Yet Another Maggie Rebuild: 185AA

[BrassTeacher]

I misread CY series caps as series connected. Sorry

Don't apologize, because you called it on the swapped plate leads!

I went out to eat to take a break (Waffle House), and while I was sitting there, I kept going through the schematic in my head. In the schematic I drew, no error is obvious. On the original schematic, the inverted signal coming off the anode of the voltage amp tube is fed into the 6V6 connected to the brown transformer lead, and the signal coming off the plate of the inverter is fed into the 6V6 with the blue lead. The feedback loop is connected to the cathode end of the voltage amp:



Here is where I screwed up: I forgot that the signal going into the cathodyne was already inverted. So, the signal coming off the anode of the cathodyne was the natural polarity, since it was re-inverted, and the signal off the cathode end was inverted. So, the way I had it hooked up the first time, the feedback was entirely positive instead of negative. I had fed the inverted signal into the blue-leaded 6V6, when I should've fed it into the 6V6 with the brown lead. Oops.

Luckily it was an easy fix, and took less than 5 minutes:



Thanks for the tip grndfix!

Now all I need to do is silence the hum.

 

[nerdorama]

Now all I need to do is silence the hum.

One thing to do is connect the power transformer HV CT directly to the first filter cap negative terminal. It appears you have it on the ground bus causing all the first stage ripple current to go though the green wire to the power supply caps. This is a basic issue for keeping noise down.
John

 

[BrassTeacher]

One thing to do is connect the power transformer HV CT directly to the first filter cap negative terminal. It appears you have it on the ground bus causing all the first stage ripple current to go though the green wire to the power supply caps. This is a basic issue for keeping noise down.
John

Ah, this will be the next step then. Thanks! Adding a bit of extra filtering and grounding the bus to the chassis has already reduced the hum by more than half.

I ran a signal through the amp this morning for the first time, and it's already far better than the 8802, even with the hum! The 8802, I retained the stock configuration with the paraphrase inverter.
Back to the soldering iron...

 

[gadget73]

another thing that will make it hum is poorly matched output tubes. Unequal current flow won't cancel out the supply ripple properly.

 

[BrassTeacher]

another thing that will make it hum is poorly matched output tubes. Unequal current flow won't cancel out the supply ripple properly.

Tried this per your suggestion. In fact, I went ahead and put my best tubes in it all around. A closely matched set of 6V6, which ironically are the originals to this amp, a set of very closely matched Bugle Boy 12AX7, and a perfectly balanced 5U4 that both halves almost pegged the meter on the 800A. There was some improvement, but it was minimal.

So, back to the bench. I added an extra 33uF cap on the 205V section of the HV supply. Again, only marginal improvement. It now has more filtering (about 28uF more) than the 8802 I just built, and it is DEAD quiet.

So, out came the DMM for some probing. 305V line measured 299V, 205V line measures 238V, so I guess that little 100K resistor isn't doing its job. Will swap it out with a stronger one ASAP. This may also explain why this 185AA is much louder than the 8802.

Anyway, back to the measurements - the HV line measured at the screens of the 6V6 shows 220mV of AC on the line. My ears tell me the audible hum is a nearly pure sine wave at 60Hz. The frequency counter on the meter confirms 60Hz (definitely not 120Hz, which when dealing with amplifiers, is rarely a pure sine wave sound) Remember, I'm a musician, and deal with analog synths all the time, I know what a clean sine sounds like. So, 220mV on the AC line sounds a bit excessive to me. Ok, a lot really.

I'm loathe to just cramming more filter caps in without more diagnosis. At this point I'm considering swapping out the power transformer with one from another 185AA I have sitting here, just to see if there's a difference (Remember I was getting 2.2VAC on the chassis with the chassis no longer grounded to the IEC connector). I'm also considering separating the ground bus between the input/cathodyne tubes and the output tubes, and grounding both ends of each bus to the chassis. Of course, all this will wait until I get the 205V line down to 205-ish from its current 238V.

As always, suggestions welcome

 

[nerdorama]

How about moving the CT wire to the first cap negative. Probably not a complete solution but the benefits can be measured.

