31H Series I amp issues

[DanielP]

I've got a 31H series 1 amplifier that had been modded when I got it (new caps, rectifier tubes taken out - solid state diodes added). See attached photos. 2652265326542655

Some of the voltages are not where they ought to be. Amp sounds great but needs attention. I'm still learning to read these schematics and am having difficulty since this amp has been modded.

Does anyone have a photo of an original unmodded 31H series 1 amp they could post so I could see how mine should look? I'm wondering if mine is missing a resistor because two of the 6L6 tubes are heating up excessively (when looking at front of amp the front right 6L6 and the left rear 6L6) whereas the other two seem to be hardly used.

Any help appreciated.

[TheAdmiral]

For one thing the diode rectifier is probably running the voltage too high and if you're not running the proper field coil speaker with it, then the voltages are running even higher, which is your overheating problem. I'll go take a picture of my original Type 1 and post it for your comparision.

[DanielP]

Field coil in there is the original 15" Magnavox as far as I know, driver is the original Jensen Hyphex. I'll check the ohms on the voice and field coil though just to be sure.

The fellow who owned it before me said he converted the rectifiers to solid state because the speaker had a considerable hum which mostly disappeared after his conversion. He gave me the original rectifier tubes and I'd be interested to switch it back to the way it was, he mentioned to do this would mean switching some of the wires where the diodes are now, I can't remember which ones, so that is another reason a photo of an original 31H series I amp would be helpful.

Nobody seems to care much about these old 31H amps, but I've got to say though that despite the slightly off voltages and overheating tube issue on this one, the amp really sounds great. In fact I'm hesitant to do much to it that would risk sacrificing the sound, but there is no way I'm going to keep running it like it is as it would wear out the really nice vintage tubes.

Thanks.

[DanielP]

Just checked and 15" Magnavox looks good, voice coil is 12.4 ohms and field coil resistance is 2450ish ohms.

[TheAdmiral]

Your readings on your speakers are fine. That leaves the amp rectifier partially suspect.
I'm posting pics of mine (cobwebs included) but it has had some work done on it before I got it 35 years ago but it also still has some original wax caps. One day I'll rebuild it. You can tell one of the rectifier tube sockets has been replaced with an amphenol, the volume control replaced and the 2 large black caps are wrong size (way too big). Somebody mis-read the schematic and put in 10mfd when they should have been .1mfd. The schematic reads 0.1 and read upside down on the part to be replaced, not noticing the decimal, looks like 10. Not uncommon for people without experience or training. The older Type 1 amps have round AC sockets like mine. Yours has the rectangular type used just before the switch to Type II amps (there is a lot of variation in these old amps.......much more than most people think). Also Type 1 amps have 3 can capacitors (yours has the holes filled with plugs). The later ones have 2. In Type 1 the power transformer does not penetrate the chassis but is mounted above it.
Measure the voltage on pins 3 & 4 of the 6L6s. It should be 350-360 vdc. That should tell you where to go from there.
Don Leslie's sister helped wire some of these early amps.
Hope this helps.

[torea]

...Also Type 1 amps have 3 can capacitors (yours has the holes filled with plugs). The later ones have 2. ...


Dunno about the 32H amps, but the 31H TypeII has 2 cans on top and one inside.

[DanielP]

Thanks for the photos Admiral. I started out by clipping out the SS diodes and moving the one thin brownish red wire from pin 2 to pin 3 of the 5YZ rectifier closest to the black caps (see photo "31H new wiring") then I connected the two really thick yellow leads coming from the big bundle of wires from the transformer which before had the orange covers over them and moved them to pins 2 and 3 of the other rectifier tube. Does this look correct? Does it matter which thick yellow lead goes to which pin?

There is another unused green lead coming from that same bundle of cords folded over at the top and covered with black electrical tape (see photo "31H new wiring2"), any idea what this was for? It makes me nervous working on an amp that someone else modded.

I plugged in the crossover and the bass speaker but not the upper driver and turned it on and as it started to fire up I saw a wisp of smoke coming from the front of the amp where the leslie cable attaches to the 6 pin male end protruding from the amp and I turned it off right away.

