Red Plating!

[Capulin Overdrive]

if anybody remembers, i've been messing with a 1987X modded by Dirk Baldringer.

anyways, went to put new tubes in it, and figured out it had a bad screen grid resistor.

replaced the bad resistors and got her fired up, and the thing will try to Red Plate with the amp properly biased up or even bias cold.

amp is also kind of wimpy, as far as volume and really has to be cranked to get it cooking, and then the red plate starts.

figure i'll end up going to a tech, but wouldn't mind trying a little more on my own.

the amp is modded like crazy, so stuff that i'm used to seeing isn't there or isn't were it was atleast. for instance it doesn't have the 5.6k resistors for oscilation on pin 5's of the power sockets. the sockets are grounded, but not with resistors. if it has the 56k resistors for the grid stuff, i can't find them. etc. etc.

thinking it might be Caps or a Cap, but don't know were to start?

anybody got ideas on what i could check on my own?

 

[Jimmie]

if anybody remembers, i've been messing with a 1987X modded by Dirk Baldringer.

anyways, went to put new tubes in it, and figured out it had a bad screen grid resistor.

replaced the bad resistors and got her fired up, and the thing will try to Red Plate with the amp properly biased up or even bias cold.

amp is also kind of wimpy, as far as volume and really has to be cranked to get it cooking, and then the red plate starts.

figure i'll end up going to a tech, but wouldn't mind trying a little more on my own.

the amp is modded like crazy, so stuff that i'm used to seeing isn't there or isn't were it was atleast. for instance it doesn't have the 5.6k resistors for oscilation on pin 5's of the power sockets. the sockets are grounded, but not with resistors. if it has the 56k resistors for the grid stuff, i can't find them. etc. etc.

thinking it might be Caps or a Cap, but don't know were to start?

anybody got ideas on what i could check on my own?

have you checked for proper voltages within the circuit?

 

[steve_k]

if anybody remembers, i've been messing with a 1987X modded by Dirk Baldringer.

anyways, went to put new tubes in it, and figured out it had a bad screen grid resistor.

replaced the bad resistors and got her fired up, and the thing will try to Red Plate with the amp properly biased up or even bias cold.

amp is also kind of wimpy, as far as volume and really has to be cranked to get it cooking, and then the red plate starts.

figure i'll end up going to a tech, but wouldn't mind trying a little more on my own.

the amp is modded like crazy, so stuff that i'm used to seeing isn't there or isn't were it was atleast. for instance it doesn't have the 5.6k resistors for oscilation on pin 5's of the power sockets. the sockets are grounded, but not with resistors. if it has the 56k resistors for the grid stuff, i can't find them. etc. etc.

thinking it might be Caps or a Cap, but don't know were to start?

anybody got ideas on what i could check on my own?

what are all the pin out voltages? this should lead you in the right direction....

 

[Capulin Overdrive]

can i check with power tubes in there, or do need to take them out like i'm starting from scratch?

 

[Capulin Overdrive]

i checked all the pins with tubes in and powered up, and i'm pretty close to what it should, with the exception that i can't read in MV's on pin 8.


it's like 460 volts where it should be, and it's 3.3 something volts where it should be. pin 5 is the negative figure like it should be.


could not having the resistor between pins 1 and 8 be screwing everything up?

 

[Jimmie]

i checked all the pins with tubes in and powered up, and i'm pretty close to what it should, with the exception that i can't read in MV's on pin 8.


it's like 460 volts where it should be, and it's 3.3 something volts where it should be. pin 5 is the negative figure like it should be.


could not having the resistor between pins 1 and 8 be screwing everything up?

I don't have a schematic in front of me, but isn't both pins 8 and 9 ground? What tubes are in there, 34s or kt77s? are they possibly toasted? I'd try a new or known to be good pair of tubes if you havent done so yet. I have a 2203 here doing almost the same thing, new tubes was the fix. If shes been run redplated for any amount of time, you might want to hit those ground points with a solder iron and some new solder. 34 mA @ 465 volts is a good goal to adjust for...hard to know for sure as to how "modded" the circuit is though...

Good luck!

 

[Capulin Overdrive]

3 deferent pairs of el-34 tubes, and all of them want to red plate.


only 8 pins on the power sockets. 1 and 8 are tied together and grounded with wire. usually they are grounded with a 1ohm resistor.


damn thing is driving me crazy!

 

[Greazygeo]

3 deferent pairs of el-34 tubes, and all of them want to red plate.


only 8 pins on the power sockets. 1 and 8 are tied together and grounded with wire. usually they are grounded with a 1ohm resistor.


damn thing is driving me crazy!

The 1 ohm resistors are there to check the bias. Does this amp have those? Stock, it should just have the two pins connected and grounded. If it does have the resistors, are they good?

Does it have diodes across the power tube sockets? They may be bad.

Does it have a ppivmv? If so, make sure it is installed correctly and none of the components have failed.

 

[Capulin Overdrive]

3 deferent pairs of el-34 tubes, and all of them want to red plate.


only 8 pins on the power sockets. 1 and 8 are tied together and grounded with wire. usually they are grounded with a 1ohm resistor.


damn thing is driving me crazy!

The 1 ohm resistors are there to check the bias. Does this amp have those? Stock, it should just have the two pins connected and grounded. If it does have the resistors, are they good?

