Blown screen resistors and more

[titser_marco]

Hi guys!

Been busy modding this crap Bassman I got and while it was working using the old GE 6L6GC tubes it had (which I presume were the stock tubes), I thought about putting in a quad of Sovteks from my Twin Reverb onto it and try out its sound.

Once I put it in, the screen resistors of 3 output tubes just lit up and burned, leaving the rightmost (when viewed from the rear) power tube's screen resistor intact. I also noticed that the last tube with the intact screen resistor lights up more brightly (which I think is the heater filaments) when I put the switch on standby. There was no sound after that so I put back the old GE tubes and there it was back again.

I then tried to measure B+ from ground to pin 3 of the tubes and I was shocked to see 559vDC on it. I know that most amp designs call for about 440-460vDC (at least the ones that I am looking to emulate i.e. D style amps) so I was bothered by this.

So this morning I put in a JJ quad that I've never used before and while it had sound, the last screen resistor burned out after I put the amp on standby mode.

I suspect that since the PT was already replaced by the previous owner that this is the cause of the problem. Today, I am looking to buy replacement 470ohm / 1W resistors to replace all four and I was wondering if there was a way that I could lower the B+ from 550v to about 450-460v without replacing the PT. My amp tech who usually works on my amps said that I could use a 5K/1W dropping resistor in series with the B+ supply as a start to lower the supply. Will this work? I read about Zener diodes being used, but I don't know what value to use and I am not sure if the local electronics suppliers in the Philippines would have them on stock.

Thanks and looking forward to hearing your thoughts on this.

[xtian]

I don't know your level of experience, but you need to SLOW DOWN! Wow. You're going to set that thing on fire.

Repair mode now. Don't try to start this amp again with power tubes in--pull them out and leave them out until we're good.

Replace all the screen resistors.

How old are the filter caps?

Get the bias voltage working at all pins 5 on the power tube sockets. Should be -50 or so vDC.

High voltage is not an issue when power tubes are removed. The voltage drops when the power tubes are drawing current.

Lots more, but gotta run.

[titser_marco]

I don't know your level of experience, but you need to SLOW DOWN! Wow. You're going to set that thing on fire.

Repair mode now. Don't try to start this amp again with power tubes in--pull them out and leave them out until we're good.

Yep, I took the tubes out. Minimal experience with tube amps --- this is actually the first tube amp I've done any real serious work on. My only other amp job was a recap for my Twin Reverb.

Replace all the screen resistors.

Yep, looking to do that tomorrow or later if I get the time to buy parts.

How old are the filter caps?

They're not stock as far as I know, but they haven't been showing any signs of leakage of or bloating.

Get the bias voltage working at all pins 5 on the power tube sockets. Should be -50 or so vDC.

Did a reading last night, and it hovers around -39vDC. Is that good enough?

Thanks for the help!

[xtian]

Did a reading last night, and it hovers around -39vDC. Is that good enough

Maybe not. What Bassman is this? Got a pot for bias adjustment? You'll want to see what the most negative voltage the bias system can make. -60 is better than -50 for startup. You don't want your power tubes to run away and redplate on you.

Only other reasons I can think for spontaneous screen resistors burning is bad tubes and, maybe, OT shorted? Best to measure resistances of OT primaries to check.

[roberto]

If you are running 6L6s with a B+ of 550 V, -39 V is not enought on the grids. I'd start with -60 then going up to something around -50.

But the problems could be more than one.

[titser_marco]

Did a reading last night, and it hovers around -39vDC. Is that good enough

Maybe not. What Bassman is this? Got a pot for bias adjustment? You'll want to see what the most negative voltage the bias system can make. -60 is better than -50 for startup. You don't want your power tubes to run away and redplate on you.

Only other reasons I can think for spontaneous screen resistors burning is bad tubes and, maybe, OT shorted? Best to measure resistances of OT primaries to check.

This is a Bassman 135 that was reconstructed by someone and it was already converted to a bias pot setup from its old bias balance config. I will measure bias voltage range as I turn the pot and get the min and max voltages. I can do this without the power tubes, right?

[pdf64]

I will measure bias voltage range as I turn the pot and get the min and max voltages. I can do this without the power tubes, right?

Yes, the amp mustn't be operational when you do this, as the power tubes may red plate at low bias voltage settings.
So either with no power tubes, or in standby mode.

[Stevem]

When you get back to the point of having the amp work please note this about Sovtec tubes.
Many Amps come with and or folks replace there 6L6 outputs with Sovtec 5881 tubes and besides the fact that these are really not 5881s by any strech of the imagination they sound like brittle crap when they are driven hard into clipping and there low end sound stage is less that what a 6L6 type should have.
Even the China made 6L6 tubes sound far better these days although there life span is still shorter than a Sovtec.
My recommendation is JJ brand 6L6GC outputs, or there better sound stage matched Groovetubes 6L6-S , which by the way is modified JJ which has been rebuilt from a 30 watt tube to a 35 watt tube which really helps out in these Fender 70 and 135 watt amps with there 500 volts of B+ on the plates!

