Power tubes running hot in a 6505??

Bardagh

Bardagh

Well-known member
I have a 6505 where the power tubes are running inexplicably hot. To try and figure out what going on I added the adjustable bias with the usual 10k pot and a 6.8k resistor in series. I currently have a quad of nominally matched EH 6L6's in it and with the maximum resistance of over 16k these things are still running hot - ranging from 45mA to 38mA. It doesn't seem to be drifting as it doesn't really change from that at idle, but what could be the prime suspect here? I have some other 6L6 sets on order but none on hand, although it seems kind of nuts to me given how cold the 6505 is with the fixed 15k resistor that the tubes themselves could just run that hot.
 
Bardagh said:
I have a 6505 where the power tubes are running inexplicably hot. To try and figure out what going on I added the adjustable bias with the usual 10k pot and a 6.8k resistor in series. I currently have a quad of nominally matched EH 6L6's in it and with the maximum resistance of over 16k these things are still running hot - ranging from 45mA to 38mA. It doesn't seem to be drifting as it doesn't really change from that at idle, but what could be the prime suspect here? I have some other 6L6 sets on order but none on hand, although it seems kind of nuts to me given how cold the 6505 is with the fixed 15k resistor that the tubes themselves could just run that hot.
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what's your plate voltage ?
 
griff10672 said:
what's your plate voltage ?
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Well, turns out its a lot higher than expected. These are supposed to be around 465 but I'm measuring right around 499 (via the pin 3 solder pads). Or I could be misremembering shit I read a long time ago on the Peavey forums.

Also I've got about 123v at my outlets.
 
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Well it's not as easy as it used to be to find info because apparently all the search engines have somehow become massively less useful in what they return compared to a few years ago, but I see different things claimed about the plate voltage of these amps. I see people saying the 5150/6505 will typically show 465-475 volts but I also see people saying up to 500v is normal.
 
Did this start as right after the bias mod was put in? Most people mod them so that they can warm up the bias and at 500V, the range you are getting isn’t overly hot.
Maybe try a 25k pot if you want more range
 
So in standby I checked the heater voltage and it's approximately 5.6v which sure seems low. Checking the grid I've got approx -53v. The chilly grid measurement isn't surprising I guess considering I am actually running the bias colder even than stock right now, but then why are the tubes running so hot? Admittedly I'm a bit out of my depth.
Aynirar27 said:
Did this start as right after the bias mod was put in? Most people mod them so that they can warm up the bias and at 500V, the range you are getting isn’t overly hot.
Maybe try a 25k pot if you want more range
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No I put the bias mod in because I noticed tubes were running hot. I got this amp used a few years ago and noticed it was running tubes hot to begin with. Those tubes eventually blew and along with them the molex connectors for the heater wires burned up on the main board. I soldered them to jumpers that are on the board right before the connector and at the same time put in the bias mod so I could adjust the new tubes if need be. I mean these things notoriously run so cold stock sometimes that it sounds shitty unless you crank them up. But these are running so hot they sound like the opposite kind of shit if you turn it up - mushy and bloated.

Ideally I'd rather just find a better value of resistor to put in that would maybe bring the average tube to the mid 20's mA or something and take the bias mod out but not until I figure out what else is going on.
 
I’d check out the region of the schem PHV2-PLV1, namely PLV3. Looks like they have tons of rectification in that amp and I’m a bit out of my element. Your low heater voltage would probably be addressed in that as well. PHV2 would also likely hold some answers to your screen and plate voltages. I’d probably try another set of tubes before diving in that though. Best of luck man.
 
Dave M said:
With Vp of 500vdc, idle current between 45ma and 38ma doesn't seem bad on paper for a 6L6. I would consider it a wide swing for a set of tubes, tho.
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Stock, people often find that the tubes on these have idle current as low as 15mA (I think I've even seen someone with 12mA). When I first opened the amp up a little while after acquiring it and having noted how hot the tubes were running, I was actually expecting to see a bias mod or a non-stock resistor in the bias supply already in it, but it was stock. Then I thought maybe it was just a weird fluke of the tubes that it had in it at the time, which were some variety of Ruby Tubes and also were all close to 40mA.
 
Chubbs said:
I’d check out the region of the schem PHV2-PLV1, namely PLV3. Looks like they have tons of rectification in that amp and I’m a bit out of my element. Your low heater voltage would probably be addressed in that as well. PHV2 would also likely hold some answers to your screen and plate voltages. I’d probably try another set of tubes before diving in that though. Best of luck man.
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I'm not sure what you're referring to there. The only schematics I've ever been aware of for the 5150/6505 are these eyesores:

https://el34world.com/charts/Schematics/files/Peavey/5150_evh.pdf
 
Bardagh said:
I'm not sure what you're referring to there. The only schematics I've ever been aware of for the 5150/6505 are these eyesores:

https://el34world.com/charts/Schematics/files/Peavey/5150_evh.pdf
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Should have clarified I was looking at a 6505 50 watt schem which used different labels. So the general area is check according to that schem is J72-86 and more specifically J80 and J72. I have to imagine it’s something in that region that’s messing with your heater voltages and power tube current draw
 
Bardagh said:
Stock, people often find that the tubes on these have idle current as low as 15mA (I think I've even seen someone with 12mA).
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That's incredibly cold, except it's not incredible since shitty manufacturers set then so stupidly low to get past the warranty period.


Bardagh said:
When I first opened the amp up a little while after acquiring it and having noted how hot the tubes were running, I was actually expecting to see a bias mod or a non-stock resistor in the bias supply already in it, but it was stock. Then I thought maybe it was just a weird fluke of the tubes that it had in it at the time, which were some variety of Ruby Tubes and also were all close to 40mA.
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40ma at 500vdc plate voltage should be plenty fine for a good 6L6.
Are any of the tubes red plating? Any noises? Have you checked your bias voltage at various points? Have you checked your heater voltages with the tubes removed, or installed?
 
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