QUAD II AMPLIFIER

[maxhifi]

Going to have one of these pass through my hands soon, am very excited to have a look at this classic amp. I want to do as little as possible to make it work properly, without harming the market value. This one is really clean, comes complete with the original box, and mono preamp, but the output tubes have been changed to 6L6GC at some point. There's even the manual, and original receipt from 1957.

I'm imagining that the electrolytic and coupling capacitors will need to be changed. Am thinking to change out the 0.1uf coupling capacitors with the Russian PIO capacitors (mostly to preserve the under chassis appearance), the 0.1uf cap between the screens of the EF86s with a metal film capacitor, and the 16uF filters can be changed to 22uF 450V electrolytics.

I'll measure all the resistors and change whatever is bad.Obviously test the tubes and change what's bad.

Does this sound reasonable? I don't want to do more than is required to make it work properly, and leave as much original as possible for the next owner. Do any of the old caps hold up OK over time, or is this an amp where I should just change them? I feel like there's some trade off here between selling an amp which doesn't work, and runing the value by making it non-original.

 

[peterh]

What needs changing is 6L6 to KT66. The rest is best kept "as is" .
See http://www.keith-snook.info/quad-stuff.html for what not to do !

 

[vaathi]

I have couple of them. I would like to see how this resto goes.

 

[maxhifi]

What needs changing is 6L6 to KT66. The rest is best kept "as is" .
See http://www.keith-snook.info/quad-stuff.html for what not to do !

Are you saying, that the old coupling capacitors aren't the type which go leaky? My fear is self destruction if the capacitors leak too much. I can get a set of Shuguang KT-66 for it.

I have couple of them. I would like to see how this resto goes.

Sure! I think it will be more of a repair than a restoration though, this one is cosmetically perfect, it will be just a matter of changing any bad parts.

 

[peterh]

Are you saying, that the old coupling capacitors aren't the type which go leaky? My fear is self destruction if the capacitors leak too much. I can get a set of Shuguang KT-66 for it.

You might destroy more by fiddeling with it then to let them continue to work.
Read the link from keith snook to learn more.

 

[maxhifi]

You might destroy more by fiddeling with it then to let them continue to work.
Read the link from keith snook to learn more.

I'm not sure it does work, as it is - it's been put away since the 1970s. I don't see a lot of risk in changing the two coupling capacitors, and testing the electrolytics for leakage. I'm hesitant to just plug it in and turn it on, it's a bit too valuable to be abused like that.

 

[peterh]

I'm not sure it does work, as it is - it's been put away since the 1970s. I don't see a lot of risk in changing the two coupling capacitors, and testing the electrolytics for leakage. I'm hesitant to just plug it in and turn it on, it's a bit too valuable to be abused like that.

Then sell it and build something else. There is a limited amount of working quad II in the world please don't reduce that number !

 

[maxhifi]

Then sell it and build something else. There is a limited amount of working quad II in the world please don't reduce that number !

I've been working on tube amps for 25 years, I'm well equipped to troubleshoot and repair, there is no need for this kind of advice - I just haven't encountered any QUADs in the wild, and wanted to check on the forum if any parts are known high failure rate.

I should add, the amplifier is not yet in my possession, once it is, I will test and check all the components properly before I apply any power. I just want to be conservative, and not do anything which will take away from the historic value of the amplifier in the course of making it work.

The reason I asked specifically about the capacitors, is that it has been my experience that while many old capacitors do become bad over time, others are just fine. I'm curious if others with experience with the QUAD know if the original parts stand the test of time well, or not. In any case I'll test them, but it's always good to do some research before jumping into a repair.

 

[Steve O]

I have three Quad IIs and all of them had component issues as found. If you intend to keep your unit for display only, you can keep it stock as previously suggested. OTOH if you intend to use it, significant maintenance is almost certainly necessary.

My experience is that all electrolytic caps must be replaced. This includes the two section unit in the PS and the cathode bypass cap in the output stage.

In my units the three signal coupling caps (per amp) were electrically leaky, off value and had high ESR so these required replacing. I used Orange Drops. Note that the two coupling caps to the KT66s were originally metal cased 0.1uF 350V Hunts brand that were clamped to the chassis. Quad made use of the stray capacitance to ground that the metal case provided to improve stability margin. This stray capacitance should be duplicated when modern plastic cased caps are used. I used 22pF dipped silver mica from each KT66 g1 to ground. Also note that the 350V rating is inadequate. I used 600V parts.

In my units most of the low power resistors were way off value so all were replaced.

Earlier in the thread it was recommended that KT66s be used for the finals and this is what Quad used OEM. However, I've used a number of 6L6 variants with very good results. The Russian 6P3CE is very good in this app. An advantage of the 6L6 types is that they take a load off the fragile PT with their reduced filament draw.

Overall, the Quad II is an ingenious and remarkably good sounding amp. It’s completely stable and well behaved into any typical load with a surprisingly extended high end. IMO, the primary sonic shortcomings are low power ~ 20W and limited low end capability.

