A 400C Transformation

A very thorough response. Thank you so much. Yes, your measurements speak for themselves. I remember now you mentioning about the impedance difference between the inputs. Wasn't sure if that was all it entailed. I should have re-read your post, the answer was right there. There has been alot to digest here (I'm not complaining). Thanks, you have saved me the trouble of opening her up again just to mess with those phono caps. There are alot of things I want to address inside the 400C now, but I hate to go without while she's tied up in the lab (kitchen table) for weeks since I am working all day and cooking at night. By the way, what would be the general sonic result of a non standard 100K input impedance? I might even like it regardless of a scewed responce curve. Might actually want one of each. I know it is customary to experiment with various settings on moving coil pre's and some have sung the praises of doing so with MM as well, though that feature isn't offered on many MM phono pre's
 
Hifi -- Cartridge loading an area I haven't worked with much, but in general, having the proper loading on a magnetic cartridge (both resistive and capacitive) is all about producing optimum response from the cartridge. For example, the capacitive element of the load is often best determined with a scope and special test records that have tracks of square waves recorded on them. The capacitive element is then adjusted in the traditional fashion of working with square waves: It is adjusted for best rise time with minimum overshoot. The R element is adjusted for optimum damping of the wave. The sound produced from non-optimum loading would be the same as any circuit that had trouble passing square waves optimally: Response could either be rolled off, or tend to have an edge to it that is not present on the recording itself.

Dave
 
HiFi some of my MM MI cartridges like a higher loading better. The thing Im not looking forward to is removing that EQ switch to reverse the 10k 100k resistors. When I rebuilt mine several years ago I must have got the message that a .047 is the way to go because that's what my coupling caps are. Also I had one of those toggles that fits in the service receptacles for power that did work out well. It wont be as convenient but a better arrangement.

Dave finding that heavy duty shielded cable wasn't as easy to find as I thought didn't find it at a store a audio buddy of mine had some. Just about finished installing that arrangement. I'm moving right along.
 
Stl -- Swapping the leads of the 100K resistor is a worth while effort since it does improve S/N performance, but that's all it addresses. Therefore, it will have the most impact on low output cartridges. With that, you can judge accordingly how important the change is with regards to the cartridge you use.

Behind the scenes, I've been working with a close friend regarding the RIAA EQ network for this unit, who is the ONLY person I trust to use modern modeling programs with vintage vacuum tube circuits. For even the generally experienced modeler, there is SO much room for error with this approach, and way, way too many who use it that DON'T check to see if the indicated results from their modeling efforts are in fact the results produced. As a result, there is at least one well known trusted name out there offering an RIAA upgrade kit for another manufacturer's product that produces anything but the improvement claimed, as measured results simply do not match the claimed results the upgrade kit is purported to achieve.

In any event, this is an area I tread very carefully in, and will report any worthwhile findings back from these efforts as the results so dictate. As I reported earlier, the components involved with the RIAA networks are accessible without removing the preamp sub-assembly.

Dave
 
Stl, that doesn't really surprise me about your cartridge results. I'm using a Grado Master into RIAA2 and a Shure V15VMRLE into RIAA1. Interestingly, that Shure with the ARXA sounds better than it has any right to considering their humble origins. I haven't looked inside yet to see what impedance the inputs are. HiFi never ceases to amaze me. To get this kind of sound from these ancient components. The old AR has to be one of the best tables ever designed, though I'll admit I am using my own DIY arm on it which made a huge difference.
Dave, I'll stay tuned for your further comments on RIAA networks.
 
Hello Dave went I rebuilt my eq I incorporated tape head and mic as phono inputs. Using original riaa specs from 400c on two inputs and very slight changes on the other two inputs. With my 62 year old ears I cant even hear a difference at all between the two. Also with having two 50k and and two 100k loading.

Dave ive been stuck finishing the shield connection I keep staring at your close up pic of the that area and looking at the 80az connections there should be only 1 and 7 power, 3 output to triple 30s, 4 feed for lamps, 5 ground. Can you tell me what else you have on yours and reason. My triple 30s are not a can but mounted under so I have a different look for the shield connection.