 

[gadget73]

try jumping the 6v6 grids to ground and see if that stops it. if it does, the problem is upstream of there.

but +1 on the trafo CT ought to be at the first cap negative. Ideally the cathode resistor and cap for the output tubes also should be at or as close to that point as possible since those will be your biggest current draws.

that resistor isn't not "strong enough", you have low current draw if the voltage is high.


other weird grounding specifics, the speaker secondary, input jack negative, and voltage amp cathode resistors also should connect to the same point. The input and output negatives are both one side of the input to the amplifier. The cathode resistor ground is the actual connection into the amp. Any noise on the ground buss between the speaker line and the cathode resistor can show up as noise pumped right into the first gain stage of the amplifier.

honestly the ground buss shouldn't be that noisy if the high voltage grounds are all together and not flowing ripple across the buss but its a thought if moving that doesn't do anything for you.

 

[nerdorama]

I have also seen feedback lines pick up noise. Since the I use a twisted pair from the speaker hot terminal ane the common and connect them directly across the cathode resistor at the first stage tube.

I notice too you have a fair bit of untwisted heater wiring. That can cause noise.

 

[grindfix]

The original Magnavox heater wiring was not twisted but I don't remember how quiet or noisy the amp I rebuilt was. I never listened to it for too long before rebuilding, just enough to confirm the sound from both channels. But, I always twist them tight, no point experimenting.

I had a heck of a time quieting down my first scratch-build amp. Gadget probably remembers that one Hammond. It's quiet as a mouse now!
The biggest help I found studying grounding techniques on this page:
https://www.valvewizard.co.uk/Grounding.pdf
From that point on, all of my builds are grounded the same way. Isolated amplifier (or preamplifier) input is grounded at the chassis. Then, I run bus from there. Power Transformer CT and first filter cap are connected to the bus at the end but not to the chassis. Single chassis connection at the signal input only. This has prevented ground loops from other connected equipment.

 

[BrassTeacher]

The original Magnavox heater wiring was not twisted but I don't remember how quiet or noisy the amp I rebuilt was. I never listened to it for too long before rebuilding, just enough to confirm the sound from both channels. But, I always twist them tight, no point experimenting.

I had a heck of a time quieting down my first scratch-build amp. Gadget probably remembers that one Hammond. It's quiet as a mouse now!
The biggest help I found studying grounding techniques on this page:
https://www.valvewizard.co.uk/Grounding.pdf
From that point on, all of my builds are grounded the same way. Isolated amplifier (or preamplifier) input is grounded at the chassis. Then, I run bus from there. Power Transformer CT and first filter cap are connected to the bus at the end but not to the chassis. Single chassis connection at the signal input only. This has prevented ground loops from other connected equipment.

I had started typing a reply to the posts immediately preceding this, and then this post popped up. Thanks again grindfix! I was actually contemplating ripping the ground bus out and going with a star ground, but I like the multiple star ground scheme laid out near the end of the article you linked. So, I'll be ripping out most of the ground bus. So, I'll need to find a couple posts/bolts that can be insulated from the chassis, and one to put near the input jacks that grounds to the chassis. I'll also rip put the HV supply caps and spread the smoothing caps around the amp, and ground them at the "local stars". The only remaining question, ground the cathodes of the input tube (the AF amp, not the side with the cathodyne) to the post that will also ground the input jacks to the chassis, or move that ground back one down the chain, to where the cathodyne will be grounded? (which will be pretty close to that point anyway) I'll re-read the article a couple times beforehand.

Funny thing though, the 8802 I rebuilt, the grounding scheme is all original, which is contrary to everything in the article, but I was able to get it quiet enough that you can't hear any noise unless you're about 1 foot from the speaker, and even then it's faint. Oh well, analog electronics seems to be part science, part art, and part voodoo...

 

[grindfix]

The only remaining question, ground the cathodes of the input tube (the AF amp, not the side with the cathodyne) to the post that will also ground the input jacks to the chassis, or move that ground back one down the chain, to where the cathodyne will be grounded? (which will be pretty close to that point anyway)

The shied of the AF grid and the AF cathode resistor should be grounded at the same point. The inverter stage you may want to experiment with trying to connect at the same point or space it further. One of my builds "wanted" it together with AF stage ground.

