Hi all,

I've been able to get my hands on a second used UFO25, and it has arrived today. I've set both up with the same selections of tubes from the stock complements that came with them (OA3 input regulator and 7DJ8 input tube). I bridged both into mono using the instructions in the manual, hooked everything up on both sides, set all four gain knobs across both amps to 14 clicks, and sat down to enjoy the fruits of my labors.

Upsettingly, I found the right channel was noticeably louder than the left, despite the gains being set equally.

In order to verify, I hooked up my left speaker, alone, up to both amps separately and played the same 125 Hz test tone, with a dB meter set up, and found that indeed, that one amp was playing much louder than the left at the same gain settings (in clicks): 68 dB vs 65 dB. Hrmph.

Thinking about it, there are of course many factors that could lead one amp to perform differently from the other: any of the 14 tubes, overall age of components (amps were built 4 years apart) etc etc.

So, I decided to do some systematic tests swapping tubes between them and re-measuring.

I made the following individual tube swaps (and undid each swap before trying the next):

-Single input 7DJ8 tubes (the one from the older amp is a Philips, the newer says Matsushita Japan)
-Pair of 6P15P output tubes
-Single input OA3 regulator tubes
-Single recifier tubes
-Pair of output OA3 regulator tubes

In each case the older amp played the tone between 64.4 and 65 dB whereas the newer amp played the tone between 68 and 69 dB. I feel like this rules out tube quality/age as being the main issue.

I also swapped the power cables between the two amps (one is an audioquest, the other came from a rice cooker), no change.

The only other somewhat major difference between the two amps is that the older amp has Jupiter capacitors whereas the newer one has Miflex. Given the caps have the same values, though, could this be the cause of a 3 dB difference?

I emphasize that both amps were set to the exact same gain on both knobs, 14 clicks. If I turn the quieter amp up to 15 clicks, they'll actually match closer than they did at equal clicks, but now the 15 click amp will be ~1 dB louder than the 14 click amp, so that's no good either.

Anyone have any ideas as to what's going on or what I should be trying? Is it wrong for me to expect that I should be able to gain match these amps so that I can use them as monoblocks?

Thanks for any advice!

Yeah it had occurred to me that this could be the case too! I actually chatted with Steve on the phone a few weeks ago back when I was still hunting for my second UFO25, and was asking him about potential design changes that I might want to be aware of when pairing two amps that were built far apart -- I think I asked him if I could assume the stepped attenuators would be identical between any two that have them, and he said yes. But I'm still not ruling that out

In that direction, I've been playing with the amps some more, even setting each to different numbers of clicks doesn't really get them to match (one's always off in one direction or another), but it seems like their output levels more or less converge when I crank the gains on both to the max (and use the digital volume control on my streamer to avoid ear-shattering SPL). So that is somewhat encouraging. Nonetheless, I would like to get to the bottom of whether there's a crucial component difference/mismatch between the amps, and if solving that will allow me to easily attain level-matched volume control on the amps themselves, without having to resort to adding a pre-amp in the chain or using digital volume control as is the case now. For example, the current solution of just maxing out both of them to level match is no good if I want to listen to my vinyl rig.

Thanks for the thoughts! I would also be surprised if burning in the caps would make a big difference. I am actually planning on replacing the Jupiters in the older unit with Miflex so that they match in that respect, but I might try and get to the bottom of this before I go messing around with anything.

14 clicks on Miflex and 15 on Jupiter does close the gap--> 68 dB and 69 dB respectively, but it's not close enough. Basically, as I try and get them close one click at a time they leapfrog each other out of sync until they converge at max gain. But even if there were some fixed number of clicks on one and the other where the levels match, that would only give me one listenable matched volume level when using an analog source

I am quite curious about the pots now, and getting in there with a multimeter isn't a bad idea. Definitely something worth trying before sending them in to Steve.

I will say that when I level match them at max gain, I can definitely tell the difference in clarity and detail from having only one amp running in stereo, they sound really great! Very motivating

Ahhhh I see what you mean. Yeah okay I was actually wondering if that could be a solution. I didn't mention it because in the owner's manual Steve says: "There are two things to remember when the amp is bridged: 1) Keep both volume controls adjusted the same 2) Keep both speaker impedance switches adjusted the same."

It's definitely not obvious to me why it wouldn't be okay for the two volume controls to vary, and had wondered why he wrote this.

Yessir -- all switches in the same position on both amps!

Ah, good thought! Just tried switching over the entire tube complement, and no dice -- the older amp is still 3dB short of the newer.

I unbridged the two amps and tested each channel individually, gain set at 14 clicks on all channels across all amps.

