good while; in fact for about a year during which time he used the amp approximately 200 times or more. Now, after this year of intensive use, Chris bought a new house, and his time became taken-up with much DIY work and other priorities. Therefore, the amp became redundant for about two years.

After he had sorted out his new home, Chris, who was now living in Nottingham, joining a bunch of guys to form a new band, and he started to use the Red Iron amp once again. After only one rehearsal, the amp suddenly “spat-its-dummy-out” and kept on blowing the power fuse! So folks……. this is where I came in. Chris contacted me and explained the above part of the story.

When the amp was brought into my workshop, I have to say I was quite surprised with the looks of the wooden case and chequer-plate metalwork. I had never seen or worked on one of these before. No big deal though… amp is an amp, and they are all pretty-much-of-a-muchness! I asked Chris to just leave it with me and bade him farewell. He also left me with the original invoice of the first repair in London, back in 2006.

OK, so here goes.........When I first took the amp out of its wooden case (which was quite fiddly), the first thing that struck me was the two SE (single ended) output transformers instead of a push-pull type. In fact, I then remembered Chris mentioning this on the phone initially. It was apparently one of the Red Iron “sales pitches" that “it uses two single-ended output transformers”………Er, why is that a good thing! The power transformer looked fine but it didn’t match the two output transformers in looks. I then turned the amp over! There was another power transformer underneath, and at the opposite end of, the chassis to the top-mounted power TX! The layout and wiring was not the tidiest that I  have ever seen but of more concern was the fact that there were virtually no rubber grommets in the drilled holes for all the transformers’ wires to pass through the chassis, and drilling burrs were visible around the holes. The mains earth chassis connection was just trapped under a nut, and the power cable securing gland was hanging off (so a knot had been tied in the cable instead). Had this left the factory and entered the UK like this? Surely it would not have passed UK safety checks and regulations if so?

Further observations under the chassis showed all the electrolytic capacitors strapped together in bunches with cable ties and simply glued to the chassis with what looks like it might be Araldite. Seeing this I suddenly remembered something that I had read a few days earlier on the invoice from the first London repair. I have photo-copied this invoice, removing any company names and phone numbers etc for obvious reasons, and it is hereby displayed below:

Let’s now take each point of this invoice in turn, as it states.

"With reference to the above amplifier, the main fault observed was instability in the HT supply due to leaking capacitors, insulation breakdown and an additional unbalanced transformer added to one half of the main transformer secondary."

What a load of bull***t! What leaking capacitors and insulation breakdown……these caps have never been changed….you can tell they are completely untouched. As for the added “unbalanced transformer” well yes…this second power TX mounted under the chassis is in series with the main power TX to get the HT voltage up-to the 560 volts, for the KT88’s but it is not unstable and would have worked absolutely fine!

2. The fault was traced to a number of components, all of which required removal/replacement.

More bull***t. There are no components that I can see that have been removed/replaced!

3. The amplifier earthing and mains/fuse wiring layout were changed to confirm to EU safety requirements. Mains input cabling was changed to Euro style moulded cabling.

Reading this could indicate that the problems with the
mains earth chassis connection and the power cable securing gland  referred to above have resulted from the "repair work" carried out in London! 

4. Repair/cosmetic work was carried out on the cabinet to restore to the original finish.

OK, well, as I didn’t see the damaged case when Chris first bought the amp, I would at least go along with this and say "fair enough". They probably did do this!

As far as the rest of the comments…..they do seem to be fabricated in order to “baffle the customer with science” and warrant charging the £88.50, plus to pull-the-wool over the unsuspecting and non-technically minded customers (Chris’) eyes!!

In all fairness though, the amp wasn’t working when Chris took it into them but, as the case had been damaged during shipping to the UK (possibly dropped?), there is also a very good chance that this may have disturbed some component underneath the chassis, and may have quite simply been the cause of the puff of smoke and the amp failing? The London repairer obviously succeeded in rectifying that problem!

Moving on, and having got all that out of the way, let’s concentrate on the amp itself now. This extra power TX mounted under the chassis is indeed wired in series with the top-mounted TX to get the 560 volts for the KT88’s…as previously stated. This transformer has two primary windings of 240 volts which can be put in series for 480 volts or paralleled for 240….which in this case it was. Once again, this transformer also looks odd. It’s not the usual type that you would find in valve (tube) amplifiers, but more like in central heating systems, and switchgear etc. All the transformers are so odd that one could be excused for believing that their choice had been dictated by whatever happened to be available, and then finally using the extra HT transformer to get the voltage right for the KT88’s. On a more positive note though, all the components were of good and suitable quality!

