Marshall JCM900 Dual Rev 100w SLXq-06 Problems Bad OT ?

[CoopDaKill]

Hello. I have my buddys' Marshall JCM 900 Dual Reverb 100 watt head. With 6L6GC output tubes. Issue SLXq-60. I think this the 4100 model? Came to me with main power fuse blown, and both output valve fuses (on back panel) blown. It also had a shorted output tube (6L6gc) which is V7 , the one closest to the power transformer.

R8 and R9 (100k) looked like they have been cooked. So, I changed them. R28 (47k), and R29 (8.2k) were both out of spec, so I changed those too. Pulled C11 and C16 (10uf 100v) and tested out of circuit. Both tested fine. I then retensioned and cleaned tube sockets. Tested remaining tubes, put together a fairly close matched quad of 6L6gc tubes. Replaced fuses. Checked all diodes. I was able to power it up and adjusted bias to -49.0 mvdc.

The amp sounds ok, but seems to be half volume ( output power switch set to high). I then noticed V6 & V7 red plating . I shut her down quick. Tried a NOS quad of Sovtek 6L6GC's. Same thing. Before that, I noticed the output transformer would rattle every time I played a note or chord. I think this amp has a faulty output transformer. I pulled the output tubes and powered it up, with standby on. I got -84, VDC on pin 5 of each output tube socket. How can I check for sure that the output tranny is bad? Thanks

[CoopDaKill]

Somebody please help?

[Helmholtz]

Schematic?

I don't think your OT is bad. But measure DCR on both sides of the primary wrt CT.

Do all 4 6L6s show an idle current of 49mA?

[alexradium]

no scope i imagine...
best thing to do first is to disconnect all wires from the amp to test the OT.
One thing you can do is feeding a low ac voltage to the secondary taps and check the primary voltage between the plate wires,it should be consistent and respect the different turn ratios,if you don't know how to calculate those i can explain better.
You can also feed the primary with mains voltage,remember to put a fuse in series,and make the measurements on secondaries.
In this case,if you have a wattmeter you can also check if there is abnormal consumption,in case of short after 30 minutes its easy the tranny becomes hot.
Another method is the neon light and a battery,it should be on geofex.com,check out R G Keen

[CoopDaKill]

Thanks for the good advise guys. I’ve been battling a head cold this week. I plan on jumping back in this JCM900 next week.

[CoopDaKill]

Finally getting over this head cold. Feeling a lot better now, I decided to jump back into this beast.

I unsoldered all of the output transformer wires. And started checking the OT resistance. While doing so, I noticed I’m getting fluctuating ohm readings. With my meter leads to the green and yellow solder tabs of the OT, the resistance goes all over the place. So, I’m assuming an intermittent winding? What you guys think?

No O-scope. Previously, all output tubes had -49 mvDC bias. I’m not sure how to apply low voltage to the transformer. But I do have a Variac.

[The Dude]

Are the two halves of your primary reasonably symmetrical DCR- brown-white and red-white? Do you measure any resistance from primary to secondary?

Also, try this:

https://www.premierguitar.com/articl...ormer-tester-1

[Helmholtz]

I unsoldered all of the output transformer wires. And started checking the OT resistance. While doing so, I noticed I’m getting fluctuating ohm readings. With my meter leads to the green and yellow solder tabs of the OT, the resistance goes all over the place. So, I’m assuming an intermittent winding? What you guys think?

For reliable readings short the secondary while measuring primary DCR. (If it works I'll try to explain why.)

[CoopDaKill]

OT DCR readings

16.6 ohms between white and brown

32.4 ohms between white and red

with the secondary shorted together

[Helmholtz]

OT DCR readings

16.6 ohms between white and brown

32.4 ohms between white and red

with the secondary shorted together

Values are perfect. Your OT won't cause the redplating.

Please post schematic and measure screen resistors/voltages.

[CoopDaKill]

Here is the schematic I'm working from

https://www.thetubestore.com/lib/the...-Schematic.pdf

I'll recheck screen resistors ASAP
Thanks

[g1]

I'm not sure how you would end up with -84VDC on the power tube grids. Suggest you re-check pin 5 voltages with power tubes removed. Check both DC and AC.

[Helmholtz]

I'm not sure how you would end up with -84VDC on the power tube grids.

^^^That!
Especially considering the rather hot 49mA idle current.

[g1]

And you need to check that pin5 voltage in 'play' mode, not standby.

[Pedro Vecino]

If I remember correctly the bias voltage is very high and when the standby is ON it goes down to the precise adjustment gradually.