ST120 KT88 bias for long life

Skizzy,
Did I not read that the current and voltage needs of those guys were pushing the VTA iron a bit to the hot side? Please correct me.
Tnx
d

deepee99 wrote:Skizzy,
Did I not read that the current and voltage needs of those guys were pushing the VTA iron a bit to the hot side? Please correct me.
Tnx
d

The 5V3 uses 3.8 amps at 5V.  The 5V3A uses 3 amps, just like the 5U4GB.  However, the 5V3A is rated at 350mA instead of the 275mA of the 5U4GB.  The 5V3A is actually a better choice as it has a lower voltage drop at the currents we use than a 5U4GB.
The 5AU4 is not electrically identical to the 5V3A.  You can use a 5V3A in place of a 5AU4. They are similar in plate current ratings.
The 5AU4 uses 3.75 Amps.


Tim

Bob Latino wrote:The Mullard CV4003 (12AU7) is a really great tube and built to standards set up by the British military...To save money, as Gregg has mentioned, you only need one for the center position. Bob

Hey Bob, would you or some other, kind and knowledgeable person, please provide a little more info about why only the center position would benefit from a better tube?  How might one center Mullard match with a pair of the Beckman 5963's I acquired with the original kit?

jasn54 wrote:

Bob Latino wrote:The Mullard CV4003 (12AU7) is a really great tube and built to standards set up by the British military...To save money, as Gregg has mentioned, you only need one for the center position. Bob

Hey Bob, would you or some other, kind and knowledgeable person, please provide a little more info about why only the center position would benefit from a better tube?  How might one center Mullard match with a pair of the Beckman 5963's I acquired with the original kit?

The center tube is the driver which amplifies the signal - as it is a dual-triode, it does both channels. The two outer tubes are "merely" phase inverters, in which one of the dual triodes supplies one of the push-pull output tubes with an exact out-of phase signal so that the output tubes can work in tandem. Thus, the two outer tubes are not critically involved with modifying anything in the signal besides phase. I hope that is accurate and it helps you understand.
Kind, I am. Knowlegeable, perhaps not so much....

You are all of the above Kentley, thank you. So...do others merely swap the center? (I suspect not...

jasn54 wrote:You are all of the above Kentley, thank you. So...do others merely swap the center? (I suspect not...

Many of us have found, IIRC, that the center driver tube is the single user-switchable element in the ST-xxx amps that has the most influence on the sound. And since every system is different, and every set of ears has different preferences, there is no predicting how this one critical tube will fare in any situation. Try a 12BH7 as well - for many it gives a slightly darker and ballsier sound. GE made a great 12BH7.
In the outer, splitter holes, just about any pair of good-functioning, matched 12AU7s will do fine. I've personally never heard and discernable differences between anything I've used there. I use RCA 5963s that Bob L. originally supplied, simply because they are heavy-duty, reliable tubes.
In short, there's a lot to be said for having several choices for that center baby. You never know what you like until you try for awhile. There's plenty of inexpensive choices on the market. Go for diversity until you find something that clicks.

Bravo Kentley! Well done! (I have Downtown Abbey reruns on right now ☺)

Tubes4ever wrote:

deepee99 wrote:Skizzy,
Did I not read that the current and voltage needs of those guys were pushing the VTA iron a bit to the hot side? Please correct me.
Tnx
d

The 5V3 uses 3.8 amps at 5V.  The 5V3A uses 3 amps, just like the 5U4GB.  However, the 5V3A is rated at 350mA instead of the 275mA of the 5U4GB.  The 5V3A is actually a better choice as it has a lower voltage drop at the currents we use than a 5U4GB.
The 5AU4 is not electrically identical to the 5V3A.  You can use a 5V3A in place of a 5AU4. They are similar in plate current ratings.
The 5AU4 uses 3.75 Amps.


Tim

I'm sorry, I was thinking of 6SN7s; there are a couple of European variants that pull way too much filament current. Brain pharte on my part.