With regards to the amp's power output. I would have expected 6550’s in the output as this amp was made in the USA, but actually it uses KT88's. With it running the output tubes in tetrode connection and at around 560 volts on the plates and screen grid, KT88’s are a better choice here as they are fine with this grid 2 voltage, whereas 6550’s would be pushed a bit hard on the screens. They prefer a grid 2 voltage of around 300 – 400 volts max.

This unit had been sold to Chris as a 100 watt head, but even with the 560 volts HT potential and tetrode connected, they were in cathode (self) bias. Running them like this as a pair, there is absolutely no chance of getting 100 watts! Once I had got the amp back to life, she was doing exactly 50 watts RMS, which is absolutely correct and spot-on with the original GEC specifications for running them under these conditions. I now phoned Chris and brought him right-up-to-date. I told him I could make the amp better by running the KT88’s in fixed bias and with a few tweaks. He gave me a free hand to do whatever was necessary. This I did and…the rest of the story now continues in the photos’ captions below. Do enjoy the rest of the article:

With no labelling on the front on this particular example, it seem to be a case of guessing which control does what!

There is no nice rear labelling on this particular example too. It is rather difficult to decide what impedance the speaker selector switch might be on!

The full underneath prior to any work.

All the smoothing caps cable-tied together and glued to the chassis. So much for the replaced capacitor bank and mounting hardware at a cost of £43.50 as charged on the invoice above. These have to be the most expensive cable-ties (sorry…mounting hardware) I have ever seen!

The extra HT transformer mounted under the chassis. And by the way…this was the cause of the fault on the amp when it was brought to me. One of the two 240 volt primary windings had gone short….hence the constantly blowing power fuse.

Thre of the unusual transformers shown here including the two single-ended output TX’s.

When it came down to rewinding the faulty second power transformer, my friend John Wood managed to get the “I’s” out in one piece but there was going to be no chance of getting the rest apart, due to the welding of the “E’s” altogether and the tar dipping. It was much easier to simply make a new one. Here you can see the new one…fitted to the top of the chassis now. The reason I did this was:

    (a) The original was a funny American lamination size and the ones that I had in stock were either too small for the required current or, 

    (b) were slightly larger and wouldn’t fit back under the chassis. 

I would have mounted it in the middle of the chassis, which might have looked better, but the coil would have been too close to the pre-amp and driver tubes. This would certainly have caused some hum issues. I ended up tucking it right-up close to the power transformer.

 A bunch of cables passing through the chassis without a grommet. Note the sharp burrs around the drilled hole!

The power cable mains earth/ground just trapped under a nut. Fair enough, it does work like this but really it should be done correctly on either a solder eyelet or spade connector. This I corrected of course.

The so-called cable restraint! I fitted a new, longer, better quality power cable. Fitted the retaining gland correctly and made everything safe and secure.

The initial stripping of the second, extra HT transformer. The square mounting plate has been cut and gouged-out, apparently in order for it to fit under the chassis in-between the pair of output transformers’ mounting bolts.

This seems to be over-kill with regard to the full-wave bridge four rectifiers, and the red/black pair of 2.5 mm cables, that brought the DC from the second power TX at the opposite end of the under chassis to be series’d with the main power supply HT. Just check-out how the thick red wire changes to a short piece of thin black with a piece of shrink-sleeving on it. This thin black wire was then connected to the centre-tap of the main TX with yet another piece of shrink-sleeving! Surely these are two un-necessary joints in less than two inches?

Here you can see that I have added a tag-strip with a few components to obtain the fixed bias supply (middle and slightly lower left). A few other minor alterations/improvements were made too.

And lastly, the big blank space where the second TX used to reside. You can also see the two gold coloured aluminium wire-wound output tube cathode resistors (these were paralleled but have now been by-passed as she is now in fixed bias).


Well everybody, I think that’s about it on this one. I’m sure you’d all like to know about the finished results? After all the work and modifications, and with all the conditions and specifications being within the original GEC data sheets for 100 watts from two tubes, she is now doing exactly 96 watts RMS. I’d told Chris that if we were to get rid of the two SE (single-ended) output TX’s and replace them with a correct push-pull one, we might squeeze a bit more from her but, Chris was well happy with the improvements, modifications and additions, so we left things as they are.  

Big thanks for looking and taking an interest in all my work, designs, and repairs, 



Chris’ Testimonial is copied below.

"Hi John. I've had my Red Iron bass amp back for a while now and have been able to give it a good test, including gigging it. I'm really pleased with it. The new transformer works fine, of course, but I'm also really pleased with the extra work that you've done to it. It was lacking in power before, but now it has power to spare, more clarity, and I'm more easily able to control the lovely overdriven sound.

It was interesting and educational to have the repair explained in detail, and helps me appreciate that your work was thorough, top quality, and good value for money. 

Thanks again, Chris."