Headroom Question (OT and PT sizes)

[ToneJunkie]

I play through AB763 amps pretty much exclusively.  I have a Deluxe Reverb pretty stock, Vibrolux with a Pro Reverb Transformer, and a Twin Reverb.  I was wondering what is more important for headroom the PT or the OT. 


Example 1: So for example I have the Virbrolux with a 180ma PT and a larger Pro Reverb OT.  How much more headroom would you expect from a Pro Reverb PT 300 ma with the same OT.  So in that scenario bigger PT.


Example 2: Vibrolux with an 180ma PT and the original OT vs a larger pro reverb OT.  Note:  My vibrolux sized amp was a scratch build so I did never experience the difference between the OT sizes. My experience says this does not make as much of a difference.


The reason I ask is I'm thinking about building 50 watt AB763 with a 300 ma PT for a super reverb and I'm wondering if I should get the Super Reverb OT or use a Pro Reverb OT I already have on hand.  I'm looking for more headroom than my Vibrolux with Pro Reverb OT. 


Thanks for you thoughts. 



 

[ToneJunkie]

Here is the stable... the one on the left is a vibrolux using Slucky's layout.  The middle one is the Deluxe Reverb that I recently rebuilt.  And the one on the right is a dumble chassis but has the usual AB763 in it.  The new build will be a Super Reverb in a 1x12 cabinet.   I guess I just like what I like :-)  Thanks for all your help.

[Latole]

WHAT IS HEADROOM?

Headroom is simply a term used to denote and describe how much power your amp can provide before the sound starts to break up and distort.



We always strive in our articles to be thorough without bombarding you with too much scientific jargon. After all, it's our hope that when you are done reading you will better understand the topic. In the spirit of keeping it simple,  the best way to understand headroom is to think of it as "clean volume." Basically in a nutshell, what we're talking about is how loud an amp can get before it begins to overdrive. As most of you are aware, one of the great things about tube amps is that they do begin to naturally compress or warm-up as you turn them up, and when they are really cranked they produce that harmonically rich overdrive that only a tube amp can deliver.  The problem is that the rate at which the amp begins to overdrive is going to be different for each amp. Different amps are going to have different levels of clean headroom.  There are a number of contributing factors, however, here are a few things that contribute significantly:

Wattage - As a general rule, there is a correlation between wattage and clean headroom. Overall, the higher the wattage, the higher the potential for clean headroom, at least as it relates to power tube overdrive. The reason for this is pretty simple. The harder you drive a tube, the more it is going to overdrive. Normally, as an amp increases in wattage, it also increases in the number of power tubes that it has, meaning that the "load" will be divided into smaller parts and each tube will not be driven as hard.

Voicing - Some amps are purposely voiced to yield more overdrive or more clean headroom, depending on the design. This can be done in a number of ways. One example of voicing an amp for more overdrive is when a designer purposefully makes the preamp section of the amplifier easy to overdrive by cascading the gain from one tube to the next in order to generate massive amounts of overdrive.  On the other hand, a designer may also choose to maximize clean headroom by either using lower-gain preamp components or even decreasing the amount of plate voltage on the power section of the amp.

Input - One of the reasons that headroom is such a difficult thing to quantify is that it's so highly dependent on the level of the signal going into the amp. Guitar pickups can vary wildly when it comes to output, and in turn, how hard they drive an amplifier. As the output of a pickup increases, the headroom typically decreases as the front-end (preamp) section of the amplifier will be driven harder and will yield more overdrive and compression.


More ;https://www.humbuckermusic.com/pages/understanding-amp-headroom-article

[Willabe]

A Fender BF SR was listed by Fender as a 40w amp. All the other 2 x 6L6GC BF Fenders were listed as 35w. The 4 x 6L6GC BF Twins were listed as 80w. Fender later had SF amps that they said got more from the 6L6GC's.

They had an amp named Quad Reverb that they listed as a 135w 4 x 6L6GC in a 4 x 12" combo cab. Freddie King and Albert Collins often used 2 on stage!   

The BF SR was the loudest and was the cleanest of the 2 x 6L6GC BF amps. Because, it had the largest PT and OT of those amps, had the highest dcv plate voltages, 460dcv 6L6 plates/270dcv 1st preamp tube and has a 4 x 10" speaker cab. Aspen Pitman, Groove tubes in his book called the BF SR "The king of klean." I gigged with 1 for years and played through many others in the Chicago blues clubs. They stay pretty clean up to ~6 or so, depending on the guitars PUPs and how hard you pick. And are pretty loud at this setting.

