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Quad 306 innards - and caps?

Discussion in 'classic' started by miktec, Aug 22, 2022.

  1. Glenn2

    Glenn2 Member

    Thanks guys. The amp isn't mine, it's a friend's spare that he inherited (he is using a 66/606 combo), and I think the Mundorfs might be overkill price-wise as I'd have to buy 4 of them. My main reason for doing this is to see what it can do. Maybe buy it off him (or get another) if it I like it.

    The originals are 35mm diameter, 30mm height but I think up to 45mm height is OK as the toroid is about 15mm taller than the caps.

    The Mundorfs would definitely fit though at 30d x 40h so it is very tempting actually, they are 125deg caps and rated as 16,000h at 85deg.

    The Nichicons are 85deg 3000hr parts, which I'm sure is fine really but the originals were very long life ones, so I'm only looking at similarly long-life ones as replacements.

    I was swaying towards the EPCOS but the Mundorfs are tempting.
    (Edit: Mouser has a typo, the 30mm diameter EPCOS part is actually 25V, would have to use 35mm, 4700uF)

    It's the 47uF cap I really don't know what to do about.
    Last edited: Feb 2, 2023
  2. Zombie

    Zombie pfm Member

    As for the C7 I put a 100uF of high quality there, smth posh like Elna Silmic II or Nichicon Gold Tune whatever. But I have a 909, might be minor differences. I also put smth over the zeners, think Martin Clark suggested that in a post a looong time ago, but not 100% sure if he.
    Glenn2 likes this.
  3. HeinzR

    HeinzR pfm Member

    I have ordered this ones a year ago when renewing the caps of an otherwise good 306 see link below. During a conversation with Keith Snook mainly about other topics he said that he uses the same caps when he restores an old amp. I put in only very slightly higher capacitance not to stress other components and the main goal was to approach long life. The size fits well.

    Re the small caps I chose

    After a year of listening I can say that this caps are good.
  4. HeinzR

    HeinzR pfm Member

    I bought three 306s used last year, one for a smaller room and two to actively drive my Klipsch Jubilees.
    I heard all three 306 first with the old PSU capacitors. They were bulging and no longer in good condition. When turning the amps on and off I heard these louder plops, But the sound o music was great. That's why I disagree with many other members. I moderately increased to 5600 mF. But I think that the original sound was very good. Many think they would improve a lot with larger capacities. I think what should I use to get the original sound back. I am happy with the 5600 uF caps, but out of curiosity I would go back a step to the original 4700 uF on the next 306. It sounded very good. Some members justify larger capacitances by saying that larger capacitances would not have fit mechanically in the past when 306 were made and otherwise Quad would have preferred larger values. But I think 4x4700 uF is a good fit for the amp.
  5. chartz

    chartz If it’s broke fix it!

    As I said earlier up this thread, I replaced the original ones with 6800 µF 105C because that’s what I had in the drawer.
    Slightly larger but they fit just fine.
    I drive an ESL57 pair, although Quad did say it was not advised!
    That said, in this context I sonically prefer the old 33/303 pair to the 34/306 one.
  6. TonyScarlett

    TonyScarlett pfm Member

    This is my 306. This was serviced by Rob at Amplabs last May.


    As you can see, he has fitted 10000uF caps. You can also see he has lifted the smaller caps off the board. Apparently they overheat due to the neighbouring resistors.

  7. Gervais Cote

    Gervais Cote Predator

    Looks like those peoples at Amplabs know how to turn something from excellent to perfect !
    TonyScarlett likes this.
  8. 337alant

    337alant Negatively Biased

    Niel Blackwood and chartz like this.
  9. Glenn2

    Glenn2 Member

    Thanks guys, Friday I ordered the Mundorf 6800/63, they arrived the next day. I chose them not just because they are frequently recommended, but because of their endurance. They are 125deg, rated 16,000hr down at 85deg.

    Interesting about Amplabs mounting C7 off the board. The heat from those resistors made me doubly anxious about fitting a boutique part like Nichicon FG/KZ or Elna Cerafine/Silmic, which are all only rated 1000hr at 85deg. Therefore I ordered Panasonic FC 68uF/100v for C7. This is a 12.5mm diameter part and the endurance for FC goes up to 5000hr at 105deg when you hit 12.5mm. The equivalent 85deg rating would be 3 to 4 times that, so should be comparable to the Mundorfs. I see people fitting both 47uF and 100uF here so 68uF will be fine. The 47uF was only 10mm (3000hr).

    The amp still has its original capacitors (dated 1991) but it has binding posts. Does this suggest it has been serviced at Quad in the past or were they ever made like this?
    This photo link isn't the amp in question, just a random photo off the net, but they look exactly like these:

    The only other differences I can see, those bright blue flat resistors next to C7 (R22) are a deep red/brown colour on mine and shaped like normal resistors. EDIT: I also don't have any of that heat-sensitive paint on the output transistors, which have what looks like a Motorola logo, not RCA.
  10. chartz

    chartz If it’s broke fix it!

    How much for the 34/FM4 plate then Alan?
  11. canonman

    canonman pfm Member

    If it has been serviced by Quad there will be a small, dated, 'Quad Service' sticker inside the amp
  12. Glenn2

    Glenn2 Member

    Ok cheers - can't see one anywhere
  13. 337alant

    337alant Negatively Biased

    £30 +P&P


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