Peavey 6505 Plus Combo B+ Voltages

[misterc57]

I am checking the health of a 6505+ combo I picked up. When I first got it the sound would fade out after 5 minutes on both channels. Even with a cable plugged into the effects loop. I went through the amp cleaning all pots, jacks, tube sockets, etc.

Now I have not seen it fade out but the clean channel does not sound clean enough to me. I have a lot more to try, like using a different pre-amp and output amp, etc.

For now I am trying to understand the B+ plate voltages. I have attached the schematic which does not show any reference voltages to check (that I could see). It shows a B+, B++, and B+++. Anyone know what VDC these should be?

Also I think I have values that should match but do not. Example

V2 pin 1 = 242 VDC, V3 pin1 = 325 VDC (these are both B+++ with a 100K plate resistor)

[alexradium]

these amps should be around 500V on B+,plates and screens.
On the preamp around 360/400V, in v3 its normal to have that 325V because its a cold biased triode,less current less voltage drop.
Check all the preamp plate resistors and screen grid power tubes,report all the dc voltages.

[misterc57]

Went to take more readings and I noticed all the per-amp plates had risen to around 440 volts. Then I realized my heaters stopped working on those pre-amp tubes V1 through v4.

V5, V6 and V7 heaters are working at 6.5 VAC.

[The Dude]

The schematic shows a separate filament supply for V1-4 from PDCH1 and PDCH2 of the transformer. Check for cold solder and/or connection problems in that supply. I've had this problem in the past and it was the actual connectors from the transformer to the PC board. Of course, yours may be different.

[misterc57]

I have 8.5 VAC at the base of PDCH1 and PDCH2. So this run gets rectified to DC and my heaters should be DC?

I have continuity from pins 4, 5, 9 to the cathode of D24 & D18 and the anode of D23 & D19.
No continuiy to any pins to the anode of D24 & D18 and the cathode of D23 & D19.

Looks like I have nothing coming out of the rectifiers D24 D18 D23 D19. I tried the diode to each in circuit but keep getting mixed and confusing results.

I have 15 VDC at U1 leg 1. So that rectifier set (D14 D4 D15 D5) is good.

Anything else to try before I pull the board up for a possible diode set replacement?

Thank you!

[The Dude]

I doubt it's rectifiers, although certainly possible. More likely a solder issue. Check the DC voltage to ground on both sides of R82. I'd believe the more likely scenario is a solder issue on that 5W resistor. And, yes, those heaters are DC.

[Enzo]

Don't confuse yourself. U1 and D4,5,14,15 have nothing at all to do with the preamp tube heaters.

FInd R82, 0.68 ohm 5 watt. With respect to ground, do you have about +12vDC on each end of it? On the cathode of D24?

You have 8vAC bewteen the two transformer wire connections? OK, do you still have 8vAC from anode D24 to anode D18? In other words right across the bridge. Having the AC at the wire posts does nothing if it doesn;t reach the bridge.

Do you have 12vDC across the bridge the other way, from anode D19 to cathode D18.

It is extremely unlikely ALL four of your diodes went open. Open on one of them is rare enough. Diodes usualy fail short far more often.

Do you have continuity from anode D19 to ground (chassis)?

It is important to always think in terms of circuits, not parts. You can change parts all day and it won;t help if the real problem is a cracked connection between them.

[misterc57]

No DC voltage to ground on both sides of R82. Well boths sides under 5 mVDC.

On the cathode of D24? No.

anode D24 to anode D18? Yes, 8.6 VAC

anode D19 to cathode D18? No DC, well 5.5 mVDC.

continuity from anode D19 to ground (chassis)? Yes at 2.2 ohms.

[The Dude]

If you have no DC voltage at R82, I suspect the rectifier is not getting AC. Back to post #4. Check those spade connectors from the transformer leads to the rectifier circuit. Check not only the connectors, but also check where the spade lugs solder into the board. I'll bet my lunch money one or both is cracked loose.

[misterc57]

I will pull the board up this weekend and check out all those solder connections in that section.

I am thinking that what I see now is the original problem. Amp would fade out after a few minutes of playing. The guy I bought it from said it would still work at the return jack. Makes sense if the pre-amp tubes are out and output tubes are still working.

[misterc57]

Spade joints looked solid. Diode solder joints looked questionable. Reworked those, also replaced one diode that looked odd when testing.

With no tubes installed I now have 10.7 VDC (with respect to ground) on the tube side of R82. Is that voltage high because no tubes are installed?

Thank you.

[misterc57]

That seemed to have been the problem. Amp is back together and working fine.

Once again, a big thank you for all the help!

[The Dude]

Small world! I got in one of these today with the same problem- no filament voltage for V1-4. I thought I'd post for anyone else who might have this come up. I resoldered the bridge diodes. Amp failed burn in. Replaced diodes (D18, 19, 23, &24) and it's been running most of the day. Diodes all check good out of circuit. Apparently not so good after they get hot. It might be a good idea to just replace them if you have this problem.

[alexradium]

Those diodes are rated 1A, they'll always fail with 4 tubes and a string of capacitors to charge, because at turn on filaments are cold and draw much more current ,it happens a lot on Marshall same way, even with 4 amps bridge.
Best solution is getting a bridge at 25A and Bolt that to chassis.

[The Dude]

That's a good point and frankly, I hadn't added up the filament current of 4 12AX7's....duh. So they use 1A diodes for a circuit that draws 1.2+ amps. Next time, I'll put in higher current rated diodes. Too late for this one. It's already out the door.