Magnavox Flea Power: Getting More Out Of The 8600 Series - A Lot More!

Hi all.

I have an 8601-20 that I recently recapped.

It's still all original circuit-wise including the tone/eq stuff in the feedback circuit.

I've yet to review voltages, etc. but it does sound very good (I actually kinda like the eq w/ my cornwalls and heresy which are otherwise a bit bass shy).

So, my question is more of a sanity thing... How HOT does your power transformer get?

After listening for about an hour I turned the unit off and noticed mine was too hot to keep my hand on, just didn't seem right.

The output transformers were a little warm and seemed fine.

As mentioned I still need to go through and check voltages, etc. I'm sure some of the resistors are out of spec and need replacing.

Also, there were no signs of red plating or anything like that.

Just wasn't sure if the power transformer should get as hot as it did.

Thanks for any feedback.

PS - For the filter caps I kept very very close to the original values and for everything else I went with exact matches using new orange drops.

Thanks
 
TLB,

I went through a HUGE phase of concern with Power Transformer Temps. Like you, I started with issues with some of my builds of the DG-SE1 circuits where the PT would get so hot my hand felt like it was being burned if I held it atop the the PT cover. In fact, it actually might have been!! However, after some research, I came up with some very interesting and somewhat 'relieving' information. First, consider that the temp of most Hot Water heaters is probably somewhere between 120 and 140 degrees. If on the high side, then when running full hot and well warmed up, if you stick your hand under there, it would probably feel like it was being stuck in boiling water. So, the first thing to realize here is: Your hand is not a thermometer!!! That said, the first thing to do is to find yourself something that is!! In my case, I did a couple of things; first, I'd purchased a couple of different multi-meters both of which had 'contact' thermometers built into them. So, I used a 'quarter' (yes, 1/4th of $1) and used it as a 'backing.....and then used some masking tape and 'taped' the contact elements to the side of the transformer with the quarter holding the back side. I did it this way so the quarter would heat up to the same temp as the transformer and assure I got a good thermo-contact and 'accurate' reading. The other thing I did was went out to Harbor Freight and purchase a small 'infra-red' contactless thermometer. I think that was all of about $15. And, this has come in handy for other things now as well. But, it worked well to 'supplement' my readings from the contact units. By the way, I put one thermo-sensor on the top of the PT, and the other on the side against the lamination stack. They averaged about 5 to 6 degrees difference with the top actually showing the hottest temp on average.

Next, I did some actual investigating as to just what the actual operating temp of a power transformer was 'supposed' to be. As it turned out, an 'average' PT when running at 150 degrees, which will seem BURNING hot to the palm of your hand....that's actually a rather cool state for the average power transformer. Most manufacturers I checked indicated that about 160 to 170 was 'normal' if not still on the cooler side. In looking at some of Hammond's spec sheet publications, they indicated that a 'normal' transformer temperature operation is appx 65 degrees (C) above ambient. So, if ambient temp in Fahrenheit is 70 or so, and this converts to 21 degrees Centigrade, and you can run at 65 degrees (C) ABOVE ambient, that means that when reaching operating temp in a 70 degree room, the transformer itself can be at 85 (C) or about 185 Celsius. And.....that's 'NORMAL'!!!

When you touch a transformer that seems warm......it's really just starting to get warm. When it seems 'hot'.....it's usually 'just' starting to get hot. When it seems too hot, it's probably at it's design spec for 24 hour operation at it's load capacity. But, NONE of this can be verified by simply 'touching' the PT. You've GOT to MEASURE it!! In the end, I found that when running Dave's circuit at the design level spec of about 100ma on the HV side, even when using the Hammond 270EX PT...which is rated for 144ma HV Rectified.....even that felt hot to my hand. But, on the infra red unit and with the contact thermometers......it was running a very 'cool' (from a manufacturer's standpoint) 155 to 160. So..... the only thing left to mention here is that the amp unit you have here in all likelihood was from a 'record player only' unit.....as I believe that was the amp that had the chassis with both amp section and 'controls' section. The other units had the amp isolated from the tuner/pre-amp. But, the record player amps that I've seen generally have a smaller power transformer, vs the Tuner console models where the PT stands about 1/2 to 3/4" taller. According to Mr. Gillespie, with the record player unit, when running his circuit, the PT is about 'tapped out' from it's current capacity level. Perhaps there's a bit more in the tuner model. But, Dave's circuit also pulls more current as it 'ups' the bias on the output tubes from what I recall as being about 38 to 40 ma on a 'stock' unit......to nearly 50 ma using Dave's circuit. So, with your amp 'not' being revised to Mr. Gillespie's changes, while your PT may seem 'hot' to you.....it should still be well within it's operating design limits. I suggest you measure it after being 'on' and playing music for about 4 to 5 hours.....and I'll bet it's not over 180 degrees. And, if not......then you're fine!!

