Peavey 5150 - Low output volume - Page 1
Forum section: electronics / repair

author: mpetro
author_rank: Newbie
timestamp: August 11, 2025, 06:16:26 pm
content: Hey everyone, let me know if this is the right subforum for this topic or not - been watching Dave/EEVblog for years and have browsed the forum a lot for work, but only just registered today. For reference, I work in the electrical technology field (industrial/commercial electrical design, industrial controls/PLCs/automation) so I'm very familiar with circuits and testing procedures - although my testing/probing is almost exclusively the caveman sort with a Fluke 87V and not an oscilloscope - but the audio side of the electrical field is a relative mystery to me.This is a bit of a long story so please forgive the relatively immense wall of text that follows because I want to be thorough in detailing the troubleshooting I've done already.Almost a year and a half ago I bought a Peavey 5150 (of the block letter variety for those playing along at home) and for about 6-7 months it worked as expected, which is to say that with the post-gain knob (master volume) at 4, it would shake the walls and damage your hearing if you weren't wearing earplugs. Some time around last October, there was a sudden drop-off in volume - compared to my bandmate's Peavey 6505+ (which is almost entirely the same amp, just with a separate bass/middle/treble EQ control for each channel as opposed to a shared EQ on mine) I would estimate it wasmaybehalf as loud. I opened it up myself for a visual inspection and noticed that the conductor/connector of the ribbon cable that carries the power tube heater supply current from the main PCB to the power tube PCB showed signs of heat damage (brown, not black/carbonized) on the power tube PCB side. If you're at all aware of amp repairs, you might know that the 5150 (and 6505 and the variants of each) are notorious for having undersized heater conductors and connections, which can cause oxidation/burning/scorching/melting of solder on any of the components on either end of the ribbon connector - more on this later.I gave it to a local tech who eventually diagnosed that the contacts on the speaker output jacks were corroded, and once he cleaned those he measured that the volume increased largely, and he also replaced a few of the pots on the front which had the shafts broken off. He charged me a very low amount considering the amount of time he had it and the parts he replaced. When I got it home, it still seemed a bit quiet, but then I remembered that I had tried a spare set of power tubes that my bandmate had bought for his 6505+. He purchased what were advertised as Sovtek 6L6GC tubes from eBay, but when they arrived he realized immediately that they were fakes, but I used them while originally troubleshooting the amp. When I remembered this, I took a dB reading using a phone app with my phone about 2-ish feet away from the speakers and measured about 99dB. I left the phone where it was and swapped the power tubes out using an almost brand-new set of JJ 6L6GC tubes from another guitar amp that I had bought not long before I bought this one. Once I powered it back up I measured about 109-110dB, so a very noticeable difference in volume level. While I didn't do a direct comparison to my bandmate's amp, everything seemed good and it seemed to be back to its original power level. I took the amp on a small tour, and upon returning it seemed to be struggling to keep up volume wise again. I opened the amp up again and revisited the damaged heater supply connector, and decided to measure the voltage across the heater supply bus on the power tube board. I only measured 5.3VAC, where the tubes are supposed to be getting 6.3VAC - I was losing approximately 16% heater supply voltage, which roughly corresponds to 30% decrease in thermionic emission, and therefore output power. I made a poor attempt at reflowing the solder around the base of the pin that suffered the heat damage in the connector which seemed to have melted and left a gap/mote around the base of the pin and the solder itself was very dull compared to the surrounding joints which you can see in the attached closeup photo of the pins, but I don't really have a good soldering setup at my house. The joint wasn't ideal, but also wasn't bridged to the adjoining pins. After doing that, the heater bus voltage was measuring around 5.45V. I sprayed some contact cleaner inside the power tube end of the ribbon connector and worked it on and off the pins to try to clear any oxidation, and the voltage went up to 5.75V, and after doing the same thing on the other end of the ribbon connector, the voltage was reading around 5.88-5.90V.For future reference, the heater supply is taken from two yellow conductors terminated in a Molex connector directly off the secondary of the power transformer. Under load (Molex connector landed on the main PCB), I read ~6.5VAC on the two pins of the Molex connector, and without load (Molex connector unplugged) it was reading ~6.7VAC. The positive leg (for lack of a better term) goes from the Molex connector, through a 10A/250V ceramic fuse in a fuse holder and then through a trace on the underside of the PCB directly to the header pin that the ribbon cable was connected to. On the other side of the ribbon cable on the power tube PCB end, it lands on the corresponding header pin, goes through a solid link on the underside of the PCB to a beefy soldered trace that goes to one side of all of the heater filaments in the power tubes (either all of the pin 2s or all of the pin 7s). The negative leg goes straight from the Molex connector to the header pin, across the ribbon cable then another solid link to the other soldered heater trace that connects all of the corresponding remaining heater pins (either 7 or 2 depending on what the other bus is connected to).After discussing it with my dad (also an electrical/electronics professional), we opted to bypass the heater conductors in the ribbon cable and install hardwired 18AWG jumpers between the boards by soldering the wire (with heat shrink) directly to the header pins on either PCB - a fairly common repair for these amps. Part of this involved fixing my previously poor attempt at reflowing the bad solder joint on the heat-damaged pin. After cleaning that up and re-soldering it to establish a good connection, my dad installed the previously mentioned 18AWG jumpers across those two pins. After doing this, I was surprised to see that the heater voltage only went up to ~5.95V. After a bit of poking around and not seeing any visible issues, we determined that from the load side of the heater supply fuse to the corresponding heater bus on the power tube board, we were dropping about 0.26-0.27V, and read more or less the same from the neutral (for lack of a better term again) side of the supply to the other heater bus on the power tube board for a combined total of around 0.53-0.55V dropped from the Molex connector to the heater bus traces, which more or less makes sense when you take into consideration the supply voltage of 6.5V and the bus measuring 5.95V (plus or minus a few 0.01V on either end). In all of our poking and prodding, we can't find where that 0.55V is disappearing, as we measure from the few points along the trace on the main PCB, we may only find a drop of say 0.002-0.004V from point to point, and there's only a couple of those points between the Molex connector and the heater bus traces. I have seen some examples online where the PCB around the pins on the main PCB becomes scorched and becomes conductive causing voltage to leak/short between the pins, but the PCB looks perfectly fine underneath mine, with no hint of burning or scorching from what I can recall but I'll take a closer look next time I'm back at the bench.Again, sorry for the wall of text but just wanted to give a thorough history on what we've tried already. If anyone has any insight/experience on these amps in particular, I'd love to hear it. Photos attached below of the initial inspection of the heat damaged connector and pin. I'll try to find a decent top-down picture of the two boards and how they're interconnected and mark them up with some test values measured between specific points.Thanks in advance!EDIT: Just wanted to add that taking out the main PCB to inspect the underside isn't really an issue, but the power tube board is connected to the bottom of the power tube sockets by 8 solder joints per tube (32 solder connections total) and the tube sockets are riveted to the chassis, so that's a huge pain in the butt to look under. Later revisions/variants have the tube sockets attached with screws, so it's a lot easier to remove on those.

