Question about Output Transformer Impedance

[Wayne Alexander]

I built my Express clone with a Heyboer HTS-8110 Output transformer, which has a choice of 6,600 Ohms or 5,200 Ohms on the primaries, and 4/8/16 ohm secondaries (with EL34's, they'd be 8/16/32 with 6V6's).

I built mine with 5200, for no particular reason. Can someone explain what the difference would be between the two? My comprehension of this is weak.

[Elcabong]

This is the reflected impedance that the tubes are going to see. We are not talking about a big difference here. It would be interesting to see if there is any noticeable difference by doing an A/B comparison.

Moose, from who I just bought an OT by the way, was wondering about the same issue in another thread... http://maximoaudio.com/forum/viewtopic. ... ht=heyboer

[cerrem]

As the plate load value gets bigger in Class AB1..... The output signal on the primary will clip sooner, sounds like a ... ALso reduced AC current swing...
The inductance would need to go up also to maintain the same low frequency response...whether or not you can hear this is another story, since you may have had an overkill of L to begin with...

Chris

[erigm]

Well, hello cerrem! Nice to have your knowledge over here.

Are these "multiple primary taps" the same as "primary and ultralinear taps". What I am asking is are they the same ratio? Are we using ultralinear designed transformers and then just using the ultralinear tap for a lower plate load impedance? Thanks.

[Moose]

Are these "multiple primary taps" the same as "primary and ultralinear taps". What I am asking is are they the same ratio? Are we using ultralinear designed transformers and then just using the ultralinear tap for a lower plate load impedance? Thanks.

It's not an ultralinear transformer. It's just tapped for wither a 6600 or 5200 ohm primary because we've seen both values in real trainwrecks, and the general consensus (my opinion included) was that having both taps would allow us to experiment and hear the difference.

[Wayne Alexander]

Chris, so if I swap the OT taps from the existing 5200 to the currently unused 6600 I may get a bit earlier breakup and possibly a bit less bass response? That would be good in the case of my amp, which is among the LOUDEST amps I've ever played (at least with EL34's in it), and currently has a bit too much bass getting through.

[Bear]

Chris, so if I swap the OT taps from the existing 5200 to the currently unused 6600 I may get a bit earlier breakup and possibly a bit less bass response? That would be good in the case of my amp, which is among the LOUDEST amps I've ever played (at least with EL34's in it), and currently has a bit too much bass getting through.

TAKE NOTES! A bass heavy Trainwreck DIY is not a common occurence, and people might want to know how to skew their builds that way.

Bear

[Moose]

That's sort of the theory. You'll lose a touch of headroom and output power with the higher loading. Looking at tube data sheets, you usually see a sweet spot of output to distortion, but as you go higher you get less output but don't significantly reduce THD. Most people don't play in this range, but KF seems to have liked slightly higher loading than datasheets dictate. I experimented a little recently and the effects of changing impedance are pretty subtle, though I could hear them.

As for it being the loudest, that's a strange perception I think we all have. It doesn't put out so much power, so it seems it shouldn't be as loud as a 50watt amp, but the voicing and touch sensetivity make it really seem loud, and really cut through a mix. I often wonder when I read the Vintage Guitar review exactly WHAT they expect for stage volume and what constitutes a medium sized club to them, because I've heard a lot of 35 watt amps that more than hold their own against a band. Needless to say, I use an attenuator even with the 6V6 when I want practice levels.

[Moose]

TAKE NOTES! A bass heavy Trainwreck DIY is not a common occurence, and people might want to know how to skew their builds that way.

You ain't kiddin!

There's some bass to be found. The problem really is less related to overall bass available, and more to perception. There's a tone of top to be had, and that overwhelms the bass balance. It's a lead amp and that means it's voiced to cut through. Upper mids cut, and that's the way expresses were made for sure.

[Wayne Alexander]

One thing I did that increased bass response was use a 1 meg bass pot instead of the 250K in the stock A1A schematic. I had the 250K first and it seemed not to have enough bass, the current setup may be too much. I might try a 500K bass pot.

[Mark]

Dear Wayne

I probably have said all this before, so if you've heard it all before please bear with me.

My initial build was the A1a circuit using 6V6's, I did this purely through practicality. I had a New Sensor output transformer with a 8.5K primary. As far as speakers went I used a Mesa Boogie Celestion in a SOB combo cab. The results were the amp was very bright and harsh.

I decided to borrow a friend‘s 4 X 12” cab loaded with 75 watt Celestions, the effect this had on the amp was to increase the bass response and to slightly decrease the treble response. In fact, all the recordings that are on this forum are with the 4 X 12” cab.

So what I’m saying here is that the speakers you may decide to use are (IMHO) an important part of voicing the amp. There are plenty of old posts by guys like Mook saying use this speaker box and that guitar and start tweaking.

I think I read somewhere that KF preferred the old Celestion H30’s, though Hogy prefers a speaker set up consisting of two alnico blues and two H30. This is an expensive 4 X 12” cab!
I think the guy at Scumbags amps was doing something with Ted Weber to make a speaker that sounds like the old H30 Celestions.

Okay, now I’ve gotten that off my chest, I can finally get to the purpose of the post, the value of the bass control. I too experimented with 250K versus 1M. I found that the 250K value gave me a change in bass response throughout it’s entire travel, where the 1M pot gave me a change in bass response only for say 80% of it’s travel. I concluded that the maximum bass response was about the same when the bass control was set at maximum.

I will have to get back into the Wreck experiments, and perhaps then I can give some sort of definitive answer.

[Wayne Alexander]

Mark, I've got lots of speaker cabs available, but the one that seems to fit the Express the best is a Mojave 2x12 "slightly" open back, with a Celestion G12H and an Eminence Red Fang currently in it. That cabinet can really pass bass along, so I might tweak my Express clone a bit to reduce some of the bass in the signal.