Time delay board, UF4007 resistor mod, KT88 GL and Tung Sol/Sovtek KT120 questions

Time delay board insights appreciated

I found the thread with the yellow sheet resister mod and ordered a couple of UF4007 to help with the rectifier tube in my ST120 and then I wondered if this modification would be sufficient enough to install the time delay board offered around here. I have a feeling it wont, but my knowledge is limited on these sorts of things. I also thought to ask if there was ever a solid state rectifier circuit schematic for the DIY people around this community put into public view? I should have searched that one out before hand, but I figured why not ask in this post since it wont clutter up the main forum page.

KT88 Gold Lion and KT120 insights appreciated

I know there have been a lot of question posts about how these 2 tube designations compare, but I hardly saw any actual comparisons of the Gold Lion KT88 against the specific Tung Sol KT120 tubes. I know some general comparisons have been made between the 88 and 120 and of course there is information from various people making claims that kind of conflict or at the very least do not "add up" from user to user. Its been stated that the KT88 is more linear, but I also saw it stated that the KT88 is supposed to have more exaggeration at the very top and very low end, as well as another claim that someone made that the KT88 was more full bodied in the bass region of the band, and another claim was that the KT120 surpassed the 88 on all fronts. Is it that the KT88 is inherently more linear? Does anyone know if the 120 has a hump in the middle or if maybe it rolls off on both ends? I would check data sheets but I figured that these characteristics can change form tube to tube considering they are manually made by human beings. If this idea is wrong please tell me as I want to know for the future pursuits. I would also love to hear from others that have direct experience using the GL 88 and the Tung 120's in the same amp(s) and same systems tell me differences with detail, staging, clarity, and linearity between the two.

Thanks for reading and any replies.

The "yellow sheet" diode mode keeps the charge in the cap from reversing into the rectifier tube if the amp is short cycled. Not sure what it has to do with the time delay board.

GL KT88 and KT120s are being sanctioned by Russia along with all New Sensor brands.

Having said that, when I was looking (pre pandemic) for GL KT88s I saw the price and got the KT120's instead. To me they sound like a 6550 with more guts. It was also mentioned that they'd live a long life in a VTA amp.

The pre-amp phase splitter (center) tube has the biggest effect on sound.

A lot has been mentioned on the forum about this. Welcome to the party!

New2Tubez wrote:The "yellow sheet" diode mode keeps the charge in the cap from reversing into the rectifier tube if the amp is short cycled. Not sure what it has to do with the time delay board.

GL KT88 and KT120s are being sanctioned by Russia along with all New Sensor brands.

Having said that, when I was looking (pre pandemic) for GL KT88s I saw the price and got the KT120's instead. To me they sound like a 6550 with more guts. It was also mentioned that they'd live a long life in a VTA amp.

The pre-amp phase splitter (center) tube has the biggest effect on sound.

A lot has been mentioned on the forum about this. Welcome to the party!

I did see this mentioned, and I think I saw the idea mentioned that one could run 12AU7 on each side of the center 12AT7, is this true? I may have some how confused that point with someone's discussion about another amp. I have a full set of Gold Lion KY88 and it seems the KT120s are no longer up for grabs other than a few Ebay sellers asking north of 300usd for 2 of them in "used" condition. I didnt realize the sanctioning of Russia affected these tubes. Now that I know this sort of answers all of my questions about the minimal differences considering the cheapest quad set is 597 usd before taxes. In my case this means 660usd for a quad set that probably cost around half of that a few years ago. Luckily I bought 2 spare kt88 tubes back 6 to 8 yrs ago. They were testing very close to new and I needed a single one from an object falling off a shelf, well, I figured why not buy 3 of them for 70 something dollars. I figured the seller had a similar issue and decided to do soemthing different, what a steal. My telefunken quad set has an EL84 that has lost the chrone-y mirror on top... yet my SC35 still sounds magnificent. Any idea if this is dangerous considering the quad has to be matched for said amp? When I see quad set prices for tele el84's I shudder with disappointment. Its mind blowing how fast the prices on these tubes and amps etc have gone up. A few years ago I sold a Marantz 2030 for a couple hundred bucks.. out of curiosity I checked the bay.. they are hard to find under 600 dollars now with a regular buy it now price of 7 or 8.

I don't think you can run 12AT7's in your ST120 unless it's an older board version. Wait for Bob's or Roy's advice before you go there.

I use a Weber WS1-t but am also using pavel's auto bias which has a built in delay so can't help you there. Prior to the AB board, I used a TungSol 5AR4 (reissue) w/ no problem. Also did the diode mod. 

YOU REALLY NEED TO GO THROUGH THE FORUM THEADS. 
That's what I did and still do if I have a question. The board design goes back to the 1980's. If there's a question, it's surely been answered.

As for the drilled down specifics on how the tubes sound, you're going to have to try them in your system with your speakers in your room, with your ears.

Bob Latino wrote:If you use the standard driver board that uses 9 pin driver tubes, then ALL VTA M-125's use 12AU7 type driver tubes. A few pairs of M-125's out there use a 6SN7 octal board.

VTA ST-70 or ST-120 amps (kit or wired) sold before April of 2012 used a higher gain 12AT7 driver board. In April of 2012 a switch was made to the newer CCS driver board which uses the lower gain 12AU7 driver tubes. Do NOT use anything other than a 12AU7 type tube in amps from April of 2012 to the present. If you try using a 12AT7 or 12AX7 driver tube in the newer amps, you can damage one or both of the LM334 current regulators on the driver board.

Besides a 12AU7 tube on the newer amps, you can also use any of the military/industrial versions of the 12AU7 like the 5963, 5814, 6189 or the Mullard CV4003.

Bob

Below is a photo of the newer VTA driver board (April 2012 - present). The two LM334's are circled in red. If you have this board, you have to use 12AU7 type tubes. If you do not see the LM334's, then you have an older board that uses 12AT7 driver tubes.

Oh man, I completely forgot there was a revised 12AU7 driver board that came along after my 12at7 variant. This other post mentioned makes sense now. I assume there is no way to run the 12AU7 in the driver board for my amp. Really not a big deal considering I have a ton of both tube types...and some how I ended up with several pairs of 6922 tubes when they only go in my SCA 35, speaking of... do you know of any decent cheap valve testing equipment that checks shorts, grids, leaks for the 12__7 and other related tubes? I would love to be able to test them and let go of some and maybe match some others.

If you want to test the nine pin 12AX7, AU7, AT7, etc. a MaxiPreAmp II is about a grand. You'll need a $50 adaptor for the 6922 family of tubes.

I built this one right after I got my VTA ST-120 in 2018, pics posted below is my variant of the original presentation:

https://valveheaven.com/2015/03/an-inexpensive-easy-to-build-diy-valvetube-tester/

https://valveheaven.com/wp-content/uploads/2015/03/An-inexpensive-easy-to-build-diy-valve-tester.pdf

KT120 draws more heater current than KT88 and amount varies by brand.
Be sure the heater windings can supply enough current... and if they can, then the voltage is likely too high for KT88.
Overvoltage shortens tube life.

I don't see how using a 12AT7 in the driver can hurt the LM334. The LM334 current is limited by the dropping resistors and 12AU7 phase splitter.
If you have more gain in the driver, the input level will be lower for the same volume out, driving the 12AU7 to the same levels.

An intermediate gain tube between a 12AU7 and 12AT7 is the 12AY7. There are industrial equivalents as well.