6505 112 v1-v4 stopped working, but PI and Power Tubes work

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rhoadsv987
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6505 112 v1-v4 stopped working, but PI and Power Tubes work

Post by rhoadsv987 » Thu Feb 21, 2013 10:54 am

Hello, I have an issue with my 6505 112 combo that I hope someone can give some insight on.

A few days ago, I powered the amp up in standby mode and let it warm up. After switching out of standby mode, I was able to play a few notes before the sound faded out. I can hear a hum coming from the amp, but it sounds like is the internal components and not coming from the speaker.

My first thought was a tube went out. I looked in the back to diagnose the bad tube, only to find that the V1-V4 preamp tubes are not lighting up. However, the power tubes and the phase inverter tube are glowing and heating as normal. I proceeded to swap each preamp tube with a new one, to no avail.

I have some minor experience working on amps and electronics, so I downloaded a schematic of the 6505 and, in line with my thinking, the V1-V4 tubes seem to be on their own power circuit, seperate from the PT's and PI. I busted out the multimeter and started testing that circuit's components, but to be honest I have never had to attempt to find a bad compenent before, so I immediately got lost. My experience in repairing amps has always been on areas that I know exactly what is wrong, not in diagnosing a bad area that is unknown...

I wanted to see if anyone else has had a similar problem like this, or maybe if someone could point me in the right direction in where to start looking to find if a component went bad. We only have one amp tech in town, and he is rediculously expensive, so any help would be appreciated. Thank you!

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by Hemi » Thu Feb 21, 2013 11:57 am

Don't have the amp schematic handy, but I seem to recall Enzo saying the preamp tube heaters were connected in series. This means if one goes out, they all go out.

Get a known working tube try substituting it in each position until you find the bad one, or check for continuity in all the preamp tube heaters, your choice.

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rhoadsv987
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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by rhoadsv987 » Thu Feb 21, 2013 1:21 pm

Great, I will give that a shot tonight. I attempted that the first night, but theres a good chance my testing tube may not have been ideal. I have a new one, still in the box, that I will try this on. Thanks! I will post back if thats what it is, in case anyone else has the same problem.

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by Hemi » Thu Feb 21, 2013 3:09 pm

course there might be a fuse associated with it too :mrgreen:
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rhoadsv987
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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by rhoadsv987 » Thu Feb 21, 2013 4:03 pm

Right, I did find three internal fuses on the board in addition to the power supply fuse, but all 4 checked out...

I also checked some of the components that, on the schematic, come before the v-1 tube but after the power, all seemed to check out. I am learning a lot though :)

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by Classic30inCincy » Thu Feb 21, 2013 5:13 pm

Amp Warranty.....

Two Year Base Warranty

Three Year Extension With Warranty Registration

Five Years Total

Amp Eligible For Warranty ?

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rhoadsv987
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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by rhoadsv987 » Thu Feb 21, 2013 5:32 pm

Unfortunately I bought this amp used, from musicians friend. Was a good deal for $400, but now I am paying the price :)

And I just got home and tested the tubes. Everything checked out there. Would there be something before the heaters get power that may have shorted out?

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by Classic30inCincy » Thu Feb 21, 2013 5:34 pm

Amp might have white ceramic fuse that cannot be visually inspected.

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by Hemi » Thu Feb 21, 2013 6:24 pm

If the schematic I'm looking at is correct, those heaters aren't in series. The power circuit is just for the tubes. Have you checked the heater voltage is there? Perhaps the tubes are heating you just can't see a glow?

Basics first, check all those power supplies are up.
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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by Enzo » Thu Feb 21, 2013 6:29 pm

OK you have the 6505+ 112 COMBO, so when you said you downloaded the 6505 schematic, that means you are using the wrong drawing, unless what you actaually downloaded was the 6505+ 112 COmbo drawing. They are not the same amp chassis.


On the 6505, the tube heaters are indeed in series. But on this 6505 112, they are not. The power tubes and phase inverter run on common 6VAC, and yours are running.

The preamap tubes run on DC. They are all four in parallel, so one dead one does not kill the others. You have l;ost your DC heater supply. My first suspect is the 0.68 ohm 5 watt R82. If that opens, no voltage to the tubes. In many amps I worry about ribbon cables and other wires that carry heater current, but this amp is all on one board, so none of that.

Your DC supply comes from a transformer winding connected to terminals PDCH1, PDCH2. That AC is rectified by four diodes D18,19,23,24 right next to those terminals COnceeivably the AC is missing, but my money os on that resistor.

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by rhoadsv987 » Wed Feb 27, 2013 8:02 am

Sorry for the delay since you posted that Enzo. I meant to say I had the combo schematic, not the head. It matches exactly to what is laid out in side my model. I can understand the confusion :D

And you were spot on regarding R82. I tested it, and the other resistors around it, and this was the only one that didnt return what i was expecting. I had to order a replacement (we dont have any sources in my small town) and will post back once I get this switched out. Hopefully thats all it was, thanks everybody for your help!

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by Enzo » Wed Feb 27, 2013 7:58 pm

Great!

I don't know if my shop has a 0.68 ohm 5w either, not a real common value. But if you can find a 0.82 ohm or even a 1 ohm, it ought to work well enough.

Or get a pair of the very common 0.33 ohm 5w, stand them vertically in the two holes on the board, solder, then twist their free ends together on top and solder those. That results in a 0.66 ohm, which is certainly close enough to 0.68 for rock and roll.


Or just wait for the real part...

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by rhoadsv987 » Sat Mar 02, 2013 12:49 pm

Thanks for the offer Enzo! Mine ending up coming in earlier than expected. Swapped out the r82, but unfortunately this did not fix the heater. I may have tested it incorrectly starting out, cause today it tested fine, but I replaced it anyways on a prayer :)

I'm not sure where else to go from here. I agree that it is in the heater supply, but for the life of me can't figure out what is going on...

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by Enzo » Sat Mar 02, 2013 2:31 pm

Well, it was the prime candidate even if not guilty.

So start at the start. That 6vDC supply comes from somewhere. Look at the schematic. Low voltage AC comes onto the board at PDCH1 and PDCH2, and then goes to the bridge made of D18,19,23,24. So is there 5 or 6 volts of AC between those two posts? If not, you won;t get any DC out of it. Assuming it is there, then look again. The tube heaters are grounded on one side, so ground your meter to chassis, and probe the cathodes of D24 and D18, essentially the left end of the R82 on the drawing. Got 6vDC or so there? Or zero? Got the same voltage on the right end of R82 then?

It is not enough that 6vDC gets to pin 9 of each socket - pull some tubes and check - but also pins 4 and 5 of each must be grounded to complete the circuit. SO pull some tubes and verify zero ohms, or close to it, from pins 4 and 5 to ground. Unless all four tube heaters are burnt out, you just need to track down where this circuit loop is broken.

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Re: 6505 112 v1-v4 stopped working, but PI and Power Tubes w

Post by rhoadsv987 » Tue Mar 05, 2013 5:55 pm

Would it make sense to be getting a reading on r82 but not D24 or D18? I am not sure if I am just doing this wrong or what, but I consistently get the right voltage out of R82 and nothing from those two diodes...

If it helps, D23 and D19 are also getting a reading. Does this mean that the power is coming through PDCH1 and working through D23 and D19, but is not making it through to the other side diodes? My thinking is that the circuit is breaking after the D19 and D23 diodes, or am I looking at this completely wrong?

I really appreciate your help, I am still learning all of this and your patience is very helpful!!!

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