The existing gap between the two amps actually widened a bit!

Amp 1 - L: 62 dB, R: 63 dB
Amp 2 - L: 67.5 dB, R: 67.5 dB

I emphasize again that all channels played to the same single speaker with the same input.

Amp 2 still dominates amp 1, but I did find a discrepancy between the two channels on Amp 1.

I swapped out the output 6P15P tubes on Amp 1 with the second pair that came with the amp, re-tested, and the two channels balanced out

Amp 1 (fresh output tubes) - L: 63.4 dB, R: 63.6 dB

But, even with the balance issue within Amp 1 resolved, Amp 2 is still clearly playing much louder with the same input, same speaker, and with the tube complements swapped between them.

Annoying. I would send them both back to Steve and let him sort it out. Better than being driven crazy.

Yeah, Bramar I've tried matching them by using different volume levels. Unfortunately none of the discrete settings match up except at maximum gain.

Example: at 14 clicks on each Amp1=65 dB and Amp2= 68 dB.

If I turn Amp1 up a click, I get Amp1=69 dB and Amp2 = 68 dB.

If instead I turn Amp2 down a click, I get Amp1=65 dB and Amp2 = 63 dB (<--- not an exact figure but it was sth like this)

I haven't found any mismatched settings that match -- closest I ever get them is 1 dB apart until they're at near max gain. But even if I did find one matched setting, that doesn't solve the problem: I want to be able to freely turn the amps up and down when listening to LPs while keeping them matched and without having to add a $$ pre-amp to my chain simply for the sake of having functional volume control.

Showme, yeah, sending them back to Steve might be the answer in the end, but I do have a non-trivial threshold for tinkering myself before I get there :p That won't be cheap. Still very open to suggestions! If I haven't figured it out by the weekend, that's when I'll probably measure the potentiometers and see if that's it.

Thanks all for your thoughts and suggestions so far

mk60, I definitely appreciate your conundrum. I recommend calling Sarah to set up a phone appt with Steve and he’ll talk you through some things….this phone contact with Steve is required anyway before an RMA is set up for sending in the amp(s).

Best of luck to you, maybe you’ll find a solution beforehand.

Brad

FWIW the stepped attenuators on my UFOs were the same. There was no intermediate, just one or the other. Which is how “stepped” attenuators should work IMO.

With all due respect this whole thread seems unnecessary. All this stuff about the decibels and the clicks. Just tune the volume by ear.

I had a pair of SE84UFOs in mono. One had Beeswax caps and the other had copper foil beeswax caps. I will be damned if the one with the regular beeswax caps sounded better.

The danger of cobbling together two different amps is that they are all unique and might sound different.

Rivieraranch -- I tried and failed to tune them by ear, and because of that started using the dB meter to validate what I was hearing. The point is that they are very noticeably playing at different volume levels by ear, and I can't get that to not be true unless I crank them to max gain. I'm not saying that the caps are the cause of the difference.

I'm just trying to figure out what needs to be done to be able to make these two amps usable together as monoblocks which is an advertised feature. I don't think that this is unnecessary, I spent a lot of money in order to be able to do so. I'm not saying that the two amps need to be atomically identical to each other, just reasonably listenable at the same volume of my choice. I understand that since both amps are used they might need to be gone over by steve to make sure they're both operating properly and play together as monoblocks. This is what I'm trying to figure out.

I have avoided entering this conversation since I do not have stepped attenuators and I use a mismatched pair of SE84, seperated by about thirty five serial numbers. One is black and one is silver.
I'm sure that dates them for some here.
I don't have anything newish to identify more closely.

I have often used them full throttle on the knobs and sometimes at the 12:00 high position depending upon varying factors. Then if necessary tweak one side or the other.

Finding that center stage and placing it correctly is critical to me. I often do not use a centered vocal to find center stage though. I use the reverberation inherent to the recording.
Obviously this does not work with many rock studio recordings which employ canned reverberation strategies. Centering duty falls to the kick in many of those recordings.

With any live recorded orchestra though, this is the most telling of ways to find true center stage and reproduce the best balanced reproduction experience.

Using my own mismatched ears to determine my own need is the only solution I have found that actually works my way. I learned long ago to ignore most number or knob settings, even if you need repeatable settings, as with stepped configurations.

The one single thing that makes more of a difference than any positioning of a gain control is the matching of tubes between the two amps.

If I missed the comment, I am sorry to repeat, but has this higher level of tube matching been looked at as a work through?

So you are adding a preamplifier?

Rivieraranch: actually I'm going to try new attenuators first. Very straightforward and cheap solution, and will make it as if the amps came matched from Steve himself. I like a good DIY, and would save me thousands.