Custom OT Impedance Question

[Steve Conner]

Yes, why would it?

We just had that thread about how idle plate dissipation and clean power output aren't related. But that's for Class-AB: in Class-A, they are directly related. The hotter you run it, the more clean power you'll get.

So, if an EL84 makes 3W clean with 12W dissipation, and an EL34 makes 10W with 25W dissipation, maybe two KT88s can make nearly 40W, if they're both cranked to their 40W maximum plate dissipation. Or maybe not. I'd use three and stay warm this winter.

[MWJB]

Yes, but bigger tubes don't seem to like being run at max plate dissipation in SE (whilst EL84/6V6 seem to thrive on it), it's more likely that the EL34 will idle at 22-23W reliably...but running at 25W will work for sturdier tubes, but not for all.

OK, OK, I know I'm playing Devil's advocate - the point I'm rather tortuously getting to, is that you would just drop plate current, before redplating & yes, I agree, I doubt that you will see 40W from 2x KT88 in SE, especially in cathode bias. However, a 2xKT88 SE guitar amp may have it's own charms...it does seem a rather expensive & impractical way to build a 25-35W guitar amp though. Probably need a hefty choke too.

[loudthud]

Loudthud, why would the KT88 redplate at 540v?

126mA is 176mA peak. That's what the idle current has to be. 540V * 176mA = 95 Watts.

SE amps really burn the watts. At least 2 watts of plate dissipation for every watt out. I've got a Hammond SE OT that has 1.25K primary. Current rating is 200mA. If I ever get around to building it, I was hoping to get 25W out of three 6L6s using a High Power Twin PT. Current 6L6s don't last long at 30W so idle dissipation needs to be 20-25W.

[MWJB]

RG I think you may have the OP confused with a representative of Dynamic Amplifiers, mistaken identity? I'm not surprised the OP came on a bit defensive.

[dynaleadamps]

Oh it was TOTALLY my bad. I must not have made it clear that I needed the impedances to be matched utilizing 2 parallel tubes. I'm not blaming them at all! they gave me exactly what I was asking for, I'm just dumb sometimes.

It does sound great, and I may pickup one of those KT120's just to see how it sounds. I have the pri on a 3pst switch so I can choose which impedance best matches a particular tube. I'll just follow your math and experiment with it.

Thanks for your help!

[dynaleadamps]

It is expensive and impractical, I just LOVE the way high-gain distortion sounds through a SE output section. All of my other amps are SE using EL34's or KT77's and I was looking to get some more clean headroom while maintaining the SE tone.

[defaced]

It is expensive and impractical, I just LOVE the way high-gain distortion sounds through a SE output section. All of my other amps are SE using EL34's or KT77's and I was looking to get some more clean headroom while maintaining the SE tone.

Thinking out lout here. Would it be possible to use a SE output stage as a cathodyne phase inverter for a pair of tubes in push/pull and get the sorta tone you're after? Or is the OT too much of the equation of SE operation to be ignored?

[MWJB]

Once you have a phase inverter and a PP transformer, then you are no longer SE...unless you use 2xSE OTs wired to a common speaker load, still less efficient that true PP.

"SE tone" is the red herring here, all amps have a degree of "SE tone" beacuse all preamp tubes in guitar amps are SE, when recorded in a studio, it can be virtually impossible to pick out SE amps from PP (W RMS/dB become less relevant), the difference between the two is only relevant in terms of the volume level at which they hit what is percieved as "SE tone". There are lots of circuit mods that can swing the final tone one way or the other, but for 2 amps using the same number of similar tubes, at the same B+ & bias method (in reality, few amps are built with these matching perameters, accross the two types) , the SE amp will make less power. Want your SE amp to sound more PP, then turn it down.

[Wilder Amplification]

Forgive me, but I'm struck by the non-sequitur nature of someone selling $2400 amplifiers, complete with press releases and advertising talking about "to engineer an amplifier from the “ground up” " and not understanding what the effect of paralleled output tubes in SE does, not understanding that paralleling tubes does nothing to the primary impedance of a transformer, nor that transformers have ratios, not impedances, nor that the SE nature of the output makes this much, much stickier than just changing the impedance ratio of the transformer.

But I should not be surprised.

