ST120 KT88 bias for long life

by **Bob Latino** Wed Sep 09, 2015 8:52 am

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Post n°26

Re: ST120 KT88 bias for long life

by Bob Latino Wed Sep 09, 2015 8:52 am

Tubes4ever wrote:

sKiZo wrote:I don't imagine anything we can do to a KT120 on one of these amps can hurt them.

I do remember Bob mentioning he'd run his up to 90mV with no issues. Also, no discernible improvement in audio.

A 5U4GB would give you the needed extra current to run at 60mA. Even better would be 5V3 or better yet a 5V3A which takes 3 amps heater current. The 5V3 and 5V3A will have lower voltage drop at 250mA than a 5U4GB will have since they are rated at 350mA and are easy to find on Ebay. You can run the 5V3 which takes 3.8A easily with the uprated 5V transformer windings. I verified this with Bob.

Re: sKiZo > I have run the Tung-Sol KT120's up to 75 milliamps each tube (.750 VDC bias voltage) for hours as a test with my own VTA ST-120. The KT120 tubes will take this no problem but the power transformer will run warmer. I never went up to the .900 VDC bias voltage point for each output tube that sKiZo mentioned above nor do I recommend doing so.

Re: Tubes4ever > The VTA ST-120 has a 5 volt @ 5 amp bias line. Technically you can plug in any 5 volt octal rectifier tube in there and it will work. That is not to say that it will work "well". A 5U4 will work fine in the ST-120 but there will be a very slight power loss due to a lowered B+. I have never used a 5V3 in these amps so I can't say how well they might work. If someone does try or has tried a 5V3, let us know in this thread how it worked out ..

Re: Biasing KT88's and/or KT120's in a VTA ST-120 - The .550 VDC bias point (for each output tube) for the ST-120 is a nice compromise for good sound and good tube life. If you want to bump the bias up a little higher, you are welcome to do so but the amp won't really sound any better and tube life will go down.

Bob

by Tubes4ever Wed Sep 09, 2015 10:12 am

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Post n°27

Re: ST120 KT88 bias for long life

by Tubes4ever Wed Sep 09, 2015 10:12 am

GreggW wrote:Amazing how much one can learn on this forum. I'm going to look for a 5V3A to try in the ST120 also. Ive got a Genalex 5AR4, WZ68, and a TDR on the way. I'm going to do an unscientific rectifier comparison with the expectation that all will sound really good. My youngest son just hijacked my 1963 vintage AR2a speakers, but I admit they sound much better in his larger room. So now I'm running a Velodyne powered sub and a pair of small Polk towers: the ST120 just loafs. I could probably blow the speaker cones across the room. Remember the scene in Back to the Future?

Gregg,

As Bob stated, you can run any 5 volt rectifier tube in the ST120. If you can't find a 5V3A you can use a 5V3 since they are more plentiful.

I am using a 5V3 in my ST70 which has a 4 amp 5V winding with no issues. The Transformer runs very slightly warmer.

For reference I get a B+ of 385v with the 5V3 and 370v with a 5U4GB with the EL34s set to 40mA. These are lower than you will get with a 5AR4, but you will gain the current capacity plus the 5V3 and 5U4GB are less prone to shorting out than the 5AR4. The voltages you will get on the ST120 will be higher, but will have a similar spread.

by deepee99 Fri Sep 11, 2015 12:35 pm

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Post n°28

KT-88 bias in VTA amps

by deepee99 Fri Sep 11, 2015 12:35 pm

Just happened to ask Jim McShane of McShanedesign.net this very bias question. While there's no "optimal" he said, for the Genalx KT-88s, "at 55 ma you are likely hearing them at their best in Bob’s amp."
Citation II drivers routinely bias the KT-88s up to .83VDC per tube, with the understanding that they're trading off tube life. (Citation owners don't care as they are the Hell's Angels of the tube audio fraternity anyway.)

by Jim McShane Fri Sep 11, 2015 1:09 pm

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Post n°29

Re: ST120 KT88 bias for long life

by Jim McShane Fri Sep 11, 2015 1:09 pm

deepee99 wrote:Just happened to ask Jim McShane of McShanedesign.net this very bias question. While there's no "optimal" he said, for the Genalx KT-88s, "at 55 ma you are likely hearing them at their best in Bob’s amp."

