Redplating Tubes in Dumble Clone

[bobshbob1]

Welp, another (hopefully not too expensive) lesson learned... Just made a headshell for an Overdrive Special-ish amp I recently built, and am having some problems with it.

At some point, I know, while moving the amp around for fitting it in the box, the bias adjustment knob got bumped out of position. While playing it for the first time inside the headshell, after about 5 minutes I looked at the power tubes and two tubes (same side) were redplating severely (almost orange-plating). Immediately shut the amp off and started checking things out. The bias voltage had gotten turned down about 10-15 volts, far enough that the two non-damaged tubes were drawing almost 75 mA each! Yikes! Got that turned down to about 30 mA each (485V on the plates so a little under 50% for 6L6s), and hoped for the best.

I'm still getting red-plating when the amp is pushed really hard, though (both gain knobs maxed, master around 5-6), on one or sometimes both sides. This is with only the two tubes that didn't more or less melt installed, and running an 8-ohm cab off the 16-ohm OT tap (correct match right?). I have tried using two known good tubes from another amp, with the same result. Haven't tried it with all 4 tubes in because (1) I'd rather not risk burning up any more, and (2) if the OT is somehow damaged I'd rather not stress it with anymore current than necessary. Since I get the same result with the two that were in it originally as with another good pair, I'm guessing the two non-damaged original tubes are still okay.

I played the amp regularly without a headshell for a month or so before this, using similar settings, and never noticed any red-plating, so I'm thinking something else has been damaged that's causing it to act up now. It only takes about a minute of heavy chugging riffs before I get some red in the creases of the plates, then I back off the volume and shut it off.

Things I've checked so far: plate and screen voltages are about 485v and 483v respectively, dropping to around 450 when driven hard. Bias is around -56v and holds steady when driven hard. Power tube cathodes are grounded through 1-ohm resistors, which read a current of 22-25mA at idle with this bias setting (I set it pretty cool hoping to keep the tubes from overheating). This current reading goes up into the 180mA range when the amp is cranked up and played through; I've only ever measured it on amps with no signal applied so I'm not sure if this is normal or not but it certainly seems awfully high.

I'm a little concerned about damage to the OT; it's a Hammond 1750R - 2k primary, and primary resistances are 14.5 ohms on one side, 15 on the other, no continuity to the secondaries. This is lower than I've seen on other OTs but I've only got a couple I can measure it on. I'm guessing the PT (Hammond 290HX) is fine since all the high voltages on the power tubes seem good/consistent.

Also...SCHEMATIC LINK

The amp I've built is basically this one here, only with 6L6s and 470 ohm screen resistors instead of EL34s and 1k's as on the schematic. Also I've eliminated the channel switching; my amp is just the overdrive channel with fixed resistors making up the second tone stack.

Now you know what I know, and I'm not sure where to go from here. The power section of this amp seems pretty simple, so I'm confident that with some help I can get it working as before. The amp still sounds great, just the power tubes are running too hot. Any advice on where to look from here would be much appreciated.

Thanks!

[gtrplayr1976]

The schematic is for two power tubes ,so the power section is the same just a duplicate ? Did you swap the tubes to the other side to see if it followed the tubes ? When you ran it with two tubes did you have two on the same side ,or did you use the two inside /outside ? If I'm not mistaken (unless these are wired different) you need to use either the two inside , or the two outside. What kind of speaker cable are you using ? There are many more on here that can help you better than I can. I'm just trying to get a little more info. Are the tubes matched ?

[g1]

This is with only the two tubes that didn't more or less melt installed, and running an 8-ohm cab off the 16-ohm OT tap (correct match right?).

You've got that backwards, so you are now quite mismatched. With 2 of the 4 tubes removed, your speaker impedance should be double what the OT tap is set for.
Do you have a 4 ohm tap you can use with your 8 ohm speaker? If not even running the 8ohm speaker off the 8 ohm tap would be less mismatch than the way you have it now.
The mismatch may be what is causing the redplating.

[bobshbob1]

Thanks g-one; I totally mixed up speaker impedance vs. OT impedance...sounds like I had a 4-1 mismatch going on...hopefully that's the cause of this issue and nothing is actually wrong except the way I have it hooked up .


Yes, I'm using two tubes in either the inner or outer pair of sockets, same effect either way, and they're pretty well matched. And it seems random which side will start to red-plate first, normally I shut it off when I first notice it but on one occasion they both started red-plating at the same time. I do indeed have a 4-ohm tap on the amp, and will use that when I get a chance to look at the amp tonight, and probably try it with 4 good tubes in at 8 ohms to be sure.

[bobshbob1]

Durh, looks like that was it. Played it loud for about 45 minutes tonight with no problems whatsoever.

Out of curiosity, are those mA readings from the cathode resistors worth looking at while the amp is being driven hard? They top out around 130-140mA on big palm-muted chords, and are steadily over 100 with normal riffing. That's a lot lower than they were when I was running it into the wrong impedance, but still seems to put the tubes at way over their maximum dissipation. Or am I missing something? Sorry to have to ask, it's just not something that's easily searched for on the net.

[gtrplayr1976]

Durh, looks like that was it. Played it loud for about 45 minutes tonight with no problems whatsoever.

Out of curiosity, are those mA readings from the cathode resistors worth looking at while the amp is being driven hard? They top out around 130-140mA on big palm-muted chords, and are steadily over 100 with normal riffing. That's a lot lower than they were when I was running it into the wrong impedance, but still seems to put the tubes at way over their maximum dissipation. Or am I missing something? Sorry to have to ask, it's just not something that's easily searched for on the net.

I think as long as the idle readings are correct the others don't matter. Idle readings are taken with controls at 0 ,and no input. When you plug in ,and increase the controls the numbers will go up ,but it's normal. More experienced folk can correct me if I'm wrong. Sounds like you have it though.

[g1]

I think as long as the idle readings are correct the others don't matter. Idle readings are taken with controls at 0 ,and no input.

This is correct, we are only concerned with the cathode current when idling with no signal present.
Although it may appear that the dissipation is increasing greatly when pushing signal through, as the current increases, the plate voltage decreases. To calculate the dissipation while signal is present, you would need to be accurately measuring the plate voltage with signal present. Best not to try this as there are dangerous high voltage spikes present.

[bobshbob1]

This is correct, we are only concerned with the cathode current when idling with no signal present.

Good to know, thanks. I guess I'm good to rock out then!