Call me a holier than thou pompous prick but I have to say I side with RG on this. While I'm no Randall Aiken, RG or Steve Connor (those guy's have probably forgotten more than I'll ever know), I've noticed an increasing trend of guys starting up amp shops who claim to be "designing" their own amps yet they don't even have a basic understanding of the elementary basics (i.e. Ohm's/Watt's Law, electron flow vs charge flow, transformer voltage/impedance ratios, basic tube theory, DC load lines, how to read schematics, etc etc).

Then everytime those of us who've paid our dues by learning all of this say something about it, those guys feel compelled to attack with personal attacks, brag about how many big name people are playing their amps, how they have "golden ears" compared to guys who actually understand amp theory, and somehow think we feel "threatened" by the fact that "hobbyists" are starting up their own shops and getting the same recognition we do.

Threatened? Not in the least.

Feel like the public is being misled to think that these guys actually have electronic basics down and really know what they're doing and WHY certain component values affect the tone of an amp the way they do in a given circuit? Feel like undeserved credit is being given to someone who hasn't paid their dues to earn it? That's more like it!

Guitarists think that anyone owning an amp shop actually knows how amplifiers work. Why? Because anyone running an amp shop SHOULD know it. Then once the cat's outta the bag as to the limitations of one's knowledge, guitarists ask "Well how can we tell which techs really know their stuff?".

I mean think about it...would you want to be going to a doctor that had no concept of human anatomy but follows the "tried and true" methods of real doctors who paid their dues by learning the required knowledge to be a doctor and actually know what they're doing? Do you take your car to a mechanic who is little more than a "parts changer"?

Most of these guys are starting shops and riding on the coat tails of those who designed certain circuits. There's another message board that has lots of mods floatin' around and guys are just using those same mods to build and sell amps. Every so often I get emails from these guys who've done a certain mod, the mod didn't quite go as planned, and due to their lack of knowledge they email me and expect me to walk them through how to fix it. And these problems they have are VERY basic...getting the OT primary/PI grid drive out of phase, forgetting to ground the bias supply, not grounding cathodes, etc etc. If your working knowledge of electronics is limited to the point that prevents you from troubleshooting even the most basic problems in a given circuit on your own via a schematic, you have no business running an amplifier shop IMHO.

Now from time to time those of us who have a genuine working knowledge of electronics will run into a problem that we cannot solve and hit up another tech for help. However, we're not expecting the other tech to "do the work for us" and the nature of the issues we typically hit up other techs for are a bit more complex than the basic "wiring the OT primary out of phase". On top of this, by the time we hit up other techs, we have voltage readings, schematics, etc etc...to supply them and we kinda go through it together...not "one telling the other how to do it".

Just my $0.02.

[defaced]

WTF?

[dynaleadamps]

It's true ... But I reject your reality and substitute my own!

[MWJB]

"Call me a holier than thou pompous prick...", ...erm, well OK, if you insist ;-)

Seriously Jon I don't think that you are a prick, but you do seem to have a sizeable chip on your shoulder. As far as I can tell the OP is an enthusiastic hobbyist, we all start somewhere. Perhaps keen on the hyperbole, but I don't see, "It does what is required satisfactorily, because that's the way it was designed", as being a great marketing tag line.

Many of the most famous & highly regarded names in the amp world didn't design their own products, they copied/bought designs, they may simply have executed those designs well. They outsourced to specialist manufacturers for things like speakers, caps, transformers...people who have the required skill sets. The amps were/are largely built by semi-skilled workers on a production line. The end result largely justifies the process. I'm not suggesting that it's not a good idea to have a thorough technical grounding, it is, but good practical skills are important too.

It's easy to design things, executing that design successfully & finding a market for it is another matter. The market place is full of products that cover all bases, some are cleverly designed & well executed, others are well marketed lash ups that are deemed a success if the light comes on, some sort of noise comes out and it doesn't hum/rattle/oscillate enough to require recall/extensive after sales care and meets someone's idea of MRP.

At the end of of the day preamp tubes have a grid, a plate, a cathode...99% of amps use the same 3 or 4 values for each, a few hours with a soldering iron and you soon know what you like the sound of. It's called empirical development and is a valid process.

If I buy a chicken dinner in a restaurant, I don't expect the chef to be a biologist & have to know how the chicken evolved. I'm happy if I get something tasty that doesn't poison me.

