New push pull triode amp - Tubeless inverter

[jsn]

Hey guys,

I just registered with the forum, and now I see the pictures :)

I wanted to give a little background to why this design seemed to make sense. It is my theory/opinion/insanity that a guitar amp needs a phase inverter that is well balanced, and can deliver a bit of grid current when the output tubes are doistorting. My first shot at this was the choke-tail pahse inverter (http://boozhoundlabs.com/bhl-15/) which I think does a good job, and the effect is apparent in the wide sweet spot of the amp.

But I have some hi fi geek friends that have been playing around with the Edcor transformers as parallel feed interstage transformers. I felt like the choke-tail does a good job of delivering grid current, but it is less that perfectly balanced, as are all of the available tube based phase inverters. But transformers are as close to perfectly balanced as you can get.

So my plan was to use the transformer to split the phase, and then use the 12au7 or 12at7 to deliver grid current.

I agree withthe comments that the transformer itself would make a good driver. I wish I had thought of that originally - it's much simpler, and I don't like the idea of having more than one tube stage between the phase inverter and the output transformer. I also agree that it needs a lower impedance driver.

And part of the fun here is that we are trying to apply Hi Fi priciples to guitar amps, because Bob and I are feeling a bit tired of the same old thing. I agree that simple is better in most cases, but it precludes the trying of actually new things just because they are more complicated or less well understood.

I have an idea for a new version of this amp that is significantly different, and incorporates all of these ideas. I'll try to get it drawn and posted when I get a free moment.

jsn

[jsn]

Here is what I was thinking. Let the interstage transformer drive the output tubes directly, and then use something with low plat impedance drive that. I was thinking initially of a paralleled 12au7 or maybe a cathode follower. But then I remembered the SRPP stage.

I really don't like the sound of SRPP for hi fi, but most of the stuff I like in Hi fi sounds like crap in a guitar amp. I like to have as much odd order distortion as possible and as little even order as possible. So a sort of push-pull driver should be perfect. And it has a low output imedance, so it should handle the transformer and output grids with ease.

I dunno. Maybe. someone build it and let me know :) haha.

[RJ Guitars]

JSN - your stuff is always unique as well as good... Thank you for jumping in on this thread. For those that haven't put it all together, JSN is Jason of Boozhound labs that created the original design for my ECC99 amp with the interstage transformer.

About that original design - well I've morphed it a bit and it is sounding better than ever. Three noteworthy changes:

1) I changed V1b to a cathode follower into the inverter
2) I dismissed the volume pot feeding the inverter and put in a two-gang pot feeding into the power tubes for a master volume kinda thang.
3) I changed out the power transformer for one with a little more juice.

With steps 1&2 this thing was sounding very good with a respectable improvement in the overall tone. I still had some farty sounding distortion when I maxed everything out and I recalled an old thread from Krash of Revelation amps where he mentioned the need to run the voltages a little hot when using triodes as output tubes. I swapped out the 180-0-180 PT for a 250-0-250 PT and Shazam! It is sounding really sweet. The best of any of my attempts at low wattage tone generators.

I'm playing with tube combinations but right now I'm using 12AU7 - 12AT7 - ECC99 in that order... I know the 12AU7 and 12AT7 ought to work better if they were swapped but doesn't sound as good.

I put the amp into a cabinet with a 10" blue alnico speaker and the tone was good but broke up maybe a little to easy and that speaker doesn't put out a lot of sound. I then swapped in a 10" Eminence Ramrod and now it is lots louder and doesn't break up so early... I think this is probably a great match for this amp.

I need lots of hours of sound testing now but this thing has some really sweet clean tones as well as some great blues tone...

rj

[RJ Guitars]

> using a direct line out

Much of the charm of a triode output is the way it interacts with the speaker. Some but not all of this is reflected at the speaker terminals (where your line out is). The more accurate recording would be a mike at the speaker. I did similar work for a living and I understand why you skipped that; whipping out a mike enormously complicates a session...

