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Discussion in 'Building the Classics' started by shakti, Feb 9, 2020.
This sounds great!
You edited today - I am waiting for a fix!
Problem solved! It was indeed that R/C combination to ground that did it. I don’t fully understand what it does, technically, but obviously it is some kind of hum filter that shunts any ground(?) hum coming from the speaker jacks back to ground and prevents it from entering into the phase inverter.
Studying photos of other 68-69 Hiwatts you see a number of different variations on the theme; sometimes just a 10n cap to ground, sometimes with a series resistor before it (100, 270 and 470 ohm all seem to have been used).
I wonder why the Slave amp setup was different, with a bypass cap across the 10 k presumably shunting high frequencies into the negative feedback circuit, but no shunting to ground.
I'm doing some stuff this weekend but am unfortunately on the 'front-line' next week, hoping the extra PPE has arrived as we were a bit thin last week and cases seem to be hotting up and our case growth (and mortality) appears to be tracking Italy. The stats are in my favour (55 and ASA 1 with normal BMI, but who kows?)
My thoughts are with you brother. People wont forget the heroes that fought COVID-19. Your moment to shine, you'll be fine.
I’m in the front line, so to speak, as well. But being a pediatrician I don’t expect things to get too crazy since children seem to be spared from the worst effects. It also looks like Norway put the brakes on at the right moment. Two weeks ago we were #3 in the world in terms of confirmed cases vs population. It looks like that was because we were testing a relatively high proportion and catching and quarantining the right people. There’s not yet any steep rise in hospital admissions or deaths. Only 7 dead so far in Norway with an average age of 89...
I am sure that is the correct technique to clamp down. I think that we missed the boat in the UK as we did have not have enough testing capacity (which did not bode well as our ITU provision is even thinner). There have only been 13 confirmed cases in our county and (as of yesterday) no deaths. London is a different story. Germany seems to be well provided for in that respect but is still on the same disease/deaths curves as both Italy, Spain and us.
We did have a young child in A&E on Wednesday with respiratory distress that needed immediate tubing and ventilation then transfer to PICU - not confirmed COVID. But then again for some reason (capacity/not having our own testing lab) tests are taking up to a week (even our ITU patients), which given the use of 6 days of our highest level PPE possibly unnecessarily to my mind borders on criminal.
At least I know our current type of masks and PPE fit me, but in 32 years of practice this is the most troubling thing I've experienced.
6 days to get a test results sounds insane, very hard to understand why. Even right from the start here in my hospital you could get a result in 8 hrs if we got the test in at the right time, and pretty soon capacity was ramped up to 3 batches of analysis per day. The problem now is actually getting enough of the required PCR agents (don’t ask me about the details).
I’m still not convinced what the most correct reaction to the pandemic is. Sweden has taken far less measures than Norway but so far seems to be doing OK. There will be more deaths, probably, but OTOH how are we going to dodge this in the long run? Drastic measures will have long-term effects on other types of disease not to mention economic consequences that will inevitably result in loss of health in the long term. Then again, not doing anything would also just about ruin the entire health system and also have long-term effects. It will be very interesting to see how it all pans out in different countries and no doubt provide valuable new insight into epidemiology, economics, social sciences and philosophy...
I saw no rush on condoms so expect a baby boom beginning next year (and many divorces)
Well, back on topic...I built amp #1 to keep for myself and #2 for a friend. So I was a little miffed to find that #2 was both quieter (less apparent hiss and absolutely zero hum) and sounded just a tiny bit better. It was more «explosive» for lack of a better word.
I built #2 with the style 2 phase inverter as per the drawing, whereas #1 used the earlier, «standard» Hiwatt late 60s/early 70s phase inverter. So I changed #1 over to the style 2 phase inverter. I am happy to report it made the tiny difference I was hearing between the two amps. The difference is very small, but style 2 phase inverter for some reason seems to sound «faster», more explosive and more «under your fingers». It appears gainier, but sounds a tiny, tiny bit cleaner or at least that’s how it came across to me. On this amp it also removed a tiny, tiny bit of hum I was hearing when plugging in without any attenuation (might have been a wire routing thing too). The change between the two styles of phase inverter is literally done in two minutes; one wire is removed and another wire is moved from V3 to V4, one resistor is moved and a jumper 100k resistor is added. That’s it.
I’ll try to update my drawing to add the «missing» 100k resistor and another drawing to show style 1 phase inverter. I still have to decide on V3 cathode R/C. I currently have it at 2k2 and no bypass cap, but I think I am going to go with 1k5 and 47n bypass cap as in the photos of Page’s amp.
An updated schem/layout would be fantastic.
I suspect I will have spare time at some stage and as long as I’m not coughing my guts up I will get this one finished!!
I'll get that XLR inlet and solid core wire out to you ASAP Neil.
Did some final tube tweaking, and came to the same conclusion that I always do with Hiwatts: it’s Mullard all the way, all the time. In my Marshalls I often like a mix of tubes, but for some reason nothing else sounds right in a Hiwatt.
Unfortunately I only have two pairs of Mullard EL34s for this amp, no matched quad. And these two pairs (one pair of xf3 and one pair of xf4) don’t match up well. One is quite on the hot side, but not redplating, and the other is about right, at 70% dissipation. It sounds killer when I pair them up. But how would you use them - mix them so there’s one hot and one cold on each side, or use the hot pair on one side and the cold on the other? Also, I forgot which side it’s better to use the hotter tube(s).
The amp now sounds absolutely killer, easily as good as I hoped. Adding the choke later will be icing on the cake.
Great thread, outstanding build !
I just modified a ROYAL JPP100 as there were some "errors" in the circuit. I used shakti's layout and the info he provided on this thread, thank you so much !
Here it is :