Marshall Artiste reverb channel sounding muddy

[Stratfordade]

I'm looking at a lovely old c1974 Marshall Artiste 2040, 50W. All looks highly original inside apart from the speaker output being hardwired to 8 ohm. EL34 anode voltages are a little low (365V but 420V on schematic) but the Normal channel sounds wonderful. Very clean and hifi which I understand is typical of this unusual Marshall.

The possible problem is with the Reverb channel which sounds much less bright by comparison, even with treble and presence maxed and minimum bass. I don't kn ow if this is just how these Artistes are, or if it's something I can correct.

I've tried a different V2 ECC83 but no difference (the Reverb channel has it's own input using V2 as preamp). What puzzles me is that V2b has a big cathode bypass cap which I thought would make than channel brighter? It's 320uF on schematic across the 820 ohm cathode resistor, but the cap in there (looks original Erie) is 47uF. There is no similar cap in the Normal channel V1 so I wonder why it's only used in Reverb channel?

Another possible cause of sounding muddier is that the cap on the anode of V2b is marked 0.047uF on schematic and actual 'mustard' component, but measures much higher 0.089uF out of circuit on two different meters. This cap looks like part of a high pass filter so maybe being high it's attenuating higher frequencies more? The 0.022uF cap connected to V1a via a 100K is similarly high reading out of circuit — 0.064uF.

Is it usual for 'mustard' caps to go high over time? Could the higher values be causing the loss of treble on the Reverb channel?

Schematic attached and photo of 'my' 2040. Thanks for any suggestions.

[Helmholtz]

What puzzles me is that V2b has a big cathode bypass cap which I thought would make than channel brighter?

No, the larger the cap value the more bass. To noticeably lower bass would require something like 0.68µ (680nF) here

Is it usual for 'mustard' caps to go high over time?

No.

Could the higher values be causing the loss of treble on the Reverb channel?

No.

What is value of the resistor between the 250pF and 0.1µ caps in the tone stack?
If it's 100k try a 220k here.

[Mick Bailey]

Another possible cause of sounding muddier is that the cap on the anode of V2b is marked 0.047uF on schematic and actual 'mustard' component, but measures much higher 0.089uF out of circuit on two different meters. This cap looks like part of a high pass filter so maybe being high it's attenuating higher frequencies more?

If you're referring to the coupling cap, it does form a high pass filter in conjunction with the 1M pot etc, though isn't intended as such and either way the corner frequency is too low to be of significance. Is the treble bypass cap on the volume control good? This is there to improve high-frequency response at less than maximum rotation. If it's open or missing you'll lose treble, more so at lower volume levels.

[Stratfordade]

Thank you for clarifying Helmholtz and correcting my mistake about the bypass cap. Maybe the channel is so much bassist just because of that 47uF bypass electrolytic?

I’ll check that tone stack resistor tomorrow.

[Stratfordade]

Mick Bailey thanks. I’ll also check the treble bypass cap. Great point too on corner frequency being too low to make a difference. Helps narrow down possible fixes. If all checks out as per schematic then it may be that the Reverb channel was intended to be less bright. Can’t find any original owner manual so hard to know what intent was.

[g1]

Maybe the B+ voltage is significant. Normally we see voltages that are too high due to modern line voltage. 365V seems quite low to me, compared to most amps putting out that kind of power. By the time it gets dropped all the way down to the preamp, I wonder what kind of voltage the preamp tubes are running at. Maybe that channel is more affected than the other.
Have you checked the idle current of the power tubes, or the amount of ripple on the main filter caps?
It could also be unrelated, but still a secondary problem that should be dealt with.

[Stratfordade]

Maybe the B+ voltage is significant. Normally we see voltages that are too high due to modern line voltage. 365V seems quite low to me, compared to most amps putting out that kind of power. By the time it gets dropped all the way down to the preamp, I wonder what kind of voltage the preamp tubes are running at. Maybe that channel is more affected than the other.
Have you checked the idle current of the power tubes, or the amount of ripple on the main filter caps?
It could also be unrelated, but still a secondary problem that should be dealt with.

Thank you. I checked idle current earlier today and it was a tad cool and unequal - 40 and 31mA so around 60% max dissipation on ‘hotter’ EL34. But that was with bias volts -32V and I’ve since adjusted it to the schematic -30V so the current will be higher now. I will check.

The preamp anode voltages I recall were pretty close to schematic but I don’t have notes in front of me. I will re-measure tomorrow with the higher power amp drain, and look at ripple.

Plenty to check! Thanks all!

[Stratfordade]

Checked my notes and the voltages I measured were:
V1 135/109 (schematic 140/105)
V2 124/97 (schematic 140/100)

V1 is normal channel, V2 reverb channel.

So V2a anode is lowest relative to spec, running more than 10% below spec. It may be even lower now I’ve wound up the power valve bias to spec.

[Stratfordade]

Had a look at the main filter caps and there's about a 10V p-to-p ripple, measured at the anode of one of the EL34s. Is this excessive?

The anode of V2b (Reverb channel) shows no ripple.

First photo is of the EL34 anode, second of the ECC83 V2b anode.

I've also found something odd with the grid voltages of the second triode in each off the preamp channels. Grid of V1a (Normal channel) measures -0.01V, but grid of V2b (Reverb channel, which has the muddy sound) measures -0.14V. What could be giving rise to this voltage, which would be reducing bias of V2b? Could this cause the muddy sound of Reverb channel?

[Helmholtz]

The ripple voltage in your scope pic looks more like 1Vpp?
Neither your low B+ nor the ripple have anything to do with the frequency response of the reverb channel.

[Stratfordade]

The ripple voltage in your scope pic looks more like 1Vpp?
Neither your low B+ nor the ripple have anything to do with the frequency response of the reverb channel.

Sorry the scope isn’t a modern ‘smart’ one so the displayed parameters don’t show I was using a x10 probe!

[Helmholtz]

Sorry the scope isn’t a modern ‘smart’ one so the displayed parameters don’t show I was using a x10 probe!

Anyway, if you don't hear ripple hum there isn't a ripple issue.

[Stratfordade]

It’s a nice quiet amp in that respect!

[Stratfordade]

I've found something which would definitely affect freq response of Reverb channel.

V2a in Reverb preamp has a 2.7K cathode resistor and a cathode bypass cap (250 something non-electrolytic — not clear on schematic). The 2.7K is present but I can't find any bypass capacitor connected to it.

Maybe this is significant?

[Helmholtz]

Try a bypass cap of 0.68µ.