jcolletteiii
Active Member
Did anyone to the G terminal ground improvement as part of the Mcshane kits?
My big Citation II rebuild questions thread
- Thread starter jcolletteiii
- Start date
targeteye
Super Member
I did not.. what kind of improvement is it supposed to ad?
I did install a star topology in the grounding but did not bring the transformer grounds to that central point. The star and bussbar type schemas are most beneficial in the low signal circuits but I guess it could hurt on the transformers as well.
Steve
I did install a star topology in the grounding but did not bring the transformer grounds to that central point. The star and bussbar type schemas are most beneficial in the low signal circuits but I guess it could hurt on the transformers as well.
Steve
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jcolletteiii
Active Member
Not sure exactly, but I think the goal is for all DC to be grounded at the bus bar and all AC to be grounded to the chassis - minimizing hum maybe? This is the excerpt from the level II sheet from Jim on the topic:
"5. The last step is to improve the output transformer G terminal ground scheme. To do this, you will need to isolate both G terminals. Anything grounded to the G terminals EXCEPT THE BLACK WIRE TO THE OUTPUT TRANSFORMER must be grounded elsewhere. You’ll probably find a terminal strip very near the G terminal you can use as a ground point. You’ll probably also find the G terminal is grounded to this same point. That connection must be removed, it’s usually just a piece of bare wire soldered between the terminal strip ground lug and the G terminal. The new G terminal ground will be at the power supply ground, either the bus bar (if you installed one), or the ground lug on the new C16 cap. Route the supplied 18 ga. brown wires carefully through the chassis, through the electrolytic bracket, and solder it in place both at the G terminal and the bus bar or ground lug."
"5. The last step is to improve the output transformer G terminal ground scheme. To do this, you will need to isolate both G terminals. Anything grounded to the G terminals EXCEPT THE BLACK WIRE TO THE OUTPUT TRANSFORMER must be grounded elsewhere. You’ll probably find a terminal strip very near the G terminal you can use as a ground point. You’ll probably also find the G terminal is grounded to this same point. That connection must be removed, it’s usually just a piece of bare wire soldered between the terminal strip ground lug and the G terminal. The new G terminal ground will be at the power supply ground, either the bus bar (if you installed one), or the ground lug on the new C16 cap. Route the supplied 18 ga. brown wires carefully through the chassis, through the electrolytic bracket, and solder it in place both at the G terminal and the bus bar or ground lug."
jcolletteiii
Active Member
After a third check of the schematic and checking off each connection point, I rotated all of the bias pots to their lowest setting, plugged into a switched outlet in case I had to turn power off quickly, and powered it up. A quick check of the grid voltages on pin 5 of the power tubes showed between -45 and -46 or so, with one on the low side around -43. I biased the tubes and watched for about 5 mins. No redplating, no issues, and most importantly, no smoke!
I adjusted the bias again after about another 6 or 8 minutes - it had drifted up a bit. I'll run it for longer tomorrow, re-adjust the bias as necessary. I want to run the tubes with a bit lower current - around 80ma - bias can be directly measured at pin 8, correct?
I adjusted the bias again after about another 6 or 8 minutes - it had drifted up a bit. I'll run it for longer tomorrow, re-adjust the bias as necessary. I want to run the tubes with a bit lower current - around 80ma - bias can be directly measured at pin 8, correct?
targeteye
Super Member
Giving this some more thought. This is a good idea since the global feedback is dependant on this ground and this would be a more reliable method of achieving the groud vs relying on a screw and washer attatched to the chassis. I'll most likely do this the next time I have my amp opened up.
Steve
targeteye
Super Member
Current can be measured at pin 8 but I would recommend that you first check to make sure that all the cathode resistors are in spec.
jcolletteiii
Active Member
Thanks for all of your help everyone - it's up and running.
Didn't do the G terminal modification - will wait until the next time it's on the 'bench'. I will likely do the turret boards and caps under them next. Let it run for an hour, checking the bias every few minutes, then put it back into the system. The bias is stable, but I've backed the bias off from the mark just a bit. I definitely need to change out the resistors on the cathode pins of the output tubes to reduce the bias current (and still use the meter, which is a convenience more than anything else).
Going to put in some NOS 12by7a's now - I have a couple gold pin Sylvania copper grid tubes that I'm dying to try in the 1 positions (they look identical to the older RCA black plate copper grid tubes, but with gold pins).
