Mig-50 PI type parasitics after a few hours

[tommy d]

I have an early Mig-50 with quality tubes, correctly biased and generally well taken care of. The amp doesn't blow fuses and sounds fine until I've given it a thrashing and then it begins to get "raggedy".

What I haven't done is replace the e-lytics and rectifier in the p/s. How big of a rectifier do I need? Is 8A ok?

Any input is appreciated.

Tom

[tommy d]

Ordered the PS e-lytics

I'll update with more info, as I test it.

[tommy d]

Throw me a bone...

Ok, I replaced 4 x 47uf 350V and used a variac to adjust the wall voltage to 110V. My mig-50 is one of the old ones with the 110/220 transformer. The problem persists and I am a little beffudled.

The amp shows some heat damage on a few of the resistors but they test ok. I am assuming that the heat is probably from being overvolted running 120+ on a 110 P/T.

Is there any reason why I shouldn't just dive in and rebuild the board with nice components that I know work and sound good to me?

[MWJB]

"Is there any reason why I shouldn't just dive in and rebuild the board with nice components that I know work and sound good to me?" There's one good reason, you haven't idntified what is causing the issue...you could rebuild the board to no avail.

You say the amp is correctly biased, what plate voltage & current do you have? The tubes get hottest, quickest...I'd want to be sure that I had eliminated these.

How do the dc voltages look? Give the amp a thrashing, when it plays up recheck dc voltages, a hot resistor pulling down voltages may be found?

[gomez63]

Have you monitored the critical voltages over a period of time? The stock transformers are prone to being faulty...add the aging process and you should be seeing fluctuations/inappropriate voltages somewhere. Check the voltages on both sides of those burned resistors.

[tommy d]

P/T = 110V 50hz

Ok, things are starting to make more sense. The P/T was designed for 110/220V 50hz. I have been running it on a Variac @ 110V 60 hz and after ~ an hour of crushing usage, the tone begins to fade rapidly.

Next time I jam, I will reduce the variac another 10-15 volts to reduce the RMS the p/t sees, rebias and give her hell. When and if the problem reappears I will re-measure voltages on the plates and rails, the PI splitter resistors.

Thanks!

[Jazz P Bass]

Variac

Why are you running the amp through a variac?
If you lower the AC mains by 10-15%, think about what that is doing to your heater voltages.
Sounds to me like the amp is biased hot and when run hard it cannot cool off.
Try new output tubes.

[tommy d]

Power tubes are new and re-biased when I hooked it up to the Variac. I have run it once on the Variac and twice on the wall voltage and I get the same result. According to New Sensor, the earliest models had the wrong p/t's. This is why I am running the variac, trying to find a compromise between a bad design and a usable amp. I don't mind taxing an amp...and variacs are just tools and not the devil. Tube amps, in my experience, run just fine +/- 20% all over the circuit. I don't sweat the small stuff.

[MWJB]

Tommy, you say that the voltage via regular wall AC is high...but you don't say what it is? You say the amp is biased correctly, again, no figures?

Don't keep reducing the AC with the variac, it'll screw your tone. Is the variac of an adequate VA rating?

"Tube amps, in my experience, run just fine +/- 20% all over the circuit. I don't sweat the small stuff." No tube amp runs +/- 20% on voltages, that 20% relates to parts tolerance, 20% tolerance on a power supply dropping resistor, for instance, equates to an almost imperceptible change in actual voltage & tone.

[tommy d]

I am trying to track down an 8 ohm power resistor so I can bench test and collect data points over time. I recognize that I am limiting the ability of others to help without data and appreciate the effort. I am currently on a six month contract job in MI and left my load and a bunch of other diagnostic equipment in TX.

The wall volatge is 122V but for the life of me I can't remember the plate voltages. Like I said, I just use the Weber Bias Calc to set for mA. I use SRS bias tools for PV and bias current. The variac is a big one: 10A 120/140V by Staco.

Give me a couple of days and I'll be able to provide the data what we need.

Thanks for all the comments, very valuable to me as I troubleshoot an amp that I alternately want to fix and chuck against the wall.

[MWJB]

"Like I said, I just use the Weber Bias Calc to set for mA" If you have biased 6L6GC to 70% dissipation, with a high plate voltage, then DON'T.

Most current production 6L6/6L6GC are only around 25W max dissipation, 15W per tube in this type of design is plenty (say 28-30mA assuming 500-525vdc on the plates?).

[tommy d]

"Like I said, I just use the Weber Bias Calc to set for mA" If you have biased 6L6GC to 70% dissipation, with a high plate voltage, then DON'T.

Most current production 6L6/6L6GC are only around 25W max dissipation, 15W per tube in this type of design is plenty (say 28-30mA assuming 500-525vdc on the plates?).

Thanks MWJB, I'll try the 15W setting for bias and run it through its paces checking voltages and heat related resistance drifts. I mostly use JJ, SED and old RCA and have never had a problem but the tubes in the Sovtek are the TAD GE small bottle reproduction and perhaps they just can't take the heat.

More info Saturday night...

[tommy d]

bad filter cap, high PV

Test results:

The resistors tightly matched Bruce's schematic even after running an hour with one exception: the post PI 82K is a 67K in the amp and is also thermally stable. (two mig-50 schematics one by Bruce Collins and one by Sovtek from '95)

I was chop sticking around and found another bad PS filter cap, the PV before the change was 550V and after was 575. Changing this cap got rid of this fantom rumble that happened occasionally. I have not replaced the 220uf filter caps because I can't find a 30mm mounting ring. Also the 1500uf/150v cap by the bias pot is a 100uf/150v. I think this is the value because the cap is Russian and says 100mk/150B. Should I replace this to match the schematic or the amp?

I reduced the power disspation to 15 watts (25ma) and the amp no longer breaks down but I am concerned that the PV is so much higher than 6l6's are rated to handle except the Sovtek 5881 which I have personally run at these plate values in a YBA-1A.

Let's say I don't go to great lengths to reduce the B+ but rather put in KT88's with the appropriate circuit mods. Is this a workable solution?

Thanks all!

[MWJB]

bias cap should be fine at 100uf.

1K 5W screen grid resistors would allow 6550 use, KT88 should drop right in - in either case, make sure that your B+ doesn't drop by more than 20vdc under a reasonable current load (start at same plate current for 6L6, probably safest to not try biasing to max for the bigger tubes, try 50%-ish?), or that your heater voltage doesn't drop significantly - either might be a sign that the PT is over stressed, due to higher heater current draw.

[tommy d]

I forgot to update the resolution of this problem. I changed all the e-caps, I had only changed the main rails before. Also I changed all the coupling caps to sozo/M150/mojo dijon , basically whatever brand I had around to make it match bruces schematic of the Mig-50. I have received great compliments on the amp by no less than Seegers/Steely Dan tour "stage sound" guy.

Been running great for a few months with no hiccups and dealt with the high PV by running 6L6 SED's biased a little cool.