Thought I would consult the knowledgeable gurus of this forum before proceeding! Any help appreciated.

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[DanielP]

Correction to my above post since I erred in my description of the pin #'s for the rectifiers. The brownish red wire was actually moved from pin 4 to pin 1, and then the two thick yellow wires (they actually look a bit orange in the photos) from the transformer were placed on pins 1 and 4 of the second rectifier tube.

Thanks

[TheAdmiral]

That's a strange place for a wiff of smoke. Are you aware 31H's changed at SN 5200 and are wired differently for rotor operation? What is your serial number? What organ is it connected to and how does the switching operate? I'm also very wary of working on previously modded amps. I try to put them back to stock. I'll have to check on that loose wire but in my picture it goes across the amp to a 2 place terminal mounted on the side wall. I think that's the AC connectioin. I also think my power transformer may be a replacement as I have a 2 color wire (taped off) and yours doesn't have the wire unless it's the same wire, different color, darker from age.

The wires seem to match my hookup OK.

After looking at my 2 Type 2's, they both have wires taped off, so I'd say it doesn't go anywhere. My 2 Type 2's have different original transformers as well as rectifier tube types. As I wrote earlier, there were many changes on these early ones.

[DanielP]

Serial number is 4485, it is an earlier model, I was told from 1948. I'm powering it with a Trek UC1A and have been using it lately with guitar. I'm wondering if the whiff of smoke maybe was the current cooking some bit of residue that might have gotten caught in the female end of the leslie plug...just out curiousity I checked my unmodded 32H series III amp and it also has a loose wire taped off in the same place, interesting.

You asked how the switching operates, do you mean how do I turn the rotors on and off? This earlier speaker has the switch on the side which you have to manually flip, the Trek pedal although it powers the speaker cannot work the motors, so what I did was buy a footpedal, added a two prong plug and plug the footpedal in where the manual switch on the speaker went. It does the same thing but I can work it with my foot.

I'm going to go over the schematic once again to see if there is anything I'm missing, after that I'll try powering up again and try to check some voltages. With luck no smoke will work its way into the equation.

[DanielP]

Fired it up again and no wisp of smoke, checking voltages power cut off, realized my cable is bad. That explains smoke. Voltages were as follows:

Pin 3 and 4 of 6L6 tubes generally ranged from 360 - 370 with the exception of one which went up to 385 on pin 4. Two of my tubes seem to have endured a bit more heat (probably from being the first two in the line of fire when the SS diodes tweaked the voltages?). I attached pics of the quad and close ups of the two which seem to heat up more regardless of what 6L6 socket I plug them into. At the base of these two you can see the silvery cloud at the base where they heated up the inside of the glass, should I try to replace these?

Other voltages:
Power supply voltages off of pin 4 on the rectifier tubes was 410VDC, a little low as schematic says they should be 430VDC
Pins 3 of 6J5 tubes were 150 and 160VDC, a bit over as schematic says they should be 125VDC
Pin 8 of the 6J5s were 4.5V which was right on
Pin 8 of 6L6 quad were 23V, a little low, should be 27V according to schematic.

Are any of these voltages far off enough that I should worry about them? At some point I will probably take another step towards restoring this to its stock state, as of now my objective is to get the voltages back in the zone where the amp is running more efficiently and not wearing out the tubes. Removing the SS diodes and putting the original rectifier tubes seems to have made a considerable improvement.

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[TheAdmiral]

Good to hear you've got it working again and the smoke problem was not the amp.
I suspect the diodes may have damaged a couple of tubes somewhat but not enough to be serious. They inject current too fast before the tubes warm up, shortening life. It also appeared the diodes were half wave or less when they should have been full wave like the later models. Also your resistors have probably drifted, accounting for the various voltage variations. All close enough tolerances to be OK. In due time, replacing all the resistors and any older caps would be a good idea.
My amp/31H is only 400 serial numbers less than yours so things like the change in AC sockets shows modifications along the development. I would think yours was made closer to 1942 being an early one. Only an educated guess though.
You answered the question about motor control just fine. Wanted to make sure you were doing it correctly and you are as the later ones, Type 2, operate like 122's with DC on the signal lines.