For what it's worth.................... Tom D.
 
TLB,

I went through a HUGE phase of concern with Power Transformer Temps. Like you, I started with issues with some of my builds of the DG-SE1 circuits where the PT would get so hot my hand felt like it was being burned if I held it atop the the PT cover. In fact, it actually might have been!! However, after some research, I came up with some very interesting and somewhat 'relieving' information. First, consider that the temp of most Hot Water heaters is probably somewhere between 120 and 140 degrees. If on the high side, then when running full hot and well warmed up, if you stick your hand under there, it would probably feel like it was being stuck in boiling water. So, the first thing to realize here is: Your hand is not a thermometer!!! That said, the first thing to do is to find yourself something that is!! In my case, I did a couple of things; first, I'd purchased a couple of different multi-meters both of which had 'contact' thermometers built into them. So, I used a 'quarter' (yes, 1/4th of $1) and used it as a 'backing.....and then used some masking tape and 'taped' the contact elements to the side of the transformer with the quarter holding the back side. I did it this way so the quarter would heat up to the same temp as the transformer and assure I got a good thermo-contact and 'accurate' reading. The other thing I did was went out to Harbor Freight and purchase a small 'infra-red' contactless thermometer. I think that was all of about $15. And, this has come in handy for other things now as well. But, it worked well to 'supplement' my readings from the contact units. By the way, I put one thermo-sensor on the top of the PT, and the other on the side against the lamination stack. They averaged about 5 to 6 degrees difference with the top actually showing the hottest temp on average.

Next, I did some actual investigating as to just what the actual operating temp of a power transformer was 'supposed' to be. As it turned out, an 'average' PT when running at 150 degrees, which will seem BURNING hot to the palm of your hand....that's actually a rather cool state for the average power transformer. Most manufacturers I checked indicated that about 160 to 170 was 'normal' if not still on the cooler side. In looking at some of Hammond's spec sheet publications, they indicated that a 'normal' transformer temperature operation is appx 65 degrees (C) above ambient. So, if ambient temp in Fahrenheit is 70 or so, and this converts to 21 degrees Centigrade, and you can run at 65 degrees (C) ABOVE ambient, that means that when reaching operating temp in a 70 degree room, the transformer itself can be at 85 (C) or about 185 Celsius. And.....that's 'NORMAL'!!!

When you touch a transformer that seems warm......it's really just starting to get warm. When it seems 'hot'.....it's usually 'just' starting to get hot. When it seems too hot, it's probably at it's design spec for 24 hour operation at it's load capacity. But, NONE of this can be verified by simply 'touching' the PT. You've GOT to MEASURE it!! In the end, I found that when running Dave's circuit at the design level spec of about 100ma on the HV side, even when using the Hammond 270EX PT...which is rated for 144ma HV Rectified.....even that felt hot to my hand. But, on the infra red unit and with the contact thermometers......it was running a very 'cool' (from a manufacturer's standpoint) 155 to 160. So..... the only thing left to mention here is that the amp unit you have here in all likelihood was from a 'record player only' unit.....as I believe that was the amp that had the chassis with both amp section and 'controls' section. The other units had the amp isolated from the tuner/pre-amp. But, the record player amps that I've seen generally have a smaller power transformer, vs the Tuner console models where the PT stands about 1/2 to 3/4" taller. According to Mr. Gillespie, with the record player unit, when running his circuit, the PT is about 'tapped out' from it's current capacity level. Perhaps there's a bit more in the tuner model. But, Dave's circuit also pulls more current as it 'ups' the bias on the output tubes from what I recall as being about 38 to 40 ma on a 'stock' unit......to nearly 50 ma using Dave's circuit. So, with your amp 'not' being revised to Mr. Gillespie's changes, while your PT may seem 'hot' to you.....it should still be well within it's operating design limits. I suggest you measure it after being 'on' and playing music for about 4 to 5 hours.....and I'll bet it's not over 180 degrees. And, if not......then you're fine!!