Very interesting on what you and your friend are doing with RIAA.
 

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HiFi I use MI and MM only I've done my run with expensive hi cost MCs. I have a table with mutable arms and have a Acutex 315 a AT 155/160 stylus, and a Signet tk10ml. The only problem like buying used old records warped skips these cartridges are in the same boat, styluses dried out boots. I've been lucky on most thankfully.
 
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Dave finding that heavy duty shielded cable wasn't as easy to find as I thought didn't find it at a store a audio buddy of mine had some. Just about finished installing that arrangement. I'm moving right along.

Single conductor shielded seems to be difficult to find. My solution will be to scrap some inexpensive stereo interconnect cables. A couple of 6 foot cables should be enough shielded wire to do an entire receiver.
 
Stl -- The connections on the EZ80 rectifier tube socket would certainly look confusing to someone used to the stock build for sure. The connections on that socket are as follows:

Pin 1 = Tube Plate. HV AC red lead from power transformer

Pin 2 = Tie Point: 10K 1/4 watt resistor coming over from output of heater bias supply at pin #6, + lead of 10 uF 100 volt bypass cap (neg lead grounded), and lead over to neg line of DC heater supply to bias it above ground by 65 vdc. I used an 11 uF cap here because that's what I had handy.

Pin 3 = Tube Cathode. Output to 3X30 uF can cap, and strap over to pin #4.

Pin 4 = Tube Heater. Lead from one end of .47 ohm dropping resistor, and strap from pin #3.

Pin 5 = Tube Heater. Green heater supply lead from power transformer.

Pin 6 = Tie Point: 270K 1 watt resistor from 320 vdc supply point, 68k 1/4 watt resistor (other end grounded), and 10K 1/4 watt resistor going over to pin #2.

Pin 7 = Tube Plate. HV AC Red lead from power transformer.

Pin 8 = N/C

Pin 9 = Tie Point: Yellow transformer lead, and other end of .47 ohm dropping resistor.

The Blue transformer lead must be well taped off with this configuration, and 28 volt panel lamps installed in all sockets, with power to all of them provided from the DC heater supply. IF YOU CONTINUE TO USE 6 VOLT PANEL BULBS, YOU MUST NOT INCLUDE THE STRAP BETWEEN PINS 3 AND 4 OF THE EZ80 RECTIFIER TUBE. IN THAT CASE, PINS 3, 4 AND 5 OF THIS TUBE WOULD BE CONNECTED AS ORIGINALLY DESIGNED, AND THE BLUE TRANSFORMER LEAD USED TO POWER THE 6 VOLT BULBS.

You can use either approach, but the modification using 28 volt bulbs and DC supply will clearly produce the lowest hum and noise. However, keep them straight, as crossing up the two approaches and using a bit of both of them will damage either the lamps, or the rectifier tube, and be none too kind to the power transformer, either.

Finally, the shielded cable coming over from the heater supply bridge rectifier should only be grounded at the filter cap end of the cable.

I hope this helps!

Dave
 
Dave have up to the shielded cable complete. I checked all work vaced and blew out. Plugged her in and all seemed well. Unfortunately that 1/2 amp agc fuse gave in after about 2 minutes. I didn't remember the fuse was only 1/2 amp that now seems low. I figured I'd start with the heaters. Disconnected the resistors for 10 ohms in the circuit. Pulled the ez80 and plugged her in again. No fuse blowing so i'm good to the heaters. I did a ohm ck of the heater pins on the 3 type of tubes installed.

This also surprised me as I thought the heaters were standard on all type of tubes.

Tomorrows another day I hate getting my butt kicked.

Mike
 
Stl -- Heater resistance varies by the size of the heater in a tube, which in turn varies by the amount of power a tube is designed to handle. As an example, each section of a small signal tube like the 12AX7, AU7, AT7, ect. operates on 6.3 volts, and only draws 0.15 amp, while the heater of a KT88 power output tube also operates on 6.3 volts, but draws 1.8 A. With these numbers, Ohms Law will then show the obvious and radically different resistance in the heaters of these two tubes.