One thing people forget while checking voltages it to check your house current first. These amps were measured on schematics at 115-117 vac and todays voltages are somewhat higher (about 124 at my house) which will account for DC differences from the schematics. Make an allowance for that.

[DanielP]

Thanks again for the photos and guidance, it is great to know there are knowledgeable people out there interested in these great old speakers willing to help out!

[johnny b3]

The silver stuff in the glass has nothing to do with over-heating - all tubes have this, called the "getter", the purpose of which is to bind any free oxygen molecules in the envelope. As a tube gets older the edges of the silver will become see-though, greyish color. If the vacuum in a tube is compromised the whole getter will turn into a white powder.

I'm glad you made it through this project okay. i'm always weary of folks who post about "which color wire goes where", it is a highly risky way to go about diagnosing/repairing an amp.

[DanielP]

I mentioned in a different thread how this amplifier has a low hum which is not affected by the volume knob, it starts off low, fluctuates slightly then stays consistent. Thought I'd bring it back to this thread since this is the amp I started the thread about.

As a basic recap this amp had been modded by a previous owner who removed the capacitor cans and mounted the electrolytics underneath, he also replaced the capacitors. I've checked voltages on the amp previously throughout and don't recall anything being too far out of line, same with the resistor values.

Questions:

1. The electrolytics in there now are 33uf 450v. If I'm not mistaken they should be 30uf 475v, is the difference anything I should be worried about?
2. Coming off of pin 8 on the left rear 6L6 tube there are 3 resistors strung together each with a value of 20 going to a ground, on the schematic it looks like this should be 75 whereas now it is just 60. I'd like to replace this with a single correct resistor, what is the exact resistor I need? (I know it is 75 but how many watts rating is it?).

Aside from the hum I absolutely love the way this amp sounds, I prefer it over my 32H series 3 and my 21H amp. The trick now will be if the hum can be eliminated without affecting the sound too much if possible!

Lastly, I was looking over the photos the Admiral posted of his type I amp and it appears there were three silver cans (all electrolytics?) on the top of the amp. Did they spread the 30/30/30/10 475v electrolytics out over three cans??


Here is a photo of my amp again for reference...

[DanielP]

Okay, I just dug in deeper and it does look like a few of my resistors are not where they should be. The long tan one mounted back near the black electrolytics if I'm reading the schematic right should be 3300, mine is measuring 1900. Then the two 220K resistors on the front 6L6s are reading 160k and 162k. Then like in my last post I'd like to get a single resistor for the 75 which goes to the ground instead of the (3) 20s rigged together in a kind of messy way. Also some of the leads going from the black electrolytics to the ground are unnecessarily long, I will shorten those, no need for radio antennae spaghetti methinks.

It looks like a pair of .1uf 400v caps were placed coming out from pins 1 and 6 of the console plug before the 150k and 130k resistors, this confused me and then I looked at a 32H amp schematic and it has them on there. Should I leave those there? Not sure if that was a later amp development that my amp would benefit from...

Any help appreciated, I'll try and pick up the resistors tomorrow or so and go from there unless I hear otherwise from any forum members finding flaws with my findings (there easily could be!). I will say these schematics are still hard for me to read, takes quite some time just to understand what is what underneath the amps!

[johnny b3]

33uf in place of 30 is fine.
Sorry I don't have a 31 around to check out but that 75ohm resistor - schematic says that line carries 27 volts. Multiply V times current (I) at the point and that will give you wattage - then it is common practice to double or triple it for 'piece of mind'.

I'm looking at the 31H type 1 schematic and it only has two filter caps - 30 and 30. You speak of the RC network as if your amp has 30/30/30/10...so if so this is no longer a 'type 1'...

the 3300ohm has nothing to do with the audio circuit - simply a buffer before the amp spits out B+ for the console preamp.

replace the 150K/130K resistors. test your output tubes to ensure their emissions are roughly matched.

