New 2550X build (will be 2555X) + soundclip + highgainclip

[mat]

I built mine using SDM's great layout as a guide. The PT I have in now has too low voltages on the secondaries (300-0-300) giving 375 on the EL34 plates. I have a Hammond 1650H as an output transformer.

The question is that does anyone have a voltage chart for a 2550 or 2555 Silver Jubilee ? I would very much like to check it out.

I will upgrade the build (after 6 weeks) with Hammond transformers aimed for the JCM800. Then I should have about 450V on the plates of the power tubes.

Its the first build that fired ok on the first try (pots, switches, footswitch ok) - the layout is very clear, Big Thanks SDM

The bias section of mine has some fault on it that I have to troubleshoot. Also when trying the pentode/triode switch on the other position I got horrible squeel. Might be the OT primaries in a wrong way. I'll try to swap the after two and half weeks (going to summercottage - no internet in there..)

I'll get back with couple photos and after upgrading the irons maybe with a sound sample.

Matti T.

[chubs]

The bias chart from a JCM800 or plexi 50 (or any 50w Marshall) would do it.

http://members.shaw.ca/house-of-jim/Html/Marshall.html" onclick="window.open(this.href);return false;

[Structo]

Those seem a little low to what I have seen here.

I wonder if the Metro PT's are higher voltage than what Marshall used?

[paulster]

I've got 470V on the plates of my 2550 clone, using a JCM800 2204 50W PT.

[mat]

What kind of voltages (50w/100w silver jubilee) should have in the V1a/V1b V2a/V2b ? How about the PI ?
Thanks,
mat

[jcmjmp]

My 1987 Silver Jubilee has approx 465v-470v on the plates.

[mat]

I have pretty much complited the iron upgrade. I used Hammond 290HX power transformer. It has a green wire on the primary side. Anyone could tell me what is it for and whether or not to connect it on a 2555X build ?

Also the secondary has only one green wire. Is that a heater center tap (I have already soldered couple of 100ohms to ground from the 3.15Vs) or should I just solder it to the ground (actually it is already soldered to ground..)

http://www.hammondmfg.com/pdf/EDB290HX.pdf

Thanks,
mat

[Structo]

Sorry can't help with the green wire.
Strange there layout doesn't show it.
Might be worth a call or email to them.

[mat]

Sorry can't help with the green wire.
Strange there layout doesn't show it.
Might be worth a call or email to them.

I think You are right. I'll get back after one week (work on mon-fri 9am-10pm..)

mat

[SDM]

The Hammond PDF shows a green wire on the primary side and one on the secondary side. From the schem also in the PDF, can see one is for the heater CT, the other the common/ground for the bias supply winding.

Both are grounded typically (which would make the 100 ohmers to ground on the heaters useless by the way, so may as well remove them if heater CT is indeed grounded). To determine which is which is pretty easy. With the green wires lifted from ground, the Heater CT will read resistance/continuity (look like a dead short basically) only to the black heater wires as it is only a part of that winding, none other. The other green wire will only read continuity (low ohms) to the white wire as it is a part of that winding only.

As to voltages, I attached a rough voltage chart of what to roughly expect in a 2550 with 460V on the plates, adjust things up or down a bit from there depending on your B+, but you should be close. Same voltages would apply to a 2555 of course, you just have 2 more power tubes.

[mat]

Thanks SDM for the explanation and for the correct voltages. One question, why would the heater center tap or the bias supply common be on the primary side ?

mat

[paulster]

They've just decided to bring it out of that side of the bell housing, probably because of space constraints caused by the thickness of the heater wires coming through the exit hole. It doesn't mean it's tied to the primary side of the winding, just that they terminated it on that side of the core stack.

[mat]

They've just decided to bring it out of that side of the bell housing, probably because of space constraints caused by the thickness of the heater wires coming through the exit hole. It doesn't mean it's tied to the primary side of the winding, just that they terminated it on that side of the core stack.

Ok, thanks for clearing that out Paulster. Can't wait to get this one going

mat

[mat]

I finally found time to swap the 100W Hammond trannies (290HX and 1750U) in.

The plates has 504 volts in them !?! What might be the issue to them to have that high voltages ? I have JJ500V can caps on my build so I'm little affraid to fire the amp right now.

I uploaded few (2.5MB) pictures of the crowdy (will neat the wiring later) build at:

http://s927.photobucket.com/albums/ad112/mattialeksi/

mat

[mat]

I have a 1ohm on pin 1 to 8 to ground on each EL34 and it reads 30mV accross the 1ohm and the bias circuit ranges at -47V to -57V so the power tubes are getting current.

I still have 511V on the plates of the EL34 - measured only one. I'm little affraid to keep the amp on with such voltages.

Any idea where to seek for the reason of the too high voltages problem ?

Thank You,
Mat