Midland Tube Amp help

So 70mA is good if that's what the amp pulls. It's not the tube redline max. at all.

Two 6BM8 running at plate limits is over 80 mA, beyond the suggest current rating for a 6X4.

Given my experience with similar amplifiers, particularly Class A, I would not be surprised if this were the case.

Higher current translates to lower lifespan.
 
Which brings up the question: does your gear exceed the rated current? Preamplifiers generally don't do this, as the circuit is inherently low current. Amplifiers, however, use a lot more current.

Tuners and preamps, so probably not. I've never bothered to actually measure it.
 
Tuners and preamps, so probably not. I've never bothered to actually measure it.

Yup. That's why the 6X4 is stable in those applications. The lower the current the longer the life.

Many of the reel-to-reel decks used the 6X4, but ran it at low current so the tube last forever. Same for a lot of the preamplifiers. It the tube isn't pushed, it doesn't fail and short.

Another factor is how the heater circuit is designed. If the heater is grounded it creates high heater-to-cathode potential difference, which stresses and eventually erodes the alumina insulation on the heater, and the eventually tube shorts cathode to heater which grounds the transformer, burning out the winding. Such heaters should be elevated to reduce the stress.

Because a preamplifier tend to be lower B+, the insulation stress is lower.
 
I gotta be honest I am lost but I do fully appreciate everyone's help. I am willing to change whatever needs to be done, I cannot read schematics yet trying to learn though. Any diagrams I can follow?
 
Best way to start this project is to find a guide to understanding tube amplifiers and schematics, do some reading, and then understand what is either present or missing in the circuit you've got.

The Merlin Blencowe and Bruce Rozenblit book are well regarded and will likely explain enough to develop an understanding of what's being discussed above. After reading you'll understand the difference between building a Class A and a Class AB, and why the low efficiency of Class A would induce a manufacturer to push the output tubes and rectifier.

You could just replace the resistors and capacitors and tube sockets, but the manufacturer cut some corners to make an amplifier able to be sold at this price point: (1) increase the gain beyond what the output tubes can safely handle (change some resistors and you can push it back into the safe range), (2) increase the current draw from the rectifier for (1) past its ratings (backing (1) off may help, using a different rectifier tube or solid state may be better), (3) derive the screen voltage from a dropping resistor instead of a regulator (easily fixed), (4) removed resistors to stabilize the amplifier (easily replaced), and (5) the components chosen were the bare minimum and often the least expensive (you're throwing them out, anyway).

All of that can be easily fixed. Metal film resistors and PP film capacitors are inexpensive. So are nice sockets which good contacts.

Sites like diyAudio.com will have well-worked out 6BM8 SEP (Single-Ended Pentode) designs which you could build using your chassis as a starting point. A Class A SEP is very, very simple to build. You literally need (as you can see from your amplifier) only a single tube. If you use the chassis as a starting point you'll end up with a far better amplifier. At some point you might want to use better output transformers, too, but headphones don't have a lot of bass response, anyway, so it likely isn't critical.

I suggest reading the books, then building a simple circuit someone has worked out for you, and adding the bulb as a fuse. That should ensure that if 6X4 fails, which if you've put the performance in the safe zone it likely will not, you won't lose a power transformer.
 
I've seen solid state replacements for many tube rectifiers.

Some even have built-in soft start circuits. My understanding is that they should be plug and play in most circuits. Less heat, not in the signal path...

In litterly a few seconds to cut and paste "6X4" into Google, I found:

https://www.rfparts.com/tubes/tubes-rec/6x4-x1dr.html
... In addition to several eBay listings.

Why wouldn't this be a good, safe solution for someone who is learning?

(I understand, he may still want to look at the power amp tube biasing, and the resistors around it, but it seems that for the rectifier, the solid state solution seems like more practical advice than telling the OP to go build a whole new amp from scratch.)

EDIT: BTW, those solid state rectifiers cost $3 !!
 
It's best to not get the cart before the horse. With no schematics or experience you're left with some basic operational tests that you can do just to establish if the amp is working OK as is.

You need speakers attached. What have you done so far to test the amp? Power on, (no one can resist) what did you hear?

DO NOT FOLLOW THIS ADVICE. IF THE COUPLING CAPACITORS ARE BAD YOU WILL PUT DC ON THE GRIDS AND BLOW UP YOUR TUBES AND YOUR OUTPUT TRANSFORMER. IF AN ELECTROLYTIC FILTER CAPACITOR IS SHORTED YOU WILL BLOW THE RECTIFIER AND THE TRANSFORMER.

IF YOU DO A SEARCH IN THE TUBE FORUM YOU WILL SEE, OVER AND OVER, TO NEVER TURN ON A TUBE AMPLIFIER UNTIL THE FILTER CAPACITORS AND COUPLING CAPACITORS HAVE BEEN REPLACED.

THE JAPANESE ELECTROLYTIC CAPACITORS USED IN THOSE UNITS ARE 100% GUARANTEED TO BE DEFECTIVE AT THIS POINT. THIS IS WELL KNOWN.
 
I've seen solid state replacements for many tube rectifiers. Some even have built-in soft start circuits. My understanding is that they should be plug and play in most circuits. Less heat, not in the signal path... In litterly a few seconds to cut and paste "6X4" into Google, I found: https://www.rfparts.com/tubes/tubes-rec/6x4-x1dr.html ... In addition to several eBay listings. Why wouldn't this be a good, safe solution for someone who is learning?