The .1uf's before those resistors are the filters for the DC relay voltage. These were not in the original design - the switching was accomplished some other way. I can't really tell from the schematic. Geo??

In my type 2 the 30/30/30 sections were in one can, and the 10 was in a second can. 30/30/30/10 is available in one can from tubesandmore.com

[DanielP]

I will do the voltage times current math to find the wattages.

I may be mistaken about this amp being a 30/30/30/10, at first looking at the schematic I could only find two 30s but then I found what I think is the other located up between the 6J5s on the schematic (this is what I'm talking about the schematics being a little difficult to understand) and then right next to that 30 is what I'm guessing is the 10. See attached scan, I circled the 30s and the 10 in red. I also circled the resistors I will be replacing in yellow for reference.

How does one measure whether tubes are matched or not? Do you do that by measuring and comparing the output voltage on certain pins? Two of my original quad I swapped out - one was redplating and died and the other was not lighting up fully. I replaced them with a pair of vintage RCAs which I had been using in a different amp.


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[DanielP]

Here are my wattage calculations for the resistors:

75 ohm resistor
Voltage (27) divided by resistance (75) = .36
Current (.36) times voltage (27) = 9.72
Wattage (9.72) multiplied by 2 for "peace of mind" = 20 watts

making this a 75 ohm 20 watt resistor?

220k resistor
Voltage (125) divided by resistance (220k) = .00056
Current (.00056) times voltage (125) = .071
Wattage (.071) multiplied by 2 for "peace of mind" = .14 watts

making this a 220k .14 watt resistor?


3300 ohm resistor
Voltage (360) divided by resistance (3300) = .109
Current (.109) times voltage (360) = 39.27
Wattage (39.27) multiplied by 2 for "peace of mind" = 80 watts

making this a 3300 ohm 80 watt resistor?

First time applying ohm's law, how do the figures look?

[johnny b3]

Not too good. Wait for Geo or Admiral. Something is way off in these calculations. I don't think you'd ever see anything higher than a 10W resistor in these amps.

Also, there is no 10uf in that diagram, it was definitely something that showed up in later on. You're right though, there are 3x 30's, I assume one per can capacitor.

I've only measured tubes with a tester but i'm sure it can be done under the chassis - just not sure what to check for. You may want to get a new matched quad for this amp - it is highly possible that these tubes aren't matched and that is causing your hum.

[TheAdmiral]

75 ohm resistor is 10 watts wirewound. 3 cans of 30uf each. 3300 ohm resistor needs to be 5 watts.

Your .1,400v on pins 1 and 6 are probably from a conversion of the amp to the DC mode of switching superimposed on the signal lines. They should stay.

[DanielP]

Thanks for the info, I ordered the parts today, 3300 ohm 5 watt they had to special order so it will be a couple of weeks before I get the stuff, I'll keep you updated...

[johnny b3]

did they have a 10W available? you could have just done that...

[DanielP]

I don't think they had the 3300 ohm resistor in any form, got it from Tonewheel General, gentlemen I spoke to even downloaded the schematic of the amp to see where it was, apparently not many people order that part. He said the type I amps are really rare and most of the ones that are out there nowadays have been modified to type II...

On another note, last night for fun I grabbed my 21H crossover and plugged in my 32H series III amp to my 31H speaker and it sounded way different. Good, but way different and it got me to wondering. Aside from the types of tubes (6L6, 6V6, 5881) and excluding speakers, cabinets, exterior factors, etc. - what are the other major factors that contribute to an amps sound?

[johnny b3]

At its most basic, a leslie amp is divided into 3 sections - the power section, the phase inverter, and the output section. (Guitar amps have a preamp, around which the tone stack is built, this makes a huge difference) So...isolating these and comparing them on the schematic may give you a better idea of why they differ. The later amps have more filtering - the added 10uf cap, adding sag to the signal but cleaning up more power supply ripple. You've already labeled the biggest contributors to differences in tone.

