Man that Tek 535A brings back a lot of memories. I started off with a Heathkit as a teenager then went to an HP 120B, a Dumont 303, the 535A, then a 547 (still have it) and now use a Tek 7904. I paid a few hundred for the 535A back in the 70's. Loved it.
Last edited:
Thats the one I started on. I took it to class so I could learn how to use it instead of the B&K ones they had at school. The instructor was a WWII radio man, so of course he didn't mind me and my antiquated equipment.
If it balances with no signal,and dosen't with a signal,it sounds like there is something wrong in the balance circuit or phase inverter.
Are both channels the same?
Are both channels the same?
Last edited:
I was actually able to get them within 250mv today with a signal... but not with the built in signal which comes off the heater tap.
I was thinking of blaming the output level. The front panel is outputting 2.2v rms. my macbook might put out 1v p-p not sure (guesstimating). I thought maybe the signal was just too high or something, I'm not Jim McShane just a college student MacGuyver LOL.
Still strange... I replaced the resistor there with a brand new one, it should be factory spec.
Still strange... I replaced the resistor there with a brand new one, it should be factory spec.
You want to set the AC Balance for a nice sine wave on the scope/ low distortion on a distortion analyzer.
You don't use the onboard meter for AC Balance...
You don't use the onboard meter for AC Balance...
First off, the nominal voltage of the test signal is .66 volts AC. It is supposed to drive the amp to about 20 watts, and that is the level where the AC balance setting procedure was supposed to take place.
Second, the AC balance adjustment is designed to maximize the symmetry between the opposing phases of the output tubes. If the amp had perfect balance/symmetry the AC balance voltage would be zero, and a scope would show only a straight line - as the phases would precisely balance each other out.
In the real world the amp is NOT perfectly balanced, so the AC balance setting procedure with a scope or a meter is designed to get the voltage to as near zero as possible or the scope trace to the minimum height with maximum symmetry.
Doing the AC balance with the little stock meter is very imprecise, since that little inexpensive (and now 50 year old) meter is trying to respond to a far from sinusoidal waveform. Remember, in the AC balance setting position of the selector switch one meter terminal is connected to the cathode of one power tube and the other meter terminal is connected to the other power tube's cathode (in the same channel of course). It is trying to measure the difference between the two. Tough duty! Hook your dual channel scope up the way the meter is (one probe to each power tube cathode in the same channel) and look at the waveform. Regarding atmasphere's comment - the only way you'll see a waveform anywhere near a sine wave is to look at the amp outputs - which is NOT how it was done on a stock amp. And it has been my expereince that adjusting for minimum distortion by eyeballing a waveform picked up at the output is really hard to do, there is little change with significant pot adjustment.
IMHO the BEST way to set the AC balance is with a good quality signal applied to the amp and the amp hooked up to a spectrum analyzer. Since that is not too common a piece of gear the second best is to use the scope as I mentioned above, and adjust the AC pot to get the closest thing to a flat line you can. Trying to adjust with the meter alone is pretty much a waste of time, just set the pot in the middle and walk away. It's a non-critical ajustment anyway, unlike bias where wrong bias setting can damage a tube.
Second, the AC balance adjustment is designed to maximize the symmetry between the opposing phases of the output tubes. If the amp had perfect balance/symmetry the AC balance voltage would be zero, and a scope would show only a straight line - as the phases would precisely balance each other out.
In the real world the amp is NOT perfectly balanced, so the AC balance setting procedure with a scope or a meter is designed to get the voltage to as near zero as possible or the scope trace to the minimum height with maximum symmetry.
Doing the AC balance with the little stock meter is very imprecise, since that little inexpensive (and now 50 year old) meter is trying to respond to a far from sinusoidal waveform. Remember, in the AC balance setting position of the selector switch one meter terminal is connected to the cathode of one power tube and the other meter terminal is connected to the other power tube's cathode (in the same channel of course). It is trying to measure the difference between the two. Tough duty! Hook your dual channel scope up the way the meter is (one probe to each power tube cathode in the same channel) and look at the waveform. Regarding atmasphere's comment - the only way you'll see a waveform anywhere near a sine wave is to look at the amp outputs - which is NOT how it was done on a stock amp. And it has been my expereince that adjusting for minimum distortion by eyeballing a waveform picked up at the output is really hard to do, there is little change with significant pot adjustment.