 

[Dandy]

If you haven't done so, I would recommend doing a search of Quad II threads here: https://www.vintage-radio.net/forum/index.php? That forum has got some knowledgeable members, with experience of working on Quad IIs.

 

[x3workshop]

Keith Snook's site is a good resource. You will be replacing the electrolytics as they're almost certainly off, and several other caps and resistors too.

 

[maxhifi]

So the quad II and QC II hit the bench today. Filter and coupling caps were shot, the filters were totally open, and coupling caps very leaky. I changed these parts, and one bad EF-86, and the result is an astoundingly good sounding amp.

Wow, no wonder these are sought after. The bass had kick, the treble has bite, but the sound is so smooth. The phono preamp doesn't really have enough gain, but with a Y patch cord from my turntable I am very impressed.

 

[Dandy]

Wow, no wonder these are sought after. The bass had kick, the treble has bite, but the sound is so smooth. The phono preamp doesn't really have enough gain, but with a Y patch cord from my turntable I am very impressed.

Darn. Another amp to add to my 'want to hear it' list. The UK mag Hi-Fi World has always dismissed these as having a 'fluid and lyrical voice' but being otherwise 'hopelessly dated', low powered and 'hard to partner properly'.

 

[maxhifi]

Darn. Another amp to add to my 'want to hear it' list. The UK mag Hi-Fi World has always dismissed these as having a 'fluid and lyrical voice' but being otherwise 'hopelessly dated', low powered and 'hard to partner properly'.

Yet it's because of reading hifi world in the 1990s, I was aware of this amp in the first place. I remember them being quite positive about it back then, maybe their opinion changed with the years.

 

[bimasta]

I want to do as little as possible to make it work properly, without harming the market value.

Doesn't the conversion to 6L6 do so already? But no matter if you wish to keep them. I had several back in the 1980s, when nobody wanted "those old things". They were my introduction to tubes and I haven't been the same since. All had the original Genalex KT66s and often came with new ones as spares. They didn't need any work that long ago, though one pair had been modified to 30W — I think the mod was designed by Tim de Paravicini but can't be sure. They all came with the Quad 22 preamp, which used to be dissed as "nothing but a doorstop", but I modified one myself, upgrading caps and resistors. The phono circuit had a row of switches for different EQ curves, which I didn't need. I eliminated it, which meant removing all switches, caps and resistors, and over a meter of wire in little 2–3" chunks and all the those solder joints. It was glorious.

I still think they were best amps I've ever had, no CJ, Audio Research etc had their unique character... but I'm sure sonic memory and nostalgia improve on what I remember.

 

[maxhifi]

Doesn't the conversion to 6L6 do so already? But no matter if you wish to keep them. I had several back in the 1980s, when nobody wanted "those old things". They were my introduction to tubes and I haven't been the same since. All had the original Genalex KT66s and often came with new ones as spares. They didn't need any work that long ago, though one pair had been modified to 30W — I think the mod was designed by Tim de Paravicini but can't be sure. They all came with the Quad 22 preamp, which used to be dissed as "nothing but a doorstop", but I modified one myself, upgrading caps and resistors. The phono circuit had a row of switches for different EQ curves, which I didn't need. I eliminated it, which meant removing all switches, caps and resistors, and over a meter of wire in little 2–3" chunks and all the those solder joints. It was glorious.

I still think they were best amps I've ever had, no CJ, Audio Research etc had their unique character... but I'm sure sonic memory and nostalgia improve on what I remember.

I'm not sure I'd consider the tubes having been replaced with a domestic substitute back in the sixties to really be a conversion per se, since it involves no internal modifications to the amplifier, circuit and since 6L6 characteristics are nearly identical to KT66. When I opened the box and saw the lack of KT-66 tubes, I admit I was a bit disappointed, just because I've never actually seen a real KT-66 before. This amp/preamp is a repair for a client, with specific instructions to evaluate and make work. Since the tube change happened before it came to me, I can't be held accountable for the lack of original tubes.

This was done long enough ago that the "new" 6L6GCs are made in Canada. The new owner can seek out and install original KT-66 tubes if he wants, but The GZ32 was also replaced with a 5AR4 - again, I'm not worrying about it.

Unfortinately no matter how I wish to keep them, these were a repair for someone else, so they aren't mine. What it has done though, is put a bug in my ear about how great Quad is. Between the construction quality, the sound, the looks and compact size I am really impressed by them. The one I repaired had a mono QC2 pre, not the 22 you have, but the build quality and layout of the preamp is one thing which impressed just as much as the amp. It's built with such attention to detail regarding layout and aesthetics, it seems like a work of art. It's also the first amp that's ever gone though my hands (there's been a lot!) which my wife complemented the looks of, and she said hey why don't you sell a bunch of your stuff, and get these? I do like the second half of that suggestion