Didn't do the G terminal modification - will wait until the next time it's on the 'bench'. I will likely do the turret boards and caps under them next. Let it run for an hour, checking the bias every few minutes, then put it back into the system. The bias is stable, but I've backed the bias off from the mark just a bit. I definitely need to change out the resistors on the cathode pins of the output tubes to reduce the bias current (and still use the meter, which is a convenience more than anything else).
Going to put in some NOS 12by7a's now - I have a couple gold pin Sylvania copper grid tubes that I'm dying to try in the 1 positions (they look identical to the older RCA black plate copper grid tubes, but with gold pins).
cbox83
Active Member
Looking awesome man! I don't recall doing the G terminal bit when I did mine either. I am using the Tektronix certified and matched GE 12BY7's in mine right now, they have a VERY high GM compared to standard 12BY7's. Few extra watts 
I am jealous of your record cabinet!
I am jealous of your record cabinet!
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jcolletteiii
Active Member
There were some tectronix tubes in the 1 and 4 positions, but when I put the gold Sylvanias in, I swear I heard a distinct increase in volume!
The credenza is by Paul McCobb around 1950 or so. It was the most awesome Christmas gift I've ever received - the wife (GF at the time) gave it to me the year after we started dating. Lol - my 'bench' for this project has been a McCobb coffee table (with layers of carboard over the top)!
jcolletteiii
Active Member
Just got some input from Don Sachs on the G terminal grounding scheme as well - it definitely makes more sense now. I think Jim's instruction sheets are a bit terse in some places. Don's explanation is pasted below.
"Jim has been out of town for a week consulting, but will be back next week. At any rate, the Citation II has two kinds of grounds, AC, and DC. All of the signal path is DC. This would be the board grounds at pins 4,9, and 13, the cathodes of the output tubes, all the power supply on the bracket, and the bias supply...and the output transformer ground. All the DC grounds should go back to the ground bus you created when you rebuilt the power supply bracket. The AC grounds are the center tap wires of the filament supply on each channel (ground to V8, V9 socket outer rings), and the shields of any coax cables you used for input from the rca jacks. If you used the original rca jack wires then the shield acts as a ground as in the original design. If you are using the better modern twin shielded wire, then the ground wire is inside the shield and connects to pin 28 on the board which is that triple ground bus. You would ground the shield of this coax wire to the chassis, usually at the rear. You only ground a shield of a cable at one end, and it should be to the chassis, so right by the rca jack is a great place. The other AC ground is the third prong (ground) of a modern power cord and the test signal circuit gets grounded at one end to the chassis in the rear by the dual rca jack plate. I generally use new terminal strips at the rear to replace the originals. I use a 3 terminal strip in place of the dual, and a 4 terminal strip in place of the triple. Then I can use the extra lug for ground that is isolated from the chassis. I run a wire from this lug to the output transformer ground (speaker terminals) and from there a wire goes under the bracket and back to the ground bus on the power supply bracket. So this becomes an extension of the ground bus that is isolated from the chassis and only grounds back at the bus. So this extra lug now grounds the 39 ohm resistor and the 4.75K resistor off the bias pot. On the other channel is also grounds that 330 ohm resistor from the meter circuit. Finally, the easy way to ground the cathode resistors is to use pin 1. Pin 1 is not used in a 6550/KT88 and can be used as a tie point on the socket. So run the 18 ohm cathode resistors from pin 8 to pin 1. Then run a wire from pin 1 to the ground bus on the bracket. Now your cathodes are grounded to the DC bus rather than to the chassis. On V7 they used pin 1 as a tie point for the 60 Hz test signal jack. Simply tie on end of that resistor to pin 7 and just let the other end hang in the air. The wire from this goes to the test signal jack and is usually brown. I put heat shrink over the connection between the resistor and the wire. You could mount a small terminal strip, but it is not really needed. The heat shrink provides security should the lead bend towards the chassis. Not going to happen though as the resistor lead is pretty thick."