For what it's worth.................... Tom D.


Holy smokes - That's got to be of of the most in depth answers I've read here on audiokarma!

You've obviously done quite a bit of research on the subject and I really appreciate your passing on the wealth of knowledge.

Great idea on the harbor freight contactless thermometer, I'll pick one up next time I'm there.

In the mean time based on what you said I felt pretty confident in powering it back up. Listened for prob 3-4 hours straight and it seemed fine. In fact it never seemed to get as hot as it did that first time.

This thing sounds so good as it is it's hard to imagine it sounding that much better w/ the mods. Thinking I might keep this one stock and pickup another to mod that way I can compare.

Here's a crummy video just recorded of it in action w/ my iPhone. Bad focus, bad audio recording and apparently echo-y sounding (in the recording but not in person) - but still fun. :p

Thanks again!
 
TLB,

I might suggest that you do just that: Keep your current unit 'as-is'......and then do a 'scratch build' of a second unit. In so doing, you can make some choices about things like 'platform' (or chassis type), type of transformers (either the P-T31s as per Dave's schematic, or something else like Edcor, or Transcendar.....or others), type of Power Transformer (again, could be some 'generic' repurposed type, a 'new' Hammond unit, a new Edcor, or other...), or even Rectifier source (depending on your PT you may have a 5 volt winding you can use for something other than the 6CA4). You can also build using the 12AX7 vs the 6EU7....and then play with tube rolling trying different 12AX tubes.......or (though NOT recommended by Dave but fun to just mess around with) you can swap in other tubes, like 12AT, or 12AU. Again, I stress that Dave has specifically pointed out that the build is NOT optimized for use with anything other than a 12AX. But, I will note that in 'matching' one of my input sources to the amp, I found that the lower gain of a 12AT7 actually mated to my Pre-amp a bit more kindly....and I was rather pleased with how that combo sounded.....though it was just an 'experiment'. Anyway..... glad you're feeling better about things. Tom D.
 
Something to consider here, for those who have used other transformers, has any stability or square wave testing been done to verify how they actually perform compared to the ones Dave selected? Different transformers behave differently, and its pretty common to need to fiddle with the feedback or HF tuning a bit to make them perform properly.
 
Gadget, I agree! In reality it's more than just 'stability' issues the come into play. The DC resistance of some of the larger OTs that I've used have had an affect on the Plate voltage seen at pin 7 on the output tubes. As such, an 'increase' in the B+ supply had to be generated in order to get the specified 255 V. The good news is that using some of the PTs I've had for some of these builds, the B+ was already too high and had to be reduced in the first place. So, a simple matter of just reducing it as much. But, as you point out, the relationship between the feedback values and the OT itself is a potential issue. Thus far I've not encountered any problems of an 'audible' nature in using other than P-T31s. And, my only 'stability' problems have been with using them.....which was probably more a function of 'build layout'.....and 'solved' by use of the Grid Stopper resistors as per Mr. Gillespie's recommendations. But, a 'test bench' evaluation of a 'scratch build' of this with something like Edcor's OTs and the Hammond PT is something that might prove interesting
 
These are tests that I ran on mine. Hopefully pics are big enough to get info out of.
dgse1new1.png dgse1new2.png dgse1new3.png dgse1new4.png dgse1new5.png dgse1new6.png
 

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so the same ones Dave used. Looks like the square response is pretty good, no ringing or other general obnoxious behavior. I'm mostly just curious how the other ones behave by comparison.
 
Gadget, maybe later tonight I can get my Transcendar OT / Hammond PT / 5AR4 unit off the shelf and run a sig gen into it at 1K and take a few square wave shots of what it shows on a scope. However, to be sure I’m doing this properly, I’d like to confirm that input voltage can vary and the process should be done where a sine wave is just short of clipping, yes?
 
