Refurbishing a Peavey Artist 240

[olliu]

Hello, first time posting here. Greetings from Finland.

I bought a 1976 Peavey Artist 240 and it's in the need of some care and maintenance. Solid state preamp & phase inverter and 4 x 6l6GC. It is nominally spec'd for 120W @ 4 ohm load and has a 2 ohm tap for use with a parallel extension cab. Currently it has a 8 ohm speaker, but the manual states 90w @ 8 ohms so I guess it should be fine for now. I have a little experience working with tube amps but I have the feeling I'm on foreign waters with this one.

Issues/thing to do I have noted so far:
- The master volume pot is busted. Makes loud pops just breathing on it. The pots are nasty plastic shafted ones so I think I'll swap them all out. They are pcb mount with unusual staggered legs so solder lugged ones & wire may be the way to go.

- Phase inverter filter cap is bulging (300V node, the black cap next to the orange one). Recap the power supply (more on that later).

- Power amp wiring is a horrible rat's nest. Neaten it up, especially the heater wiring. One side of the heaters is grounded. Add an HT fuse.

- There are no screen grid resistors. Each pair has a 100R 5W between the screens but the supply comes straight from the PSU node. (I've heard that the 100R's are to keep high frequency gremlins out?)

- BIAS! I don't have a bias probe and being a quad output, the familiar way of measuring voltage drop over the OT secondary resistance doesn't really cut it. Signal grids read -65V idle and it's not redplating so maybe it's fine for now.

- Voltages all around are higher than what the schematic says. The amp is old enough that I think it was made for 220VAC, but the wall voltage has since been upped to 230VAC. (At the moment it reads 237VAC). The nominal voltages are 500V, 490V and 300V. On idle I get 534V, 528V and 280V. The heaters sit at 7VAC.

- The power transformer core stack seems to droop to one side, as if the amp was dropped on it's corner and the laminations slipped a bit on each other. Is this a concern?

- Make sense of the input wiring / channel switching and make a footswitch

- Once the amp is otherwise healthy, tinker with the preamp. An FX loop would also be nice.


The schematic given the available manual (in addition to being nearly illegible) seems to be for the later revision with pcb mount tube sockets. In it the screen grid filter caps are two 100uF 350V in series plus equalizing resistors, identical to the reservoir cap arrangement. In mine, it's a single 20uF 600V (the orange cardboard cap). I'm not gonna pay half the amp's price for such a cap. I'm thinking of modifying it to match the later revision, two MOD 350V caps back to back should fit on the board. On a cold start with standby off the voltage at the cap reads ~560V, settling to ~530V once the tubes start to conduct, so a single 500V cap won't do.

Since the heater voltage is a bit high and I'll be redoing the wiring anyways, I think I should swap the pilot light from a mains one to a 6.3V one. Adding an artificial center tap should bring the voltage down even more. The current pilot light is a 125V one wired with one wire on the mains switch and the other on the junction of PT primaries which makes it physically impossible to twist the wiring neatly.

So, what would you guys recommend for the course of action?

- Olli U.

Link to the manual/schematic:
https://www.manualslib.com/manual/121177/Peavey-Artist-240.html

[PRR]

Welcome.

So you don't even know if all the tubes are conducting? If 1 or 2 are dead, that could be the high idle voltage.

The grounded heater circuit is fine. There's no low-level heaters here. The signal level at the 6L6es is SO high (over 30V) that heater-hum won't happen.

I don't see how artificial heater CT reduces heater voltage enough to care. Another 0.032A on top of the 3.6A is 1%.

If there is a 240V primary it should be used. Or get a 12VCT or 24VCT 1 Amp bucking transformer against the primary.

Has anybody ever figured out that input switching maze? I've seen it discussed but it never made sense to me.

[Latole]

Build you own bias probe. it is a must.
Read bias across 1 ohms cathode resistors to ground on each power tubes.

You will know how each power tube conduct.

[olliu]

I measured and the PT-side pair draws 56 mA total and the OT-side pair 52 mA through the transformer. Plate voltage was 538 V. IR thermometer pointed at the plates seems to show that the far OT-side is running hotter than the rest, ~200 C whereas the others are ~150 C. And I just noticed that the hot tube seems to be redplating after all.

The local toob-amp supply store has a DIY bias probe kit but it is out of stock 

[thetragichero]

let me advise against changing out all the spider pots on the peavey because then your preamp board likely won't have nothin` to hold it to the panel (edit. not the spider pots i'm used to seeing in the heads but i still would use board mounted pots)

peavey pots seem like good enough quality. take cleaning well, can take em apart, etc

[olliu]

Update: The mains transformer is now set for what I believe to be 240VAC. It was a matter of swapping one wire. With 238,5 V wall voltage I'm getting 490V B+, 487V on the screens, 261V on the phase inverter and 6,4 VAC on the heaters. I had swapped out the bulging PI filter cap since I had a F&T one at hand.

The hot tube is still slightly redplating and the bias pot is cranked. Guess I'll have to loosen the purse strings and pay up 140€ for a new quad.

[shooter]

I've seen it discussed but it never made sense to me.

looks like my pinball machines ladder logic, think I'll go back outside and play in the poison Ivy