Rectifier compatibilty

[graaudio]

Hello all, I've bought a couple of mono's that look to be essentailly Mullard 5-20 design but have been wired as triode. The OPT's (Australian Ferguson's) have no UL taps. The standard rectifier is a GZ34, however I have a couple of CV1264's that are in good condition. These are quite high-power handling devices, is there any reason I can't use these for this amp? The CV1264 is also referred to as NU12, U18 or FW500. It's just such a great looking tube I'd love to use them.
I'm new to tubes but can have a good electronics background, so I will understand any replies I get.

Thanks, Gra.

[barretter]

The U18 needs 3.75 amps of heater current compared to the GZ34's draw of 1.9 amps so it might tax the amplifier's mains transformer. Also the U18 is directly heated so the HT will come on much faster.

[timpert]

Hi Gra,

I found some sparse data on the CV1264 here:
http://www.tubecollector.org/cv1264.htm
From this you can see that they are not compatible. Major differences: CV1264 has a 4V/3.2A filament and the GZ34 a 5V/2A filament. While you can drop the voltage using a power resistor, the current draw will likely overload your power transformer.

But more importantly, the GZ34 allows for a buffer capacitor up to 60uF, and the CV1264 only 4uF. Most probably, the input cap of your power supply will be 50uF, so if you put the CV1264 in your amp, it will fail quickly unless you modify that too. Another issue is that the GZ34 is (in terms of technology) a newer tube, and there are very few other rectifier tubes that have a voltage drop as low as the GZ34 has. I have no data on the CV, but its voltage drop will probably be three to four times as high, which of course affects the performance of your amps.

So unless you decide to do some modifications to the amps, I suggest you leave the GZ34 in them, and make something new using your CV1264 tubes.

But by all means check the datasheets of rectifier tubes yourself. Great places to find tube data are Duncan's TDSL:
http://tdsl.duncanamps.com
and Frank Philipse's page:
http://www.tubedata.info

Jurgen

[graaudio]

Thanks for the info. I guess there are different specs as per which manufacturer's tube you are using. I have found another set of specs relating to the Mullard FW500 (which is what my tubes actually are) that state a 3a heater current and 16mf maximum resevoir. My power trannys have a 5V 3a heater winding which would be right on the limit, is that unaccepatable? There are also 2 x 6.3v windings, one at 2.5a and one at 4a. Could I perhaps use the 4a winding with the appropriate dropping resistor if the 2.5a is enough for the other heaters? The resevoir cap in the 5-20 is 8mf, so no probs there. Although the choke is a rather large 12H 200ma job which I would like to replace with something smaller like 7H 150ma which I could fit under the new stereo chassis I am rebulding all this into, so maybe that would mean I need to be able to have a larger resevoir?
In relation to the HT coming on faster, I understand this may stress my outputs, could this be helped by installing a standby switch?
I think I will probably use the NU12's for another project but all this info would be very helpful for that when it happens, and I may very well use these same trannys if possible.
Sorry about the long post, Thanks again for your help in advance.

Gra.

[Eli Duttman]

For the CLC filter, 15 muF. in the 1st position will keep the rail voltage up nicely while staying within spec. for the rectifier. A 7 H. inductor is fine. Pile the energy storage up in the 2nd cap. position. The choke protects both the rectifier and the power trafo from the large "final" capacitance.

[timpert]

Hi Gra,

I second what Eli sait about the first cap reservoir and inductance.

The 5V/3A winding will be safe to connect your rectifier to. I'd save the 4A winding for the EL34's and their drivers, because an EL34 draws 1.5A heater current. The 2.5A winding will not be adequate for 2 pcs. plus preamp/driver tubes. Also, it is better to drop 1V at 3A than 2.3V, because of the amount of heat generated under the bonnet. A standby switch is a good idea, but it is wise to use it only for applying HT after the tubes have heated up. If you use it in the fashion of a TV stand-by, your tubes may suffer from a phenomenon called "sleeping sickness". So if you're not using the amp, it is better to turn it off completely. An electronic turn-on delay using a relay might be a better idea in this light, and also a bit more user friendly.

Regards,
Jurgen

[graaudio]

Thanks for your advice. I think the message here is stick with the GZ34's if I want to remain reliable and well within tolerance. I've decided the 1264's will be best left to another project with a single rectifier for 2 channels of amp, and the appropriate transformer.
Thanks again. Gra.