For starters, most of those rectifiers do not have a delayed start for B+, nor is any regulation used to compensate for the loss of the voltage drop across the tube. Not at that price. So rapid start, bad for the tubes, coupled with higher voltage which not only shifts the load line but pushes the already hard driven tube even harder.

Delay requires a MOSFET, a few resistors, a diode, and a capacitor. Not at that price. Most also use noisy diodes as these are less expensive.

(I understand, he may still want to look at the power amp tube biasing, and the resistors around it, but it seems that for the rectifier, the solid state solution seems like more practical advice than telling the OP to go build a whole new amp from scratch.)

In summary:
(1) The OP has no schematic. How would you suggest someone who doesn't understand component values rebias the tubes into the safe range? One of the Class A amplifier designs that's been worked out would be suitable. This is why I suggested first reading some books and looking at diyAudio.

(2) The corners cut to make amplifiers at that price point were outlined above. Do you disagree with my assessment? I own a bunch of those gold-paint amplifiers in push-pull configurations and they all have issues. Most run the tubes far too hot to meet a marketing power point, and they arc over and take out the output transformers.

(3) A Class A amplifier is very simple to build, so starting from a known good configuration and replacing a dozen or so passive components on each side is not difficult. Better to start with a design which works and is stable.

(4) If the tubes red-plate because of the design issues in (1) and (2), would the OP even realize this before a meltdown? The whole amplifier can become a smoking ruin because of (2). I know this because I own two with blown output transformers and scorch marks on the chassis. You know how how much current must pass through a tube before the envelope softens, the chassis ends up brown on the underside, and an output transformer has brown/black marks on the paper with wires burnt through? Lots!​
 
6DZ7 - I don't know what it is between you and Retrovert, but STOP IT!

Amcrebelfan is trying to diagnose a problem and your snipping back and forth is ignorant and really annoying.

It can get you Banned, if that is your plan ...

and yes, I did wipe all your posts in this thread, just for being combative.

Suggest you find another thread to chat in ...
 
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I haven't powered it up yet, I do not have a dim bulb tester.

No need for a dim bulb tester as long as you simply replace all the caps and any out of spec resistors first. If you have, in fact, rebuilt a Magnavox then this amp will not be anything you can't handle.

I'm not familiar with this particular amp but it looks like a very basic circuit with very few parts. Yes, the electrolytic power supply caps and all the other gray caps in these Japanese amps are notorious for being out of spec. All of those caps need to be replaced before you go any further. Measure the resistors to see if they're in spec. Or, just order all new ones since there aren't that many.

Here's a link to a thread I posted a while back which explains how to get free shipping from Digi-Key. http://www.audiokarma.org/forums/in...hipping-on-parts-orders.739513/#post-10018562 Shipping charges can raise the cost of small parts orders significantly so, if you're not in a big hurry, you can save a bit of money.

Yes, the 6X4 rectifiers are prone to failure when they are run too hard. Adjusting the circuit so that the current draw doesn't overstress the rectifier tube is not that difficult. I'm sure the output tubes use cathode bias which consists of a resistor and (possibly) a bypass cap on the cathodes of the output tubes. It's easy enough to solder in a different (higher) value resistor. If the current draw is too high in stock form this will reduce it. No more difficult than changing any other cap or resistor. If you're ordering new resistors just order a few values that are a bit higher than the stock cathode resistors so that you have options.

I would suggest that you replace all the caps first, put together a dim bulb tester (simple and cheap) and use it to power the amp up in otherwise stock form. Then you can use your meter to measure the actual current draw, instead of speculating on how high it might be. If the current draw is too high, simply solder in some different cathode resistors until it is no longer stressing the rectifier tube.

You could also replace the rectifier tube with some diodes (UF4007 should be fine) which are very cheap. Adding a part called a thermistor (CL80 or CL90) to limit the inrush current on startup would be a good idea if you go that route. You may also need to change a resistor or two in the PS to adjust the voltages too. I'm sure there are more sophisticated circuits that will provide a slower startup but thermistors work pretty well. Keep in mind that most vintage amps that were designed with SS power supplies didn't use anything to limit inrush current and voltage.

Yes, there are some issues surrounding the use of the 6X4 but the sky is not falling. The amp is, what, 60 years old. If it hasn't blown up by now it probably won't blow up immediately the next time it's powered up and by making a few parts adjustments you can avoid that scenario entirely.

Perhaps someone has a schematic and will post it. That would be helpful.
 
I cannot read schematics yet trying to learn though. Any diagrams I can follow?

Schematics look intimidating at first but if you can read a road map you can read a schematic. Just think of the lines (which could be wires, or the leads of a resistor or cap, etc) as roads and each component (resistors, capacitors, pots, transformers, etc) as towns along the road. If you follow a particular "road" you will, at some point, come to the "town" shown on the schematic. The actual distance between "towns" will vary considerably so forget about whether the "road" between them is short or long on the schematic. Distances are not accurate. If you follow the road, though, you will eventually find the town.

That amp looks pretty simple. I would suggest that you draw your own diagram of the wiring before you replace anything since you don't have a schematic. Detailed close-up pics are a good idea too.
 
To the OP. Might invest in a component tester, under $20 normally. Tests caps, resistors, diodes etc. When you pull a tube or unhook a device look to see if that isolates another component that can be tested without removing it. Simplifies the work quite a bit sometimes. Looks like a great starter amp too. Be careful with those capacitors, discharge them before touching the wires. Store some pretty high voltage sometimes.
 
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