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.

 

[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.

Okay, I finally got around to opening the amp up again. I am getting 428v for the B+, 425v before the resistor on G2, and 418v on the pin4 side of G2 (after the resistor). It's the same on all 4 power tubes. I thought these things were supposed to run closer to 460v on B+?

Anyway, I also checked a bunch of test points that were published on an old 2203 circuit. Perhaps not valid for the reissue if the B+ is supposed to be lower, but I was getting 20v-30v lower at all the higher voltage test points than what was shown as nominal on the schematic.


www. drtube.com/schematics/marshall/jcm800pr .gif (it wont let me post a link because I'm new. So I had to break it.)

1) 2.56v (nom 2.6v)
2) 230v (nom 250v)
3) 1.64v (nom 1.75v)
4) 194v (nom 210v)
5) 256v (nom 280v)
6) 265v (nom 290v)
7) 0.9v (nom 1v)
8) 155v (nom 165v)
9) 156v (nom 167v)
10) 301v (nom 330v)
11) 22v (nom 24v)
12) 195v (nom 220v)
13) 192v (nom 210v)
14) 23v (nom 25v)
15) 36.4v (nom 37.5v)
16) 36v (nom 36.5v)
17) 11v (nom 11v)

You did mention the reissues might be running at a lower voltage. Not sure if that explains some of these discrepancies.

 

[Jon Snell]

The voltages all look OK for the newer version.
Input a low frequency signal into the input of the phase splitter ... what do you get?

 

[cof_3pc]

The voltages all look OK for the newer version.
Input a low frequency signal into the input of the phase splitter ... what do you get?

You mean like a sine wave to check on an oscilloscope?

 

[cof_3pc]

Forgot to mentioned yesterday. I think I had erroneously stated that my plate voltage was 457v last week. I believe I was mixing it up with my early 90s 1959SLP. This 2203x is a 2017.

However, when I was running tests yesterday I noticed my bias seemed to have gotten colder. It was around 34mA when I checked it, which is about 58% dissipation, and I am almost certain that I had set it to 70% last week. I was unable to get it back to 70%. I had to max the VR1 pot out to even get it close.

Perhaps this is an indication that something has gone bad in the bias circuit? Caps maybe?

 

[Pete Farrington]

Perhaps the mains supply voltage (at your wall outlet) is running lower this time than it was last week?

 

[cof_3pc]

Perhaps the mains supply voltage (at your wall outlet) is running lower this time than it was last week?

I have a AmpRX Brown Box I use to keep the voltage consistent. When I bias my amps I usually set the bias so that it's somewhere between 65%-70% at an input range of 117v-122v. I had done that last week, and this time, at 119v, it was at like 58% and wouldn't even got back to 70% with the pot maxed out.

The tubes are new, but I suppose that doesn't mean they couldn't be the issue.

 

[NickKUK]

Ok, switch off an let everything drain - measure the resistance of the output transformer.

My thinking here is if low frequencies go then something is preventing that:
* low power reserves - filter caps are going
* bad tube response - duff tube not supporting current flow
* duff coupling caps shorting out - reduces capacitance
* bad output transformer - some windings have shorted reducing the amount of flux it can make
* shorting NFB - reducing the output
* duff potentiometers