Have your re-capped the crossover yet?

[DanielP]

I ordered the crossover caps the other day, actually ordered 2 sets - figured might as well recap my 21H crossover while I was at it, still waiting for them to arrive. Any chance that the old crossover caps possible gone bad in the 31H are causing/contributing to hum? The hum is coming primarily from the woofer, maybe it is just the frequency of the hum is low so the crossover sends it through the woofer. When I plugged in my 32H amp and the 21H crossover it was dead silent so that means it has to be an issue with either the 31H amp or the 31H crossover.

I power my speakers with a Trek UC1A which has a single tone control, when I play the 31H I have to turn the tone control to the most trebly possible position otherwise the speaker is lacking in high end. At that setting though it sounds glorious. When I play through my 32h amp or 21H amp I have to go almost all the way in the other direction as to roll off high frequencies otherwise they are too trebly for my taste. I've tried all three amps in my 31H cabinet and the 31H type I trumps the others by far. The test isn't a straight shoot because the crossover is different, so that is also a factor.

I'm assuming by recapping the 31H this will bring back more trebles and I won't have to crank the UC1A treble tone. Have to admit that though I ordered the parts I'm considering not recapping it just yet because like I said it sounds glorious. On the one hand, recapping it could make it sound even better right? But other words of wisdom such as "if it ain't broke, don't fix it" are also coming to mind! Still on the fence about it. First thing I'll do is replace the resistors and attend to the hum and afterwards see how I feel about the crossover.

[DanielP]

I was just browsing ebay and there is this really nice 31H up there from that seller out in Runnemede, original magnavox FC and a series II amp, in the description though they mention that there are 6L6GC tubes in it. Are these amps supposed to run GCs? Those have a higher power rating than the normal 6L6 tubes. Maybe the amp has been modded to accept the higher rated tubes?

[johnny b3]

The leslie will still sound fantastic after you fix the crossover - it IS broken, in the sense that it is not at the specs Leslie chose to use - some have calculated them to drift over 200%. If technically nothing is broken, then soon enough your fragile Jensen will be.

I wouldn't be banking on recapping to remove your hum. When the crossover point moves as the caps age, more bass frequencies (those under 800hz) start to leak to the driver. The Woofer still handles everything from its lowest rated frequency points up until the 800 point (depending on where the crossover point has drifted too), so installing fresh caps won't fix the hum, although it may better hide those frequencies you hear a hum on - it'll simply redistribute the signal and keep all bellow 800 to the woofer, and all above to the driver. If the hum is in the 600-800 range, it may be less noticeable since it will only be emitted by the woofer after the recap.

Try the 21H amp through the 31 crossover and cabinet and you'll see the hum disappears - its the amp almost for certain. The crossover may help a bit with hum, but it certainly shouldn't be causing it in the first place.

The 6L6 (metal envelope) was rated at 19 watts and the 6L6GC is rated for 30, and all the other variations fall in between these two versions (although, if they are glass, they're probably closer to 30). They should not be backwards compatible - meaning a new high wattage guitar amp designed with GC's in mind should only be run with GC's, not older tubes like metal jacket versions, which may not handle the higher power specs of the amp. Think of the tube ratings as "maximums", like voltages on a capacitor - it can handle anything up to and including that point, so you can always use a 6L6GC in an older amp running less than 30W - it just won't be pushed to its limit (possibly suffer some tone loss?). FWIW, I believe later 31H amps were all 30W, so it would be on the safe side to run these GC's...I'd be happy with any glass envelope 6L6 though...there's lots of tube hype out there...

[DanielP]

I take a meter and measure the AC voltage across the filter caps. Yes, it's a DC voltage there but some AC can be measured. The first stage from the rectifier will have several volts but the rest should be under 1VAC.

Also, out of balance output tubes will cause hum.

