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mushy Marshall. Used to be firm. Have changed tubes, re-biased.

gtrnstuff

Member
Messages
2,568
It's a 1987 50w reissue "plexi" from the early 90's.
Usually new power tubes will make it tighten up and sing. Not this time.
I always re-bias.
Tried swapping some known good 12AX7 types in all 3 holes.
Filter caps have been replaced.

The sound is similar to a fuzz face, actually. Like a tube stage is really saturated.


Any ideas, or is it time to admit I'm over my head and take it to a real tech?
 

Blue Strat

Member
Messages
30,243
Could be lots of things including non tube related items. Make sure the phase inverter tube is good. If one half is out the amp will break up really early. Same for all components in the PI.

What you need to do is measure all voltages and compare to the schematic. If this is out of your league you need to punt and take it to a pro.
 

gtrnstuff

Member
Messages
2,568
Voltages seem all normal, maybe little high but in expected range. AC line reads about 125 at the moment.

From Pin 1 ascending
V1 179 0 1.16 H H 234 0 2.2
v2 193 0 1.49 H H 342 193.6 195
V3 268 27.6 45 H H 249 29 45
V4 0 H 484 482 -44.9 484
V5 SAME as V4

Bias readings for V4 and V5: 36.2ma and 33.7ma at idle.
 

Blue Strat

Member
Messages
30,243
Voltages seem all normal, maybe little high but in expected range. AC line reads about 125 at the moment.

From Pin 1 ascending
V1 179 0 1.16 H H 234 0 2.2
v2 193 0 1.49 H H 342 193.6 195
V3 268 27.6 45 H H 249 29 45
V4 0 H 484 482 -44.9 484
V5 SAME as V4

Bias readings for V4 and V5: 36.2ma and 33.7ma at idle.
How does it sound at first? Still ok? Play until it gets mushy and check bias readings again.

Since you'll need new power tubes eventually you should just try new ones.
 

gtrnstuff

Member
Messages
2,568
How does it sound at first? Still ok? Play until it gets mushy and check bias readings again.

Since you'll need new power tubes eventually you should just try new ones.
This is a new pair of EL-34s, replaced after noticing the mushy thing this week.
Good advice on the bias check. I'll report back.
 

gtrnstuff

Member
Messages
2,568
Running it on my Furman regulated AC, plates are at 461v.
Bias at -41.7
current at idle V4 38.9 V5 36.8
same after playing 10 or 15 minutes, then turning amp volume off to keep it from trying to amplify noise, otherwise current reads in the 40-50 range.
pin 5 reads consistently where it's set, so I don't suspect bias supply

I heard a little bit of the bad distortion, adjusted bias about 1ma cooler and it's not sounding as bad now. It's never been this finicky before. I'm pretty sure I have been this finicky before :)

FWIW I have changed speaker cables, different cabinets, tried guitars with different humbuckers that I use frequently.

Could an OT sound or behave like this on the way out?
 

gtrnstuff

Member
Messages
2,568
Probably not.

Did you change BRANDS of tube?
For grins, yes. The new pair is Ruby EL34BSTR from Doug's.
I usually run =C= in this amp, and that is what just came out of it. I think my last batch of those came from Mike @ KCA. Both are excellent vendors, IME.

After reducing idle current about 1ma, it hasn't done the over-saturated thing, but I'm not convinced it sounds as good as it used to. Older recordings bear this out. I just don't remember 1ma sounding *that* different before.

On the preamp side I rolled several in each position.
 

big mike

Cathode biased
Platinum Supporting Member
Messages
13,681
Those are good tubes, but they are different than SED's. Doug and Mike are both great vendors. But that doesn't mean you like the sound of this tube as much.

Could be the deal.
 

gtrnstuff

Member
Messages
2,568
Those are good tubes, but they are different than SED's. Doug and Mike are both great vendors. But that doesn't mean you like the sound of this tube as much.

Could be the deal.
Well, no, the problem first showed up with the most recent pair of =C=. They are usually great, but tubes can fail.
And yes, OK, maybe the Rubys don't have my favorite tone, but the over saturation hasn't showed up again, yet. But it was there for awhile after first putting in the Rubys and biasing.

So I'm still trying to get an intermittent problem to stay long enough to fix it, or take it to someone who can.
 

RussB

low rent hobbyist
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11,157
Have you replaced all the electroltyics, including the bias supply caps and the cathode bypass cap(s)?

Have a close look at all the solder connections, especially the one's on the replaced filter caps
 

gtrnstuff

Member
Messages
2,568
Have you replaced all the electroltyics, including the bias supply caps and the cathode bypass cap(s)?

Have a close look at all the solder connections, especially the one's on the replaced filter caps
bias supply, no. I noticed those were original when I opened it up and made a note to order them.
And only one or two cathode bypass.
Yeah, I also wondered about cold solder joints, especially since I did them. A friend who used to be a TV tech would just go through an entire board, reheating solder joints if nothing else seemed to be curing a problem. And I did fix an intermittent Focusrite mic pre after a tech failed, by finding a bad solder joint.
 

gtrnstuff

Member
Messages
2,568
Haven't changed anything yet.
After a play test, the super saturation came on after 10 minutes. Gave it a rest of about 3 minutes. Started fine, then the sat came on again after about 10 minutes, then it CLEARED UP. Didn't mean to yell..
During this, I lifted the light end of the chassis about an inch off the case it's lying on and dropped it to see it anything popped or otherwise sounded like a loose connection.
Tapped all the tubes, filter caps. Nothing.

Well, at least I think I can now rule out a tube problem, since I have changed all of them since the problem first showed up.

I would think something heat related would stay after initial heating up and blasting at vol 7, instead of coming and going like that.

OK, so when the caps arrive I'll change all old e-lytics, check solder joints. Test again.
 

cap47

Member
Messages
2,276
Did you jumper the efx loop?
A dirty contact in the switching jack can cause problems.
 

gtrnstuff

Member
Messages
2,568
OK, update.
I re-did all the solder joints on the power tube sockets. Some old repairs looked suspect.
Played it over an hour, and no symptoms.
Sounds as firm, crunchy, singing as it ever did.
And it makes sense that an iffy contact on pins 3, 4, 5 or 6 especially would affect the operation of the power tubes, and would be influenced by heat build up.

I will probably go back to =C= EL-34, for that thing they do, but now the amp is usable again.
 
Last edited:

jcs

Member
Messages
8,093
Fwiw, in my 71 Smallbox 50, i prefer real Mullard El34 xf2's over most any tube but next would be RFT/Siemens EL34 followed by early Tesla EL34.

A 12at7 in the pi slot can sometimes work wonders as well.
 






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