Most importantly, forums like this are where people go to have their questions answered, not to be judged/insulted...if they knew it all already, the forum wouldn't exist. Even the knowledgeable have better specific skills in some areas, more than others. If you think that it is beneath you to share, then don't, put a price on your intellectual property... shit, or get off the pot.

[Steve Conner]

It's true ... But I reject your reality and substitute my own!

Adam from Mythbusters actually has a T-shirt with that slogan.

Jon, calm down. You know that solid engineering doesn't sell amps... advertising does! Whether the amp actually works is irrelevant, you just have to move to a different town and set up again under a new name before it catches fire and kills your customer.

And you think Dynalead is bad (I'm saying nothing, I haven't even seen his amps) you should see some of the crap that gets sold over in the hi-fi world. They take things that we wouldn't even play guitar through and sell them for $10k.

Would it be possible to use a SE output stage as a cathodyne phase inverter for a pair of tubes in push/pull and get the sorta tone you're after?

This was where I was coming from with that thing that uses a SE EL84 to drive a PP transistor output stage through a transformer. It really does work, it sounds like the same dirty little EL84 amp, just a lot louder.

Well, to be honest, the last time I really cranked it, a bit of smoke came out, but my marketing guy thinks he can dress that up as a "feature"

[R.G.]

As Steve guessed, I did have a bad day - in fact, it's been a bad couple of months. And I did know that my remarks would bring out comments that I was being snotty. Sorry - I couldn't help it. I even pre-apologized; that was not rhetorical.

I don't mind in the slightest giving away any technical knowledge I have. I think I can show the history on that. I have consulted for some of the bigger names in the boutique amp industry quietly on occasion. In fact, what set me off was the question appearing over the obvious advertising for an amplifier builder. If "DYNA-LEAD AMPLIFIERS THIS IS NOT YOUR DADDY'S AMPLIFIER" had been replaced with "Joe", "Fred", or "Alphonse", I'd probably have simply blatted out the techie info. But the ad did cause me to go to the web page (as it was intended to do), and I read the business about engineered from the ground up. As I say, I was having a bad day, for which I request forgiveness.

If I buy a chicken dinner in a restaurant, I don't expect the chef to be a biologist & have to know how the chicken evolved, either. But I do expect him to have either known or looked up how to order chickens and not ducks (I don't like duck) and to not put an ad for his restaurant over the posted question about why chickens have webbed feet, while pointing to a web site which says he developed new chicken dishes from the ground up.

Sorry. I guess I'm having another bad day.

To the OP, what I should have said earlier:
- SE at 40W, with tubes, is technically impractical and very expensive, as you have later said.
- The detailed technical info on how to do this is contained in the RDH4, which is on line at ax84.com; I'll point you to the chapters if you like.
- The fact that SE OTs have to have the full peak DC current (plus a bit) for one side of the output swing going through them at idle makes them much less flexible as regards the magnetic design than PP OTs, in addition to several times the mass and cost. This carries through to the power supply and cooling requirements for the output stage.
- A quick phone call to Heyboer and the guy who designed that transformer would pretty quickly get your options set out clearly. He's the one with the design sheets on that transformer and he can tell you how much of the iron's B-H curve he used and what changing the impedances would do to you. I could eventually get there if I had that info, but the designer at Heyboer has the info and is current with that design.
- I'm guessing that you may have to get another trannie made to do what you want. Or if it's a purely fun project, dink with it. There is a sweet spot in the output loading where you'll get the max power out of whatever tubes you put on the primary at that given DC idle current. Mucking with the loading will tell you what that is.

[MWJB]

Eveyone has bad days, sorry to hear that you seem to be having a protracted time of it. As Steve also said, you have contributed much over the years, forgiveness is not required.

I don't mean to draw this out, but I haven't once been able to access "dyna-lead.com", nor to read any statement made by the OP regarding "designed from the ground up", I have only seen such a statement in the Dynamic Amps Co. blurb, who make a Dynalead amp (no hyphon) in Arizona (OP is from MI). I can see what irked you, but it seems it was further compounded by mistaken identity.

I'm always reading amp blurbs that explains how the builder "invented" this or that, how it is a "first", when in reality it is a rehash of a 70yr old idea! At that point I generally lose interest...shame really, some of them probably don't sound too bad!

Best regards, Mark.