small vs large OT

[Smokebreak]

I've got two builds here I'm A/B-ing.
Both are AB763 2 x 6L6 variants, and circuits are very similar
Amp A is using a large Hammond OT, amp B is using a (much)smaller "40W" OT from Mag Comp. The only other mentionable differences for this purpose is that amp A has an extra node of filtering for V1, and amp B is BF style(everything before the PI on 1 node). Also , amp A has 250V on V1 plates, amp B 190V on V1.

Amp A sounds great all the way up the dials. Amp B makes the speaker fart/rattle sound when the bass and mids are up, along with the volume. Note that in amp B , bypass caps are quite low and outputs are .02 .
I'm getting the same results for different speakers.

Now assuming the small OT is the stinker, this is the opposite of what I expected. I figured the smaller OT, with less bandwith, wouldn't even pass fartable frequencies to the speaker, right? Or is just getting so saturated that it's just representing those lower frequencies as a belch?
Where is this creamy, sweet OD that these smaller OTs produce?

I'm sticking in more filtering, then a large OT, but I'm curious what you guys think.

[johnnyreece]

Maybe try swapping them for fun (and science)?

[Phil_S]

I agree, different amps. You've got to swap the OT's and see if it's the amp or the OT.

[matt h]

(deleted)

[lord preset]

Selecting output transformers strikes me a dark art, at least as it is practiced for guitar amps. Beyond matching primary impedance to power tubes, picking a manufacturers wattage rating appropriate for the expected output and eyeballing the physical size of the OT what is there? Notwithstanding physical size what are the specs that electrically describe the difference between a "30 watt" AC30 OT and a "35 watt" Vibrolux OT that is tiny in comparison? Put another way if one were to have a custom OT wound for a given amp and wanted the OT to provide low bandwidth/high compression or the opposite, how would you specify it? I realize that size is indeed a factor but how big is big?

Enquiring minds want to know.

[Smokebreak]

This little guy is the CT 40-18006. It sounds great until the amp is halfway up. I just stripped it down to a bandmaster(I've got a hot 3rd stage in there), thinking I may have just been pushing it to hard, but it's the same....not in that "every stage is 25u bypassed and the bass in on 6" kinda farty way, but like it just gives up at a point and goes blat. If the bass and mids are turned all the way down it can hang. Granted I have to beat on the guitar for this to happen. This is through a gold and a Jensen Falcon(which I'm really diggin btw)

[JazzGuitarGimp]

Hey Smoke, As I read your post, i was thinking "swap the OT's and see if the farts follow the OT', only to find out that has already been suggested not once, but three times! Oh well, what can I say - late to the party yet again!

[sliberty]

Are the PTs the same in both amps?

[pdf64]

Maybe the Bandmaster coped with a seemingly underspeced OT because the PT was similarly underspecced, and, with a tube rectifier, the B+ sagged enough to avoid the OT getting saturated?

A stiffer B+ may result in the OT being able to be pushed into saturation?

The system consists of the 'wood' and the 'trees' that comprise it, so a system designer needs to be able to focus on both!

[Smokebreak]

I'm gonna drop in a new Hammond 1750M in a few days and see what happens. That's what's in the other one, and that OT always does me right! If I get a chance I'll swap the lil guy in the other amp, but I'm gonna work on the amp as-is, as I'm curious what it's gonna take, if anything, to get it workin with the rest of the amp.
The PTs are different. The one with the lil OT is GZ34 and 460V plates, and the other is SS and 475V IIRC, so rectification aside, pretty close @200mA rating.
I recently fixed up a '66 Bandmaster that sounded incredible(that's what got me buying the MC lil OT). SS rectified w/460V plates and 280V V1 plates! There wasn't a single harsh thing about that amp. All to say I don't think it's a voltage issue, as I've had the preamp from 180V-280V in this current build. I do think it may be a filtering issue, that is , if the OT is indeed up to snuff in the first place.
Not quite the same thing, but I did use the teeny 6V6 version OT in another amp and that thing does the creamy compressed thing really well

[pdf64]

The method I use to characterise an amp's power supply compression is to check the VB+ at idle, at max clean sine, and at max square.
You may find that even though two amps have a similar static VB+, the effective series resistance of the B+ winding etc differs, resulting in one sagging more under heavy load. That may be what's happening with the Bandmaster (thereby avoiding its OT saturating)?

[Smokebreak]

Thanks for the thoughts guys. To follow up, the OT swap didn't affect any change. I had been culling from a handful of brand new prod HG++, which has been my go-to for a while. Looks like the most recent batch(or at least the ones I got) weren't up to par...and I thought I had swapped enough tubes on this amp to put that possibility to rest. Then a couple of new AT7s were stinkers.
Things started to get better, then I upped the PI filtering and bingo, the thing bloomed.
I just got to crank it and it's absolutely glorious. Just the kind of Fender OD I love. Two thumbs up for the MagComp teeny OT 40-18006.

The more tubes that come in and out of here, the more I think I'm finally gonna start sending the stinkers back. 20% of this batch I just got sound bad, and I've only been through about 1/2 of them. Ridiculous.

[martin manning]

The more tubes that come in and out of here, the more I think I'm finally gonna start sending the stinkers back. 20% of this batch I just got sound bad, and I've only been through about 1/2 of them. Ridiculous.

Yes! Everybody should do that. If the dealers get them back they will put pressure on the manufacturers to improve their QC.

[Colossal]

The more tubes that come in and out of here, the more I think I'm finally gonna start sending the stinkers back. 20% of this batch I just got sound bad, and I've only been through about 1/2 of them. Ridiculous.

Yes! Everybody should do that. If the dealers get them back they will put pressure on the manufacturers to improve their QC.

There is certainly enough demand to warrant better QC.

[pula58]

I'm gonna drop in a new Hammond 1750M in a few days and see what happens. That's what's in the other one, and that OT always does me right! If I get a chance I'll swap the lil guy in the other amp, but I'm gonna work on the amp as-is, as I'm curious what it's gonna take, if anything, to get it workin with the rest of the amp.
The PTs are different. The one with the lil OT is GZ34 and 460V plates, and the other is SS and 475V IIRC, so rectification aside, pretty close @200mA rating.
I recently fixed up a '66 Bandmaster that sounded incredible(that's what got me buying the MC lil OT). SS rectified w/460V plates and 280V V1 plates! There wasn't a single harsh thing about that amp. All to say I don't think it's a voltage issue, as I've had the preamp from 180V-280V in this current build. I do think it may be a filtering issue, that is , if the OT is indeed up to snuff in the first place.
Not quite the same thing, but I did use the teeny 6V6 version OT in another amp and that thing does the creamy compressed thing really well

I like the hammond 1750M too!