Within a given tube type, it is not uncommon for there to be slight differences in heater resistance between two of the same type of tubes -- even to the point that there can be slight differences in the resistance between the heaters in the two sections within a given tube like the 12AX7 or its cousins. This is why when the heaters of such tubes are operated in a series string circuit as they are in the 400C, the mid way point between two tubes is not always exactly equal to one half of the supply voltage to the string. As a result of these differences, proper operation of series connected heaters has them typically adjusted for correct CURRENT draw, rather than for the correct voltage across each individual tube heater.

The 1/2 amp rating of the 400C fuse is marginal, and particularly so if you have upped the size of the filter caps in the DC heater supply for the audio tubes. I lost the fuse in mine twice during its many bench tests, but that was during some relatively quick power cycling events. In normal operation however, the 1/2 amp rated fuse has held up fine.

Dave
 
Hello Dave I've got my fuse issues solved holding and all is well in the heater circuit. Many thanks on the explanation on ohm values between the types of tubes. My half amp fuse is the one I bought the 400 with. Hoping the new one is good for three years.

I was satisfied with all voltage readings well within range so I buttoned her up and was ready for a night with the Beatles. Back in the system and go figure there's a hum in the phono eq positions. Determined by loudness in the volume level very low volume no hum high volume very high hum. All other selector positions no hum at all till max volume and very slight then. Ive had it connected bottom off to system and tried probing all through the components. No luck with the blind method though as probing had no effect to reduce or change the hum. Could you throw any ideas my way to get to the bottom of this. If all else fails and as much as I would hate doing it bring it to
Richard Gray.

Hopping you had a good weekend
Mike
 
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It would be helpful to know if this is "new" hum that wasn't there before. If that's the case, then this should be a rather easy problem to find, as the phono preamp section by itself is not prone to produce hum in the 400C. If it's "old" hum that still hasn't been killed yet, then other issues may still be present.

Assuming it is new hum, first verify that it is absolutely from the phono preamp section by selecting the low level preamps, but reducing the individual level controls for this section to zero. If the hum is gone, then it is in fact being generated by the phono preamps. If not, then selector switch wiring should be examined.

If it is from the phono preamps, there is little that can cause hum in the actual preamp module itself, with only one audio ground in the module, and heavy shielding for most of the components. Therefore, I would first inspect the DC heater supply to make sure that a good ground reference still exists for this circuit.

In the original design, this was provided by the fact that this supply had its negative side connected directly to ground. However, if you installed the bias network to raise this supply by about 65 volts above ground, then the 10 uF 100 volt cap in the new network providing this bias voltage now provides the ground reference for this supply. Therefore, if the cap is defective, or improperly connected, then the DC heater supply will not be properly referenced ground, and the phono preamps will likely produce hum in that scenario.

Properly installed, any direct connection between the negative side of the DC heater supply and ground from the original design should be removed, and the output of the new heater bias network connected to the negative side of the heater supply. In the output of the heater bias network is the above mentioned 10 uF cap, with the positive side of this cap connected to the negative side of the heater supply, and the negative side of this cap grounded. So connected, this cap then maintains a reference to ground for the DC heater supply to minimize noise.

Check to make sure that the negative side of the DC heater supply is in fact biased by about 65 volts above ground, and that the cap mentioned is correctly installed. If not, that is the likely problem, and if so, it eliminates this possibility before proceeding on.

Dave
 
Good morning and many thanks for the reply Dave. Yes this is a new hum and it gets much louder as you turn up the volume. No I have not installed the new bias for the heaters. I have been all over the grounds in the heater circuit and will back check that.

I do know its wired correct with the in series tubes. I don't believe theres any way that the mod, your first for the heater supply could cause this.

I fear problems happening that's why I try and not do to many changes at one time. This time the fix has eluded me hopefully its a high resistance ground going to the EQ. Last night just looking and probing and I thought you are going to have to come out. Its got to come out anyway because I want to do the 10k 100k swap of the input resistors.