"Jim has been out of town for a week consulting, but will be back next week. At any rate, the Citation II has two kinds of grounds, AC, and DC. All of the signal path is DC. This would be the board grounds at pins 4,9, and 13, the cathodes of the output tubes, all the power supply on the bracket, and the bias supply...and the output transformer ground. All the DC grounds should go back to the ground bus you created when you rebuilt the power supply bracket. The AC grounds are the center tap wires of the filament supply on each channel (ground to V8, V9 socket outer rings), and the shields of any coax cables you used for input from the rca jacks. If you used the original rca jack wires then the shield acts as a ground as in the original design. If you are using the better modern twin shielded wire, then the ground wire is inside the shield and connects to pin 28 on the board which is that triple ground bus. You would ground the shield of this coax wire to the chassis, usually at the rear. You only ground a shield of a cable at one end, and it should be to the chassis, so right by the rca jack is a great place. The other AC ground is the third prong (ground) of a modern power cord and the test signal circuit gets grounded at one end to the chassis in the rear by the dual rca jack plate. I generally use new terminal strips at the rear to replace the originals. I use a 3 terminal strip in place of the dual, and a 4 terminal strip in place of the triple. Then I can use the extra lug for ground that is isolated from the chassis. I run a wire from this lug to the output transformer ground (speaker terminals) and from there a wire goes under the bracket and back to the ground bus on the power supply bracket. So this becomes an extension of the ground bus that is isolated from the chassis and only grounds back at the bus. So this extra lug now grounds the 39 ohm resistor and the 4.75K resistor off the bias pot. On the other channel is also grounds that 330 ohm resistor from the meter circuit. Finally, the easy way to ground the cathode resistors is to use pin 1. Pin 1 is not used in a 6550/KT88 and can be used as a tie point on the socket. So run the 18 ohm cathode resistors from pin 8 to pin 1. Then run a wire from pin 1 to the ground bus on the bracket. Now your cathodes are grounded to the DC bus rather than to the chassis. On V7 they used pin 1 as a tie point for the 60 Hz test signal jack. Simply tie on end of that resistor to pin 7 and just let the other end hang in the air. The wire from this goes to the test signal jack and is usually brown. I put heat shrink over the connection between the resistor and the wire. You could mount a small terminal strip, but it is not really needed. The heat shrink provides security should the lead bend towards the chassis. Not going to happen though as the resistor lead is pretty thick."
cbox83
Active Member
It amazes me just how much you can make little "tweaks" to this already astounding piece of equipment. I am going to do this next time I pull the Citation out of it's home in the shelf. Right now my preamp is in dire need of some rework (has a hum), but this is now in my list of "to do".
Thanks for posting/sharing the very well explained write up on the procedure. Makes perfect sense.
Thanks for posting/sharing the very well explained write up on the procedure. Makes perfect sense.
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jcolletteiii
Active Member
I'm going to do that one last grounding scheme update next time I have it opened up - probably when I start on the caps that are on the undersides of the turret boards. I need to find one more NOS or ANOS Tung-Sol 6550 first and want to give the amp some run time to see how it sounds with just these updates before I dive in deeper.
I'm just amazed at how much this thing weighs, especially when moving the ST-70 around in comparison - the difference is staggering.
I'm just amazed at how much this thing weighs, especially when moving the ST-70 around in comparison - the difference is staggering.
cbox83
Active Member
My back pops every time I have to move it, and I'm only 31. I can't imagine how it must feel to some of the older guys. Its not just heavy, its AWKWARD. That stack of brown painted lead bricks on the back just makes it an utter pain. I try to avoid moving mine at all costs, I've even brought the solder iron to it. LOL
jcolletteiii
Active Member
Just rolled in a quad of the Genelex Gold Lion reissue KT-88's. Those tubes are large compared to the 6550's that I have. They seem a bit 'soft' compared with the Tung Sols - the bass is maybe a bit tighter, but the mids an uppers seem a bit muffled almost. But a lot of people tell me they really need 40-50 hours before they start to sound really good. Imaging is great though, very holographic. What are you guys running for output tubes?
cbox83
Active Member
I switch between my Tung Sol's (The original ones that came with the amp) and a set of Winged C's. I am curious to know what they sound like after a break in period myself. I am happy with the SED Winged C's for the most part, but quite frankly, the Tung Sol's blow them out of the water and I find myself pained when pulling them out. There's just nothing like old tubes, sadly... and second best just never cuts it. Therefore, I refuse to listen to a set of original Genelex - I don't want to know how they sound because I couldn't afford a set.
s-petersen
Scott
I've had mine on and off te bench more times than I can imagine.I'm 55 and it's tough.
