Marshall 4150 Bias current changes on half pair after 20 min

[kevania]

1) I have a marshall 100w amp with el-34's. I've had it since the late 80's, and used it occasionaly but never found it very inspiring tonally. Thought it might be a decent candidate to change into a 1959 with a metro amp board kit and an additional preamp tube (the 4150 only has two) but haven't got that far yet. I wanted to would check the basics first, such as proper output tubes and biasing.

2) Measured 451v on all plates, -42v at bias pins. Power tubes were a mismatch set of various brand el-34's (2 ruby's and 2 service master's which say made in w.germ). I haven't found much info on the 4150 but i have a schematic and it shows resistor values for el-34's are present in my bias circuit, not for 6550's.

3) Cathode resistor bias measure** with the old tubes was 21ma, 25ma, 9ma and 13ma. after installing a matched set of EH tubes, I measured 24-25ma on all tubes. i'm pretty happy as it seems all the measurements are much more consistent and i believe in the right range.

4) I did turn the bias current up to 28-29ma before pluggin in. It sounded much nicer than with the old tubes, more detail. No funny noises. Plate voltage dropped by about 2 volts after the bias current was adjusted but was still virtually identical accross each tube.

5) After about 15-20 minutes playing on half to three-quarter volume in very low light, I see that the right two tubes are running too hot and starting a small glow spot on the plates in each right side tube. The left pair does not. I shut down right away.

6) I backed off on the bias trimmer a little, Voltages have not changed but now the bias current on the new tubes measures 25-26ma on the left side and 10ma and 15ma on the right. Not too different than with the old tubes! I haven't had a chance to measure any resisters yet but none look burnt.

7) the amp appears all stock.

**I used a 1 ohm 1 watt resistor from ground to pins 1 and 8. I am not entirely sure pin 1 should be connected to pin 8 before going to ground through the cathode resistor but the metro amp 100w assembly manual showed it that way as a revision to the previous metroamp assembly manual and claimed it was more accurate this way.

Any thoughts?

Kevin

[billyz]

Swap the tubes around and recheck the bias current. If it follows the tube then it is the tube. If not check the 220k bias feed resistors to each pair.

[Firestorm]

Swap the tubes around and recheck the bias current. If it follows the tube then it is the tube. If not check the 220k bias feed resistors to each pair.

Yes on swapping the tubes to see if the problem stays put or moves. And check the bias feed/grid return resistors (but these probably wouldn't account for the problem unless they were seriously stressed.) If the problem stays with the sockets (doesn't follow the tubes), the most common fault in my experience is a leaky coupling cap betwen the PI and the output tube grid. This lets through positive voltage that swamps the negative bias voltage, letting the tube run away.

[billyz]

If the problem stays with the sockets (doesn't follow the tubes), the most common fault in my experience is a leaky coupling cap betwen the PI and the output tube grid. This lets through positive voltage that swamps the negative bias voltage, letting the tube run away.

Yes, that is a good one to remember.

[kevania]

It appears to be the tubes....which is dissapointing since they are a brand new "matched" quad. The bias current reading followed the tubes, even after I played at half throttle for 15 minutes or so.

After the test, I switched the tubes around so that the sum of the bias current readings on left is closest to that of the two on the right. Compared to the previous tubes the amp sounds pretty good.

I'll check the readings again after a few hours of playing and look at replace the coupling caps and feed resistors if it happens again. Btw, the two feed resistors measure 217k and 226k, with 226k on the side that got hot. I'm keeping a notebook on the amp now.

Thanks for the guidance...... Now, on to the 1959 conversion....

Kevin

[jaysg]

I will admit that I don't know much about those Club & Country models, however, they're from a decent enough era.

1) From whom did you buy the quad? Imo, they should go back.
2) If I understand exactly what you've done, then you are running the tubes pretty cold. For a B+ of 450V, I guess I'd expect to set the bias for about 35 to 38mA per tube across each 1 ohm resistor. I don't think you shouldn't have seen redplating spots.
3) I would recommend a quad of SED (=C=) tubes matched by Lord Valve or someone else with a good reputation.

