Help with JTM45 build

[sct13]

Hi everyone,

It's been a while, but I had a knee replacement, knew I needed something to do for six weeks and decided on another JTM45 build.
I had a JTM 45 reissue OT Lying around and bought the rest of the parts and went at it....pain meds and all ...LOL

But on first power up, the voltages were way off...I followed the instructions I found here, and I thought I was pretty thorough....

But on turning on the Standby I got a horrible Howell, and I switched it off immediately and began looking for errors, reflowed some solder, checked grounds, and orientation....I changed out the can caps (I had some lying around) I hooked up the output to an OX so I can hear it through headphones and not destroy my cabinets, scare cats, wife etc...But after days of debugging I get a "Motorboat" noise....can't seem to find the problem. Also, the volume controls do not work. and the bass and presence pots act like phase shifters.


While debugging I pulled the Pi tube and the noise stopped, I also pulled out the NFB resistor. No Sound....

Voltage at the plates is 475VDC

I can record the noise, or get voltages for anyone that has debugging skills....

[Tazin]

The "howling" & "motorboating" could mean that the OT primary wires (Red & White) are reversed. It also could be from bad lead dress too. The 475 volt dc is concerning since it's rather high....Is this with the valves/tubes installed?

[sct13]

thanks

I will give that a try tonight....The 475VDC is not with the valves installed....

the OT is used from another JTM45 the red and white wires are cut to size, meaning the the red might not reach....I will need to splice someting on to it.

Thanks for responding, I'll report the findings ....

[sct13]

OK, I reversed them, got nothing now......418VDC at the plates Its dead silent. and I can smell the tubes heating up. Could this be a bad OT?

[Tazin]

Interesting...OK, what are the OT wire colors for the primary side?

[sct13]

Primary side is Red, White and black,

Secondary side is Orange, Grey, Yellow and Green

I don't have any documentation on this transformer, except that it has a label on it that reads 784-389 and 7-89

I was able to measure resistance in the red and white separately at 175 ohms ......

[Tazin]

As far as I can tell the JTM45 reissue OT's (the Drake 784-389 & Dagnall D2505) both use the same primary wire colors.

Primaries: Red, White, Black (center-tap).

As for proper phasing, the Red wire goes to V4 (closest to the Speaker jacks), and White goes to V5. Valve socket #4 (V4) should trace back to the 82K plate resistor for the Phase Inverter (V3).

As for a bad OT...You could use your Multimeter to check the resistance on the primary side by the reading of the Red & Black wires against the White & Black.

[sct13]

OK will do,,,,

I'll give that a try

[sct13]

OK so,

Red against White 150.9 ohms

Red against Black 76.6 ohms

White against Black 74.4 ohms

So it checks out, unless those values would be incorrect. I have no idea....

Now the bias....I checked that and the lowest setting is 49.0 mV and the highest is 89.6 mV
So something is very wrong here.

Pin 5 on V4 and V5 are around -43

If I pull the Pi tube the noise will stop....or not occur at all because I don't leave it on long enough...I just pull each tube and then switch it on again, there is no change on V1 or V2 if I pull the tube.

I pulled off the board de soldering and triple checking the routes, colors and lead r\dress, in fact I found a few errors that I fixed ,,,,but there is no change.

I also put all the tubes on my 1960's tube tester....and those tested fine.....

[Tazin]

Those OT primary resistance measurements are probaby good...The Drake -139 OT for EL34's runs about 45-ohms per side. Also, since the two measurements are very close (as they should be) that's another indicator that they are probably correct.

Regarding the OT secondaries...I assume you installed an Ohms Selector and you correctly wired the individual taps (i.e. 16-ohm, 8-ohm, 4-ohm)? Did you verify continuity between each setting on the Selector and the speaker jacks? Did you install the output wire from the Selector to the correct solder lug on the speaker jack(s) so that the tip of the 1/4" mono plug is positive? Assuming that the Orange colored OT secondary wire is the negative which connects to the speaker jacks did you also run a link wire from that Orange wire connection at the speaker jack to ground? Typical grounding location for this link wire is the solder lug attached to one of V4's octal mounting screws....Same place where the cathode of V4 get grounded.

[sct13]

I'll post a detailed photo of the area.

I will also check those selector points....

I didnt have a diagram for this OT and used an "assumed" wire scheme of a 50 watt transformer. But I will get on that checks right after dinner.

the grounds are run as you said but I'll double check.

[Tazin]

Since the 1989 Marshall JTM45 reissue OT (Drake 784-389) has 4-ohm, 8-ohm, and 16-ohm output taps I can only assume they used the same wire colors as the 784-139 OT (Gray = 16-ohm, Green = 8-ohm, Yellow = 4-ohm, Orange = negative).

[sct13]

Well this is interesting, there is continuity on all the connections at the selector switch, regardless the position of the switch. could that be a problem? Maybe I should unhook the green and yellow and see if it works. ?

[Tazin]

That is normal since the secondary is one winding with leads/taps soldered at certain points which correlate to 4-ohm, 8-ohm and 16-ohm. What's important here is that the OT secondaries are connected properly (continuity) through the Selector switch to the speaker jacks.

[sct13]

Ok … yes I pretty sure these are correct then.

How do I post pictures from my phone ?