MRL_Audio
RIP 2022
First - thanks for the thread and nice work on the rebuild. Curious where you decided to set bias at. I have had a set of EH-KT88's in my Cit II for some time. I have them biased at 75ma/1.35 VDC and have been pleased with the output. When I read your comments on the 80ma and that it was considered a "bit low" made me re-think where I am at. I think Jim reco's EH's set to 80-85ma or 1.45-1.55 VDC. Thanks.
jcolletteiii
Active Member
I saw that Jim recommends running everything but the KT-88 reissues at 75mA, but I've settled on 80-85mA. I've got some 18ohm 2w resistors that I need to put in there when I get a break, perhaps later in the week. Power here in ND is pretty stable right at 120v, so I figured 80mA would be a good value, especially when running my no-hole Tung-sol 6550's - still need to find one more at a reasonable price to complete my quad tho.
poconoman
Active Member
Hello all! Not to hijack this thread. Figured why bother starting one when this is fine?
Ok, I'm a new proud owner of a CII and would like some clear answers on bias and AC balance.
1. Jim recommends that any new issue tube be biased less than 'normal', 80-85ma. So, does that mean the needle on the dial come slightly before it hits 'BIAS'? And how do you get the voltmeter readings? Is there a bias pot to stick in the voltmeter somewhere?
2. AC balance. For the life of me, I'm having a hard time understanding this. Can someone please be kind enough to this CII newb, step by step on HOW to do this?
And yes, I have to careful on picking this brute up. My lord, that's some heavy iron!
Thank you!
Ok, I'm a new proud owner of a CII and would like some clear answers on bias and AC balance.
1. Jim recommends that any new issue tube be biased less than 'normal', 80-85ma. So, does that mean the needle on the dial come slightly before it hits 'BIAS'? And how do you get the voltmeter readings? Is there a bias pot to stick in the voltmeter somewhere?
2. AC balance. For the life of me, I'm having a hard time understanding this. Can someone please be kind enough to this CII newb, step by step on HOW to do this?
And yes, I have to careful on picking this brute up. My lord, that's some heavy iron!
Thank you!
jcolletteiii
Active Member
I've not done the AC balance - although when I get the time, I'll probably do it following Jim's instructions on his website, so I can't help there.
The stock bias circuit applies 100mA to each tube when you are at the line on the bias dial. That is pretty hard on the tubes (especially if your wall AC is above 120v), and especially some of the valuable vintage tubes. It'll run very nicely at that current, but the tubes will apparently have less useable lifespan. Right now, I'm just backing the bias off for each tube maybe 2 or 3mm below the bias line. I have not measured what that translates into in current, but you can do this by taking a voltage reading on pin 8 of the power tubes VERY CAREFULLY! There is easily enough current going through the amp to kill you. With the stock 15ohm resistors on your output cube cathode resistor, Ohm's law says if you want 80mA on each tube, you should be measuring 1.2v at pin 8. if you want 85mA, you should measure 1.275v at pin 8 on the power tubes.
Now, if you want to set up your bias meter to read perfect on the bias line AND have 80mA bias current to the tubes, you have to replace the stock 15ohm 2 watt resistor on the power tube sockets with an 18ohm 2 watt resistor for all four output tube sockets. So, using Ohm's Law again, that should give you I = 1.5 / 18 or 0.083 amps (83mA). Hope i did that right...
The stock bias circuit applies 100mA to each tube when you are at the line on the bias dial. That is pretty hard on the tubes (especially if your wall AC is above 120v), and especially some of the valuable vintage tubes. It'll run very nicely at that current, but the tubes will apparently have less useable lifespan. Right now, I'm just backing the bias off for each tube maybe 2 or 3mm below the bias line. I have not measured what that translates into in current, but you can do this by taking a voltage reading on pin 8 of the power tubes VERY CAREFULLY! There is easily enough current going through the amp to kill you. With the stock 15ohm resistors on your output cube cathode resistor, Ohm's law says if you want 80mA on each tube, you should be measuring 1.2v at pin 8. if you want 85mA, you should measure 1.275v at pin 8 on the power tubes.
Now, if you want to set up your bias meter to read perfect on the bias line AND have 80mA bias current to the tubes, you have to replace the stock 15ohm 2 watt resistor on the power tube sockets with an 18ohm 2 watt resistor for all four output tube sockets. So, using Ohm's Law again, that should give you I = 1.5 / 18 or 0.083 amps (83mA). Hope i did that right...