Hi there,
I've just recapped and replaced various resistors in the first of 2 1950s Quad II amps. I checked voltages just after after C4 and I'm getting around 375V DC where the schematic suggests 340V is to be expected - I'm using a DMM rather than the AVO meter the schematic was put together with, so is this about right?
I haven't gone any further yet, but I noticed one of the KT66s was too hot to touch after just 30s of operation, which I'm guessing suggests a fault somewhere, so I'm leaving it off for now!
I've just recapped and replaced various resistors in the first of 2 1950s Quad II amps. I checked voltages just after after C4 and I'm getting around 375V DC where the schematic suggests 340V is to be expected - I'm using a DMM rather than the AVO meter the schematic was put together with, so is this about right?
I haven't gone any further yet, but I noticed one of the KT66s was too hot to touch after just 30s of operation, which I'm guessing suggests a fault somewhere, so I'm leaving it off for now!
Last edited:
KT66 gets too hot to touch, can get over 200°C.
The set is probably for 220V and with 230V today everything get higher and hotter.
With a bit of luck there is a 240V position you can use.
Mona
The set is probably for 220V and with 230V today everything get higher and hotter.
With a bit of luck there is a 240V position you can use.
Mona
Thanks. I tried a different KT66 in the position of the cold tube and now both get hot so I guess I have a dead KT66! Damn.
I've set it to its highest voltage, 230v, and I think our mains here is around 240.
I've set it to its highest voltage, 230v, and I think our mains here is around 240.
Last edited:
Well, you took a much greater risk then. I assume you don't do this mistake again. You now have an amp with lots of unknown instead of
testing first to figure out known good and bad parts ( and documenting this)
I've been told by many people with experience of these amps that the Hunts capacitors are reliably unreliable and should always be replaced, so it seemed prudent to go through and replace them - there are only 6! I checked every resistor and replaced the ones that were beyond 10% out of spec. It's a completely unmolested amp so I don't think I've taken any risks so far...
I agree with petertub,
And this is not aimed at anybody personally, just a general rule of advice, to newbies and seasoned amplifier people:
Unless an old amplifier is in danger of exploding old electrolytic capacitors, and full of leaky coupling caps, etc. . . .
Then use a variac or an incandescent lamp to power it up.
Next, check the voltages . . .
After doing that, begin restoring your amplifier.
(follow the same rule as for doing a modification to an amplifier; get it working first, then modify it).
Otherwise, when you restore it or modify it, and now it does not work, was it not working in the first place, or is the problem due to the new parts, solder connections, wiring changes, etc.
And this is not aimed at anybody personally, just a general rule of advice, to newbies and seasoned amplifier people:
Unless an old amplifier is in danger of exploding old electrolytic capacitors, and full of leaky coupling caps, etc. . . .
Then use a variac or an incandescent lamp to power it up.
Next, check the voltages . . .
After doing that, begin restoring your amplifier.
(follow the same rule as for doing a modification to an amplifier; get it working first, then modify it).
Otherwise, when you restore it or modify it, and now it does not work, was it not working in the first place, or is the problem due to the new parts, solder connections, wiring changes, etc.
Last edited:
I went to a class that taught all about a 500 Watt low frequency transmitter.
After coming back to my US Naval Destroyer, I found that the rest of the electronics technicians had already installed the new transmitter for me.
I started by testing the transmitter's 3 Phase power supply pull-out drawer.
Phase 1 voltage good, Phase 2 voltage good,
Phase 3 voltage was extremely high, not good!
The electronics crew had installed 2 Phase power mains connections.
They did not even know about 3 Phase power mains.
The un-connected phase 3 transformer tap in the transmitter was seeing the reflection from the other 2 phases; that caused the un-usual high voltage.
Assume anything, and anything will bite you!
After coming back to my US Naval Destroyer, I found that the rest of the electronics technicians had already installed the new transmitter for me.
I started by testing the transmitter's 3 Phase power supply pull-out drawer.
Phase 1 voltage good, Phase 2 voltage good,
Phase 3 voltage was extremely high, not good!
The electronics crew had installed 2 Phase power mains connections.
They did not even know about 3 Phase power mains.
The un-connected phase 3 transformer tap in the transmitter was seeing the reflection from the other 2 phases; that caused the un-usual high voltage.
Assume anything, and anything will bite you!
This is good reading ...
Turner Audio Quad Mods
There is a schema with voltages, and some simple mods (such as seperate cathode bias resistors)
The most important voltage with a KT66, which is quite tough, is the heater. That will affect tube life. I have my amps hard wired to the 240v tap, to keep voltages just on the low side of the right voltages.
Turner Audio Quad Mods
There is a schema with voltages, and some simple mods (such as seperate cathode bias resistors)
The most important voltage with a KT66, which is quite tough, is the heater. That will affect tube life. I have my amps hard wired to the 240v tap, to keep voltages just on the low side of the right voltages.