[Colossal]

I will admit that I don't know much about those Club & Country models, however, they're from a decent enough era.

An interesting note about the Club & Country: Alex Lifeson used a pair of 4140 combos with a pair of 100w Hiwatt heads and 4x12s for studio work and live for Rush's Exit Stage Left/Moving Pictures tour in 1981. Many would argue that this was a great period sonically, if not the peak, for Alex's live and recorded sounds. Live versions of The Trees, Xanadu, Limelight, Jacob's Ladder, Red Barchetta, Freewill, and La Villa Strangiato were just <i>bone-crushing</i> using this setup. I would include Tom Sawyer in this list, but alas, it is so cliche' when mentioning Rush. Still, with the Howard Roberts fusion guitar, Alex's lead on that song is definitive prog!

See here: http://www.youtube.com/watch?v=JTD1QW3SM60 Quick glimpses of the rig are at 4:59-5:16, 7:03

I don't know how he used the 4140s, whether specifically just for clean or distorted tones, but effects included an MXR distortion, a MXR Microamp preamp, and Advanced Audio and Roland digital delays.

Regards,
Dave

[kevania]

I bought them from antique electronic supply but am hesitent to sling smelly stuff at them because i can't say with certainty that my amp didn't damage them. I purchased a few other things at the same time that seem to be fine. I will probably try Svetlana's next time. I have their 6550's in my 2104 which I am very happy with.

The amp still sounds way better than with the tubes that were in it before and the bias current seems to be stable now. What really confused me was that they measured fine when i check the bias the first time and thought they shouldn't drift 'that' much. I was really expecting something to be wrong with one side of the bias circuit......

This is my favorite period for Rush though I had no idea what amps he played. I bought the 4150 for about $175 figuring, since it worked, it probably had good iron and i wouldn't feel bad tinkering with it since it wasn't prized vintage stuff. It was an open back bass combo with 4 horrible sounding 10" "marshall" speakers, cheesy plastic hand grips and castor wheels which i ended up stealing to use on a 1960a cabinet. The chassis is very 800'ish but the layout is odd. Can't quite figure out why there are so many fuse. (PT tag says DE 4145, choke says drake t100 1471-263, cans are Daly's and OT is C272(?), last digit isn't legible.)

[Colossal]

This is my favorite period for Rush though I had no idea what amps he played. I bought the 4150 for about $175 figuring, since it worked, it probably had good iron and i wouldn't feel bad tinkering with it since it wasn't prized vintage stuff. It was an open back bass combo with 4 horrible sounding 10" "marshall" speakers

The 4140s are 2x12 100w combos (and what Alex used on Moving Pictures and live). They had high wattage Celestions in them (I believe). SRV also used a 4140 for clean tone in the 80s. I did not know the 4150 was a 4x10 until you mentioned it. Good luck sorting your amp out, sounds like the iron is good stuff and the price was right.

Dave

[Firestorm]

I'll never say anything bad about AES or CE Distribution; but I can't totally vouch for their tube matching services. I usually pay for it because it eliminates DOA tubes, but I don't think they burn the tubes in at all before they match them. A lot of new manufacture tubes don't settle in until they've been on at voltage for at least an hour, usually more.

If you have a good tube tester, you can buy a bunch at the volume discount, burn 'em in and match them yourself. Or you can call Willie/Lord Valve who does that himself, but for a price. He's got to make some money for it to be worth it, right?

[kevania]

Sounds like the Lord has a few disciples. Even with the mismatch, the amp sounds way better, so I don't feel to put out. Now at least, I can move on to the '59 project.....living and learning as I go.

Will probably post a pictorial of the '59 conversion project as I get psyched up for the NY amp show.

Kevin