 

[BrassTeacher]

The amp is belly-up on the bench, and surgery will begin soon. Had to purchase some supplies first:



Brass screw, washers, and nuts for the star that will be grounded to the chassis. I will put this right in between the input jacks. Nylon screws, nuts, and washers for the insulated stars, with some metal nuts to go on to squeeze the ring terminals together. That's 8 gauge copper wire to bridge between the stars. Yes, I am not playing around this time. I'm hoping I can get away with just two star grounds, one at the input, and one insulated one near the rectifier. That said, it's a relatively long way from the output transformer grounds to the star at the rectifier.

If you don't have this book, I highly recommend it. It tells you everything you need to know, even the stuff you didn't think you needed to know. Funny thing though, it dedicates an entire chapter to the various types of power supplies and how to build/improve them, but it spends a little less than one page on grounding. The valvewizard article has 10 times the information than the JLH book. Put them together, and you have a formidable reference book.

 

[grindfix]

If your PS caps are separate and not a part of the can, grounding should be simple. You should only have as many grounding points as the amount of PS nodes in the unit, plus the chassis connection. You can just run a wire from one point to the next. No real need for an elevated copper bus.
In commercial units that used can caps, prototypes were tested and I'm sure engineers and lab techs spent days finding the quietest points for grounding using the chassis as a fat wire.

 

[BrassTeacher]

Well, progress has been made -

Ground buss ripped out (figuratively speaking)



The brass screw for chassis ground has already been installed, but nothing else.

The front end re-wired. I soldered all the leads to lugs so that if I need to move the chassis ground somewhere else, it won't be nearly as painful as if I had soldered all the leads directly to the post.



Note the ground wire. 14-gauge insulated. I started to reply to grindfix's admonishment about the 8-gauge wire with, "There is no such thing as overkill!!!". When I tried bending that 8-gauge wire to my will (pun intended), I decided that the roll of 14-gauge insulated green wire that I saw in Lowe's the next day was the prettiest roll of wire I had ever seen. I'll save the 8-gauge for the two W4-AM amps I'll be rebuilding soon, as they are more straightforward (Also pun intended).

The whole thing wired back up:



Well, it still hums, still 60Hz. This time though, it is half as loud as it was before. Another difference is that as the HV line comes up, the humming gets a good bit softer as the filter caps charge up. I find this encouraging as the 8802 did the same thing, and adding more filtering caps shut it up. Even more encouraging, this 185AA has about half the filtering capacitance as it did when I first tried getting rid of the hum. The amp still plays well considering the hum.

Photos are enlargeable, so if anyone finds an area that could be improved, please don't be shy about speaking up. I have another one of these to build, and I'm considering putting the power switch on the back wall (may do it with this one also), and putting the input jacks on the front wall of the chassis to make the routing of the front-end easier.

 

[gadget73]

Last one I did with a ground buss I don't remember if I used 12 or 14, but it was a piece of something I had handy either way. Too big and its just too difficult to work with.

what did you do with the heater circuit? Is it balanced to the cathode with a pair of resistors? Kind of looks like that on the second tube from the right, but just confirming.

Does it shut up if you pull the 12ax7 tubes?

any special reason the AC wiring is so close to the screen supply? Looks like the power switch lands on the terminal right next to the DC for the screen. The wiring in that area looks generally pretty mixed. I might route the wiring to/from the AC socket and switch over around the transformer and rectifier socket so its as far from the DC and signal wiring as possible. Would also be worth pushing the feedback wire from the output speaker terminal over so its as far away from the fuse holder as practical.


also whats the white wire on the terminal strip up in the top right corner for? Doesn't appear to connect to anything there, and it looks like it connects to the input jacks? Is there a connection from the input jacks to the grid to ground and cathode to ground resistor at the 12ax7?

 

[grindfix]

The left channel driver tube heater wires look elevated above all components. I'd reposition that twisted pair away from the tube socket in the corner of the chassis. That PS cap next to the input stage may have something to do with the noise. Why did you move it from the PS corner?
Also, the input grid resistor body should be as close to the grids as possible to prevent the pick up of any noise.

 

[JonL]

Couldn't find outlet checker, then I remembered I had a $100 DMM! At outlet, Ground to Neutral - 100mVAC, Hot to Ground - 121.4VAC, Hot to Neutral - 121.4VAC, so that outlet is just fine.