IMHO the BEST way to set the AC balance is with a good quality signal applied to the amp and the amp hooked up to a spectrum analyzer. Since that is not too common a piece of gear the second best is to use the scope as I mentioned above, and adjust the AC pot to get the closest thing to a flat line you can. Trying to adjust with the meter alone is pretty much a waste of time, just set the pot in the middle and walk away. It's a non-critical ajustment anyway, unlike bias where wrong bias setting can damage a tube.
Last edited:
Agreed! This is why I recommend at or near clipping. Or the use of a distortion analyzer, should one be handy.
Thanks for the response Jim... I have the Bias reading VERY well, and the deuce is sounding EXCEPTIONALLY good. I feel a lot better knowing this is a minuscule adjustment, I didn't know if the AC balance being off was damaging the tubes. I plan to order some GL KT88's or EH KT90's in the near future and I can save this for then. Maybe I'll get some Deoxit (and time) to chase down my scope with too, the knobs are a bit iffy.
Jim> I saw you have EH KT90's and GL KT88's on your page. Also didn't see a whole lot said about cryo-treating though (I know this is a dangerous subject). Wanted to ask if you'd share your feelings about both in reference to a Citation II, and about cryo-treating. I know how the process is done (the dealers that sell them want you to read a novel about it, black sable tubes have a 2 page write up just about cryo-treating and a short paragraph about the tube)... hoping instead for your 30 years experience and what your ears (and sense) tell you. I'm auditioning a cryo'd set of 12ax7lps right now in the preamp and I think they sound pretty sweet (albeit they aren't run in yet).
I'm still a LITTLE worried about the separate Bias transformer putting out 86v. Everything seems to be working fine...
The preamp seems to finally be working as it should too. The pair sound pretty frickin amazing together.
Jim> I saw you have EH KT90's and GL KT88's on your page. Also didn't see a whole lot said about cryo-treating though (I know this is a dangerous subject). Wanted to ask if you'd share your feelings about both in reference to a Citation II, and about cryo-treating. I know how the process is done (the dealers that sell them want you to read a novel about it, black sable tubes have a 2 page write up just about cryo-treating and a short paragraph about the tube)... hoping instead for your 30 years experience and what your ears (and sense) tell you. I'm auditioning a cryo'd set of 12ax7lps right now in the preamp and I think they sound pretty sweet (albeit they aren't run in yet).
I'm still a LITTLE worried about the separate Bias transformer putting out 86v. Everything seems to be working fine...
The preamp seems to finally be working as it should too. The pair sound pretty frickin amazing together.
Both the KT-90 EH and the Genalex KT-88 are excellent tubes, but I definitely prefer the Genalex in the Citation II. We bias them at about 83 ma, and they sound really great!!
On cryogenically treated tubes... cryogenic treatments are a known enhancement to many materials, but I'm not sure the process is particularly useful for tubes. It seems to be darn near a 50/50 split on the question of whether it does the tubes any good. And even those who claim to hear a difference genrally describe it as small. That difference could be simply because there is a different tube playing, it may have nothing to do with the treatment.
My personal view is that while it MAY be of some value in some cases, the cost-benefit ratio is not good. I don't use nor offer any cryo tubes at this point. But I'm willing to change my mind...
On cryogenically treated tubes... cryogenic treatments are a known enhancement to many materials, but I'm not sure the process is particularly useful for tubes. It seems to be darn near a 50/50 split on the question of whether it does the tubes any good. And even those who claim to hear a difference genrally describe it as small. That difference could be simply because there is a different tube playing, it may have nothing to do with the treatment.
My personal view is that while it MAY be of some value in some cases, the cost-benefit ratio is not good. I don't use nor offer any cryo tubes at this point. But I'm willing to change my mind...
^^ IMO/IME cryo for tubes is a bad thing- they may sound alright (as Jim points out probably more to do with the individual tube than the process), but its a good bet they won't last as long.
If you want to do something for a new unused power tube, place them in the circuit and light up the filament for 3 days and nights then let them cool off. Do not apply any B+ during this period! This process (called 'preconditioning') can double the life of the tube and reduce the possibility of early failure due to arcing damage.
If you want to do something for a new unused power tube, place them in the circuit and light up the filament for 3 days and nights then let them cool off. Do not apply any B+ during this period! This process (called 'preconditioning') can double the life of the tube and reduce the possibility of early failure due to arcing damage.
Hi Ralph,
Interesting. It sounds good but also reminds me of stuff I read years ago about cathode poisoning. Stuff like the following:
"If the tubes in the amp are allowed to operate for considerable lengths of time with the standby switch off (no B+/high voltage) the cathode material itself tends to build up a destructive process within itself, and this is referred to as “cathode poisoning.”
