CHAMP ELECTRONICS -" THE VINTAGE VALVE AMP HOSPITAL"

NOTTINGHAM, ENGLAND

SORTING OUT AN AMPLIFIER HEAD BASED ON THE HIWATT DR405 400 Watt





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This is a cloned, home-made copy of the Hiwatt DR405 model (400 watt amplifier head) that I have recently been asked to sort out. Please do enjoy everything that follows.



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So, here we go. I have a friend who runs a second-hand music shop which is about 15 miles from my home here in Nottingham, England. One day, a young guy called Luke had taken this amp into my friend’s music shop for repair, with the following story:

Luke had purchased most of the components from various dealers on the Web, the chassis he had made himself, and lastly the power and output transformers together with the choke were all purchased from somewhere in Yorkshire….Leeds, England I believe. (?) He was apparently having lots of problems with it…….mainly the bias caps “blowing-up” and so…..this is where I came in. My friend advised Luke that I was the man to sort this out for him and duly gave Luke my contact details.

After Luke had contacted me, we arranged for him to pay me a visit to see if I could sort out the amp. On first seeing the amp for real, I was gob-smacked with the state of the wiring of this thing…….as can be seen in the photo above. However, Luke did explain that originally it was a little neater but that he’d had a problem with the bias smoothing caps exploding and couldn’t understand why. This being the case he had removed many wires and done many check-ups trying to find this problem, but all to no avail. This had also left the amp looking pretty awful too.

The following four photographs taken from various angles show how it looked when Luke first brought it round.




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Luke and I struck a deal, which was that I would get the amp back to life for him but……he was to sort-out any pre-amp/drive problems, if any. Thinking that the amp was far too untidy/scruffy/un-reliable to be trying to find the problem, I decided to simply rip-out the power supply together with the output stage/drive wiring and start-a-fresh. In the photo above, I have fitted all new output tube bases. (Luke had burnt many of them with his abuse of a soldering iron and, in-fact, one of them was actually broken.). I have also rewired the heaters with solid copper [silver-plated] wire, covered with Silicone sleeving. All of Luke’s ground connections were done with crimp-on eyelets, most of these I was able to simply pull the wires out, plus they weren’t even securely tightened down!



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This was Luke’s original attempt at the underside wiring of the power supply/output and drive board!



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The original four jack sockets; which were adjusted for the 4, 8 & 16 ohms by a rotary switch. However, this switch was broken so I did the best thing here by removing it altogether and replacing it with a single jack socket. I then hard-wired this single socket to the 16 ohm winding on the output transformer. The remaining four sockets were then divided into two groups of two. One pair for 8 ohms output, the remaining pair for 4 ohms. Also, look at the “less than bell-wire” that Luke had used for the wiring of these output jacks! God, 400 watts @ 4 ohms is far more current than this wire could possibly carry!





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Here I have rewired all the underside of the main board….again using sleeved solid copper wire.



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And now here is where things really take a turn for the worse!! Once I had finished re-doing the amp (...my way!), I connected up the bias supply (being Luke’s original problem) and this all came on fine. Next was to connect the lower-half of the main HT supply, and this too came on fine. Finally I connected the second (stacked) HT and on switch-on the bias caps immediately started to fizzle & smoke! This was in-fact the exact same problem that Luke had experienced from day one….when he had initially finished the amp! What the hell was going off here? It took me about 10 seconds to realize that the problem had to be a shorted winding on the power transformer! And, I was right! The power transformer had a short between the second HT winding to the bias winding! No wonder Luke had struggled with this!



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Well…….just look at the state of this winding! This has to be one of the worst-state transformer windings I have ever seen! God knows who wound this thing…….an apprentice I would think!

Luke now contacted the company were he had purchased the transformers and explained to them the problem that had been there all along…..a faulty power transformer! Apparently, they were not prepared to replace the said transformer. As Luke had made the amp himself, they said that he must have made a mistake and caused damage to the transformer. This in-fact wasn’t correct. Although Luke’s wiring was rough, and he had made a good few silly mistakes in building this amp, he had nevertheless not wired the power supply incorrectly! I also approached the company on Luke’s behalf, to try and reiterate this to them and also with a view to advising them of my findings on the disgusting winding of this transformer. They informed me that the same type and make of transformer had been used for many years worldwide and that there had been little or no problems with them before! It was obvious by now that Luke wasn’t going to get this transformer replaced. I struck a further deal with him, as I had now to rewind the transformer too.




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The transformers that Luke had purchased for his amp come with what at first-glance looks like a Partridge sticker. However, on closer inspection of the sticker, it states “Original Partridge Designed Transformers Made In England”. It wasn't made by the Partridge company at all! Somehow, I don’t think Partridge would be very pleased at having their name on this particular transformer! You can clearly see windings piled on top of one another, hanging out the side, soldered joints hanging out & not insulated too…..the list goes on!



