I totally
agree ! The advice to change tubes was more of a "daily usage hint" but i totally forgot the value of saving the original amp.
I'm ashamed
Hi Eli!
Changing to PCL86s would certainly be a less radical modification, only requiring the addition of a suitably rated filament transformer.
On the other hand, a set of four NOS Mazda ECL86 valves would cost £120 + VAT from Watford Valves, which is actually small beer in valve circles!
Watford Valves :: Valves - ecl86
Changing to PCL86s would certainly be a less radical modification, only requiring the addition of a suitably rated filament transformer.
On the other hand, a set of four NOS Mazda ECL86 valves would cost £120 + VAT from Watford Valves, which is actually small beer in valve circles!
Watford Valves :: Valves - ecl86
£120 is a sizeable sum. A quick check on EBone shows several attractive PCL86 listings. Multiple matched sets of PCL86s could be acquired for less money. Even if the Mazda NOS is more durable, the OP would (IMO) do better in the long run with PCL86s. The initial funding of a new filament trafo is taken care of and PCL86s are more likely to be available, over time. Stocking up now is also an option.
Thanks Eli. I have no quibble with your argument for the substitution of the PCL86. It makes absolute sense when you take cost and long term security of supply into consideration.
Just to put the price of a NOS ECL86 into perspective, I submit the price of a NOS EL84, also from Watford Valves.
EL84-TUNGSRAM 1970
Just to put the price of a NOS ECL86 into perspective, I submit the price of a NOS EL84, also from Watford Valves.
EL84-TUNGSRAM 1970
At the same vendor a JJ EL84 cost 9.25. New production, warranty and matching available. And i do not think the sound is different ( as long as the tungsram is performing as it should )Thanks Eli. I have no quibble with your argument for the substitution of the PCL86. It makes absolute sense when you take cost and long term security of supply into consideration.
Just to put the price of a NOS ECL86 into perspective, I submit the price of a NOS EL84, also from Watford Valves.
EL84-TUNGSRAM 1970
The risk with NOS is that this is tubes that might have been left over or put back in the box for any number of reasons. It might be lemons, or to be positive, one does not know as there is no traceable documentation.
I don't want to get into the merits and demerits of NOS valves.
However to be fair, the vendor offers excellent after sales service and matching of the Tungsram EL84s is not only available but is mandatory!
But Shirley the ECL86 is a triode/pentode in one envelope, and the EL84 is a straight pentode. any substitution would involve extra sockets etc., so my integrated Cadet and Leak ST20 tell me. I rate the Rogers highly, but the bass is not as explicit as the Leak - boof rather than badaboom. Still a lovely thing.
Cheers Steve
Cheers Steve
That's true Steve. Go back to post #5.
Also, I was only comparing the cost of a NOS ECL86 with that of a NOS EL84.
Any information you can offer about your Cadet, Steve, would be most appreciated.
Valve experiences, component upgrades etc? Cheers!
Note that one of the can caps in the power supply first filter is at a raised HV level - so preferably don't touch that directly - perhaps easier to use a cheap thermal IR spot gun.
The simplest way to install new can cap replacements is to remove leads from under the can caps and solder on a small tag strip to one of the old cap terminals, to mount the new cap to and reconnect the existing leads. The aim is to retain the old can caps in situ for the same look, and allow any future can cap to be easily installed if needed. Some may go to the hassle of coring out the old can cap and hiding a new cap inside.
Crusher, are you ok to probe all the circuit voltage levels? It is important to confirm that all circuitry is operating as it should, and not just assume. That includes confirming that vintage coupling caps are not leaking some current through to raise the next stage's grid voltage. And also goes for confirming the output stage valves are operating with about the same anode current level (or cathode current level - depending on which of many techniques you end up using to check this important aspect).
If everything is at nominal operating level, then that usually leaves you with the fun of confirming that all switch and socket connections/contacts and pot wipers are ok.
The simplest way to install new can cap replacements is to remove leads from under the can caps and solder on a small tag strip to one of the old cap terminals, to mount the new cap to and reconnect the existing leads. The aim is to retain the old can caps in situ for the same look, and allow any future can cap to be easily installed if needed. Some may go to the hassle of coring out the old can cap and hiding a new cap inside.
Crusher, are you ok to probe all the circuit voltage levels? It is important to confirm that all circuitry is operating as it should, and not just assume. That includes confirming that vintage coupling caps are not leaking some current through to raise the next stage's grid voltage. And also goes for confirming the output stage valves are operating with about the same anode current level (or cathode current level - depending on which of many techniques you end up using to check this important aspect).
If everything is at nominal operating level, then that usually leaves you with the fun of confirming that all switch and socket connections/contacts and pot wipers are ok.
trobbins makes an important safety point. The Cadet III employs a full wave voltage multiplier to provide 295V dc from a 115V ac transformer winding. The metal can of one of the two main 100uF power supply capacitors is not grounded but sits at 115V with respect to ground. This capacitor (C10 in the power amp schematic) will have its metal can covered in insulating plastic so must be the red one in your photograph. As trobbins says, do not touch any exposed metal area of that capacitor while the amp is live.