Citation II drivers routinely bias the KT-88s up to .83VDC per tube, with the understanding that they're trading off tube life. (Citation owners don't care as they are the Hell's Angels of the tube audio fraternity anyway.)

Actually the .83 volts is not accurate. Not every amp uses 10 Ohm cathode resistors - the Cit II uses 15 Ohms stock, but we recommend 18 Ohms for the KT-88s. 83 ma bias current through 18 Ohms gives you 1.494 volts.

FYI - the stock setting was 1.5 volts over 15 Ohms or 100 ma.!! Back when a quad of Genalex could be bought relatively cheaply and easily that might not have been so bad. But even H-K found 100 ma. just ate tubes - so they changed the spec to 90 ma later. And our recommended 18 Ohm resistor gives us just about 1.5 volts at 83 ma. - so the stock bias meter in the amp can be used.

by GreggW Fri Sep 11, 2015 2:22 pm

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Post n°30

Re: ST120 KT88 bias for long life

by GreggW Fri Sep 11, 2015 2:22 pm

Well, I've got a new 5AR4, WZ68, 5V3, three 5V3As, and a TDR in the mail. The TDR will be installed first, then the unscientific rectifier test begins. Is there an easy measurement point to get the B+ voltage with the different rectifiers?

by deepee99 Fri Sep 11, 2015 2:24 pm

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Post n°31

Re: ST120 KT88 bias for long life

by deepee99 Fri Sep 11, 2015 2:24 pm

Jim, my apologies. I forgot that other variable in Ohm's Law and shoulda checked with you first.

by Tubes4ever Fri Sep 11, 2015 6:21 pm

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Post n°32

Re: ST120 KT88 bias for long life

by Tubes4ever Fri Sep 11, 2015 6:21 pm

GreggW wrote:Well, I've got a new 5AR4, WZ68, 5V3, three 5V3As, and a TDR in the mail. The TDR will be installed first, then the unscientific rectifier test begins. Is there an easy measurement point to get the B+ voltage with the different rectifiers?

Gregg,

I just installed a TDR on my ST70. While I had the bottom off, I ran a wire from pin 3 on the left front tube socket to pin 2 on the left front socket where you measure the bias voltage. Pin 2 is at 7 o'clock. Makes measuring B+ very easy. Otherwise you have to partially insert the left front tube so that you can reach pin 3 with a probe.

Good thing about using the 5V3 or 5V3A, you are running it well below its maximum rating. It'll probably last a very long time.
I won't use a WZ68 because I have a tube amp and the rectifier should glow!

Why 3 5V3A tubes? Just curious.

by GreggW Fri Sep 11, 2015 7:40 pm

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Post n°33

Re: ST120 KT88 bias for long life

by GreggW Fri Sep 11, 2015 7:40 pm

Thanks for the tip about running a B+ wire. I bought a small lot of tubes which included the three 5V3As. Hopefully I only need one....

by Jim McShane Fri Sep 11, 2015 9:21 pm

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Post n°34

Re: ST120 KT88 bias for long life

by Jim McShane Fri Sep 11, 2015 9:21 pm

deepee99 wrote:Jim, my apologies. I forgot that other variable in Ohm's Law and shoulda checked with you first.

No apology required. And we all make mistakes - I'd tell you about mine but I'd use up every bit of Bob's bandwidth!!

by arledgsc Sat Sep 12, 2015 1:07 pm

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Post n°35

Re: ST120 KT88 bias for long life

by arledgsc Sat Sep 12, 2015 1:07 pm

What a timely post to follow. My KT120s starting popping in the left channel yesterday. After three years of heavy use it is time to retire them. I put in my broken in Gold Lion KT88s and impressive! Not as much bass grunt but still plenty of good bass. Mids and highs are good with great imaging.

