Peavey EVH5150 problem

[MarkusBass]

I have a problem with 13-years old Peavey EVH 5150. It was noisy - valves were probably never changed. I checked that 6L6 in the power amp are not so good as they should be. After replacing them with Russian equivalents the amp is still noisy. I looked at possible sources of the noise and found out that when the amp was new, there was most probably a kind of metal foil on the bottom of the enclosure, which disappeared somehow. I can see signs of a glue but there is foil any more. Can someone confirm (looking at the attached photo) that there should be a metal foil there? I know that Enzo knows Peavey amps very well, or maybe someone else can help.
The amp has several other problems, e.g. it has US version transformer so I have to supply it with 230V->117V transformer. This causes that the amp is not grounded but grounding it does not help much.
PS: changing preamp valves does not change anything regarding noise.

Mark

[Chuck H]

There almost certainly should be a shield there. I use rubber cement and heavy duty aluminum foil. Steel is better, but heavy and difficult to adhere even in the thinnest stock you'll find. Missing this shield can be a source of noise in a noisy location. But you can usually move the amp around and the noise will stop or at least change. Do install a shield, but I don't think this is your main problem.

Noisy is a sort of general term. Can you describe the noise? Is it hum? If yes, can you determine the frequency (ie:50hz or 100hz)? Is it buzz? If yes is it the same with both single coil and humbucking pickups? Is it hiss? In any case does the noise happen with AND without an instrument plugged in? Do any of the controls affect the noise?

[MarkusBass]

Thanks for the replay. I also think that there was aluminium foil. Maybe Enzo can confirm this.
Yes, when I move the amp (e.g. pick it up 1 meter above the floor and turn it) the noise decreases. The noise is a hiss but there may be just a little bit of hum (I can't tell the frequency because the hiss is so strong) and in this case I think that the hum is decreased.
All controls influence the hiss. The amp has two switchable channels (rythm and lead) and there are two volumes and one gain. With gain and volume on 12 the hiss is acceptable but when I turn the volume pot fully clockwise (gain still on 12), it becomes unacceptable. With volume pot fully counterclockwise the amp is perfectly quiet. Also equalization pots (especially treble) influence the hiss.
The problem applies to both channels only on the louder one (LEAD) is much more painful.
The tests were without instrument plugged in but when I plugged in just a short jack-jack cable (without instrument) nothing changed.
PS: can you see the signs of glue on the photo? This line in the middle.

Mark

[Chuck H]

The 5150 is a real high gainer and prone to hiss. Hiss is a byproduct of the sheer gain of those amps. a little hum is usually to be expected too. The amp almost surely won't ever be used with the controls full up as you are testing. Also, the lower impedance of a guitar pickup will reduce the input sensitivity. Since there is no change with a cord plugged in I suspect an input jack ground fault due to incorrect wiring, a bad jack switch or broken board connections that is leaving the input lead floating at a full 1M impedance. If you plug a cord into any amp and leave the other end floating there will be unacceptible noise at high gain settings. Try plugging in an instrument and turning the instrument volume to 0. Any change in the problem? The fact that moving the amp affects the noise some means that the missing shield is also a problem (and that your in a noisy environment). I would start by replacing the missing shield and looking for input jack problems.

[MarkusBass]

I added the shield (copper foil - see attachment). I check the input jacks but there is nothing wrong with them. They are soldered directly into the PC board. The amp has 78W both when the signal is injected into the effect return jack, or into input jack. It distorts easily. I check it with bass guitar and this is not the sound I like - any Fener sounds much better. But of course this is not a bass amp. I will check it withe owner of the amp. The amp has huge gain - 5 stages instead of typical 3 in other amps. Maybe this is how it is designed.
Thanks Chuck for help.

Mark

[Chuck H]

Actually, most amps have only two gain stages and then one for signal processing (ie:tonestack, cathode follower, etc.) So cascading four or five stages is A LOT more gain.

Copper is better than aluminum foil for shielding of course. So nice upgrade there.

