Steve Farris ODS #075

[talbany]

It also seems when some let the math rule the design too much, it doesn't make that great of a guitar amp anymore but more likely a high fidelity amp used to amplify recordings.

Tom
SO TRUE..

Guitar amps are not designed to be highest-efficiency, lowest distortion linear amplifiers, they are tone generators.IMHO

Tony

[jelle]

In addition to what Tony and Tom stated, I'd like to add this attachment:
Jelle

[Max]

Tony, Jelle,

thanks a lot for the basics.

But from a more practical kind of view:

If you take into account what you know about these original Dumble combo amps with a 4 ohm Bassman or Twin OT and the single Altec 417-8H Series II and EVM-12L Series II http://archives.telex.com/archives/EV/S ... %20EDS.pdf speakers Alexander has built into these combo amps:

Would your recommendation in regard to these original combo ODS then be similar as the one I reported of this EV engineer or different?

And if different:

What precisely would you recommend and why?

Can those, who play these combos, feel safe to use them as "mismatched" as they have been delivered by Alexander, if they like their tone this way, or would you urgently recommend to use an 8 ohm extention cabinet, if they do not want to risk that their amps break down at some point because of this "mismatch"?

Thanks again and all the best,

Max

[talbany]

Max

Those Bassman/Twin Fender output transformers Dumble used in those combos are by comparison tough ass transformers,, Look at how many are still around..Couple that with the heavy (well designed) higher wattage speaker he used as well as an amp that used some sort of Global negative feedback to help with dampening... A 4-6 ohm mismatch is really nothing to be concerned about with a single 12 combo amp..where you need to be cognizant of impedance mismatch is when you begin to add more speakers and the demand for current increases(Generally more than 2 w/ Bassman)and play more loudly as the cause and effects of my previous post become more of a concern, due to increased current draw on the secondary and Back EMF intensify's along with the phasing issues the engineer mentioned..The amp will tell you..

As a Side note!!
Being an amp manufacturer we need to discourage the practice of speaker mis matching because most out there don't understand the general rules (or in the case of impedance matching more of a guideline) principals associated ... Like in music it's ok to break a rule or to just know what the rules your breaking are and famalirize yourself with the cause and effects of an inproper load before the damage is done.. Such as
Chewing through tubes
Blowing Fuses
un-natural distortions
Phasing/Ghost note
Excessive OPT Compression/Regulation
Overheating/ Arcing

My opinion!!..All the Best Back!!
Tony

[odourboy]

IIRC, Kevin O'Connor makes light of impedance mismatch as well, describing the impedance switch as another tone control.

[jelle]

I have to agree with what Tony said. And if you look at fig 8 of the EVM specsheet you'll see the frequency/impedance plot which clearly shows that even with good matching of OT and speaker, this matching is only valid at certain frequencies. But do we play at only these frequencies? Of course we don't! We also play at frequencies at which the impedance will be higher. This will cause stronger flyback currents to occur at those frequencies. To complicate things.... the impedance plot was made with the speaker in a TL806. The plot will be different for a EVM in an open back cabinet. I cannot find it right now but I recal that WeberVST had plots of many speakers brands in both open and closed back on their website, but I digress.

The key thing is that it poweramp design is a balance between tone, power and reliability. Those Fender (Schumacher?) OT's are extremely durable. My comment about blowing OT's pertained to bassman OT copies made by a small European transformer manufacturer around 2000-2005. Those OT's arced at the primary and poweroutput dropped to a couple of watts.

In the graph I posted you'll see that there is a wide peak where the powertube will generate it's power. In the tube datasheets you'll often find recommended plate loads that are a lot higher than what you'll see in tube guitar amps. Fender was one of them and they probably did this to increase power output from their designs. It also reduced back EMF. However, the tubes will often redplate when dimed at this setting because they put out more power than the were designed to do.

In the case of the ODS 70's combo's, HAD must have decided in favor of operating the powertubes closer to the datasheets recommendations (I always call them 'guidelines' ) because, and I'm guessing, he liked the tone better of the higher Z, did not want to redplate the tubes at max settings and relied on the quality if the Fender output transformers. I haven't heard of too many 70's ODS amps blowing up, so these OT's pretty much hold up. I would not worry about it. Since materials age and the insulation on the OT wires may get brittle at some point causing failure causing any amp to get issues at some point. Plus, these bassman OT's can still be found on ebay, etc. So if I would own an amp like that, I'd use it because for me, amps are tools to generate tones and if it breaks, I'd fix it and continue to play it. On the other hand, if you want to preserve a museum piece....put it in a glass box.


jelle

[Max]

One more question in regard to the "classic" ODS amps:

Why are the 100k plate resistor "classic" ODS, like #093 e. g., called "low plate classic" and the 220/150 plate resistor "classic" ODS like #094 e. g. called "high plate classic" here at ampgarage?

