Jon Mark Rocket

[RJ Guitars]

TAG folks,

I got this set of photos from a friend that identified it was a Jon Mark Rocket? As near as I know it is authentic but I couldn't support that assumption with anything I know?

It is curious:
1) extra transformer inside to provide the 5 volts?
2) Orange drops rather than the Panasonic coupling caps?
3) Something besides Mallory filter caps?
4) Authentic looking chassis?
5) Authentic looking circuit board?
6) Green pilot light?

does any body recognize this build?

rj

[redshark]

Yes, it looks like a JM trainwreck reissue. The transformer inside to drive the rectifier tube heaters is something that Ken used in the past in another rocket.
Yes, it surprises me he is using orange drop caps instead of the panasonic. I have a bunch of those!! I should sell him some 0,047's!
The electrolytics actually look like F&T's.
Oh well, it's obvious Ken didn't keep a huge stock of certain parts and if some are obsolete or not ready available parts he needs to find replacements.
The board looks legit and the chassis too.
The green pilot light and the power cord's strain relief looks like his signature to make a diference between reissues and originals but still you can see is a genuine trainwreck.
His lead dress looks clean. Good looking build!!

[Mark]

I really don't like the Norlin switch for the impedance selector. I've known people to have issues with those switches.

[RJ Guitars]

Am I correct to say the power transformer looks like a Pacific and can we assume that it's the same tranny used in the Express or Liverpool?

Has anyone got info or a good guess on what output tranny he is using?

[Mark]

Have a look at the location of the rectifier tube, it is in line with the last output tube and it is further away from the power supply caps.

I also get the feeling the board MIGHT be a little shorter too, (not a big deal but nice to know.)

[redshark]

Am I correct to say the power transformer looks like a Pacific and can we assume that it's the same tranny used in the Express or Liverpool?

Has anyone got info or a good guess on what output tranny he is using?

Yes, PT may be a pacific. As far as OT for rocket I know that Ken had a few diferent prototypes so it is dificult to know exactly which one is it.
There are rockets with Heyboer and pacific PT's and all diferent kinds of OT's, but I think never a Heyboer OT was used in any trainwreck.
Anyway at least one rocket has pacific OT too. Of course diferent spec from the express and liverpool's.

[M Fowler]

Trainwreck Rocket amp called Jovanna used that same power cord strain relief.

I think the Stancor P-8356 PT was used in some.

[redshark]

Jovanna was the incomplete rocket that was on Ken's bench and that John Mark finished wiring power supply.

[wsaraceni]

first JM gut shots i've seen. looks like a clean build

[njwrecks]

I believe the OT is a Dynaco A-470 from Triode.

[RJ Guitars]

I was gonna guess that... it's nice and shiny and uses the end bells that go all the way to the chassis so I figured it was a Dynaco clone. Heyboer never sent me one with the highly polished end bells and they typically would have coated it with a heavy coat of varnish... they sounded great but occasionally I wondered if the varnish guy was really trying??

[Mark]

Given the rectifier does have a hard time and the four notch Mullard seems to be the only rectifier capable of handling the 80F filter cap, why not look at changing the secondary winding so additional resistance can be added and hopefully make the rectifiers life a little easier?

http://www.valvewizard.co.uk/fullwave.html

Indigo colour text denotes info from the link above.

Edit: For example, when supplied by a transformer rated at 300-0-300Vrms, the EZ80 specifies Rlim(min) = 215 ohms per anode. The total impedance presented to the rectifier by the transformer is given by:

Rt= Rs + (n^2)* Rp
Where:
Rs = DC resistance of one half of the transformer secondary winding.)
Rp = DC resistance of the transformer primary winding.
n = Secondary to primary turns ratio (equal to the secondary voltage divided by the primary supply voltage).

If we were using a mains transformer with a 240V; 80R primary and a 310-0-310V; 50R per half secondary:
Rt= 50 + (1.29^2)* 80 = 183R
The EZ80 requires at least 215 ohms, so an additonal 215 - 183 = 32 ohm resistor must be placed in series with each anode (so we would probably use 33 or 47 ohms).
Because the limiting resistors will have to carry the ripple current of the reservoir capacitor it is best to use high wattage resistors. Even 7W resistors will usually get quite warm. Remember that the voltage drop across the limiting resistors will cause the HT fall proportionately.
Alternatively, a single limiting resistor could be placed between the transformer centre tap and ground, although its power dissipation will be doubled.

Moving on the the Rocket.

I have a Classic Tone Trainwreck compatible power transformer, the resistance between the centre tap to either end are 16 and 17 ohms respectively.

The primary resistance is 1.5 ohms for 120vac operation and 6 ohms for 240vac operation.

The turns ratio for 120vac operation is ([260X2]/120)=4.3333333

The turns ratio for 240vac operation is ([260X2]/240)=2.1666666

For 120vac operation Rt=17+([4.333333^2]X1.5)
=17+(18.7777777X1.5)
=45.166666ohms

For 240vac operation Rt=17+([2.1666666^2]X6)
=17+(4.69444444X6)
=45.166666ohms

Now time to look at valve specs.

First lets look at the common stuff.

http://www.drtube.com/datasheets/gz34s-jj2003.pdf

I may have it wrong here, but I'm looking at the capacitor input section at 2 X 300vac (Utreff), it states Rt = 2X75 ohms.

75-45.166666=29.833333 ohms

So a 33 ohm resistor per plate would assist the operation of this valve. I'm not sure but I'd guess- imate that the 33 ohm resistors would drop the rail voltage by about 10VDC.

Hopefully we can kiss the four notch rectifiers goodbye.

[overtone]

Given the rectifier does have a hard time and the four notch Mullard seems to be the only rectifier capable of handling the 80F filter cap, why not look at changing the secondary winding so additional resistance can be added and hopefully make the rectifiers life a little easier?

http://www.valvewizard.co.uk/fullwave.html

Agreed, the Rt min of transformers seems to get overlooked. Much easier to talk about notches!
The thing about the four notch rectifier is starting to get out of hand. Do we need another internet myth to sky-rocket the prices?
tony

[RJ Guitars]

How far can we go with kiss the 4 notch rectifiers goodbye? Is it reasonable to think we could get Rocket tone using SS diodes? I know that is heresy but the specs on a really pristine EZ34 are not so far from solid state performance??

Also, I'm curious how many folks are having melt downs with the Sovtek and JJ EZ34 tubes? I have heard a few rumors but don't know anyone who has actually lost one in a Rocket style build?

[njwrecks]

The OT has really thin wires on both sides- they are Teflon - same as the ones that Triode sells-