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This is the total amount of winding insulation I removed from the WHOLE of the transformer…….nowhere near enough!



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Here is a list of all my findings on rewinding this power transformer:

1. Disgusting winding with layers lying on top one another (overlapping).
2. Far too little insulation between windings.
3. The inter-winding screen was not actually connected to anything!
4. The primary winding gauge size was actually too big, making it harder to get the rest of the windings on the bobbin.
5. Both the bias & second HT windings had slipped down the side of the bobbin & were shorting. (In fact they were both lying on the mains primary. It was a good job that they hadn’t shorted to this!).
6. The bias winding had been wound on last….over the top of the thick heater winding! This is stupid as it results in the winding going up and down with humps and hollows, as well as being very untidy. The thick heater winding should really be the last winding.




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Phew…..OK…….having got all the transformer problems out of the way, I finished the amp and at last it came to life! Not the neatest thing I have ever done, but far superior to when it first arrived. It was at least working now, to some extent!



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Above I have just used the term “to some extent” and there’s a very good reason for this. Although the amp was working fine now, I was only seeing about 170 watts at the 8 ohm output! I could also tell by the picture on the ‘scope that the output tubes were not pulling the expected amount of current. I have seen this many-a-time before and it is usually a plate load issue. With this in mind, I now plugged the 8 ohm load into the 4 ohm socket and yes…we now had about 250 watts and also I could see that this was now matched much better. This basically means that at 8 ohms, properly loaded, the plate load is too low for the six output tubes. By putting the 8 ohm load across the 4 ohm output in effect raises the plate load by a factor of 2 and this was a far better match. Bottom line here…..I think someone has got their maths wrong when designing this output transformer, and I wasn’t at all impressed! I couldn’t squeeze any more power than the 250 watts from the six bottles (KT88’s) and of course this is far short of the 400 watts that Luke had hoped for. I advised Luke that if we were to put six separate bias pots in her (rather than just the two) and……if I wound the output transformer correctly, we could expect to see around 300 watts. As for 400……no chance! Luke decided to simply accept this and “put the whole thing down to experience!” Even at just 250 watts it still sounds nice on bass guitar.



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Since first meeting Luke we have become friends and I see him quite often. He is now working on another Hiwatt project based on the 100 watt DR103 model. (This time, I have supplied him with everything he needs…..including the transformers!) Also, though Luke did have a small amount of knowledge on what he was doing, I did give him a "rollicking" for attempting to copy, with very little experience, such a big beast as the DR405 for his first tube project. (Although I gave him 10-out-of-10 for trying!)

I have since taught him a few “tricks of the trade”, such as how to solder better, how to do ground buss rails, etc. I have told him to slow down, be more patient, and to take his time over amp building work! He took the amp away and said to me “I am now going to re-do the pre-amp and take on board all that you've taught me”. The photo above was the result! Nice effort Luke…. It looks so much better now!




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Finally, just one more thing to bring to light here. Although this particular amp doesn’t make the 400 watts, it will be interesting if and when a genuine Hiwatt DR405 comes through the workshop. I’m looking forward to being able to make the comparison between Luke's copy and the "real thing".

If one looks at the majority of amps that use KT88’s as either a pair or a quad, being 100 watt or 200 watt respectively. It’s quite “the norm” to run them with a “stacked HT supply” Usually around 400 volts on the bottom half for the screen grids (grid 2) and a further 300 – 320 volts stacked on-top of this for the plate supply of around 700 volts+. The Hiwatt 200 watt amp does in-fact follow this type of design and gives the expected 200 watts (please see this 200 watt power supply schematic below).



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200 watt Power supply schematic…..courtesy of Mark Huss.


And now, please take a look at this 400 watt power supply schematic (on which this amps’ power supply is modeled on).


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400 watt Power supply schematic.......courtesy of Mark Huss.


You can see that we have a slightly better bias supply arrangement but……everything else is the same, all voltages are the same, and we have more current available from the larger transformer obviously......but there are only six output tubes! In my mind (and experience) the use of the same voltages and six tubes would be, well, 300 watt and not 400! I have built quite a few KT88 amps, and the only way that I got a true 400 watts RMS from six tubes was to run them with 450 volts on grid 2 and 800 volts plate. This way they will give the 135 watts a pair, or 400 watts plus as a sextet and….they are quite happy at this!


NOTE:

ALL MY FINDINGS RECORDED ABOVE REFER SPECIFICALLY TO LUKE'S AMPLIFIER, AND ARE BASED ON HIS ATTEMPT AT COPYING A HIWATT MODEL. THEY DO NOT REFER TO A GENUINE HIWATT AMP.

I do hope you enjoyed the story….I have attempted to tell it in the way that the events unfolded. Cheers, John (and Luke).


(By the way, although I am actually aware of the company that manufactured these transformers and the company who supplied them to Luke…..for obvious reasons I am not prepared to disclose these company names!)
 




 

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