author: mpetro
author_rank: Newbie
timestamp: August 11, 2025, 06:55:38 pm
content: Here's a quick and dirty markup of the best picture I could find of the PCBs. The one I found is a 6505+ so it's not a perfect match - notably the heater traces on the power tube board are routed a little differently - but I think it's close enough, so I sketched it out to be the same as mine just for illustration purposes. To reiterate, the voltage drops measured from source to end of line on each end match up with the voltage measured across the bus on the power tube PCB (6.5V across source, 5.95V measured across heater bus, ~0.26-0.27V dropped from start to end of each leg of the bus, 6.5V-0.54V = 5.96V), but following point-to-point from the source along each leg I measure no appreciable voltage drop.

author: Swake
author_rank: Frequent Contributor
timestamp: August 13, 2025, 11:41:04 am
content: I'm an expert in swapping tubes that are not glowing. And that is about all I know about tubes, which is probably not even entirely correct.Anyhow.... herewith 2 versions of the schematics related to the Peavey 5150 and the 5150 Van Halen edition.

author: mpetro
author_rank: Newbie
timestamp: August 13, 2025, 12:06:51 pm
content: Quote from: Swake on August 13, 2025, 11:41:04 amYeah that's not much less than I know myself - I know in broad strokes how they work in isolation (I was going to say "in a vacuum") but as far as in-circuit, I don't really know much. I did already have those schematics, but appreciate you sharing them - the 5150_2 is actually the schematic for a 5150ii, which is equivalent to the 6505+ mentioned in my original post. Basically the same amp as mine, but where each channel has a dedicated EQ (bass/middle/treble) as opposed to sharing one. I've been working off of the 5150_evh.pdf the whole time, but again, a lot of it is above my skill level.Thanks for the reply!

