Marshall 4145

[gbono]

I have a 4145 (club and country 4x10) that is claimed to have and issue with the heater circuit. All the heater fuses have been removed from the amp and it looks like someone replace the power switch. I have found a schematic on the net but it is a pretty bad copy. Anyone have a readable schematic they can post?

[g1]

All the heater fuses have been removed from the amp

Is this a CSA version with the fuse board? If it has 4 x 5A fuses for the heaters, they were supposed to be changed to 6.25A SB fuses.
Of course, this is after any actual faults in the heater circuit are corrected.

[gbono]

Yes it does have a fuse board and the correct fuse is marked as 6.25A (slo blow). Is there another Marshall model that is similar to this amp? I would really like to get a readable schematic - see attached.

[g1]

That appears to be the only version of the schematic available for that model.
Here is a power amp schematic that shows the heater fuses and a power switch that is probably more in line with what you have:
http://www.drtube.com/schematics/marshall/78331-3.gif

[g1]

Here you go, this is nice! pg.14 of pdf
Lots of data for lots of JCM800's, haven't seen this service man. before!

[gbono]

Thanks that is just what I was looking for

This amp has 1.5K resistors on the the control grids of the output tubes. They look stock but this amp is definitely not the bass model (spring reverb). I was expecting 5.6K a la lead version.

[Enzo]

Oh, cool.

[gbono]

The output tubes in this amp are EL34 - several are old Mullards. The schematic/service manual calls out for 6550s (US models). Pin 1 is tied to 8 on all the sockets (V4/5//8/9) so the EL34 becomes a beam power pentode? Is this common tube arrangement in Marshalls? I have a glut of 6L6's so I'm going to use these in the amp unless someone can enlighten me on a better way.

[Jazz P Bass]

Pin 1 is the suppressor grid.
It should be tied to ground.
On a EL34 tube, there is no internal connection.
On a 6L6 there is.

The EL34 cannot 'become' a Beam Pentode.
It's a Pentode, pure & simple.

Now the internal construction of a 6L6 makes it a Beam Tetrode.

[gbono]

Duh - the wire mesh of the suppressor (EL34) is replaced with beam forming plates = beam power (6550). Still the question - what tubes to use in this amp?

[Jazz P Bass]

Here is a Marshall file on converting between tube types.

[gbono]

Thanks for the doc. Was the 4145 a unique design that used 6550's? Most of the designs in the doc start out with EL34s.

[Enzo]

Marshall made EL34 amps. When they started exporting large numbers of them to the USA, they made the simple conversion to 6550s for them to use the MUCH sturdier tube.

[g1]

That doc is basically for the JCM900 series, they were having trouble getting good EL34's so switched to 5881's (sovtek, not 23W type). Later they went back to EL34 and many people wanted to convert their 5881 versions to EL34's. It also shows conversion for JCM800 to 5881.
For the JCM800's (and some prior models), some of the US amps had 6550's. The schematic for yours shows KT77 (Marshall used the term interchangeably with EL34), and 6550 (US).
As yours has EL34's, it may not have been a US version, or may not have had 6550's. No need to convert it to 6550's unless your customer wants that (some metal guys like them). It would probably require a bias circuit change as shown on the 4145 schematic (resistors in bias circuit).

[gbono]

I spoke with the owner and he said the amp came from the UK - that explains why the amp was not converted to KT88/6550s. The power switch needs to be replaced and I'm looking at this Switch - Lighted Power, used in Marshall JCM Series & Others | Antique Electronic Supply

Heater circuits appears to be functioning once the correct fuses were in place. The amp needs some of the caps for the front panel knobs, bias adjusted and possibly some new tubes.