By PNPs do you mean T7&8?
Some say use BD242C; others say use BD244C. The one amp I’ve got working with no distortion uses MJE15033s
The C suffix denotes much higher 100VDC ceo and cbo ratings for TIP42C than standard 40VDC. With nominally 100V available across the rails in the 405, don't plan on trying anything less there. I've used them as replacements in 306 models and I admit that I picked up this confirmation for the 405 model from a couple of sources including Dada's website and home of much 'sus' Quaddery 
. https://www.onsemi.com/pdf/datasheet/tip42c-d.pdf
. https://www.onsemi.com/pdf/datasheet/tip42c-d.pdfThe C suffix denotes much higher 100VDC ceo and cbo ratings for TIP42C than standard 40VDC. With nominally 100V available across the rails in the 405, don't plan on trying anything less there. I've used them as replacements in 306 models and I admit that I picked up this confirmation for the 405 model from a couple of sources including Dada's website and home of much 'sus' Quaddery
606 not 405.The C suffix denotes much higher 100VDC ceo and cbo ratings for TIP42C than standard 40VDC. With nominally 100V available across the rails in the 405, don't plan on trying anything less there. I've used them as replacements in 306 models and I admit that I picked up this confirmation for the 405 model from a couple of sources including Dada's website and home of much 'sus' Quaddery
I have been advised that distortion in the 606 is caused by current source CR1 (LM334Z) and can be ‘tuned out’ by changing value of R8 (270R)
Correct about the different models I referred to. I probably should have been clear about it needing to meet the requirements of both models since the 100V rated 40872 is fitted to the 606 model as well. In fact, according to available schematics, Quad specified the same driver transistor in all their consumer and pro. models up to and including the 707.
It's interesting about tweaking the shunt filter across the LM334 current source. I don't think it would be a simple job, though. You would very likely need and know how to use at least an oscilloscope to see what is actually happening there and whether you fixed or reduced an apparent problem. It's all too easy to imagine that a problem is the same as suggested by others or you fixed something, when there is no suggestion or evidence to the contrary.
It's interesting about tweaking the shunt filter across the LM334 current source. I don't think it would be a simple job, though. You would very likely need and know how to use at least an oscilloscope to see what is actually happening there and whether you fixed or reduced an apparent problem. It's all too easy to imagine that a problem is the same as suggested by others or you fixed something, when there is no suggestion or evidence to the contrary.
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The second amp sounded fine on headphones, but exhibited oscillation and distortion on speaker. I tried the idea of tuning R8 with a 500R trimpot, and it appears to have worked. At a very rough guess the value of R8 is now around 240 ohms, which tallies with the suggester’s own experience of one value up or down from 270R. The amp has been on test for two hours now.
Good that it works (your advisor was correct
) and you know the reason is a resonant response peak. I don't like the idea that it still lurks there, though. As suggested, this needs to be seen on a 'scope and optimised so that the problem doesn't return under different operating conditions like temperature, signal strength, dominant program frequencies etc. When you've got it under control in cold and fully warmed up in 20C ambient temp conditions, swap the adjustable combination for a single, nearest E12 or E24 value resistor. Don't leave adjustable pots to fail or for others to play with when you decide to sell for something else.
) and you know the reason is a resonant response peak. I don't like the idea that it still lurks there, though. As suggested, this needs to be seen on a 'scope and optimised so that the problem doesn't return under different operating conditions like temperature, signal strength, dominant program frequencies etc. When you've got it under control in cold and fully warmed up in 20C ambient temp conditions, swap the adjustable combination for a single, nearest E12 or E24 value resistor. Don't leave adjustable pots to fail or for others to play with when you decide to sell for something else.Yes I plan to replace the trimpot with a fixed resistor after several hours testing; and then carry out the same procedure on the other clone, which, I suspect, will also exhibit oscillation and distortion.Good that it works (your advisor was correct
Thank you for the compliment of implying someone might wish to buy a piece of home made audio kit from me - I would not feel comfortable putting someone else’s home at risk of burning down.
I purchased the PCBs in 2014 from China; my “alter value of R8” advisor is, I believe, in The Netherlands; I’m in England, and, I assume you are in The States. A worldwide endeavour may be drawing to a conclusion.