Will post findings and getting that hum fixed before I continue on.

Thank you again
Mike
 
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Within a given tube type, it is not uncommon for there to be slight differences in heater resistance between two of the same type of tubes -- even to the point that there can be slight differences in the resistance between the heaters in the two sections within a given tube like the 12AX7 or its cousins. This is why when the heaters of such tubes are operated in a series string circuit as they are in the 400C, the mid way point between two tubes is not always exactly equal to one half of the supply voltage to the string. As a result of these differences, proper operation of series connected heaters has them typically adjusted for correct CURRENT draw, rather than for the correct voltage across each individual tube heater.
.....

I have worked on TVs with no power transformer and the tube heaters were in a complex combination of series and parallel. Madman Muntz type design. Works ok from the factory, but one you start replacing a few tubes, the entire thing can go out of balance and some tubes barely light.

Muntz also did similar with the B+, putting one group of small signal stages in series with another group to total the same B+ as the horizontal and vertical stages. This eliminated a large voltage dropping resistor and also reduced required B+ current. But, it suffered from the same balancing problems as tubes started to become weak or replaced with better tubes.
 
Hello Dave, No fix yet for my hum problem still trying though. I hate to give up without a good fight.

Having a difficult time finding 8384 lamps. I did come up with a
Eiko #335 35211-0 Lamp Miniature Bulb 28V 1.12W E5.5 Base Light NOS
Is that wattage ok its higher than the8384.

Mike
 
You should be able to find the 8384 bulbs with a search engine. It is best to keep the lamp wattage down not only to have the jewel brightness appear as it always did, but also to minimize heat, and draw from the DC supply. Remember, the lamps (two of which draw about 75 ma together) draw not only the current flowing through them, but also increases the peak charging current required by the filter caps, meaning the effective draw of the two lamps is closer to 100 ma. The power supply has taken no particular notice of this increase, since much of it is balanced by reduced draw on the rectifier heater winding, but still, it is best to minimize the increased draw that the lamps present to the DC supply.

Without trying to add to your frustrations, I am sure that your hum problem is likely quite simple -- particularly if it is in both channels. There is just nothing to create much hum from the phono preamps in this unit. This must be an open or ineffective ground, or heater reference issue as I mentioned earlier. What about if you short the phono inputs. Does that reduce the hum? And is it in both channels?

Stay at it -- I have no doubt that you will get to the bottom of it!

Dave
 
Dave your encouragement is a moral builder. And I do apologize for high jacking your thread.

Your understanding of the description by me of the hum is right on. That does not happen much putting into words my problems. Will be away for a few days. Maybe letting that bad girl sit will be good for me. Will start with grounding the inputs.

I did a web search and came up with a wholesaler with not wholesale prices plus a 40.00 min. purchase. The ones I found were on ebay searching 5.5 base I will do this mod. Would making the 6.5 volts used now dc work?

The good part a very good friend lent me his Infinity preamp a early fet design that is very easy to listen to. I could easily listen to this sweet pre daily.
Mike
 
You could not really use the 6 volt bulbs from the DC power supply, as:

1. Those bulbs pull 100 ma each for 200ma to light to bulbs, and

2. Besides over loading the power supply, the wasted energy lost in the use of an appropriate dropping resistor would just add nearly 6 watts of heat under the chassis. In fact, the dropping resistor would be dissipating more heat and energy than the lamps themselves would be.

I will try to determine where I got my bulbs to see if that source is still available.

Dave
 
Hello Dave thankfully I found the bad ground connection and it was self inflicted. She's is quiet than before for sure. I still have a very slight ear to the speaker to hear junk coming from the left side. Its a constant thumping with a quack every 5 secs. Not going to worry about this for now.

I want to get back to your upgrades next and its going to start with pulling the eq for the swapping of the 10k 100k resistors. Dave if you have any luck coming up with your supplier for the 8384 lamps please let me know.

Hoping your weekend is a good one.
Mike
 
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