I usually set the output to about 1v p-p. It doesn't want to be too high or you might get funny results.

If you could get 1khz and 10khz that would really give a nice picture.
 
I am wondering how much better the bass would be with better transformers. At one watt output I get 4% distortion at 100Hz increasing to around 40% at 20Hz.
 
Well I resolved my sputtering issue! After some success experimenting with grid stoppers on the power tubes (820 ohms) I added some screen stoppers (47 ohms). This largely worked, but I switched from my iphone to my macbook on the bench for playback purposes, and the issue totally went away. There was lots of listening involved in this since it became so intermittent .. and after that it became nonexistent. That started off some googling ...

Apparently in iphones the wifi, bluetooth and cell phone are all one one chip and share an antenna. Some of the RF was leaking into the audio, so I guess combined with a sensitive input caused this noise. Putting the phone in airplane mode makes it go away. Since I don't hear it with headphones or in my car, I assumed the problem was a small issue in my construction of this amplifier and it was driving me crazy.

It's a really textbook case of how one can overlook certain things when trying to isolate faults.

The time was not wasted. I learned a lot about input stages, grid stoppers and the Miller effect along the way.

I ended up taking out the grid stoppers on the power tubes because they seemed to roll off the highs. The screen stoppers are there still and seem to make the amplifier behave nicely when inserting inputs and damping the "thump". They seem to not color the sound as far as I can tell.

Thanks for listening!
 
I’m back with more questions. I built out a scratch build, more or less, from a single ended rca chassis. I wasn’t able to use the transformer that came with the rca build, a lesson I learned a page or two back in this thread. A couple days ago I traded an empty console for a bunch of junk and in that junk was a tuner/amp with a transformer that I’m pretty sure is going to work for me. It only has a single 6.3v heater winding, so no bucking, and it was rectified by a 6X5, so only 70ma output. I installed it into my DGSE and the voltages were a little over but awfully close. Using my variac to lower the voltages, everything is spot on and it sounds good, but it must be starving for some power(70ma)? Not sure it works this way? I quickly converted it to use an EZ81 and now my voltages are high; 300, 293, and 281 with a wall voltage of 121v AC. Using the variac I can drop the wall voltage to 110 and my numbers are perfect at 271, 265, and 255, so I definitely built it right. I am going to try and see if a 6AX5gt rectifier will get me closer to correct voltages, plus it has 125ma output, I have one on order.
Here are my questions. A 6AX5gt is only supposed to have 10uf of filtering connected to it and my first cap in this build is 23uf. Is that a big problem? I know nothing about the power transformer as far as specs, other than that it is physically slightly larger than the magnavox tranny. It did not get over 133* when using it at 110v AC and not over 150* at 121v AC with the ez81, and that’s while cranking it hard for 30 mins or more. It originally powered stereo single ended 6v6’s and 7 other 6 volt tubes in a tuner. Does it sound like it has the power for this new job? Anyway to tell other than the fact that it doesn’t seem to be working hard, at least as far as the heat it is creating? As always, thanks guys!
 
tubetime -- Thanks for taking the time to post the performance results of your build. I always marvel at all the test result graphs and curves that can be generated today with a computer and appropriate software. I'm old school in that regard, with all analog test equipment, a camera, measured results hand drawn out on graph paper, and hand drafted schematics. Something about this approach that just seems more romantic to me, as it clearly could not have anything to do with my unwillingness to submit to a digital age! But the digital presentation is clear and precise, so thanks for adding it to the mix. My only comment of consideration is that it is typical and customary to examine square waves at 10kHz rather than 1 kHz, as then you can see how good (or not) the transient response and stability performance of a design is up in the supersonic region where such problems can exist. At 1 kHz, the harmonics produced in that region are so attenuated as to yield little if any useful information at those frequencies. Some 43 pages later, I never had any idea this little project would generate so much interest, analysis and enjoyment by others! Thanks again!

indo -- If the alternate transformer can produce the necessary voltages, and remain relatively cool over time while doing so (< 150˚F), then its giving every indication of being up to the task. You would definitely want to use the 6AX5 or preferably the 6CA4/EZ81 however, as the steady state B+ current draw of the amplifier is 100 mA. Understand too that this current draw is nearly constant as it is in any properly designed Class A output stage amplifier, so the power output level you operate the amplifier at has no bearing on the temperature that the power transformer will reach.

