Peavey Ultra 60 head producing squealing when it shouldn't

[HerniaFlange]

Hello! I was directed by a friend to seek advice on an amp repair job that I'm having trouble with. The only successful repair job I've done in the past is replacing blown capacitors and a bad power transistor in a Sunn Concert Bass head, so I took on this job to try and gain some experience with tube amps.
The specific problem is that the amp produces a loud squealing with occasional crackles and pops when set up the way the owner uses it (everything at 10). Adjusting the master volume knob can stop this, but anything above 12 O'clock produces this noise. He has to use it at max volume due to being the lead guitarist in the band but having to compete with a rhythm guitarist with a 400W head (!!!). As recommended by the people at Peavey, I plugged the guitar into the Effects Return jack to check if the noise went away, which it did. The next suggested action was to swap every preamp tube (which is a brand new set of 12AX7s) to see if there was a specific tube causing the problem, but the noise continued to occur. When talking to a friend, they asked me to measure the voltages of each of the preamp tube pins with the tubes plugged in and the speaker plugged in as well. I've attached the chart of results to this post, and I'll attach a schematic as well. According to them, the heater voltages look wrong. He told me to measure them as AC against Pin 9, and those were the results. Would a lower heater voltage cause this issue? I would hugely appreciate any help, as would my friend who would like to avoid having to buy a new amp even though he knows he really ought to. Thank you!

[alexradium]

in this amp the 4 preamp tubes are all in series,and its not AC but DC voltage,there should be 12V on each one,the main line is a dual 24V.
you measure between 4 and 5.

[HerniaFlange]

in this amp the 4 preamp tubes are all in series,and its not AC but DC voltage,there should be 12V on each one,the main line is a dual 24V.
you measure between 4 and 5.

Ah, okay! Remeasuring across pins 4 and 5 of each tube, I get:

V1: 12.83VDC
V2: 12.96VDC
V3: 12.16VDC
V4: 12.78VDC

So that doesn't seem to be the issue. I should also add that I had to replace C55, that was so hot at some point that the outer label was peeling off. Everything connected to it seemed to be okay though. I guess I should double-check that though.

[alexradium]

Ah, okay! Remeasuring across pins 4 and 5 of each tube, I get:

V1: 12.83VDC
V2: 12.96VDC
V3: 12.16VDC
V4: 12.78VDC

So that doesn't seem to be the issue. I should also add that I had to replace C55, that was so hot at some point that the outer label was peeling off. Everything connected to it seemed to be okay though. I guess I should double-check that though.

bad filter caps are one of the main reasons for squealing,you could replace them all in the preamp,they're all lowest quality.also check the resistors connected to them.

[Enzo]

In any tube amp, it\f it amplifies and makes sound, then teh heaters are working, regardless of voltage readings.

Before going nuts... Does it do this with no guitar plugged in? If so, then yes it is oscillating. If not, then plug a guitar in and set it up to scream. I assume it is now screaming. Turn the guitar volume to zero. Still screaming?? If it stopped, it is your pickups feeding back. IN a high gain amp, they can do this even at low volumes. also, while it screams try turning side to side - aim the guitar different directions. If that make ANY CHANGE in the scream, then it is pickups.

Amp knobs all go to 10 - or the equivalent on other amps - but that doesn't mean the amp will function well with them ALL on 10. You can't drive your car with the gas pedal all the way down all the time.

He doesn't HAVE to play too loud. If he can't be heard, he should mic it into the PA system.

Just my opinion here, but is he playing in a band or in a group of individuals all playing at once? As a band, it is everyone's function to create a cohesive sound. One guy playing a way too loud amp and ignoring the needs of everyone else is not an effective band member. Everyone in a band should cover a part of the sound spectrum. If there are three guitarists but all use the same amp with the same tone, then it all turns into one guitar sound, no depth.

[The Dude]

Not saying it is the issue. It could be many things. That said, I've run into this same problem several times on the Ultras and it's been connection issues on the reverb tank wires. There is a molex type connector where the tank plugs into the board. Try retensioning the pins in that connector and giving them a good clean. That will normally fix it.

Edit: I'm assuming this is a new symptom. If not, I would certainly expect some feedback with the amp dimed if the guitar player is in close proximity to the amp.

[Enzo]

There you go, an alternative scenario. A good focus here though is not to assume it is going to be a "bad part". it may be, but so often it is not.

[HerniaFlange]

Not saying it is the issue. It could be many things. That said, I've run into this same problem several times on the Ultras and it's been connection issues on the reverb tank wires. There is a molex type connector where the tank plugs into the board. Try retensioning the pins in that connector and giving them a good clean. That will normally fix it.

Edit: I'm assuming this is a new symptom. If not, I would certainly expect some feedback with the amp dimed if the guitar player is in close proximity to the amp.

...you mean the reverb tank I've been leaving unplugged while testing? Welp, that'll teach me to test things without absolutely every part of the system attached. I'll retension the connector sockets and report back after trying it with the reverb back in. And yes, it's a new symptom. The reason why he handed it off to me is that while it works while they're playing (I took it off of him after a gig he just used it for) it made practice a hassle because turning the guitar volume all the way down didn't resolve it, so he'd have to stand by the amp all the time.

