Author Topic: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?  (Read 1522 times)

Offline chrisc

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Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #15 on: August 07, 2021, 08:14:15 AM »
Is the hum volume dependent, or fixed despite volume level?   If fixed, probably smoothing capacitors in power supply
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Offline Family_Dog

Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #16 on: August 07, 2021, 09:18:16 AM »
If the hum is constant, it will mean the filter caps need replacement. In fact, you would be well-advised to replace all electrolytic caps. paying attention to their voltage & capacitance ratings as well as polarity. Not a big issue.

Also, as previously mentioned, look for a cherry glow on the anodes (large plate surfaces) of the KT88 valves. If there is any hint of glow, switch off - the coupling caps need urgent replacement.

Spend 30 minutes or so and watch this video, will explain a lot.
https://www.youtube.com/watch?v=2rQ3llxBR6g&t=8s


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Offline AudioJunkie

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Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #17 on: August 07, 2021, 11:12:38 AM »
Is the hum volume dependent, or fixed despite volume level?   If fixed, probably smoothing capacitors in power supply

No its a constant fixed hum
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Offline chrisc

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Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #18 on: August 07, 2021, 12:06:06 PM »
Good advice from Eric above.  Electrolytic caps do not last that long and are usually not expensive
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Offline AudioJunkie

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Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #19 on: August 07, 2021, 01:29:47 PM »
So this is what I see in the one amp. The large caps are 47uf 500v, is that correct? I've read articles that say it's 2 16uf caps.



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Offline chrisc

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Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #20 on: August 07, 2021, 01:50:03 PM »
That is quite a high voltage

Mantech have some 350V models.  Do you know what the circuit requires?
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Offline Family_Dog

Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #21 on: August 07, 2021, 03:16:16 PM »
That Quad has been totally rebuilt, albeit not very neatly. All resistors and caps have already been changed. If the amp still has the original GZ32 rectifier, the 47uF/500V caps are fine.

This is what the original underside looked like (pic from the 'Net)


Previously I mentioned KT88 output valves - this is wrong of course, they should be KT66, a lighter version of the KT88. Same comments apply, however.

Regarding the hum problem you experience, I would suggest they you strip that amplifier completely and rebuild it - it's a total mess!


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Offline AudioJunkie

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Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #22 on: August 07, 2021, 05:56:22 PM »
If a rebuild is the best option I'd rather sell them off as is to someone with the time, patience and funds to do so.
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Offline Family_Dog

Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #23 on: August 07, 2021, 06:51:01 PM »
Please let Contrabass have first option, he has been looking for a second unit for sometime, not sure if he has come right since.


-F_D
« Last Edit: August 07, 2021, 06:56:52 PM by Family_Dog »



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Offline Contrabass

Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #24 on: August 07, 2021, 07:08:17 PM »
Please let Contrabass have first option, he has been looking for a second unit for sometime, not sure if he has come right since.


-F_D

Give that man a Bells ! Yes I would definitely be interested. As Eric has mentioned I have been looking to buy or build up a second unit. The single unit that I have (in those olde days people had mono hifi systems !) belonged to my father and was lovingly restored. 
Audiojunkie  - I think we corresponded last year about some Quads - PM me if you keen to sell.

Offline Moonraker

Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #25 on: September 28, 2021, 10:08:09 AM »

Also, as previously mentioned, look for a cherry glow on the anodes (large plate surfaces) of the KT88 valves. If there is any hint of glow, switch off - the coupling caps need urgent replacement.



The output tubes are KT66.  Try KT88 if you want to burn out the power transformer in short order.

Also, a bias problem causing the KT66 to glow will cause that transformer to overheat and soon suffer a very probable fate.

Offline Family_Dog

Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #26 on: September 28, 2021, 12:35:38 PM »
You're right, of course. I pictured the shape of the valves and immediately 'KT88' sprang to mind. They are KT66 - but the intended comment remains the same.

-F_D



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Offline charles

Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #27 on: September 28, 2021, 02:09:46 PM »
The following output valves can be used in the Quad II power amplifier:

KT66
6L6G
5881

Source: Quad original Instruction Book/s  for e.g.: Issue 4  3m/3/62

Offline Ampdog

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Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #28 on: October 18, 2021, 01:59:05 AM »
chrisc:
The original power supply capacitors were 16F + 16F 450V types in a double-wound fashion. Capacitor voltage can never really be too high; the only penalty is the price!

But that re-wiring! !  Who in heaven's name thought that something was wrong with the original wiring in the first place . . . . (I suppose the difference 'blew him away . .)

Then A.Junkie: You only mention the main amplifiers. Did you power them up without a suitable pre-amp, i.e. without a signal input 'shunt'? The input resistor here is 1,5 megohm; not low enough to avoid stray hum pick-up from the surroundings - and particularly with the haphazard wiring! If you know which Jones-socket pins are the input you can short then out.  (Not trial-and-error please! There is 330V d.c. also on the Jones socket!)

Then the power supply electrolytics: Yes, one prefers to put in new ones, but of dozens of restorations over the decades, I only found one capacitor ever to be leaky; the TCC's lasted quite well! i would test them properly and leave in if not faulty. The can looks quite sophisticated in there (unless one takes the trouble to open the can and replace by modern caps, which will fit in quite well since modern caps are quite smaller than ones from the era. I have also replaced with the sophisticated JJ 47 + 47F/ 500V types.)

Also the Quad IIs could be powered up without a loudspeaker load, the internal nfb resistors of < 1K being enough of a load to avoid instability. Other matters should be attended to with restoration but I will not turn this thread into full-blown  restoration advice.

Lastly, i am also in the market for to-be-restored Quad IIs. kindly mention a price if so inclined, A.Junkie? 
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Online El Sid

Re: How Safe Is It To Switch On A Quad Valve Amp After Being In Storage?
« Reply #29 on: October 18, 2021, 06:43:43 PM »
chrisc:
The original power supply capacitors were 16F + 16F 450V types in a double-wound fashion. Capacitor voltage can never really be too high; the only penalty is the price!

But that re-wiring! !  Who in heaven's name thought that something was wrong with the original wiring in the first place . . . . (I suppose the difference 'blew him away . .)

Then A.Junkie: You only mention the main amplifiers. Did you power them up without a suitable pre-amp, i.e. without a signal input 'shunt'? The input resistor here is 1,5 megohm; not low enough to avoid stray hum pick-up from the surroundings - and particularly with the haphazard wiring! If you know which Jones-socket pins are the input you can short then out.  (Not trial-and-error please! There is 330V d.c. also on the Jones socket!)

Then the power supply electrolytics: Yes, one prefers to put in new ones, but of dozens of restorations over the decades, I only found one capacitor ever to be leaky; the TCC's lasted quite well! i would test them properly and leave in if not faulty. The can looks quite sophisticated in there (unless one takes the trouble to open the can and replace by modern caps, which will fit in quite well since modern caps are quite smaller than ones from the era. I have also replaced with the sophisticated JJ 47 + 47F/ 500V types.)

Also the Quad IIs could be powered up without a loudspeaker load, the internal nfb resistors of < 1K being enough of a load to avoid instability. Other matters should be attended to with restoration but I will not turn this thread into full-blown  restoration advice.

Lastly, i am also in the market for to-be-restored Quad IIs. kindly mention a price if so inclined, A.Junkie?

These are going on Vleisboek, Ampdog: Quad II vintage amps

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