You should not listen to "many people". Measure and figure out yourself.
Don't trust what you read on the net ( not even this post) until you have
verified it yourself.
Always test an unknown amp before any changes, then do needed repairs.
Avoid wild changes of good components. Use fresh ( new or known good) tubes.
Older amps were built and designed for the line voltage that was common AT THAT TIME.
With current voltages being increased, those amps are now a bit stressed to perform under today's line voltages.
A mere 5 to 10 volt line increase will obviously become 20 to 40 volts ADDITIONAL B+.
Run any vintage amp designed for 115 volts at 125 volts - and see the substantial difference in things.
Some modifications and re-adjustments are in order, if the amp is to perform reliably today.
With current voltages being increased, those amps are now a bit stressed to perform under today's line voltages.
A mere 5 to 10 volt line increase will obviously become 20 to 40 volts ADDITIONAL B+.
Run any vintage amp designed for 115 volts at 125 volts - and see the substantial difference in things.
Some modifications and re-adjustments are in order, if the amp is to perform reliably today.
My high school electronics teacher (circa 1980) assumed the picture tube in a large color TV had been discharged. The two knuckleheads working on it assured him it was safe.
A couple minutes later, the instructor was flat on his back in the floor. He was okay but it could've been much worse. Trust but verify.....
A couple minutes later, the instructor was flat on his back in the floor. He was okay but it could've been much worse. Trust but verify.....
And that's what I think of the internet and its plethora of suggestions and answers to questions...
Not all, mind you....
But you really don't know who is at the other end of your screen, and I simply don't care who they are...
I only trust my life and my own "real life" measurements and tactics.
The internet is what it is.... but accepting it as the absolute remedy is foolish.
Not all, mind you....
But you really don't know who is at the other end of your screen, and I simply don't care who they are...
I only trust my life and my own "real life" measurements and tactics.
The internet is what it is.... but accepting it as the absolute remedy is foolish.
There seems to be a bit of an over-reaction here for the poor guy, seeking some guidance?
I just recapped a Tandberg 74, and without fail the Rifa's were split, and the Hunts were mostly cracked. When I finished it I was sure I had screwed it up because it was so quiet when I switched it on - even the potentiometers had stopped scratching (a sympton of a leaking coupling capacitor).
The danger of starting up with failed coupling capacitors is that you can do a lot of damage to tubes and OPTs if DC is passed, so why not just fix a handful of capacitors for insurance? A mod would be changing the design in some way, or using different values than originally specified, which is not the case here?
However, I would not start an old tube powered device from the 60's or 50's without a variac or a dim bulb tester, or ideally both to be safe the first time.
A variac can be picked up for £50 from auction sites, and is the second most important device after a DMM, IMO.
I just recapped a Tandberg 74, and without fail the Rifa's were split, and the Hunts were mostly cracked. When I finished it I was sure I had screwed it up because it was so quiet when I switched it on - even the potentiometers had stopped scratching (a sympton of a leaking coupling capacitor).
The danger of starting up with failed coupling capacitors is that you can do a lot of damage to tubes and OPTs if DC is passed, so why not just fix a handful of capacitors for insurance? A mod would be changing the design in some way, or using different values than originally specified, which is not the case here?
However, I would not start an old tube powered device from the 60's or 50's without a variac or a dim bulb tester, or ideally both to be safe the first time.
A variac can be picked up for £50 from auction sites, and is the second most important device after a DMM, IMO.
With both KT66s working I'm getting about 378V where the schematic says 340V. There is a note on the schematic to take account in your measurements depending on the impedance of your volt meter, and the voltages written on the schemo were taken with a low impedance coil meter vs my high impedance DMM.
I replaced the caps before testing because I don't have a stash of replacement KT66s available or spare OPTs and I assumed the coupling caps were probably very leaky - I don't have a caacitor tester. I understand the point about starting with a working amplifier though.
My mains voltage is a bit high then - I measured 5.15v on the 5v winding and 6.6v on the 6.3v winding, under load.
I'm happy to report though that it is passing audio and sounds great, no hum and almost no hiss even with my ear near the speaker.
I replaced the caps before testing because I don't have a stash of replacement KT66s available or spare OPTs and I assumed the coupling caps were probably very leaky - I don't have a caacitor tester. I understand the point about starting with a working amplifier though.
My mains voltage is a bit high then - I measured 5.15v on the 5v winding and 6.6v on the 6.3v winding, under load.
I'm happy to report though that it is passing audio and sounds great, no hum and almost no hiss even with my ear near the speaker.
Last edited:
Worth checking that heaters are close to 6.3v. Do you have a tube rectifier, and if so, which one? And what values are the smoothing capacitors?
That does seem a tad high - mine is about 320v. I did add a CL90 inrush limiter on the primary side to add a little resistance.
That does seem a tad high - mine is about 320v. I did add a CL90 inrush limiter on the primary side to add a little resistance.