Cesar Dias put BF Twin OT's in SRV's BF SR's. But he also put EV speakers in his amps. The larger OT was mostly to hold the bottom bass end together. But too much bottom will sound muddy. And SRV was in a 4 piece band, with guys who knew how to not step on the other guys toes. So he could use a little extra bottom end. Carlos Santana said that guitar players always had the high but it was SRV who showed them to remember the bottom end.   

A smaller OT shaves the far 2 ends of the frequency spectrum. The smaller the OT the more it shave the highs and bass end. If you go large enough the OT won't shave any or extremely little from the high/low end. This can be good or bad depending what you want from your amp.

If you would hold a SR and Pro verb amps OT's in your hands or find a pic of the 2 together you will se a fairly large difference between the 2. Larger and more lams and larger core, so more winds/heaver gauge wire(?) with the SR OT.

But you can only go so far with a bigger PT and OT before you can't get any more from the 6L6GC's, that's a limiting factor.   

Louder dB rated speakers will greatly help with staying clean. 

[mresistor]

Cool info Willabe..    I read that Carlos Santana was really partial to 12" Altec speakers back then and they are the predecessors to JBL.   I had a pair in a BF Fender cabinet and they sounded really fine.  I also think the BF SR is the cats meow when it comes to clean and chime.  I overhauled an extremely abused one a few years back for a friend from TX. It had seen water damage and the cab and the original alnico speakers were history. He found it on the way home when he saw a guy with his pickup tailgait down and selling it out of the back. He bought a new replacement cabinet and I installed 4 x 10"  Emi alnico speakers.   Holy cow!   That thing was one of my very favorite, low volume or higher volume. It was just "super". I played my Pro Tone strat through it all the time.

[acheld]

you can only go so far with a bigger PT and OT before you can't get any more from the 6L6GC's, that's a limiting factor.

My opinion is that the "headroom" in the AB763 series is limited by the pre-amp section.   

You know, when you turn the volume up to 4-5 and the amp comes alive, but after that you don't get much increased volume but do get added distortion.

Once upon a time, I built an amp for a friend who wanted "NO DISTORTION" but built on an AB763 platform.  This was doomed from the start, but I was curious and a friend is a friend . . .   

The only way I could get to no distortion with adequate power was to use either 12AY7's, 5751's, possibly others as well, I don't recall, in some combo in the pre-amp.  I was able to get to 30W output with no visible distortion (standard 100mV, 1KHz input signal).   So yeah it was as clean as sterilized hemostat in an OR, but as utterly lifeless as rocks from the moon.

Prior comments by Latole and Willabe are spot on.  And that's a nice stable!

[Willabe]

My opinion is that the "headroom" in the AB763 series is limited by the pre-amp section.

I meant any tube can only dissipate so many watts before it red plates.

And I said that part of the SR being clean was the preamp plate dcv. Look at several of the BF 2 x 6L6GC line and compair their preamp and PI tube plate dcv's.

 

You know, when you turn the volume up to 4-5 and the amp comes alive, but after that you don't get much increased volume but do get added distortion.

I think the 35w BF amps act like that, the SR goes a little further before you hit that breakup. I have an old friend, Curt Obeda, up in Minneapolis Min. He lived in Chicago for some years to hang with the blues guys. He still plays up in Mini. in the Butane's. He played a SR in Chicago, but now he told me he likes his Pro verb better, because he want's it to bust up some sooner. 

[ToneJunkie]

Awesome guys!  Thanks for all the info :-) Sounds like the PT makes much more of a difference due to the higher B+ on both the power tubes and preamp tubes.  I think I will build it with a larger PT the pro reverb OT.   I turn the bass down to 3ish often and don't need the high end either.  Also if I need more speakers just use two amps.  Here we go.

[Willabe]

 

You know, when you turn the volume up to 4-5 and the amp comes alive, but after that you don't get much increased volume but do get added distortion.

I think the 35w BF amps act like that, the SR goes a little further before you hit that breakup. I have an old friend, Curt Obeda, up in Minneapolis Min. He lived in Chicago for some years to hang with the blues guys. He still plays up in Mini. in the Butane's. He played a SR in Chicago, but now he told me he likes his Pro verb better, because he want's it to bust up some sooner. [/quote]

It's both the higher PT dcv and the larger OT.

A BF SR OT will stay cleaner longer than a BF Pro verb OT, but it is NOT a full band width OT like used for a stereo that won't shave the high/low ends. It would have to probably 2x the SR OT size to do that.