That amp cannot be evaluated like any PA, bass amp or vintage guitar amp. Though it should have a "clean" channel that should behave more like you would expect.

If there is noise with no cord plugged in that is the same as noise when a cord is plugged into the amp but not plugged into anything on the other end then something is wrong with the input ground or the jack. Even if you can't see it. There could be a fault at a solder joint or PCB pad or a jack switch that is dirty or sprung loose.

[MarkusBass]

If there is noise with no cord plugged in that is the same as noise when a cord is plugged into the amp but not plugged into anything on the other end then something is wrong with the input ground or the jack.

Wow, this is a very complex sentence - let me think. The noise with no cord plugged in is kind of a hiss. With a cord plugged into the amp but without guitar there is a hum (much louder). So it seems that this is not the case mentioned above. The input jack are secured with a metal nut so I think that there is direct contact between the jack and the enclosure. But I may check it tomorrow. Maybe there is something on the bottom of the PC board. Jack switch is OK - I check is with my multimeter.

Mark

[Chuck H]

That sounds right then. I must have misread something you mentioned before.

Does the amp have a clean channel you can activate? If yes, is there more noise than you would expect from that channel for the amount of gain?

[MarkusBass]

Yes, it has. The clean channel is called RYTHM and when activated the hiss is only slightly more than in simmilar amp like e.g. Fender. This "to high" hiss is mainly on the other channel LEAD and with volume pot turned up above 50%.

Mark

[Chuck H]

The hiss is probably normal then. If the amp has been modified or otherwise messed with you could check the schematic for any top end bleeder type caps on the signal chain. Make sure they're in there and haven't been snipped out.

[soundguruman]

Well yes there is foil in there, that's for sure. Copper is probably better.
The "Illinois Capacitor" filters 100 uF X2 and the 47 uF X2 tend to vent when they get this old. The caps puff smoke, but afterwords, tend to keep working somewhat and don't really short out, they start to open . The 22 uF filters don't seem to puff at this age. The caps read perfectly good at room temperature, but then after the amp is heated up, they puff.
I have just been replacing those 4 with F&T caps.
Look at the + end of the (100 and 47 uF) caps, you may see some white residue that looks like dust, on the black insulator cap. It's actually fine white ash, that puffs out of the vent hole. Some Fender amps with Illinois Capacitors have been doing the same thing. It puffs smoke, but funny, the amp keeps working seemingly just fine.

[J M Fahey]

5150s *are* noisy, period.
They are known for that.
Quite normal, considering the huge gain.
My customers who use them, are always near the footswitch, to turn the lead channel off the very second they are not actually playing.
Sorry.

[MarkusBass]

5150s *are* noisy, period. They are known for that. Quite normal, considering the huge gain.

Thanks, this is exactly what I thought after looking at the schematic.
The capacitors look fine although they are 13-14 years old. Also the input jacks are OK.

Mark

[Enzo]

There is no clean channel. They call it Rhythm because it is not clean. The drawings may reference CLEAN but that is only in comparison to the Ultra and Crunch. The clean channel on this amp is only less dirty than the other channels. Not clean.

These do hiss a lot. Gain monsters usually hiss.

Bottom shield should affect hum, not hiss.

Peavey doesn;t use foil a lot, they have a conductive black paint they use inside the cabs. Paint won;t peel or tear or snag the chassis.

[soundguruman]

Both 5150s here in the shop have aluminum foil. One says "patent pending" on the faceplate. I think that was the first model year.
Later, they had aluminum sheet instead of foil, installed with staples. I saw that in the 5150 combo amp.
Hiss monster. Not my idea of a well designed amp. There is a lot of noise and intermodulation in the signal when the amp is working correctly.
As much as I would like to mod it to get rid of the noise, I have not designed that yet. It's scary.
The Illinois Caps do tend to puff smoke a bit. The 100s and 47s. Been using F&Ts to replace them. PV techs at factory advised that filter caps
are opening and causing oscillation in preamp, about 3 months ago when I talked with them.