6J6 has positive-grid data and with Mu=38, the grid voltage needed is quite small. The plate current available without grid current is small, but with a step-down transformer you can deliver the small grid voltage and high grid current from a modest-current driver.

And 6J6 is just $4 each!!! It was produced in vast quantity, rarely dies in its normal uses, most of its gear has been obsoleted. It is a bit awkward for audio due to common cathode, but is a really good tube. You could probably score a bakers-dozen for the cost of one ECC99 and a spare.

The condition I see is 100V supply voltage, zero bias, 10Kpp load, around 2 Watts output. Grid drive could come through a 10K:600CT transformer from another $4 6J6, both sides parallel. This bottle needs about 2V drive. Since 6J6 are cheap and you like paralleling, two more in resistor-coupled mode (or one 12AT7) could give enuff gain to come up from guitar pickup and through a lossy tone stack. (You could throw in another interstage transformer, get a little more gain, but not enuff to eliminate a bottle, and frequency response of three cascaded transformers could be less than you want.)

The downside is that 6J6 heater is hungry. Four of them will suck 1.8A at 6.3V. A Fender Champ will do more than double the speaker watts at less than half the heater power. OTOH, 150V-200V power caps today are far cheaper than the 450V caps needed for a Champ.

Yeah the line out was mostly taking the quick way out... It was actually pleasing to me that the line out actually worked well. I shouldn't be shocked at that but somehow I wasn't expecting it to be that easy to get good sound from a direct out... The lead parts were miked and that actually went O.K. as well. I tried to do those on the line out and they sounded kinda buzzy.

I'm a bit intrigued by the 6J6 talk... I know I have a few of those. I'm not really afraid of the current draw because there is a local technology salvage vendor that has gobs of filament transformers that I can pick up for a reasonable fee... Do you have a basic schematic available for a build like you mentioned? I have a long queue of ideas to follow up on but as JSN mentioned I tend to be drawn to something a bit off the mainstream...

rj

[bcook]

Another alternative I tried once is to place cathode followers between a standard fender PI and the power tubes. It was too smooth really, it didn't sound like a guitar amp fixin' to explode. Of course I was younger then...

What's going on up on the hill? If there's some kind of amp party going on, I want in!
-Bob

[RJ Guitars]

Another alternative I tried once is to place cathode followers between a standard fender PI and the power tubes. It was too smooth really, it didn't sound like a guitar amp fixin' to explode. Of course I was younger then...

What's going on up on the hill? If there's some kind of amp party going on, I want in!
-Bob

Too smooth... sounds intriguing. In addition to the search for the grail of electric guitar tone, I have a back burner idea for a silicon free acoustic guitar amp. Smooth might be part of that equation.

Bob you're in for sure... we've got a local club that's a mix of hifi and Geetar amp builders. It's about time for an amp fest so we'll be in touch and make sure we plan so you can join us. Are there any other New Mexico folks interested in an amp fest?

Look into the Boozhound lab sites and maybe Jason will hook you up with the "Audionerdz"... a lot of innovative folks that love to explore new territory (or rediscover in some cases) tube technology.

rj

[jsn]

I think it is a really interesting point about "loudness" in an EQ sense and quiet amps. One thing that has revolutionized my thinking on speaker building is the notion of baffle step losses and how to EQ for them. Basically in a speaker you get a 6db loss of frequencies below a point determined by the average width of the front baffle. This sucks out all of the lows below about 300 Hz.

Conveniently, if you look at the equal-loudness countours, the perception of bass begins to get bad at about 300-400 Hz. So the baffle step circuit can be used as a sort of loudness control. It also has the added advantage of being a step function rather than a low boost type thing so that you don't end up with lots of low frequency cone-flapping type boost.

I want to try it in a guitar amp. Hey Bob - wanna cliplead this into the EQ section of that amp you are working on?

