Jolida 502 KT120 tube swapping

Having been told by the nice people at Jolida that the 502 transformer can handle the KT120's "filament requirements", I wondered if any Jolida-philes out there have tried them in either the 502BRC integrated or 502P (what I have) power amp, as they're apparently similar circuits. I'm tired of reading about how groovy the KT120 is and plan to try 'em eventually just to stop the madness and have some extra tubes around.
The Jolida is basically a Mullard 5-20 design with different tubes. Don't spend the money on the tubes. IMHO. Replace the 15k resistor in the 12at7 tail with a Compact Cascode current source kit from

Then replace the Plate resistors on the 12at7's. Match them at either 48k or 56k. Adjust for about 3mA per side or 6mA current across each resistor. Remember do this to both channels.

Look on the net for the Jolida schematic and you will see the resistors I'm talking about. You will thank me. This will make a larger improvement than the output tubes.
I think that's a done deal as all the important (seemingly) resisters in the signal path were replaced when Jolida MD did an "upgrade" mod before shipping the amp to me. The tube "rolling" is simply for my own amusement as every review I've seen of the KT120s (which are fairly inexpensive as these things go) has mentioned there is a significant audible difference between those and other types, and if your transformer can take the "heat" you can drop 'em in...I am merely fishing for owners who've tried them in a Jolida. I have no issue with the current Tung Sol 6550s as the amp sounds absolutely great with the supplied "stock" tubes (Tung Sol 12AX7s, EH 12AT7s).
The resistors I mentioned was not part of the upgrade. I'm sorry I was just trying to help you improve the amp.

I would be careful putting the KT120's in this amp. First they draw more current and this could hurt the power transformer with extended use. The current draw from a KT88 is 1.6A. According to the Tungsol data sheets they draw between 1.7A and 1.95A. It doesn't sound like much but it could be enough to fry the transformer over time.

Secondly these are not the same tubes. KT88 and KT120 have different operating conditions. Yes they both have 6.3v heaters and the tube pins are the same. But there is more to a tubes operation. If your amp is not set-up to run the KT120 it will not be running these tubes at their best operating point.

There are enough new KT88's to choose from. Also your input tubes can both be improved on. IMHO Roll a few and find out.
Don't be sorry. I appreciate the feedback and respect your technical tweak choices, although you assume I have the ability to install, match, and adjust this stuff when I rarely venture past soldering something that looks unsoldered. That said, I'll look into your suggestions and see if I trust myself to do the surgery. I have spoken to everybody but the janitor at Jolida at length about the KT120 having enough current available to not melt my amp, and they assure me it's overbuilt to easily handle the 120s. If it blows up they'll fix it (at least for the next 2 years). Along with the KT120 appealing to me as an amusing tweak, I'll certainly be trying whatever KT88s seem interesting also. I do plan to mess with the input tubes (I roll tubes in my guitar amps from time to time and do notice obvious differences)...I mean really...this is the entire point of this amp for me...I can mess with it! In any case, thanks again for the feedback...nice to know there's a braintrust out there.
SO...I put the KT120s in the 502P..and they work swimmingly. It is a LARGE tube in every way. Just more of everything...power (hard to say how much, but it's there), better, sweeter treble...more overall tonal range across the board. They do run hotter of course, but highly recommended at least until my amp blows up! (Talked to Jolida again and Mister Allen says no worries...and he's the warranty man!).
Hey Wolf

In order to get the KT120 into the Jolida 502P did any special modifications have to be done to it so that KT120s could be used in place of the 6550s? Was it just drop 'em in run the EZ-Bias? Also where did you get the KT120 tubes? The more I look at the Jolida 502P and reading about it I keep thinking about getting one.

Other than the "upgrades" they did on the amp before shipping there are no mods...I just dropped the 120s in and I bias them to the 502P specs and check it with my trusty Radio Shack meter just for the hell of it (a compulsion, but I'm getting better at not burning my fingers while poking the probes into the test port), even though the "ez bias" works well...In fact, now that the KT120s have settled in it sounds better than I noted in my last post, just fuller and brighter. Highly recommended! I bought the 4 "matched" tubes from "Ctech" on Ebay mostly because of their price, and they're on the East Coast so they shipped really fast. I don't think there's anything to risk buying one of these amps as they seem to be an insane bargain for something that sounds this good.
Upon reading this thread I tried the KT120. Its almost 2 weeks now and I really like it a lot!

Just wondering what pre-amp tubes should I use? I got a recommendation from a Jolida tech that SED (Winged C) is a good option. Have someone tried it?

Thank you for sharing!
There are a LOT of "small tube" suggestions around...I'm using "preferrd" 7025s (12ax7) from thetubestore, and recently added a pair of Mullard 12at7/4024s. Jolida's Jerred likes GEs, Michael Allen likes Phillips Mini Watt tubes...I like whatever sounds good and doesn't cost a fortune.
Hi Rattosecond

Welcome to the club. Like Wolf said there will be LOTS of preamp tube suggestions from owners to tube sellers.

Here are some combos that others have recommended to me and some that I have tried.

Tungsram 12AX7 tubes platinum or gold matched paired with Mullard CV4024 12AT7 type tubes.

I too also liked the Preferred 7025 from the tubestore paired with the Mullard CV4024 tubes.

If your source is a bit laid back or mellow to get more detail out of your music I liked the combination of Penta Labs 12AT7 tubes paired with Groove Tubes Gold Series GT-12AX7-C.