How important is originality? Is remaining faithful to the designer's concepts, but taking advantage of better performing modern parts acceptable?
The OEM SS diodes rate to be fairly noisy. Replacement with quiet, low cost, UF4007s is (IMO) a good idea.
Even if they "re-form", the equivalent series resistance (ESR) of the can caps. is suspect. Sourcing replacement can caps. is frequently a PITA and modern electrolytic caps. are superior in all sorts of ways. Can caps. can be replaced by clamp mounted, radial connection, parts that are placed in the existing chassis holes.
The power amp schematic, with the doubler stack directly feeding the "finals", clearly shows cost containment. Better filtration while not overheating the power transformer is (IMO) something that warrants careful consideration. The schematic shows 100 μF. parts in the doubler stack. Increasing the stack parts' value to 220 μF. should be safe enough, provided the increased turn on surge is tamed. A CL-140 inrush current limiter inserted in the line between the unlabeled fuse and the "center" of the doubler stack disposes of the matter.
One of the photos in the OP shows corroded RCA jacks. Treatment with DeoxIT D5 (#D5S-6) may save the day, but future corrosion is reasonably certain. Replacement with corrosion resistant gold plated parts makes sense to me. This black part and this red part (of UK origin) could be a good fit for the available space.
The OEM SS diodes rate to be fairly noisy. Replacement with quiet, low cost, UF4007s is (IMO) a good idea.
Even if they "re-form", the equivalent series resistance (ESR) of the can caps. is suspect. Sourcing replacement can caps. is frequently a PITA and modern electrolytic caps. are superior in all sorts of ways. Can caps. can be replaced by clamp mounted, radial connection, parts that are placed in the existing chassis holes.
The power amp schematic, with the doubler stack directly feeding the "finals", clearly shows cost containment. Better filtration while not overheating the power transformer is (IMO) something that warrants careful consideration. The schematic shows 100 μF. parts in the doubler stack. Increasing the stack parts' value to 220 μF. should be safe enough, provided the increased turn on surge is tamed. A CL-140 inrush current limiter inserted in the line between the unlabeled fuse and the "center" of the doubler stack disposes of the matter.
One of the photos in the OP shows corroded RCA jacks. Treatment with DeoxIT D5 (#D5S-6) may save the day, but future corrosion is reasonably certain. Replacement with corrosion resistant gold plated parts makes sense to me. This black part and this red part (of UK origin) could be a good fit for the available space.
I Agree with Eli. As stated earlier, I replaced the solid state rectifiers, as well as all power supply associated capacitors in my Rogers HG88 III.
I used 1kV 3A fast recovery diodes to good effect.
I replaced the 100 uF capacitors in the voltage multipier circuit (or 'doubler stack' as Eli terms it) with ones of the same value as I was afraid of inceasing the inrush current.
The result of these changes was a hum free amplifier, better than it had been when new when I wrote to Rogers complaining of the hum level. Their customer service was excellent and they sent me an apologetic letter along with two new rectifiers and two new capacitors. These replacements helped, but my recent replacements outshine them!
P.S. When choosing replacement capacitors for a voltage multiplier, one is advised to use ones of at least twice the ripple voltage rating compared to normal.
The two piece Rogers Cadet III was intended to reside within a home made cabinet. The valves, power supply capacitors and transformers were never intended to be on display, unlike those of a modern valve amp. Only the control unit facia would be visible.
Similarly, the 'workings' of my Rogers HG88 MKIII are concealed within its original teak formica case. Consequently, I was not overly sympathetic to the original look and happily substituted smaller, modern electrolytic capacitors as suggested by Eli. I just wanted my amp to work safely and efficiently for my own pleasure!
However, I did retain all the original capacitors for a future owner who may wish to restore the original look by emptying the old cans and stuffing them to their heart's content!
A clang associated with magnetostriction, when power is applied, is disturbing. That's why you install an inrush current limiting thermistor. The thermistor also slows B+ rise down a tad, which gives the heaters a bit of time to work.
Both cost and space mitigated against larger valued caps., back in the day. The demands of switching PSUs work to our advantage in making prices lower, volumetric efficiency greater, and performance vastly superior.
Increase the value of the stack caps. excessively and 2 bad things happen: the power trafo overheats and HF/RF noise is introduced into the B+ rail. Jim McShane increased the doubler stack cap. value in the H/K Cit. 2 from 200 μF. to 820 μF., without incident. While Rogers did not use magnetics as good as H/K did, there's little reason to believe that increasing 100 μF. to 220 μF. will cause trouble.
Yes indeed - I did consider upping the capacitance at the time of my HG88 III restoration, but decided to play safe!
Would upping the capacitance from 100uF to 220uF definitely require an inrush limiter and, if so, can you recommend a suitable thermistor?
I presume it would be connected between the transformer secondary and the intersection of the two capacitors?
Yes, the thermistor goes in the line between the power trafo and the doubler stack's "center".
While its presence may not be absolutely necessary, the themistor can only do good. I like draw odds. This part will do the job and should be available from Farnell.
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