No problem Deepee.  It happens to all of us.

It's been awhile since I've posted regarding the 5V3A I installed in my ST120 in October 2015. Well, the same rectifier is still in operation and I can't find any faults in the way the amp sounds. I do check bias every couple months and make only minor adjustments. Now have a pretty good collection of 5V3A spares; they're still reasonable on eBay. I agree that the big bottle Mullard rectifiers have a definite cool factor, but after almost three years on the same rectifier, it's hard to argue with the results. There is a variable coming up in the near future. I'm getting back a set of Infinity RS6 speakers that I bought new in 1984. Looking forward to how they sound with the ST120..... All is well.

Nice to hear Gregg! I'm sure the 5v3A will be doing fine 5 years from now. You definitely aren't pushing it hard at all.

My center tube is 12BH7 EH gold pin with matched/balanced sections from Guru Jim McShane  ....and I'm a happy camper!

deepee99 wrote:

Tube Nube wrote:Dave, are you saying you can hear a difference related to rectifier used?

Absolutely. It pertains to the B+ voltage drop under a load, or what guitar amp players  refer to as "sag." 5R4 tubes have a lot of voltage drop, or sag; 5AR4 types, and GZ33 and GZ 37 Mullard tubes and the solid-state Webers have much less of a drop and are better suited for our purposes, as we are into reproducing sound, not creating it, with hi-fidelity, "fidelity" to the source material being the operative word.
As to the difference between solid-state copper-tops and and an antique Mullard tube, well, that's one them intangibles. But the Mullards look cooler.

Ever since I switched to GZ37's in my VTA ST120s, I don't want to use anything else.

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.

I (as well as a few others) find they develop more PUNCH at 60mV, and the ST120 has plenty of reserve to drive them properly at that level. I'd think that would definitely have an impact on a 5AR4's longevity though.

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.

Well, I have 5U4GB (NOS), 5µ3C (Winged C Svetlana  https://www.watfordvalves.com/product_detail.asp?id=72) and Mullard GZ37.   In all honesty, I hear no differences in sound (93 dB/W easy to drive speakers) .  The 5µ3C Svetlana is as beautiful (big bottle) as a GZ37 at 1/3 rd of the current price.   Only have 1 GZ37, but 4 5µ3C, so, they should serve me for the rest of my life .   And then , in worst case, I still have the Weber 68     ;-) ;-)

I forgot to mention the TAD solid state rectifier : http://www.tubeampdoctor.com/en/shop_TAD_Premium_Tubes_TAD_Rectifier_Tubes_SELECTED/TAD_Solid_State_Rectifier_389

The first part of this thread was on output tube idler current. Now mostly rectifier info. All good comments though.

On output tube current. I have always lowered the bias setting on all my Dyna vintage amps since the first one I owned in 1972. From this groups comments I lowered them further and it should help even more with tube longevity. One thing it did do that was not widely discussed was change the noise level in the amps. They run cooler but the biggest difference was the lowering of the magnetostriction noise from the power transformers. I think that is the proper term.

The higher the current the louder the noise. Lowering the output tube idle current helped lower my vintage amps noise level. Mark III KT88s are at 53 mA each and the Mark II 6CA7s at 50 mA each. It may not help a bit with the modern VTA amps. Anyone else find this to be true?

After four years and a few months, the 5V3A in the 120 quit quietly, no fireworks, sparks, or smoke. I turned the amp on and the rectifier filament didn't light. Went through my cache of 18 spares and selected a Tung Sol 5V3A, plugged it in, and all is well. I'll wait for 10 minutes or so and check the bias. Sockets and tube pins were all cleaned last year for normal maintenance.

Gregg,

Did you check the 5V3A to see if the filament is open? Just curious if it might have been a bad connection at the base pins?

Yes, the filament was open. None of the tube pins were loose, all had a shiny appearance. The solder connections looked good, but I did not reheat them. Since these tubes are all old, I wonder how much they were banged around in a tech’s tube caddy.