Quad II classic - help with tips, valve rolling and what to look for?

[frankmcvey]

I have another query regarding using 4,6 and 8 ohm speakers. Would i have to change the resistors based on this? They were originally setup for 15ohm from what i have read? Either way i have them connected to my DIY built Planet10 mini onken builds with Mark Audio Alpair 7.3 and they sound lush.

Hi, Vish,

No, you don't have to worry about changing resistors. All you have to do is to set the desired impedance (8 or 15 ohms) on the output transformer. The method of switching impedance on the Quad II isn't very user-friendly and involves desoldering a couple of links on the output transformer and connecting another couple. You'll see the tags just below the resistor board.

The details are given on this schematic

I'm not familiar with the speakers you mention, but they look to have a 5-10 ohm impedance over most of the audio range. You can swap over the links in the Quads to 8 ohm if you want, but you certainly won't hurt anything by leaving them in the 15 ohm position.

Cheers,

Frank

Edit - crossed with David this time!

[PJL]

The amplifiers were designed around the MOV KT66 and changing types and even manufacturers can cause damage to the amplifier as it will change the quiescent current.

The Quad II also uses a common cathode bias resistor, R12 (180 ohm), which requires the output valves to be fairly well matched. With this arrangement, if one valve takes less current the other takes more.

It looks like the output valves are new but it is still worth checking the voltage on R12 which should be about 26V. If it is significantly more than this report back as it may be worth changing the value.

[vishalk]

This is excellent guys thank you so much! I have ordered brand new resistors for the amplifiers. This weekend i will attempt to test the capacitors, I think i will leave it at 15 ohms.

I also have an oscilloscope, I wanted to test the output power of the amplifier, from what I read I can't just put the leads on the output, with valve amps I need to put a load? Is this correct, forgive me I'm getting back into my hobby of electronics and testing.

Thanks for all the info and help! Frank if i get unsettled dealing with 400V and testing can I come see you?

[frankmcvey]

Hi, Vish,

You'll be very welcome to come and visit; however, I'll be visiting family over the Bank Holiday weekend, so make it any time after Sun 27th Aug. I'll PM you my phone number.

What kind and specification of resistors are you going to use? Be aware that if you're planning using NOS Erie Type 8/9s that, unless they're the hi-stab variety, it is likely that they will also have wandered in value during their time in storage. Avoid NOS Eries unless the seller states that they're in tolerance. If you're going to use modern resistors, check their voltage rating and that the leads will be long enough if you intend to follow the wiring routing pattern used by Quad.

If you're going to replace the resistors, then work very carefully - the eyelet tags on the resistor tagboard are VERY fragile, and it doesn't take very much bending or pulling at them to snap them off at the base, turning a fairly straight-forward job into a either a tricky repair or an unsightly bodge.

Take a few photos first, and/or draw a colour-coded diagram of the resistor locations and connections, plus any wire shorting links hiding underneath. You'll be glad you did! You might also want to ident the connecting wires that you remove with a bit of masking tape or somesuch, since Mr Quad had an irritating habit of using the same-coloured wire in different places...

However tempting it might be to try to remove the original resistors intact, it will be much kinder to the tags to snip the resistors out, leaving just the tails for you to desolder and wiggle out.

I have all the kit you'll need to test the amp's performance afterwards; and yes, you do need dummy loads unless you want to deafen yourself and **** off your neighbours!

Cheers,

Frank

[vishalk]

The resistors and caps finally came today, a very nice gentleman named Stephen "405man" on the diyaudio forum sent these to me, he has built and reconditioned many pairs and sent me these.

Time to get my soldering skills engaged and ready!

[Radio Wrangler]

Practice on something cheap first

David