Output Transformer Primary Impedance

[vibrostrat43]

How does primary impedance affect the tonality of the amp? For instance are there any generalizations about how a 6.6k CT OT sound compared to 4k CT OT if both were rated for the same wattage etc? Would a higher impedance make the amp break up earlier or later? Make the sound "browner" or have more air?

On a related side note, has anyone had good luck with Hammond transformers, or are they too hifi at their design ratings? They have an offering that I believe would work perfectly for my Tweed Bassman with reverb design (not really a clone but kind of inspired by it) that's unavailable anywhere else. For what it's worth I'm looking to get a tighter low end, and a little easier breakup which I know usually don't go hand in hand.

I'm currently using a stock '67 Bassman OT the Hammond I'm looking at is a 1650PA which you can see the specs for here

http://www.hammondmfg.com/1608A.htm

I'm also looking at 1650HA which is rated for 40 watts, but as of right now I'm more concerned with tight than easier breakup. Of course I may just be completely barking up the wrong tree for tightening the low end. Another related question would be what can I do to tighten the low end other than reducing coupling cap, and cbc values? and what would make the biggest impact?

[TweeDLX]

Heavier filtering will tighten low end, but at the cost of easier break-up. Bigger/higher rated OT will also tighten low end. The two examples you cited do not tighten, but REMOVE low end/decrease gain. Try the heavier filtering and decrease any negative feedback to get a bit rawer sound. Might be what you're looking for.

[vibrostrat43]

Heavier filtering will tighten low end, but at the cost of easier break-up. Bigger/higher rated OT will also tighten low end. The two examples you cited do not tighten, but REMOVE low end/decrease gain. Try the heavier filtering and decrease any negative feedback to get a bit rawer sound. Might be what you're looking for.

Ahhh! Thanks, for the information! I did not want to drop the money for a transformer if it wasn't going to help me achieve my end goal. I've had trouble finding much information on the web about what effect primary impedance has on tone, other than that you need a certain range of impedance for certain tubes.

[DGDGBD]

The OT with the 6.6K primary Z will result in less clean power output than with the standard 4K primary, and hence earlier breakup. This assumes you are talking about a 6L6 push pull amp design.

[GrungeMan]

The Hammond "Guitar Amp Transformers" are not hifi, they are spec'd for the amp model stated.

Check the spec's on the Hammond site.

Grungy :D

[pdf64]

The two examples you cited do not tighten, but REMOVE low end/decrease gain
Mike, what's your reasoning there?
My thinking is that plenty of iron (for the 1650PA) and the 30Hz roll off should eliminate risk of OT saturation (which would cause collapse of primary inductance) for high power low end.
Pete.

[TweeDLX]

The two examples you cited do not tighten, but REMOVE low end/decrease gain

Mike, what's your reasoning there?
Pete,

I was referring to the lowered values on CK bypass and coupling caps. :)

[pdf64]

Doh!

[TweeDLX]

If it's any consolation, I realized there might be some confusion as I typed it.

[vibrostrat43]

If it's any consolation, I realized there might be some confusion as I typed it.

Oh you were referring to the caps. My bad. Anywho I'm using smaller values to reduce the chances of blocking distortion of course, and to prevent there being so much low end going to a tube that it causes flub. Maybe what I'm experiencing isn't actually low end flub and something else entirely...

I was referring to a 6L6 amp...the topology is pretty much

preamp stage>cathode follower>twd dlx tone&volume control>gain stage>cathode follower>tone stack>reverb recovery>phase inverter>power tubes...for the dry signal anyway,

and this is why I said it was pretty much a tweed bassman inspired design.

If I had a clip I would show you guys how it sounds...good but not quite the tightness in the low end that I want at higher levels of distortion. The cleans are great however, and sound pretty blackface-esque...although that may just be the reverb.

I think I misunderstood what I read about Hammond's transformers...I believe that it said something to the effect of "their ratings are meant for clean undistorted performance and 50% more power than the rated power could be used with no ill effects in guitar amps" just paraphrasing of course...I took it to mean that their transformers were more hifi in design, when in reality guitar amps are rated for a certain wattage CLEAN, but that doesn't mean that a guitar amp is designed to be hifi.

So I would perhaps like a little earlier onset of distortion, but I don't want the amp to lose tightness in the low end...would going from 4k 50 watt OT to a 6.6k 60 watt OT (that probably has higher quality iron in it) tighten up low end while simultaneously lowering the breakup point, or am I trying to solve problems in the wrong section of the amp and would be better doing something else (or a combination of something elses) that would have a larger effect on the tightness?

The breakup point is rather unimportant point right now, but if it will be lowered then great, if it won't get raised considerably then I'll probably be ok with it.

I've already determined that I can't get more filtering without going to solid state rectification, so that is off the table (for now). Anything else that can help me increase the tightness in the low end?

Thanks for all of the responses so far.

[Tone_Terrific]

No answers, but couldn't you change spkr impedance and see if the reflected load altered the sound?
And/or, move to a bassier spkr and keep the bass control lower thereby driving the power stage less hard in the flubby low freq area?

[vibrostrat43]

No answers, but couldn't you change spkr impedance and see if the reflected load altered the sound?
And/or, move to a bassier spkr and keep the bass control lower thereby driving the power stage less hard in the flubby low freq area?