So I biased the GL KT88s at 54mA and sound good. The KT120s like about 56mA in the ST120.

by Tubes4ever Thu Sep 24, 2015 9:03 pm

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Post n°36

Re: ST120 KT88 bias for long life

by Tubes4ever Thu Sep 24, 2015 9:03 pm

GreggW,

How's the rectifier tests going? Question

by GreggW Thu Sep 24, 2015 9:39 pm

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Post n°37

Rectifiers and a little more

by GreggW Thu Sep 24, 2015 9:39 pm

Just got the GCFIs in the mail today and the TDR board is in and works great: easy retrofit by the way. I did a little maintenance including cleaning all tube sockets and tube pins with Deoxit. I also moved the on/off switch to the stereo/mono slot and added a wire to check the plate voltage on one of the KT88s. I will hook everything up and flip the switch. The first tube will be the Genalex GZ34 I got from Jim McShane. If I'm not back in two hours, something smoked.....

by GreggW Thu Sep 24, 2015 11:37 pm

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Post n°38

Sovtek 5AR4

by GreggW Thu Sep 24, 2015 11:37 pm

$%^&*# Dropped my Genalex GZ34. Sovtek 5AR4 installed and bias set at .530. 494 volts on pin 3 of KT88. Looks like the voltage is right in the middle according to the ST120 voltage chart. Amp sounds great. I'll try the 5V3A and WZ68 in the morning. The tubes are a little warm...

by deepee99 Fri Sep 25, 2015 1:07 pm

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Post n°39

Re: ST120 KT88 bias for long life

by deepee99 Fri Sep 25, 2015 1:07 pm

GreggW wrote:$%^&*# Dropped my Genalex GZ34. Sovtek 5AR4 installed and bias set at .530. 494 volts on pin 3 of KT88. Looks like the voltage is right in the middle according to the ST120 voltage chart. Amp sounds great. I'll try the 5V3A and WZ68 in the morning. The tubes are a little warm...

First time you've dropped a tube? You must be new here Razz

Lemme tell you about the bank-shot one of my KT-88s (also Genalex) took from the amp to a shelf to the hardwood floor some time. Maybe it's a brand thing. Funny thing is, I stuck it back in the hole, glass unbroken, and two years later it's still rockin'.

by Tubes4ever Fri Sep 25, 2015 1:17 pm

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Post n°40

Re: ST120 KT88 bias for long life

by Tubes4ever Fri Sep 25, 2015 1:17 pm

GreggW wrote:$%^&*# Dropped my Genalex GZ34. Sovtek 5AR4 installed and bias set at .530. 494 volts on pin 3 of KT88. Looks like the voltage is right in the middle according to the ST120 voltage chart. Amp sounds great. I'll try the 5V3A and WZ68 in the morning. The tubes are a little warm...

Ouch!! My wife bounced a 45 off a hardwood floor awhile back. affraid

It survived and still works. I take it the GZ34 broke?

Let us know how the 5V3A and 5V3 work out. Bob wants to know! Laughing

by GreggW Fri Sep 25, 2015 6:43 pm

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Post n°41

Break time...

by GreggW Fri Sep 25, 2015 6:43 pm

Maybe it's time to back off for a few days. Fat fingers here broke one of the leads on the TDR board, capacitor C5. The local Rat Shack only has 50V .022 caps and I don't know if that's a high enough working voltage for those two transformer leads. When I dropped the tube, it didn't break; it hit the corner of my Vox guitar amp, then hit the floor. Nothing seems to have broken loose but I'm not sure I'll try it. The amp is worth a lot more than a $40 tube!

by **Bob Latino** Fri Sep 25, 2015 7:11 pm

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Post n°42

Re: ST120 KT88 bias for long life

by Bob Latino Fri Sep 25, 2015 7:11 pm

GreggW wrote:Maybe it's time to back off for a few days. Fat fingers here broke one of the leads on the TDR board, capacitor C5. The local Rat Shack only has 50V .022 caps and I don't know if that's a high enough working voltage for those two transformer leads. When I dropped the tube, it didn't break; it hit the corner of my Vox guitar amp, then hit the floor. Nothing seems to have broken loose but I'm not sure I'll try it. The amp is worth a lot more than a $40 tube!

Hi Gregg,

A 50 volt .022 uF cap will work in that spot because those two .022 uF caps only look at about 3 to 4 volts AC. Those two .022 uF caps help ground the center taps of the two 6.3 volt AC filament lines. The working voltage of the brown/yellow and green/yellow wires in relation to chassis ground therefore is only 1/2 the 6.3 volts AC or about 3.15 volts. Use the Rat Shack cap 50 volt rated cap in there. It will work fine.