Yes, even more worldwide than that! Curiously, my profile only says (hover over the icon to the left), that I'm from NSW i.e. New South Wales - actually a state division of Oz; the big pink blob dividing the South Pacific and Indian oceans, just west of New Zealand 🇦🇺 
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I raised safety with repairs as an issue because private trading in antiques, old wares and memorabilia has become so popular in the UK where apparently there is also as much, if not more trade volume in used than new, traditional style audio equipment but the standard of necessary repairs is often seen here to be poor or unwise due to limitations of the obviously unskilled amateurs sourcing the replacement parts, doing the work and also when adages like "If your only tool is a hammer, every problem looks like a nail." become reality.
Good that you are raising the game a little and talking to others in similar situations. It helps us all to know what to expect and what's expected of us to get things working reliably.
.I raised safety with repairs as an issue because private trading in antiques, old wares and memorabilia has become so popular in the UK where apparently there is also as much, if not more trade volume in used than new, traditional style audio equipment but the standard of necessary repairs is often seen here to be poor or unwise due to limitations of the obviously unskilled amateurs sourcing the replacement parts, doing the work and also when adages like "If your only tool is a hammer, every problem looks like a nail." become reality.
Good that you are raising the game a little and talking to others in similar situations. It helps us all to know what to expect and what's expected of us to get things working reliably.
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Please accept my apologies for assuming you are American - some folk might take offence at that.
Although I can remember many electronic projects I abandoned because I could not make them work, I have to admit I could not have made this one work without advice and encouragement to persist.
My original plan was to re-imagine the Quad 606 in a more conventional (and, in my opinion, more attractive) housing - a 2U 19” rack unit. Quad use a 650VA transformer in the 606; and I have sourced an 800VA which is low enough to fit.
Although I can remember many electronic projects I abandoned because I could not make them work, I have to admit I could not have made this one work without advice and encouragement to persist.
My original plan was to re-imagine the Quad 606 in a more conventional (and, in my opinion, more attractive) housing - a 2U 19” rack unit. Quad use a 650VA transformer in the 606; and I have sourced an 800VA which is low enough to fit.
No offence taken and hopefully our US friends won't see any of this as intended comment either. It's all too easy to goof up in global communications. Sometimes we need a degree in diplomacy to get a point across without offending someone but thanks to the power of movies and music as culture, we can usually communicate well enough to avoid unpleasantness.
By coincidence, I'm working on a 2U rack case project too. The steel case came from RS and though the website images showed top ventilation, the only vents are perforated side panels which would most likely require fan forced cooling because that's about all you can do for medium to high power dissipation in slim, plain cases. I don't like doing it and I've spent enough already so I'm going to try cutting vents aligned with the channels between the vertical heatsink fins instead.
By coincidence, I'm working on a 2U rack case project too. The steel case came from RS and though the website images showed top ventilation, the only vents are perforated side panels which would most likely require fan forced cooling because that's about all you can do for medium to high power dissipation in slim, plain cases. I don't like doing it and I've spent enough already so I'm going to try cutting vents aligned with the channels between the vertical heatsink fins instead.
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Hi Ian,
Are you, like I, wanting to use a 300mm deep case?
I too want a fully vented type, but the RS one I’m looking at (their item number 810-4387) appears to have a plain top and vented sides.
My eBay supplier can do a fully solid sided case, or one with vented sides, but are waiting for stocks of base/top panels. They expect supplies in 10 days.
Yes, I'm looking at ways to modify the RS case because the heatsinks I'm using are a very close fit in the case and that leaves few options for improvement. Actually, the mocked-up assembly looks like the parts were made for each other and it might all work fine but the airflow volume is less than 30% of a similar but properly ducted arrangement with open top and bottom vents aligned to the fins. No need to say it but as it stands, its just unacceptable.
In other words, the sink gets hot because the awkward, side-stepping air flow through the perforations doesn't allow enough fresh air to efficiently cool the heatsink. If the amplifier only used a single pair of output transistors, there may be less of a problem since even the low amount of bias used in the 3 pairs of output transistors of the 606 model can be difficult to control together, when initially setting them up.
In other words, the sink gets hot because the awkward, side-stepping air flow through the perforations doesn't allow enough fresh air to efficiently cool the heatsink. If the amplifier only used a single pair of output transistors, there may be less of a problem since even the low amount of bias used in the 3 pairs of output transistors of the 606 model can be difficult to control together, when initially setting them up.
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