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Quad II

Monophonic Valve Power Amplifier

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Quad II


Designed for use with the Quad II or 22 pre-amplifiers

The function of a main amplifier is that of amplifying the output from the control unit or other source with the highest possible standard of accuracy.

This is achieved in the Quad II to a degree unexcelled by any equipment known to be offered to the public.

The amplifier is further unique in that this performance is obtained with stability which is complete.


Power output: 15W into 8Ω (mono)

Frequency response: 20Hz to 20kHz

Total harmonic distortion: 0.25%

Input sensitivity: 1.4V

Signal to noise ratio: 80dB

Speaker load impedance: 7Ω to 15Ω

Valve complement: 2 x EF86, 2 x KT66, 1 x GZ32

Dimensions: 13 x 4.75 x 6.5 inches

Weight: 18.25lbs

Finish: stoved steel grey


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Reviewed Sep 28th, 2013 by dominiquemichel

I put a comment on the description of that amp, so I will not repeat it here.

With its original sound, I will put it 7 or 8 stars on 10. After the modifications I made (see the comment), I put it 10 stars on 10. It just have an outstanding sound.

I also own a pair of philips motional feedback studio monitoring speakers with integrated amplifiers. The sound of the mational feedback gears is more precise than the sound of the quad, and I use them for music productions with my computer, but for regular listening, I just prefer the sound of the Quad.



re: Quad II

I have a Quad II/22 stereo combination from many years now. The II is a very good amp that was relatively cheap to buy at that time, in comparaison with other brands of the same sound quality. That imply it have 2 defaults:

1) the electrolityc capacitors are of relatively small sizes, and the amp like its 22 preamp use the feedbacks to get a correct bandwitch in the basses. The sound is outstanding for classical music and rockabily, but the basses are too soft for jazz(-rock) music.

When changing these caps, I made all the bandwitch calculations based on the schematic and the tubes datasheets, and put new values. The result is outstanding, the basses are just outstanding with any kind of music, and the trebles are as good than before. Another advantage to do this, as the feedback get less work to do, is that the overall sound is cleaner and more precise than before. I made the same calculations/modifications in the Quad 22 preamp.

2) this amp heat too much for 2 reasons:
a) accordig the KT66 data sheet, the Quad II doesn't respect the minimum distance between 2 tubes ans they heat too much.
b) if the output transformer is of outstanding quality for a so old amplifier, the main transformer is a cheap one that use a very old technology and heat a lot, and when it heat too much, a liquid get out of it, making it to become hoter and hoter. As the amp is too compact, everyting is heating everything, and it become just too hot after a few hours of utilisation.

From the shematic of the amp, I commanded new transformers to a transformer's maker, and I mounted them outside of the amps. They heat a lot less and it is really a good investment for these amps. I keeped the original transformers, maybe they work now as coolers, and I painted the amplifiers in black.

If you don't do that, I would recommand you to not use such an amp more than 2 hours consecutively.

I also replaced the GZ32 by a GZ34. The final result is not only a better sound and much less heat, but also a little bit more output power.

marcuswilson's picture

re: Quad II

The QUAD II was first released in 1953. The UK was still in a great deal of debt as a result of World War 2. Steel was expensive and had been rationed for a time after the war. High quality transformer steel was very expensive.
Probably for this reason QUAD used high quality steel for the output transformer and low quality steel in the mains transformer where it would have no effect on the sound quality.
As far as transformer heating is concerned, the steel deteriorates over the years and loses some of it's magnetic qualities. So today these old mains transformers probably heat up a lot more than they would have when new.
Many other UK manufacturers (eg Leak) had poor steel in mains and output transformers.
On the other hand, PYE seems to have higher quality steel in their transformers, both output and mains. Also the PYE transformers do not seem to deteriorate much. My friends and I suspect PYE had access to good steel because they had military and infrastructure contracts.
I think the PYE and Radford are the best UK made domestic amplifiers of the period.

re: Quad II


I am using this Amp over a past decade and have same observation. I have to switch off the amp for about 3-5 Minutes to get it cool for my next LP operation. Also Bass is not great and hence was looking at mod, can you suggest me anything on caps / circuit which should help it reduce defaults. I am interested in knowing more on Point No 1.


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