Does it have diodes across the power tube sockets? They may be bad.

Does it have a ppivmv? If so, make sure it is installed correctly and none of the components have failed.

it doesn't have diodes across the sockets.


you may be onto something with Master Volume? have read about guys having red plate trouble after adding the Lamar over at Metro.

i don't know if it's ppivmv or not?

somewhere (most likely now! ) it's gonna be over my head and patience level.


it won't red plate at idle, but as soon as i start to play and increase volume the tubes slowly start turning red. does it on the dirty channel, or on the added clean channel, so guess that might let it be some other problem?

 

[Nigel Tufnel]

Post some pics man, I'll give it a look.

 

[wolfeman28]

Just look at the MV pot, if it's a single pot it's a standard MV. If it's a dual pot, you have a PPIMV.

So are you only getting the red platting when you turn the volume up, or are they red platting from the get go? What voltage are you getting for the bias(should be somewhere around -35 to -50)

Edit- Never mind, you say it doesn't do it at idle.

My guess would be you have a short somewhere, or some crazy oscillation.

 

[glpg80]

some crazy oscillation.

if it does not do it at all times this would be a likely culprit assuming the tubes are up to par.

they joys of electronics - half the fun is in fixing problems

 

[MrDan666]

Post up a pic and i'm sure one of us will spot the culprit!

 

[Jimmie]

3 deferent pairs of el-34 tubes, and all of them want to red plate.


only 8 pins on the power sockets. 1 and 8 are tied together and grounded with wire. usually they are grounded with a 1ohm resistor.


damn thing is driving me crazy!

The 1 ohm resistors are there to check the bias. Does this amp have those? Stock, it should just have the two pins connected and grounded. If it does have the resistors, are they good?

Does it have diodes across the power tube sockets? They may be bad.

Does it have a ppivmv? If so, make sure it is installed correctly and none of the components have failed.

it doesn't have diodes across the sockets.


you may be onto something with Master Volume? have read about guys having red plate trouble after adding the Lamar over at Metro.

i don't know if it's ppivmv or not?

somewhere (most likely now! ) it's gonna be over my head and patience level.


it won't red plate at idle, but as soon as i start to play and increase volume the tubes slowly start turning red. does it on the dirty channel, or on the added clean channel, so guess that might let it be some other problem?

If you have the patients to stay in there, you'll be wiser once you figure this out. Hard to armchair technical issues like these, but with every post, you're providing more of the puzzle and getting more familiar with the circuit.

Someone suggested posting some gut shots, that'll help a lot.

 

[Capulin Overdrive]

gut shots in a couple hours.

 

[Greazygeo]

3 deferent pairs of el-34 tubes, and all of them want to red plate.


only 8 pins on the power sockets. 1 and 8 are tied together and grounded with wire. usually they are grounded with a 1ohm resistor.


damn thing is driving me crazy!

The 1 ohm resistors are there to check the bias. Does this amp have those? Stock, it should just have the two pins connected and grounded. If it does have the resistors, are they good?

Does it have diodes across the power tube sockets? They may be bad.

Does it have a ppivmv? If so, make sure it is installed correctly and none of the components have failed.

it doesn't have diodes across the sockets.


you may be onto something with Master Volume? have read about guys having red plate trouble after adding the Lamar over at Metro.

i don't know if it's ppivmv or not?

somewhere (most likely now! ) it's gonna be over my head and patience level.


it won't red plate at idle, but as soon as i start to play and increase volume the tubes slowly start turning red. does it on the dirty channel, or on the added clean channel, so guess that might let it be some other problem?

Just look at the area the phase inverter is located. If there are extra wires and a dual pot attached to it, that's where you will find it.

Start by making sure that 1-8 pin connection is actually grounded with a good connection. I'd reflow those to be sure.

 

[Capulin Overdrive]

sorry for no pics yet.

i dicked off and didn't get them taken during the day, and went to take them tonight and my cam battery is dead. so i'll get some up some day.

 

[Black Cat]

Without knowing what was done to the amp, Mods, Hacking, etc... it becomes pretty hard to narrow in any potential problems. Red Plating under signal conditions can be the cause of more than just one thing. One scenario is that the load the output tubes see breaches the maximum dissipation rating of the valve over a very big portion of their operation. If this is the case increasing the reflected load, and biasing a bit colder may help you. for example If you are normally running a 16ohm cab, try selecting the 8ohm tap on the back (likewise 8ohm cab, and 4 ohm tap), and see if this cures the red plating.

Also how are you measuring your quiescent bias?

Be safe,

-Black Cat

 

[glip22]

You say they want to redplate. Sounds like you are getting some screen glow. It could be several things . I recently had a similar problem. Unfortunately for me it was my OT which was replaced and all is well. Mine didn't sound right either. Wimpy as you said and the tubes would glow like they were going to redplate under volume only. You could have a minor short or open winding in your OT that comes on under volume. Could be a leaky PI cap causing an imbalance in the bias under load causing one tube to draw too much current. The second tube usually follows and will redplate also. As others stated could be the master volume affecting the PI tube.

 

[Capulin Overdrive]

finally got off my ass and took some crappy gut shots. if see something holler, but figure it's going to tech.