Bob

by GreggW Sun Sep 27, 2015 11:39 pm

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Post n°43

What happened?

by GreggW Sun Sep 27, 2015 11:39 pm

All was well with the Sovtek 5AR4 until I turned the amp on again, then no plate voltage and no bias voltage. Pulled all the tubes and checked the voltage at pins 4 and 6 on the rectifier, 418vac. Installed the yellow sheet diode mod and got 216 and 217 volts DC on pins 4 and 6, I think as expected. (I've got a Klein Tools meter, not sure how well it measures half wave rectified DC.) Put the Sovtek and the rest of the tubes back in, still no plate or bias voltage. Found my fire extinguisher and installed the Genalex, same result. Temporarily removed the delay board, checked the voltage pre and post the diode and got 418, 217 respectively. Again installed the Sovtek and Genalex with same result. Haven't tried the WZ68 or the 5V3A yet. The filaments all light and the filament voltage is correct at all tubes. Think I'll review the troubleshooting guide and inspect every solder joint......

by GreggW Wed Sep 30, 2015 12:00 am

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Post n°44

5V3A

by GreggW Wed Sep 30, 2015 12:00 am

Back in operation. I went through Bob's checklist and reflowed a few solder joints. I think Murphy's Law got to me on the 5AR4 and the GZ34, no plate voltage or bias voltage with either installed. Dropping the GZ34 probably didn't do it any good. The 5V3A was installed and amp turned on. Bias on all tubes was around .380, so a quick rebias to .525 was done, checked again 10 minutes later and rebiased again; bias stayed stable. Plate voltage, KT88 pin 3, was 457 (AC line voltage 121.2). The voltage charts calls for 470-520 on pins 3 and 4 of the KT88, so I'm not sure how much, if any, power is lost. If there's a difference in sound, I couldn't hear it. The 5V3A is going to stay in it for awhile just to see what happens. It's time to get an IR thermometer and check some temps. The 5V3A heater draws 3.0 amps vs. 1.9 for the 5AR4. I noted in Kentley's post on his Weber WS1 adventure that the position on his bias pots changed significantly. Mine did too, two positions on the clock with a 12AT7 board, 1 o'clock to 11 o'clock.

by Tubes4ever Wed Sep 30, 2015 9:18 am

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Post n°45

Re: ST120 KT88 bias for long life

by Tubes4ever Wed Sep 30, 2015 9:18 am

Gregg,

The amount of power lost will be minor. Unless you regularly run the amp at window rattling levels, you won't notice the difference. Plus the 5V3A isn't being pushed hard at all.

by Tubes4ever Sun Oct 11, 2015 8:27 pm

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Post n°46

Re: ST120 KT88 bias for long life

by Tubes4ever Sun Oct 11, 2015 8:27 pm

GreggW wrote:Back in operation. I went through Bob's checklist and reflowed a few solder joints. I think Murphy's Law got to me on the 5AR4 and the GZ34, no plate voltage or bias voltage with either installed. Dropping the GZ34 probably didn't do it any good. The 5V3A was installed and amp turned on. Bias on all tubes was around .380, so a quick rebias to .525 was done, checked again 10 minutes later and rebiased again; bias stayed stable. Plate voltage, KT88 pin 3, was 457 (AC line voltage 121.2). The voltage charts calls for 470-520 on pins 3 and 4 of the KT88, so I'm not sure how much, if any, power is lost. If there's a difference in sound, I couldn't hear it. The 5V3A is going to stay in it for awhile just to see what happens. It's time to get an IR thermometer and check some temps. The 5V3A heater draws 3.0 amps vs. 1.9 for the 5AR4. I noted in Kentley's post on his Weber WS1 adventure that the position on his bias pots changed significantly. Mine did too, two positions on the clock with a 12AT7 board, 1 o'clock to 11 o'clock.

Gregg,

How are your tests going?

by GreggW Mon Oct 12, 2015 2:19 am

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Post n°47

Re: ST120 KT88 bias for long life

by GreggW Mon Oct 12, 2015 2:19 am

The Genalex GZ34, the one I dropped, and Sovtek 5AR4 are history and I haven't ordered a replacement yet. The 5V3A in the amp now has more than 40 hours on it, bias holds steady, and runs with no TDR board. KT88 plate voltage varies only slightly with small changes in line voltage. The tube temperatures, after a one hour of run time, are as follows: 5V3a 310-340, KT88s - front 280-300, back 300-340, 12AT7s 110-140. The power transformer runs between 130-145, the higher reading measured when the room temperature was 85 degrees. I didn't have the IR thermometer when the 5AR4 was in use. The yellow sheet diode modification was done and the entire system is connected to a TRC GCFI (manual reset).