Update, parts came and swapped out the 3.3K 5 watt wire wound, the pair of 220 1/2 watt resisitors and the 75 ohm 12 wire wound, also just swapped out the old crossover with a recapped 21H crossover I had backup (though the caps in there are polarized and I'll have to replace those). The hum is still there, though it does sound a little different probably because of the new crossover, same basic thing though, swells a little when first powered up then is constant.

I checked for AC voltage like geoelectro had advised on a different thread, not sure if I checked in the wrong places or what because I was getting readings of between 600-800 in most places except on the negative leg of the electrolytics which was 0.

Johnnyb3 mentioned also to change out the 150k and 130k resistors, when I read it the first time I thought he was referring to the 220k pair I was planning on changing, just read the post too fast, changing those two out would be a good idea?

It is looking like the next wave of things to test are the things I was kind of hoping I wouldn't have to test because they cost $ (i.e new tubes, electrolytics) and then if it turns out they don't fix it and the hum is still there I have a hum, less $ and parts I don't really need. In terms of tubes, no doubt my tubes are mismatched, thing is though I've tried numerous combinations of (mismatched) tubes and the hum is always the same. If it was caused by mismatched tubes, wouldn't it change at least a little depending on how mismatched the tube combination was?

Suggestions?

[DanielP]

Update 2, I decided to swap out the electrolytics because the ones in there were some ruby brand cheapos, got some big blue sprague 30uf 500v atoms and put those in. Also I went ahead and swapped out the (2) 150k resistors I mentioned in my last post. Hum is STILL there.

I did a test and put the 5881 tubes that are in my 32H amp which runs quiet in the 31H amp and the hum was identical to what it was with the 6L6s I am running in there now. I'm not convinced it is my tubes creating the hum and am hesitant to throw the $100 bucks a new quad would cost at it in hopes it would be the answer.

I have a question for the Admiral if he is reading this: I was looking at the photos you posted of your amp and then I was looking at the wiring and the schematic and was curious if the previous owner wired one of the electrolytics incorrectly. Can you tell me where the yellow wire coming off the 30uf cap can farthest to the right (see photo where I circled it in yellow) connects to? I can't tell from the photo. In my amp, that wire goes all the way back to pin 4 of one of the 6J5 tubes and is linked by a 56k resistor connecting to pin 3 of that same tube. I'm wondering if your does the same.

Also, is the resistor directly above the yellow circle I drew in your amp the 10K?


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[TheAdmiral]

I have a question for the Admiral if he is reading this: I was looking at the photos you posted of your amp and then I was looking at the wiring and the schematic and was curious if the previous owner wired one of the electrolytics incorrectly. Can you tell me where the yellow wire coming off the 30uf cap can farthest to the right (see photo where I circled it in yellow) connects to? I can't tell from the photo. In my amp, that wire goes all the way back to pin 4 of one of the 6J5 tubes and is linked by a 56k resistor connecting to pin 3 of that same tube. I'm wondering if your does the same.

Also, is the resistor directly above the yellow circle I drew in your amp the 10K?


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That Type I amp (only TI that I have) is way out in my garage and it raining now. I'll have to look tomorrow.
I can tell by the image that the resistor in question is not 10K but probably 3K or 5K.

[DanielP]

For the 10k in my amp there are a pair of 5W resistors (one 5k and one 4.7k). See photo.

That section in the schematic right around the top 6j5 and top left 6L6 just confuses me, there is so much going on, it is like the Times square block of the 31H amp! That section mixed with all the ancient type faded color spaghetti wiring and previous owner mods is a crash course in leslie amp schematic understanding for me. LOL.

Whether or not I get to the bottom of the hum it has been a learning experience. Part of the reason I think it is more noticeable is because with the chillier fall weather here in MI I've got my speakers inside rather than out in the garage, my fingers were getting too cold out there playing. Indoors I have to play quieter so now I hear the hum whereas before I just turned it up. Speaking of cold, does colder weather have an adverse affect on these speakers? Tubes, woofers, etc.? Obviously things like moisture are going to wear on your equipment, rust, but what about temperature?

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[DanielP]

Are the 150k, 220k, 84k and 56k resistors in the amp 1 watt or are they 1/2 watt?