author: MathWizard
author_rank: Super Contributor
timestamp: August 13, 2025, 01:39:51 pm
content: How much have any power resistors, or filtering caps drifted I wonder, and be lowering the 6.3VAC ? I have to remember tho this 1990's tech, not some 1950's tube amp w/ big brown resistors and paper caps.Have you tried inputting audio, preferably a constant volume and frequency, like 500Hz or 1kHz, and follow the signal path ? There's a lot of caps or other little things that could be going wrong, and result in loss of output volume.If you don't have any signal generator, you can record mp3's from youtube of test signals, or probably download 1 directing somewhere. Or you can make some basic BJT oscillator circuits, or use op-amps, or 555 timer IC, etc. Or use a keyboard.

author: mpetro
author_rank: Newbie
timestamp: August 13, 2025, 02:24:51 pm
content: Quote from: MathWizard on August 13, 2025, 01:39:51 pmHaven't gotten to the point where I'm tracing the audio signal yet, luckily I do have access to a signal generator - I've seen amp techs on YouTube input a 1kHz signal at a low P-P voltage (2V or less) for signal troubleshooting/tracing purposes.Possible that the filter caps need to be replaced, not exactly sure how badly they would affect the heater voltage as it's derived from a different tap on the PT, but there is (for some reason unbeknownst to me) a resistor tying it to the 24VDC+ output of a nearby diode rectifier circuit, so it's not exactly isolated from the rest of the amp either. Attached a marked up screenshot of the schematic showing this.

author: mpetro
author_rank: Newbie
timestamp: August 15, 2025, 12:16:22 pm
content: Hey everyone, just a quick and hopefully penultimate update - we ended up soldering a parallel run of 18AWG jumpers directly from the line side of the heater supply fuse and neutral terminal solder pad underneath the Molex connector to the heater bus trace on the power tube board. Tubes are now reading steady 6.3VAC on the heater trace/pins, haven't had a chance to test out the volume of the amp yet, will hopefully do that this weekend and compare it to my bandmate's 6505+.

author: David Aurora
author_rank: Frequent Contributor
timestamp: August 15, 2025, 01:27:10 pm
content: I work on these a lot, the ribbon cable issue is super common. Another very common issue is oxidisation in the effects loop return connector causing intermittent signal loss.Other than that they're usually pretty solid, it's not often I need to recap one.You do really need to do some actual diagnostics though (or have a tech familiar with tube amps do it). Quieter than another similar amp doesn't mean shit unless you're talking about plugging the same head into the same cab and using all the same gear. On the bench into a dummy load you can take a real power measurement and see whether or not it meets spec and work backwards from there.You also need to test all the tubes, check screen resistors and check bias. These don't have a bias adjustment pot, you need to play with bias supply resistors to bias them properly, so the vast majority of them out there are so mis-biased it's ridiculous.But yeah, they're a pretty straightforward amp, albeit a pain to work on with the rivets and stuff.

author: mpetro
author_rank: Newbie
timestamp: August 15, 2025, 02:00:43 pm
content: Quote from: David Aurora on August 15, 2025, 01:27:10 pmYes, I was originally comparing to the other amp through the same cab and speaker cable, same guitar and same patch cord. I'm not overly suspicious of the tubes as they have low playtime on them from the original amp they were in (brand new EVH 5150 Iconic bought in 2023), but if the volume issue isn't fixed then we'll be looking into the tubes. I hope we don't have to break out the oscilloscope and dummy load, but my dad does have a scope on his bench so it is something we can do if it comes to it.With regards to the effects loop, I did spray inside the jacks with contact cleaner and insert/remove a patch cable several times and it didn't make a difference - also tried jumpering the send to the return with no effect. If the jacks are oxidized beyond being cleaned in that manner, then I think I'd have to replace them because I think they're mostly enclosed in plastic.Thanks for the insights! I'll post again after I get a chance to compare volumes with the other amp. I know it's not super empirical, but I'll at least use the dB meter app on my phone to see if they're in the same ballpark.