2203x Loss of Bottom End

[cof_3pc]

Hello All,

New here. I've been lurking and reading through threads on here trying to troubleshoot an issue with my jcm800 reissue, but haven't found any threads yet that solved my issue. It seems to have suffered a loss of bottom end after blowing an HT fuse.

Quick recap on the sequence of events and what I've tried so far.

-Amp blew HT fuse immediately after being switched out of standby mode at a friends house. Admittedly I had gotten side tracked and left it in standby for longer than anticipated.

-I replaced the fuse and tried it again to see if it would blow again. It did immediately.

-Later I tried throwing some used tubes in it from my 1959SL plexi reissue to see if that would resolve the fuse blowing issue. I put them in and rebiased. The fuse would no longer blow after this, but now it sounded thin. At this point I figured maybe those tubes were shot from being cranked through an attenuator in the plexi for so long.

-Most recently I got a brand new set of JJs from Eurotube and replace all of them in both the preamp and power amp and rebiased. Still thin.

I'm thinking that initial blowing of the fuse took another component or components with it. This thing used to smoke my plexi when they were put side by side, and I hardly ever had the bass above 4. Now with the bass on 10 it sounds thin and harsh. The general marshall crunch is there, but it's got no oomph.

I'm wondering if anyone has any ideas or knows some components or parts of the circuit I should check for correct resistances and voltages. Any help would be appreciated. Thanks.

 

[Jon Snell]

What value did you set the bias current to?
Check R38, 39, 40 & 41.

 

[cof_3pc]

What value did you set the bias current to?
Check R38, 39, 40 & 41.

I set the bias to about 38mA for a 457v plate reading. I used a Eurotubes Pro One on one pair, and a bias probe hooked to a fluke 87 on the other pair.

You want me to check the resistance values or the voltages after each?

 

[Jon Snell]

That is 19mA per valve? ... should be 35 - 38mA per valve for 70% dissipation.
Usually when the valves fail short circuit, thay can take out the G2 feed resistors.
Accertain there is equal voltage on each G2. Looking for around 350 - 400v on 2203 older amplifiers, slightly less on reissue amps. They must all read the same withing a few volts.

If all checks out, feed a low frequency signal into the input of the phase splitter. If still no power but OK above say 1kHZ. The output transformer is suspect, or of course your speakers.

 

[cof_3pc]

That is 19mA per valve? ... should be 35 - 38mA per valve for 70% dissipation.
Usually when the valves fail short circuit, thay can take out the G2 feed resistors.
Accertain there is equal voltage on each G2. Looking for around 350 - 400v on 2203 older amplifiers, slightly less on reissue amps. They must all read the same withing a few volts.

If all checks out, feed a low frequency signal into the input of the phase splitter. If still no power but OK above say 1kHZ. The output transformer is suspect, or of course your speakers.

That was 38mA read off of one tube at each pair. I just use the fluke to double check the Pro One, but have a bias probe hooked to one tube per pair.

I will try checking the voltages at G2 when I get a chance. Might be closer to the weekend before I get around to it. EDIT: I was actually planning on testing a bunch of voltages this from an old Marshall schematic with 17 test points and a voltage table.

Unfortunately, I don't have equipment to feed a signal into anything unless there is a DIY trick I can throw together.

Speakers are okay. I use this cabinet with other amps. It's a 1960BV.