Enzo: I've done a lot of that testing, and sometimes it would stop, and sometimes it'd do it with the exact same setup, which is annoying. I thought about just telling him to play with the master down (the cheapest fix of all!), but they're basically a Motorhead-worship band, which means that you aren't playing right if you don't have everything all the way up. Sometimes they play gigs without PAs as well (Elks Lodge DIY spaces, things like that). You're right that I shouldn't have assumed it to be a bad part at first, the fact that it was a new issue and that the amp is old enough that caps could start dying if everything is dimed made me skip a bunch of steps I should have done first. Ah well.

Alexradium: Could an electrolytic still potentially have a noise issue if it reads the capacitance it's listed as? I know I should just recap the whole thing, but he's trying to keep the repair job under $100 and it sounds like he wants me to leave in the new tubes I put in, which were $80. Maybe another time.

Thanks for all the advice so far! I'll let you know what happens when I can try some of this.

[Enzo]

The actual capacitance is probably the least interesting aspect of a cap. Your meter puts a volt on the cap, the amp puts several hundred volts on a cap.

If you suspect a cap, just clip another one in parallel with the existing one. That will tell you if it makes a difference or not.

I've done a lot of that testing, and sometimes it would stop, and sometimes it'd do it with the exact same setup,

Don't follow. Did the control on the guitar or the direction the guitar was facing affect this or not?

[HerniaFlange]

The actual capacitance is probably the least interesting aspect of a cap. Your meter puts a volt on the cap, the amp puts several hundred volts on a cap.

If you suspect a cap, just clip another one in parallel with the existing one. That will tell you if it makes a difference or not.



Don't follow. Did the control on the guitar or the direction the guitar was facing affect this or not?

Sorry, I should have been more specific. I did finally retest though, so that data's no good, sorry. I tried it again with the reverb tank attached after cleaning the contacts with emery cloth and tightening the socket contacts in the Molex-style connector. The amp still produced squealing with the volume above a certain level, and it did this with the volume knob on the guitar all the way down. So, I'll check that there's continuity through the contacts in that connector, but otherwise I'll have to sit down and pick out all the filter caps in the circuit to start trying to test them. This will probably take a while, so would it be better to comment in this thread again or to start a new thread explaining the issue again if it's not the caps?

[glebert]

Similar to The Dude's comment: This amp has several PCBs connect with cables and Molex type connectors? Have you cleaned all of those contact points? I have a Peavey Triumph (which I think is similar, yet different) that had this terrible hissing, and a little contact cleaner on one of these board interconnects is all it took to fix it. It was quite remarkable how big the improvement was. Maybe worth a shot (of Deoxit) if you haven't already.

[HerniaFlange]

Okay, things have gone downhill since the last time I posted. Based on suggestions, I cleaned the contacts of every internal connector I could find with electrical contact cleaner spray. I also noticed that a different capacitor (C56) seemed to have emitted some kind of burnt waste from the hole around one of the leads. Remembering what everyone said about it being for the best to just replace the electrolytics, I bought a few replacements and replaced C56, C57 and C301, then tested the amp to see if there was any change. Now the amp produces absolutely NO sound. There speaker reads the correct ohmage, the guitar is outputting, I double-checked that the caps are installed in the correct polarity and are contacting the rest of the circuit, none of the fuses are blown, and all I really did was use the contact cleaner and replaced those 3 caps.
Alongside this, checking the voltages on the tubes yield even more bizarre changes, as there is now no voltage when measuring across pins 4 and 5 of the 12AX7s, but measuring each against ground reads -27VDC. Pin 1 and 6 now reads ~500VDC on all of them, and 8 reads close to zero on all of them as well. It's looking like replacing the caps, making them work correctly again, popped something else. I'm going to read the schematic some and try to track down other spots that might have blown, but in the mean time does anyone have any suggestions? Thank you, you guys are awesome!

[HerniaFlange]

Okay, things have gone downhill since the last time I posted. Based on suggestions, I cleaned the contacts of every internal connector I could find with electrical contact cleaner spray. I also noticed that a different capacitor (C56) seemed to have emitted some kind of burnt waste from the hole around one of the leads. Remembering what everyone said about it being for the best to just replace the electrolytics, I bought a few replacements and replaced C56, C57 and C301, then tested the amp to see if there was any change. Now the amp produces absolutely NO sound. There speaker reads the correct ohmage, the guitar is outputting, I double-checked that the caps are installed in the correct polarity and are contacting the rest of the circuit, none of the fuses are blown, and all I really did was use the contact cleaner and replaced those 3 caps.
Alongside this, checking the voltages on the tubes yield even more bizarre changes, as there is now no voltage when measuring across pins 4 and 5 of the 12AX7s, but measuring each against ground reads -27VDC. Pin 1 and 6 now reads ~500VDC on all of them, and 8 reads close to zero on all of them as well. It's looking like replacing the caps, making them work correctly again, popped something else. I'm going to read the schematic some and try to track down other spots that might have blown, but in the mean time does anyone have any suggestions? Thank you, you guys are awesome!

[g1]

I can't make out the heater circuit on the 'schematic' but if they are in series, any bad connection along the way means none of them will work. Sounds like that may be your problem.

[HerniaFlange]

Okay, I figured out part of it at least: R301 and 302, 22 ohms each in parallel to make 11 ohms, which was between the filament pins and the +24VDC line, were both toast. The only 22 ohm resistors I had weren't high enough wattage and popped (they didn't have the wattage written on them and they were a similar size, so I figured it was worth a shot), so I need to buy some to replace those. The 24VDC line reads about 23VDC, so I"m assuming this is the last hurdle to getting sound coming out of the amp again at least.