2xEL84 Rocket

[Littlewyan]

I’m finally looking to build a Rocket! However I need chassis space for an extra channel and so it will have to be a 2xEL84 version, which is fine as the normal version will probably be too loud for my needs anyway! I’m looking to use an Edcor OT, wondering if anyone has used the 8.6K OT with a Rocket? Seems most use the standard 8K OT but given the 4xEl84 version uses a 4.3K I figured this might be better?

[Colossal]

I’m finally looking to build a Rocket! However I need chassis space for an extra channel and so it will have to be a 2xEL84 version, which is fine as the normal version will probably be too loud for my needs anyway! I’m looking to use an Edcor OT, wondering if anyone has used the 8.6K OT with a Rocket? Seems most use the standard 8K OT but given the 4xEl84 version uses a 4.3K I figured this might be better?

I think this would be fine.

[joeboo88]

Hi Ryan. I believe my Edcor OT is a 8.4k 15 watt , but I used it on my Xits X10 ( Thanks Dave) which is a SS rectified Liverpool ish type amp
I used a 270 0 270 Pt (Edcor Too)
I really like the sound and yes the amp is pretty loud

I also have my full Rocket and that thing is loud So I built the RJ power broker attenuator to go with it
hope this helps a bit

[Littlewyan]

Thanks guys, I think I will go with the 8.6K OT from Edcor. For the PT I'm looking at the Hammond 370JX (250-0-250), used this in another amp and I have a solid 350V HT on that. Going to leave out the valve rectifier though and may use a 100-180Ohm resistor in place instead for sag.

[Littlewyan]

Interested in people’s opinions on the rectifier for this amp. After a bit of reading it seems you can’t really replicate a valve rectifier with a resistor due to the way a valve rectifier’s resistance decreases under load. However, what one do you use for a half powered rocket? I see JM still uses the GZ34 but Dr Z uses a 5Y3 in his Z Wreck Jr.

[martin manning]

Interested in people’s opinions on the rectifier for this amp. After a bit of reading it seems you can’t really replicate a valve rectifier with a resistor due to the way a valve rectifier’s resistance decreases under load. However, what one do you use for a half powered rocket? I see JM still uses the GZ34 but Dr Z uses a 5Y3 in his Z Wreck Jr.

Looks like an EZ81 needs almost exactly half the current to produce the same voltage drop as a GZ34.

[Littlewyan]

Thanks Martin, looks like the EZ81 is the way to go. Wonder why JM still uses a GZ34 in the half power rocket...

Another thing that's been bugging me is the voltages on a Rocket. Looking through various posts and at various voltage charts I see the transformer for a Rocket has a 250-0-250 secondary, in the voltage chart I can see 256VAC measured, but only 294VDC rectified. I can't get my head around how this can be as even with a GZ34 it shouldn't drop that much voltage. Can anyone share any insight on this? I feel like a 225-0-225 PT would be more appropriate for Rocket voltages.

I actually used a 250-0-250 PT in my last build, it's solid state rectified but I see 345VDC after rectification.

[sluckey]

Depends on the PT. If you have two PTs rated for 250-0-250, but one has a 200mA current capacity and the other has only 100mA current capacity, the PT with the higher current capacity will produce a higher B+ voltage simply due to the higher current rating (lower internal loss).

[Littlewyan]

So I believe the Rocket PT is pretty beefy, 250-300mA current rating. Also the voltages given are in operation, so 256VAC on the rectifier and 294VDC coming out the otherside. I think for a GZ34 you multiply your AC by 1.3 so it should in theory be 325VDC coming off the rectifier.

[sluckey]

I think for a GZ34 you multiply your AC by 1.3 so it should in theory be 325VDC coming off the rectifier.

That 1.3 multiplication factor is a relative number that can have meaning when comparing different rectifier tubes in the same amp. It accounts for the voltage drop across the tube for a certain amount of current flowing through the tube. But, the B+ voltage still depends on the load current, voltage drop across the rectifier, and the current capacity of the PT winding. If you check several different amps you will soon discover that "1.3" does not hold true for all cases.

[didit]

Looks like an EZ81 needs almost exactly half the current to produce the same voltage drop as a GZ34.

A cool chart, thanks Martin. Had a scattering of datasheets and an old ascii file someone in peak Usenet era had compiled, and then scribbled interpolations 'til arriving at roughly that image in my mind's eye. A EZ81/6CA4 is the answer but do note it requires a 6.3VAC secondary, and that then offers up another question to answer.

Best .. Ian

[martin manning]

A EZ81/6CA4 is the answer but do note it requires a 6.3VAC secondary, and that then offers up another question to answer.

Only 1A heater current, and the heater is isolated from the cathode, with a 500V Vh-k max. Could it be powered from the main heater supply? Otherwise, and maybe better, a Marshall 18W PT (two 6.3VAC windings) would do nicely. It is 290-0-290, and a lay-down mount, though. FWIW, the 18W schematic does not show any connection between the rectifier cathodes and its heaters, which are left floating. There are others that show only one 6.3VAC winding with a grounded CT.

[Littlewyan]

Thanks guys. Looks like an EZ81 is the one if I go for an 18W Rocket and you're right about the multiplication factor sluckey. I'm on the fence right now though, I'm only really going for an 18W so I have room in the chassis to install a 2nd channel, but by doing this I feel like I'd be compromising the Rocket circuit. So I'm leaning towards splitting this into two amps! A standard 30W Rocket and an 18W High Gain Monster of my own design.

One slight issue, does anyone sell the correct transformers for a Rocket nowadays?

[didit]

A EZ81/6CA4 is the answer but do note it requires a 6.3VAC secondary, and that then offers up another question to answer.

Only 1A heater current, and the heater is isolated from the cathode, with a 500V Vh-k max. Could it be powered from the main heater supply? Otherwise, and maybe better, a Marshall 18W PT (two 6.3VAC windings) would do nicely. It is 290-0-290, and a lay-down mount, though. FWIW, the 18W schematic does not show any connection between the rectifier cathodes and its heaters, which are left floating. There are others that show only one 6.3VAC winding with a grounded CT.

Agreed it can be sensible and just fine to share the 1A@6.3V from the single common filament winding. As a subscriber to the idea the Rocket recipe involves a lower B+ my personal approach would avoid the stock Marshall 18W, which at 290VAC is going yield well above desirable. A small separate filament transformer as was stock in the original Rocket and match it with suitable 260VAC main power transformer is the model I'd probably look at.

Best ..

[didit]

Thanks guys. Looks like an EZ81 is the one if I go for an 18W Rocket and you're right about the multiplication factor sluckey. I'm on the fence right now though, I'm only really going for an 18W so I have room in the chassis to install a 2nd channel, but by doing this I feel like I'd be compromising the Rocket circuit. So I'm leaning towards splitting this into two amps! A standard 30W Rocket and an 18W High Gain Monster of my own design.

One slight issue, does anyone sell the correct transformers for a Rocket nowadays?

Rocket done with classic Trainwreck level B+ voltage and a suitable >=8K Ohm output transformer will be 15W. Would look at sourcing a current production clone of the Dynaco z565, which is the suitable "little" version of the a470 used in 30W Rockets.

Best .. Ian