Here are some links for the curious:
http://sound.westhost.com/bafflestep.htm
http://en.wikipedia.org/wiki/ISO_226

For guitar, you don't really get much output below about 100Hz right? But still, you probably get harmonics and whatnot. And that range, like from 100-300Hz really adds a lot of "weight" to the sound. I think it might do wonders for a small amp. And of course if you do it with a knob, you can dial it in or out to suit the situation.

[klingo]

Hello!

I've tried this passive BSC (schem) in a blues Jr between third stage and PI (with a switch from cap to ground)

It was interressing but i guess it will work better with an amp which has some bass ability (have you ever seen a Blues Jr output transformer? )

it has to feed a high Z input such as cathode follower/cathodyne or even schmidt PI

not sure we need a lot of bass in a guitar amp, or only for low volume setting...

anyway i have a similar BSC (with dif value 10K/10K/22nF and 1nF bypass cap) between my CD player and my Gainclone with FE164 full range speaker TQWT, and with this set up it is essential.

olivier

[RJ Guitars]

Hello!

I've tried this passive BSC (schem) in a blues Jr between third stage and PI (with a switch from cap to ground)

It was interressing but i guess it will work better with an amp which has some bass ability (have you ever seen a Blues Jr output transformer? )

it has to feed a high Z input such as cathode follower/cathodyne or even schmidt PI

not sure we need a lot of bass in a guitar amp, or only for low volume setting...

anyway i have a similar BSC (with dif value 10K/10K/22nF and 1nF bypass cap) between my CD player and my Gainclone with FE164 full range speaker TQWT, and with this set up it is essential.

olivier

Well... I'm still studying your schemo and sorting out what you've done (now that I look it over it was probably an amazing feat for me to make it complicated)...

But as far as needing bass, I say lets get all we can and we could throw some away later if we need to. If we can enhance the bass and treble response like a loudness control for an old hifi... we might just have one key piece to great tone a t low volume...

Thanks for the note.

rj

[klingo]

hello!

in fact the BSC is a voltage divider (about -5 db in the schemo), and the cap set the frequency where this divider begin to operate.

IIRC in Blues Jr there is a 220K resistor between third stage and PI for reverb send & return. as i relocate the reverb, i just wired 270K/1.5nF? at the output of this resistor with a switch to ground for comparison.

It lessen the overall volume ( -5db means roughly voltage divided by 2 ) except for the bass.

If you like Math you will take the surrounding impedance into account ...but there is not a lot of components to solder and try.

once again for better explanation and real english:
http://sound.westhost.com/bafflestep.htm

hope this help

olivier

[RJ Guitars]

Thank you... I really enjoy this stuff.

I've got a trip planned for the next week but I'll read up and see what else has been posted when I get back.

Thanks again,

rj

[novosibir]

What trannies you're using as for the OT & PT?

Larry

[RJ Guitars]

What trannies you're using as for the OT & PT?

Larry

Larry, thanks for the note - sorry to be so slow to get back to you, I've been away for a bit...

I started with Hammond iron - OPT=1608 and PT=269EX (190-0-190). I am very happy with the OPT, but I changed the PT to a 250-0-250 that I had on the shelf... unfortunately I don't know much about it. It looks like an old TRIAD or STANCOR - I say this because it's painted gray. EDCOR also makes one that looks almost identical so it might be theirs. Neither of my PT choices had a 5V tap so I use a SS Diode based Rectifier.

I switched to the 250-0-250 based upon a suggestion that Josh from Revelation amps posted a long time back. He said he got a much better sound from triode outputs if he pressed the voltages very high (300V plus)... based upon my one experience I agree with him. The ECC99 tube is very stout tube and can handle a lot of voltage so I think all is well, time will ultimately tell. I may someday kick the voltage even higher just to hear it.

I've made several triode output amps and it seems that they all get kinda of a farty compressed sound when they are cranked up... the higher voltage seemed to take a lot of the bad stuff away and leave the good stuff... sorry that isn't too technical of an explanation.

The ECC99 will make about 5 watts in push pull so what I have is something like a champ level output, but in push pull configuration... I'm quite pleased with it and it has become my primary gig amp.

rj