A week ago, the amp was really cranked up for a solid two hours, louder than I've ever played it. My son commented several times how clean the music sounded at the high volume levels. The tube temperature variation from the above was nil.

With the 5V3a and the above changes, I'm going to leave the amp "as is" for another 150-200 hours just to see what happens. It sounds so good I don't want to mess with it!

by GreggW Tue Dec 22, 2015 2:23 am

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Post n°48

ST 120 bias, 5V3A, Mullard CV4024

by GreggW Tue Dec 22, 2015 2:23 am

Since 10/11, I've put at least 250 hours on my ST 120 using a 5V3a rectifier. Bias was set at .500 and remained very stable. Last week I installed one of Jim McShane's Mullard CV4024s in the center position of the driver board, replacing a perfectly functional EH 12AT7 (now a spare reverb driver for a guitar amp). There is a noticeable improvement in dynamics as it's easier to place a guitar on the stage and vocals are a little more front and center.
I don't think 300 hours on a particular rectifier type means much, but 3000 hours on one tube without failure might mean something. I'll be watching for more reports on the newest generation of 5AR4s. And may the Sovtek KT88s live a long life because I don't have any spares.....

by **Bob Latino** Tue Dec 22, 2015 10:13 am

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Post n°49

Re: ST120 KT88 bias for long life

by Bob Latino Tue Dec 22, 2015 10:13 am

GreggW wrote:Since 10/11, I've put at least 250 hours on my ST 120 using a 5V3a rectifier. Bias was set at .500 and remained very stable. Last week I installed one of Jim McShane's Mullard CV4024s in the center position of the driver board, replacing a perfectly functional EH 12AT7 (now a spare reverb driver for a guitar amp). There is a noticeable improvement in dynamics as it's easier to place a guitar on the stage and vocals are a little more front and center.
I don't think 300 hours on a particular rectifier type means much, but 3000 hours on one tube without failure might mean something. I'll be watching for more reports on the newest generation of 5AR4s. And may the Sovtek KT88s live a long life because I don't have any spares.....

Just a note here that Gregg has one of the older VTA ST-120's that use the higher gain 12AT7 driver board. The newer VTA ST-120's use the lower gain CCS board. The comparable tube to a Mullard CV4024 (for the 12AT7 driver boards) for the newer boards is a Mullard CV4003 (12AU7 type tube). The transition on the VTA ST-70 and VTA ST-120 between the older higher gain board and the newer CCS board took place on 4/1/2012. As a side note > The VTA M-125 monoblocks always used a lower gain board.

The Mullard CV4003 (12AU7) is a really great tube and built to standards set up by the British military. It is a great sounding tube that gives a nice holographic soundstage. To save money, as Gregg has mentioned, you only need one for the center position. Sometimes these tubes, as well as the CV4024's, are not marked "Mullard" but are marked "Valve Electronic" instead. Rumor has it that in WWII the British military did not want the brand name of parts that they used marked on the part for fear that the Germans would bomb the factory that made the part.

Bob

by sKiZo Tue Dec 22, 2015 11:35 am

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Post n°50

Re: ST120 KT88 bias for long life

by sKiZo Tue Dec 22, 2015 11:35 am

Probably been mentioned, but worth repeating, the 5V3A is electronically identical to the 5AU4, which is a lot easier to find. Same tube, basically the same price. Not many of the boxes were labeled with both, so it's easy enough to forget.

vta st120 - ST120 KT88 bias for long life - Page 2 5v3a-5au4

vta st120 - ST120 KT88 bias for long life - Page 2 5v3a-5au4

Quite a bargain compared to a GZ33 or GZ37 ... if you can find one. But them old coke bottles IS purtier ...

PS ... my test voltages are down in the low end of the expected range due to using the bucker (wall in around 116vac) AND a CL90 thermister on the line in, but it sounds great. If anything, it sounded a bit pinched in comparison when I was running it at 124vac-ish - pretty much standard here. Much more relaxed now, but still plenty of room shaking bass, especially with the KT120's installed.

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