As to achieving the proper operating voltages, you want the AC line voltage entering the amplifier to be such that the AC voltage as measured at the output tube heater terminals is 6.30 vac +5%/-0, which will then achieve maximum tube life relative to the heat applied to the cathodes of the tubes. Once that is achieved, then any extra B+ can be adjusted by adding an appropriate resistor between the output of the rectifier tube and the first filter cap. Use of a resistor here is possible because of the near steady state current draw by the amplifier under all conditions of operation. The resistor will also waylay any concerns of using a 23 uF first filter cap as well. The cap's value is a concern not in steady state operation, but during any "hot switching" events that might occur (instances where the power is removed and then quickly reapplied before the tube has had a chance to cool below a level of current conduction). When using a larger than recommended first stage filter cap, the use of a resistor between the rectifier and first cap will inherently limit the (damaging) excessive peak currents that can occur during such occasions. Finally, such a resistor will also act to limit the peak ripple current drawn by the first filter cap, and aid in maintaining the coolest possible operation of the power transformer as a result.

When a resistor is required between the rectifier tube and first filter cap, it's value is best determined by experimentation. Understand that simple Ohm's law will be deceiving in trying to determine both value and wattage rating in this instance, as the resistor is not only passing the steady state current draw of the amplifier, but also the ripple currents drawn by the first filter cap. The value then will typically be smaller than simple Ohm's Law would indicate, and the required wattage larger.

I hope this helps!

Dave
 
Thanks Dave, that def helps. I bought a stash of stuff from an old timer a few weeks ago and one of the things he had was hundreds of large high watt resistors. I should be able to find something that works in there. Not sure how the resistor should be incorporated into the circuit so I will include a picture so you guys can help me. I understand where it’s supposed to go, just not how. A or B, but I am thinking A, just like the one I have circled is used to drop voltage? I should have my 6AX5gt rectifier by Monday so I will be able to see what kind of voltage drop I will need.
Also, can the same principle be used to drop AC voltage going to my heaters? Not sure that’s how it works? I am over recommended heater voltage at 6.8v with a wall voltage of 121v and no way to buck the transformer.
I am excited about this amp because of the output transformers that I found to incorporate into the build. I got a console pull from a sylvania, 803-1. It was running single ended 7868’s. The transformers are 8ohm secondaries with primaries very close to 5000ohm. They weigh 50% more than the AES p-T31’s, so I knew they could potentially be a good match. So far they sound great. I don’t have the test equipment to give any hard data on them, wish I did. I will post a pic once I have completed the build. I would have just kept that sylvania se 7868 integrated but the finished DGSE1 kicked its butt.
 

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B59F42C9-4996-4EA4-A6C6-8A6F204FC6F1.jpeg ADC9303D-0D88-4CA7-8EC2-856112130C70.jpeg D8D03770-69D4-4F93-9071-B398A2B73B07.jpeg B59F42C9-4996-4EA4-A6C6-8A6F204FC6F1.jpeg ADC9303D-0D88-4CA7-8EC2-856112130C70.jpeg Hi,
I’ve been a long time reader of this tread but never post anything. I first try this build on a 8600 amp, and with lots of help from Dave, I manage to make it work. I then thought to try to scratch build one from a 270ex Hammond with a 5ar4, with again lots of help but from wharf creek this time, I ended up to a point where I thought I was good to go. Today I hook the thing up and the fuse instantly blew. I’m posting here some pics so maybe some one could help me since I bite way more then I could chew. Thanks
Fred
 
I can't tell from the pic but what pin is the white, 5 watt resistor connected to on the rectifier socket? Is there anything else connected to it? A closeup shot of that rectifier socket would be helpful..
 
I can't tell from the pic but what pin is the white, 5 watt resistor connected to on the rectifier socket? Is there anything else connected to it? A closeup shot of that rectifier socket would be helpful..
On pin 8 and there's two 10 uf 250 v caps solder to ground
 
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