If your source is forward or has a high gain I liked pairing reissue Tung Sol 12AT7 and Mullard reissue 12AX7 tubes together.

Again this is all system dependent and in the systems I had these last 3 combinations is what I found after weeks of listening. I had Tung Sol KT-120 tubes in my Jolida at the time.

Best of luck on your search. I hope I have not confused you further.
I should have said "ordered" the Mullards...they don't get here until Monday or something. The only tube Jedinite24 seemingly doesn't have (and I do have) are the el cheapo Sovtek KT88s I bought a while back...and damn if they don't also sound great in the Jolida, although the KT120s are in there currently. I nominate Jedinite as the official 502p tube information and testing source...I might send him my KT88s for a review! Somebody should make a little LED lighted tube holder so you could enjoy extra tubes as visual art, instead of sticking them in a drawer like I do.
Morning Wolf

I'll pass on being a testing source. Doing the swapping I did and keeping notes of findings and combinations was getting annoying. That and like everything else findings are subjective and system dependent. I'll definitely keep compiling info from others though about different tubes used in the Jolida 502P as I get them. I'll definitely share although most of the tubes I got were based off of your recommendations. You get lots of better information than what I got when I called Jolida or the Tube Store. I'm even trying JAN 12AT7 type tubes but from GE now but am finding them to be the same as the Mullard CV4024 tubes. Will give it more time though.

I bought way too many 12AT7 and 12AX7 tubes when I saw deals on various sites. I did try to get the Sovtek KT-88 tubes based on your thoughts about them but my local pro audio shop couldn't get them for me. Speaking of Sovtek from talking to Jim McShane he was suggesting a real quiet Sovtek is the 12AX7LPS. I was tempted on that one too but decided to pass.

I'm going to cry if Tung Sol makes the Gold Pin uber rugged version of the KT-120s. Another set of tubs to get. I think I need to be put in a straight jacket.
HA...yeah...the Sovteks were an impulse purchase from an electronics/hobby shop so I was clueless about them...really bought because they were so inexpensive (about $100 for all four), but they look cool when glowing. I had to return the JAN Phillips 12at7s since one failed in the harm to the amp ...and they suggested the Mullards so I bit. Warranty! Since KT120s have no "vintage" relative to re-issue, we can assume they're safe in the current "matched or cryoed or shaken and not stirred" world...I do wonder how many of these are being made relative to other tubes as they show up everywhere now.
Follow up: The Mullard 12at7s sound great. And the frightening glow explosion flash when the amp powers up is a beautiful thing.
When I first saw the Mullard CV4024s flash so much on the initial turn on I was a little alarmed but then read that flashing is normal for them. It is cool looking. They glow brighter than the Tung Sol 12AT7 re-issues. Very pretty.
Yeah...I had heard about Mullard Flashing and also was calmed when I looked up specific information about it. It adds a dramatic moment to amp start-up that otherwise would be a non-dramatic event. Maybe my threshold of entertainment is too low.
Thanks Jedinite24! Will try those combination if I get a chance.

Someone is recommended me to use Telefunkens. Dont know what to expect till I have it.

How did you do guys the biasing? My speakers sensitivity is 86 so I am thinking the Jolida is struggling. I am thinking of making it a little hotter at 60, would it cause some problems?
Hello again Rattosecond

For my Jolida 502P with the KT120s or any of the KT88 or 6550 type tubes in place I would always bias the amp at least 515 to 525 millivolts. One time by accident when I was swapping out NOS 6550 tubes for newer KT88/KT120 I had forgotten to lower the bias settings and found when I checked with my meter some tubes were getting 625 to 712 millivolts. I didn't notice anything odd with the tubes or any additional heat coming from the Jolida but I also didn't have the 502P at these bias settings for long.

I think if you increase the bias you will be taxing the tubes more and the transformers. My speakers are mostly 88db sensitive. Only the Ohm Walsh 2s I have maybe are the most difficult load for the Jolida. I used the 4 ohm taps on the 502P with the Ohms but kept the bias at around 515 to 520 millivolts. The music was relaxed and not so forward. Really good for nearfield listening. When I swapped out the Jolida and put in a Class D Audio amp that put out just 120w/channel into 4 ohms it made the Ohm Walsh 2 go quite a bit louder, more forward but the treble became a bit harsh till I made adjustments.

What do you feel is lacking with the Jolida 502B and your 86db sensitive speakers?

I have a Jolida 801. I like it well enough but having to turn the amp over to bias the tubes is a pain in the arse. Thinking about maybe going with the 502. I lose 10 watts (70wpc in the 801 vs. 60wpc) but my speakers are efficient enough that I probably won't see too much difference. I gain a Remote, which purists may condemn, but hey, I am getting older, and HT pass-thru.

Will you describe what mods Jolida MD actually performed? I was looking at Underwood Wally but would like to have some options. Thanks.
I was advised (and my manual recommends) to bias any sort of tube to the stock 500mv and it seems to work fine. There is 50mv wiggle room built in.
Oops...missed the mod query. Here's what Jerrod sent to me:

2 - clarity esa caps .47uf/630v
4 - jolida custom caps .22uf/630v
4 - hexfred diodes
4 - ohmite little demon ceramic resistors 10k/2w
array of resistor changes within the signal path

An inexpensive installation of some seemingly worthwhile items, and, after almost a year and a half of tube rolling, this little amp has yet to sound anything but wonderful.