CHAMP ELECTRONICS -" THE VINTAGE VALVE
AMP HOSPITAL"
NOTTINGHAM, ENGLAND
SORTING OUT AN AMPLIFIER HEAD BASED ON
THE HIWATT DR405 400 Watt
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.
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.
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!
This was Luke’s original attempt at the underside wiring of
the power supply/output and drive board!
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!
Here I have rewired all the underside of the main
board….again using sleeved solid copper wire.
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!
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.
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!
This is the total amount of winding insulation I removed from the WHOLE
of the transformer…….nowhere near enough!
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.
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!
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.
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!
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).
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).
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!)