'The “old timers” used to call this phenomenon “SLEEPING SICKNESS!” When a tube is at full operating temperature and no current is being drawn (as is the case with no B+/high voltage) a high resistance layer is gradually formed at the “interface” between the oxide coating and the nickel cathode structure.
'This layer of barium orthosilicate causes tube performance to degrade significantly. It is important that the standby switch NOT be left in the “off position” (no B+) for more than 15 to 20 minutes at a time to avoid “poisoning” the tube’s cathodes."
This isn't meant as a challenge , just a question. I believe you know what you're talking about but it would still be foolish for anyone not to check this apparent contradiction before setting out to try it on their NOS tubes. Thanks.
^^ Dang! Not heard of that one before. We have found in our own amps that preconditioning (which you can read about in the Audiocyclopedia if memory serves) does indeed improve tube reliability.
It does not serve if the tubes have already been run. It only works if the tubes have not had B+ applied in the past!
It does not serve if the tubes have already been run. It only works if the tubes have not had B+ applied in the past!
11.55 What is the purpose of perheating vacuum tubes?
He continues,
Hope this helps. Next time I'm just going to scan it.
And that's a whole nuther world of fun.
NOTE: Regulated power supply +- 1% 6.3AC
He continues,
Audio Cyclopedia. Tremaine, Howard M. Howard W. Sams & Co., Inc. Indianapolis, Indiana. U.S.A. 2nd ed., 1st Printing,1969. pg. 468,469.
Hope this helps. Next time I'm just going to scan it.
And that's a whole nuther world of fun.
I might have found a local lead on some Ei KT90's... He says he's had them for a while but doesn't know what brand they are. (hasn't dug them out of storage in years) Brand new in boxes. Crossing my fingers...
I REALLY wish I could find an extra 10, maybe 15watts out of the Citation. I am already using a separate bias transformer, but the main power transformer is still getting QUITE warm! This thing could be a space heater! However, as I predicted, the electrostats are EXTREMELY power hungry! With the extra available headroom in the KT90, is there anything that can be done?
For those who aren't yet electronic guru's (like me), I have found a bias calculator
Tube Bias Calc
I figured out today that I have about 75ma of bias across the original 6550A's.
Can't remember if I posted this, but I found someone locally with a big stash of the Techtronix matched 12BY7's. I bought a quad and replaced all the RCA's except for the V1/V4 (per Jim's usual recommendations for JAN 12BY7's - I think the Techtronix pieces are made the same way, I know they are RIGOROUSLY matched by Techtronix). They definitely perked it up a bit and sound frickin AMAZE. Occasionally one of the channels picks up a radio station, will be hunting that down after exams in a couple weeks.
After much work the Fisher 400C is working about 95% correctly. I have the earliest known version of it. With some help at audio asylum, I made some improvements and got the hum to practically nothing. Also replaced some caps around the unit to flatten out the midrange hike, rebuilt the tone controls with components (they were PEC's).
Was not a fan of the Sovtek 12AX7LPS cryotreated. First set lasted 20 hours and started testing really low. Second set are testing 1350mmhos on Hickok 600A. All the black plates that I have test 1500-1550. They sound ok, but they don't hold a candle to the old fashioned black plate unless you want to tone your setup down a bit. I'm enjoying the crazy vividness and life that tubes can produce so I'm not a fan. Will probably be getting rid of them.
Oh and finally... I added a remote control to the Citation and the Fisher. No more reach around 🙂
http://www.walmart.com/ip/Generic-WARC4-Practecol-4-Outlet-Wall-Adapter-with-Remote/33078466
HIGHLY recommended! (not to mention, the on-off switch for the Fisher is on the volume pot and they are known to go bad and be un-replaceable)
I REALLY wish I could find an extra 10, maybe 15watts out of the Citation. I am already using a separate bias transformer, but the main power transformer is still getting QUITE warm! This thing could be a space heater! However, as I predicted, the electrostats are EXTREMELY power hungry! With the extra available headroom in the KT90, is there anything that can be done?
For those who aren't yet electronic guru's (like me), I have found a bias calculator
Tube Bias Calc
I figured out today that I have about 75ma of bias across the original 6550A's.
Can't remember if I posted this, but I found someone locally with a big stash of the Techtronix matched 12BY7's. I bought a quad and replaced all the RCA's except for the V1/V4 (per Jim's usual recommendations for JAN 12BY7's - I think the Techtronix pieces are made the same way, I know they are RIGOROUSLY matched by Techtronix). They definitely perked it up a bit and sound frickin AMAZE. Occasionally one of the channels picks up a radio station, will be hunting that down after exams in a couple weeks.