Now, I just this morning got in another 185AA to rebuild. Here is a photo of its power transformer, followed by a question -

View attachment 2842812

The arrow is pointing to a thin, braided steel wire that is obviously attached to the bell and lying against the laminations. Is this some sort of drain wire to limit stray inductance? The transformer on the amp currently being built had one of these, but it wasn't lying against the laminations, but sticking out a bit. I was wondering why this random piece of wire was sticking out, and I clipped it off. Did I screw up?

Going ahead with HV supply installation, but again, any ideas on the 2.2V leakage, please chime in. It may be a normal thing...

I recognize that amp! Glad it got there safely!

 

[BrassTeacher]

The left channel driver tube heater wires look elevated above all components. I'd reposition that twisted pair away from the tube socket in the corner of the chassis. That PS cap next to the input stage may have something to do with the noise. Why did you move it from the PS corner?

That's the filter cap for the AF stage. I figured the best place for it was right at the AF stage.

Also, the input grid resistor body should be as close to the grids as possible to prevent the pick up of any noise.

Noted

what did you do with the heater circuit? Is it balanced to the cathode with a pair of resistors? Kind of looks like that on the second tube from the right, but just confirming.

Your surmise is correct

Does it shut up if you pull the 12ax7 tubes?

It actually got worse

any special reason the AC wiring is so close to the screen supply? Looks like the power switch lands on the terminal right next to the DC for the screen. The wiring in that area looks generally pretty mixed. I might route the wiring to/from the AC socket and switch over around the transformer and rectifier socket so its as far from the DC and signal wiring as possible. Would also be worth pushing the feedback wire from the output speaker terminal over so its as far away from the fuse holder as practical.

Yeah, was thinking about that. I noticed in the 8802 build, I kept anything with high-voltage AC well away from everything else. I'm going to move the power switch to the back panel, with all accompanying paraphernalia. That terminal strip just happened to be convenient, I wasn't thinking clearly.

also whats the white wire on the terminal strip up in the top right corner for? Doesn't appear to connect to anything there, and it looks like it connects to the input jacks? Is there a connection from the input jacks to the grid to ground and cathode to ground resistor at the 12ax7?

Yes, all grounding you mention is there, it's just hard to see because it's all so tight. The white wire is the B+ line to the AF stage. Bad photo angle.

 

[BrassTeacher]

I recognize that amp! Glad it got there safely!

HI there! Actually, this amp was one I got a few weeks before the one I got from you. I'm not that fast! The one I got from you is still sitting in the cue. If you're not going to use that 7025, let me know!

 

[BrassTeacher]

Update: Ok, Based on advice from you guys, and reading things on other threads, I have done the following:

1. Moved the power switch to the back of the amp, and got all high-voltage AC wires well away from the rest of the amp
2. Added a little more filtering to the 305V line
3. The shield ground wire that I clipped off because it was sticking out of the transformer has been re-attached and grounded to the chassis. When I took the bell off the top of the transformer, it wasn't connected to anything

Result, hum dropped by a little more than half again. Almost there, but before stuffing caps in it until it shuts up, I did some reading. I was considering going with DC heaters, and was trying to figure out how much resistance it would take to drop 9.06V to 6.3 (Heaters currently running at 6.41VAC), I ran across a reference on one site (can't remember which one) that mentioned elevating heater voltage should reduce hum, especially on a split-load inverter. Hmm, ok, did some more digging, and found this (Stolen from Valvewizard.UK):



So, curious after looking at the bottom graphic , since this was already done in the amp, I measured DC voltage across the cathode bypass resistor. 17.5VDC. Then I measured from the cathode of the inverter to ground: 96.8VDC! Since that pushes the voltage differential from heater to cathode close to the absolute limit, I figured I may have found a big part of my problem.
So, assuming it is ok to add the voltage divider scheme to the amp, I assume it is ok to attach the voltage divider at the junction of the hum balance resistors. I have no problem calculating a voltage divider, but I do wonder about the maximum heater-to-cathode resistance mentioned above (Rhk9max)). I have yet to find a 12AX7 or 6V6 data sheet that lists this as a parameter. Any ideas, or just play it safe and stay under 100K for the value of R2? Also, with this basically being connected to the cathodes of the output tubes, I would assume that 17.5VDC will be added to the value at the junction of the voltage divider?

Thanks in advance for any input!