[titser_marco]

Well I do have a JJ quad and when I put it in, it eventually blew out the last remaining screen resistor while on standby mode. I think that the real problem here is the huge B+.

[Stevem]

You have huge B+ due to having none , or only some of the output tubes working!
When each output tube is at least idling it will pull down the B+ voltage by 10 to 15 volts or more dependant on if the amp is biased at the minimum 50% or 70%.
You can at least test your output tubes for standing shorts with a ohm meter, between pins all the pins on the tube the only two that should have continuity are 2 and 7, if pin 3 is shorted to 4 you will burn up those screen resistors, any other shorts will blow the fuse.
While trouble shooting this issue if you have not made a load limit lamp I would at least derate the amps fuse by 25% to ease that load on the PT if things continue to smoke out on you!

[titser_marco]

I plugged the Sovtek back into my other amp (Twin Reverb) and they all seem to be working. I dialed in a cold bias (about 30mA at 475v B+) just to be safe. So my guess is that it isn't the tubes per se. The screen resistors burned when I put them in the Bassman. My theory is that the original tubes were an overspec'd set that tolerated the higher B+.

I've dialed in -50v on pin 5 now, and will probably replace all four of the burnt screen resistors between tomorrow and the weekend. I'll keep it at -60v and see how it works with the GE set.

If that works, I'll do a swap again with the Sovteks and see if they still burn up. If they do, I guess I'll really have to find a way to reduce the B+ down to 450-470v.

[Stevem]

The GE tubes will take the 480 to 500 volts that these Amps run at , but they may not stand up to the vibration of 135 watt amp!

When The GE tubes dryed up Fender had Sylvania develope a more rugged 6L6 tube to handle these 70 watt and 135 watt UL output stage amps.
The amp will sound like crap with a very cold bias voltage of -60 and that will only make the plate voltage soar!

[titser_marco]

I guess then, I'm left with two options

A. Lower the B+
B. Get tubes that can take the high B+

For Option A, I can think of the following:

A1. Use a dropping resistor to drop the B+ to about 440-460v.
- What value should I use? What wattage? Can I just put it just before the standby switch?

A2. According to some sites, I can also use a Zener diode.
Again, what value should I use? I saw some using 5W Zeners from CT to ground, but I was wondering what voltage rating the Zener should be. And could someone confirm where exactly to put this? And are there any disadvantages to this method? My local tech said that this generally doesn't work for high-voltage environments and that I risk having high voltage swings.

A3. Get a new PT.
Getting a new PT is definitely my long-term solution, but I would want to be able to use the amp while waiting for the new PT.


B1. Get KT66 tubes.
Heard that they were essentially more robust 6L6GCs but I wanted to verify this.

B2. Get KT88 / 6550 tubes.
Given the high B+, I looked around for datasheets on these tubes and it seems that they may be able to take the high B+. Do I need to do any mods before putting these in?

Thanks!

[titser_marco]

Was doing some research and found this discussion somewhere else:

http://www.fenderforum.com/forum.html/searchthreadgateway.php?topic_number=740085

Based on this, it seems to be that a way to alleviate this problem without having to lower B+ is to use a higher screen resistor (1k instead of 470ohms) but still keep it at 1w so that the resistor gives instead of the tubes.

Thoughts?

[Stevem]

I am having a hard time understanding your logic here!
If you have made what I concider the better choise out if the things you listed, that being replacing the tubes, then why go with even more expensive 6550s, or KT88 outputs when JJ brand 6L6GC tubes will work out just fine.
They can take the plate voltage, and are rugged !
If you do not want to go this route then there are new 6L6 tubes listed as the STR series of tubes.
That stands for special tube request and referes to the better built tubes tgat Slyvania made for fender just for these amps .
The only thing is that these tubes are Russain made and I can not vouch for how they sound nor how well they truly hold up!

kT66 outputs are 30 watt tubes just like most 6L6 tubes other than the Groovetubes/ JJ 6L6-s which has been up rated to 35 watts by a manufacturing change.
You will loose some output power if you run a 6550 or KT88 but the amp will be
The KT66 will handle more plate voltage, but I feel that most of the tube failures in these amps is not due to the 25 volt increase in plate voltage over the blackface amps, but like I posted vibration!

Last but by no means least is the condition of the output tube sockets. If they have arced over and left a carbon track behind you will be blowing up new tubes right off the bat, so be sure to take a good hard look for signs of that.