After much work the Fisher 400C is working about 95% correctly. I have the earliest known version of it. With some help at audio asylum, I made some improvements and got the hum to practically nothing. Also replaced some caps around the unit to flatten out the midrange hike, rebuilt the tone controls with components (they were PEC's).
Was not a fan of the Sovtek 12AX7LPS cryotreated. First set lasted 20 hours and started testing really low. Second set are testing 1350mmhos on Hickok 600A. All the black plates that I have test 1500-1550. They sound ok, but they don't hold a candle to the old fashioned black plate unless you want to tone your setup down a bit. I'm enjoying the crazy vividness and life that tubes can produce so I'm not a fan. Will probably be getting rid of them.
Oh and finally... I added a remote control to the Citation and the Fisher. No more reach around 🙂
http://www.walmart.com/ip/Generic-WARC4-Practecol-4-Outlet-Wall-Adapter-with-Remote/33078466
HIGHLY recommended! (not to mention, the on-off switch for the Fisher is on the volume pot and they are known to go bad and be un-replaceable)
Last edited:
Citation with e'stats
What stats do you have?
For me when I had my magnaplanars I bridged my Citation IIs.
I had a pair of them. I had to work on it. Then I borrowed a
buddy's modded adcom and bi amped it. I used the SS
for the lows and the Citation's for the highs.
I finally had my buddy build me a pair of electronic crossovers.
I fooled around with it all for about another few months.
and finally ended up selling the Maggies, cross overs, one
of the citations.
If you find some of the KT90s let me know. I'm up for
a few quads for my remaining CitationII.
What stats do you have?
For me when I had my magnaplanars I bridged my Citation IIs.
I had a pair of them. I had to work on it. Then I borrowed a
buddy's modded adcom and bi amped it. I used the SS
for the lows and the Citation's for the highs.
I finally had my buddy build me a pair of electronic crossovers.
I fooled around with it all for about another few months.
and finally ended up selling the Maggies, cross overs, one
of the citations.
If you find some of the KT90s let me know. I'm up for
a few quads for my remaining CitationII.
Have a pair of Martin Logan Aeon i electrostats.
I'll let you know if he has more than 1 quad left, and what he's asking for them (or if they're too much for me we'll work something out to get them to you). Still awaiting a response though.
I'll let you know if he has more than 1 quad left, and what he's asking for them (or if they're too much for me we'll work something out to get them to you). Still awaiting a response though.
I went ahead and got them, but they are Type 3. They came from Upscale Audio, NOS. We put them in the Citation and listened to it for a while - ran fine, no red spots. He seemed very knowledgeable about tubes, and builds amps himself (directly heated triodes were his cake and butter). V4 screen was getting a little red, and I've been hearing a radio station come thru that channel. He advised that I change out screen caps (10,11,26,27) and I can see why - they're original. Think I'll go ahead and replace all the turret board top side caps while I'm in there.
I just inherited my late dad's Citation II.
It's always been kind of noisy. Old carbon-comp resistors. It seemed to need new sockets even when it was new (yes I can remember that far back). He did a partial recap probably 10 years ago. It might have less background noise with something besides the complicated Citation I preamp feeding it too.
I'll live with it a while before I do anything besides replace tube sockets.
But I'm leaning toward the McShane power supply kit, and I already have another filament transformer suitable for supplying the smaller buffer/gain/PI tubes and I'd raise those heaters, reference that new heater transformer centertap to a voltage divider off their B supply node, which really gets rid of the hum. I also have a pair of KT120 and I'd just get another pair...maybe rebias a bit...
I come from a novice background of tube guitar amps, a similar yet dissimilar field LOL.
It's always been kind of noisy. Old carbon-comp resistors. It seemed to need new sockets even when it was new (yes I can remember that far back). He did a partial recap probably 10 years ago. It might have less background noise with something besides the complicated Citation I preamp feeding it too.
I'll live with it a while before I do anything besides replace tube sockets.
But I'm leaning toward the McShane power supply kit, and I already have another filament transformer suitable for supplying the smaller buffer/gain/PI tubes and I'd raise those heaters, reference that new heater transformer centertap to a voltage divider off their B supply node, which really gets rid of the hum. I also have a pair of KT120 and I'd just get another pair...maybe rebias a bit...
I come from a novice background of tube guitar amps, a similar yet dissimilar field LOL.