I hadn't thought about changing the speaker impedance, but I am currently looking for speakers that hold low end better.

I have tried turning the bass knob to 0, and there's still the sound that I'm associating with low end flub when the amp is cranked...may have something to do with tone stack I implemented...I never ran the Duncan Tone Stack calculator on it because I previously didn't have it. It might also just be something else entirely...I really don't know

I have an extra Twin Reverb OT from Heyboer I might try out to see if it tightens things up.

[pdf64]

My 6L6 BF type amp has a Hammond OT with a toggle switch to select 8 or 4 ohm taps.
With an 8 ohm speaker, switching to the 4 ohm tap makes the tone more 'harmonicy', compared to the 8 ohm tap which seems cleaner. Similar to switching between pickups on a strat, but not as pronounced a difference.
An 8 ohm speaker on the 4 ohm tap would present about an 8k primary load to the 6L6, compared to the 'proper' 4k load.
So changing to a 6k6 primary (from 4k) may not make a massive difference.
To get rid of flub, coupling caps are best reduced in value. Bear in mind that the 5F6a was intended to be used clean as a bass amp rather than an overdriven guitar amp.
As a starter, reduce the 0.1uF caps between the phase splitter plates and power tube grids to 0.047uF, and the caps that feed the 1M vol pots to the range 4n7 - 1nF.
My preference is to wire the Mid control as a variable resistor rather than a pot, ie link the wiper and hot terminal; that will help to cut more low end at low Bass / Mid settings.
Pete.

[donnyjaguar]

You guys completely missed the effect that NFB has on the low-frequency performance of an amplifier. In short, the more NFB you use the better the damping factor and hence less flub you'll have on the bottom end.

[pdf64]

If hifi rather than guitar amps then I take your point.
However, guitar (power) amps get overdriven and global ac NFB becomes ineffective when that happens.

When the power amp is operating in its 'clean' range, the increased damping factor resulting from global NFB would have most effect at the bass resonance of the speaker, so to benefit from it, the user must be able roll off any unwanted low end, before it reaches and overdrives the power amp. Unfortunately even with the 'Bass' control set to minimum the problem still exists, so it seems logical to roll off more low end by reducing pre-amp coupling cap values.

Once the power amp does become overdriven, power tube grid conduction can result in bias shift / blocking distortion, which can be described as 'flub'. The bias shift will take time to get back to normal after the overdrive condition subsides. An effective method of mitigating this is to reduce the RC time constants, eg reducing the coupling cap values to the power tube grids.

The JTM45 has a virtually identical schematic to the 5F6A, however the power amp NFB feed is taken from a 16 ohm tap, rather than the 2 ohm output of the 5F6A. It therefore has considerably more (2.8 x higher) NFB but is nonetheless rather prone to flub.
So it can be seen that for overdriven guitar amps, more (power amp global) NFB is not effective flub mitigation, whereas reducing coupling cap values does resolve (or at least bring it into acceptable levels) the issue.
Pete.

[vibrostrat43]

Sorry to revive, an old thread, but I've read on Aiken's site that a really good way mitigate blocking distortion in the power amp is put a direct-coupled cathode follower between the phase inverter and power tubes (or two to be exact, one for each side) in order to lower the output impedance of the signal from the phase inverter to the power tubes, and basically create a buffer to stop grid clamping. I've tried it, and it seemed to cause my amp to lose a considerable amount volume when it should have actually gotten louder according to the site...I attribute it to user error rather, but I was wondering if others have tried and if they have any tips to offer.

[schmidlin]

First off, you need a low gain high power tube in there like a 12AU7.

What is your resistor from cathode to ground? You should have zero gain, really.

Besides, if Randall would GET HIS FREAKIN' SITE BACK UP we could have a better discussion. /rant. ;)

[Hwoltage]

Sorry to revive, an old thread, but I've read on Aiken's site that a really good way mitigate blocking distortion in the power amp is put a direct-coupled cathode follower between the phase inverter and power tubes (or two to be exact, one for each side) in order to lower the output impedance of the signal from the phase inverter to the power tubes, and basically create a buffer to stop grid clamping.

No kidding.... That's a pretty cool concept.

[vibrostrat43]

I was using a 12ax7 which I now realize was limiting the swing from the 12at7. But I had an extra 12au7 laying around that I've since popped in.

Some side info, I've since abandoned this build as it wasn't my style, and had a bit more compression than I liked. The signal path changed to

preamp>cathode follower>tone stack (brown Fender)>tone stack recovery>phase inverter>cathode follower>power tubes.

I had forgotten the values I used on the cathodes, so I opened it up, and found that the jumper to the grid of the cathode follower before the tone stack looked like it had been soldered in, but was actually able slide in and out of the pin as the solder had melted all around it but not quite adhered to it...just need to do a better job to make sure I get the parts heated before I take the iron off lol. That's what was causing the low volume. Everything sounds great to my ear now. The cathode values are 220k's to minimize compression.

[schmidlin]

No kidding.... That's a pretty cool concept.

Yep, and I won't build a bass amp without it. Eliminates farting and gives you grind. Guitar amps are a little more forgiving.

220k seems very high. As you understand, the idea is to give low impedance to the power tubes; I usually see 47k in there and it works well for me, but I'm glad it all worked out.