Why these two different "names"? Just because 100 is a "lower" numerical value than the "higher" numerical values 220 and 150? Or is there any other more substantial reason for these two different "names" for these two variants of the "transition generation" and "4th generation" ODS, built between approx. 1979 and approx. 1988 and originally all with a "classic" tone stack?

Have a nice week,

Max

[jelle]

It's just because 100 is a lower numerical value than 220 and 150. Of course it gets confusing when values like 120k and 180k are mixed into the circuit.
BTW, I hope our explanations on the bassman OT were useful to you.

[Max]

It's just because 100 is a lower numerical value than 220 and 150. Of course it gets confusing when values like 120k and 180k are mixed into the circuit.
BTW, I hope our explanations on the bassman OT were useful to you.

Yes, very helpfull indeed, as I would make sure, that I did not make some nonsense out of what this engineer has explained to me. So thanks a lot.

In regard to the terminology:

Are the terms "high plate classic" and "low plate classic" associated with a specific different kind of tone/response/etc.? And if so, why (from a technical point of view)?

Thanks again and all the best,

Max

[odourboy]

Are the terms "high plate classic" and "low plate classic" associated with a specific different kind of tone/response/etc.? And if so, why (from a technical point of view)?

Here's my take: Although maintaining a plate resistor to cathode resistor ratio of ~67:1 maintains a similar bias and thus operating point, the higher plate results in a higher gain transfer function. (I think this also gives you more compression.) The higher gain seems to result in a smoother OD and more headroom. The lower gain, a grittier tone and a more 'staged' transition from clean to dirt as the signal level is boosted - i.e. a more 'responsive' amp.

[Max]

[1] the higher plate results in a higher gain transfer function.

[2] (I think this also gives you more compression.)

[3] The higher gain seems to result in a smoother OD and more headroom.

[4] The lower gain, a grittier tone and a more 'staged' transition from clean to dirt as the signal level is boosted - i.e. a more 'responsive' amp.

Can you or someone else explain why all this is the case with very similar operating points and plate voltages? I really would like to understand the technical reasons (if this is possible without a degree in physics).

Cheers,

Max

[odourboy]

Hard to explain without resorting to tube characteristic curves and load lines. You can google 'vacuum tube load line' and have yourself an afternoon of reading.

Here's a nice calculator though:

http://amps.zugster.net/tools/triode-calculator

[Max]

Hard to explain without resorting to tube characteristic curves and load lines. You can google 'vacuum tube load line' and have yourself an afternoon of reading.

Here's a nice calculator though:

http://amps.zugster.net/tools/triode-calculator

Thanks, will try at least.

From a more practical point of view and to give me an idea in regard to the amount of difference in tone you (and/or others here) perceived between the original high plate and low plate classic amps you know:

Here Doug Doppler demonstrates a 4th generation "classic" Overdrive Special (different guitars and playing styles, clean, OD lead, shred etc.):

Part 1: http://www.youtube.com/watch?v=34Y3lHpKXWY

Part 2: http://www.youtube.com/watch?v=SNSAj6AzWGo

Part 3: http://www.youtube.com/watch?v=-qvMwCte8TE

Part 4: http://www.youtube.com/watch?v=pGPxrt7d6xk

Is the amount of difference in tone between the original "high plate" and "low plate" classic amps that you know of a kind, that you (or others here) can tell for sure from just listening (and not because someone should know per chance the guts of this amp), if this "classic" ODS is a "low plate" or "high plate" classic?

Cheers,

Max

[talbany]

Max
I'll give it a shot...You could write a small paper on the technical cause and effects on plate load resistors..But the 3 main ones that you should know are.. higher plate load resistors and therefore lower plate currents equate to a higher gain. Thus 100K/1K5 gain is less than if you use 220K/2K2.

Cause!!
Sets the Rails..
The higher the value resistor the larger the voltage drop across the resistor this translates into a wider voltage swing or operating point before clipping..w/corresponding cathode resistor ratio of ~67:1..and of coarse voltage being the equal!!
This of coarse effects headroom/Frequency response ...Since our ears don't really hear the effects of the headroom all that much( in the way of volume increase) what we do hear are the effects of when where and how the tube begins to clip or is clipping.. Plot it on a graph Transconductance curve..

Effects!!
When= By increasing the headroom (like anything else) the tube doesn't clip as early so the anomalies of of a tube about to clip or in clipping are less prominent.. Things like less compression , tighter low end..Generally more focused tone..

How= has more to do with how the tube clips ..How the tube clips determines the harmonic structure generated..Low plat amps generally produce harmonics in the 2nd order (.. High plate's generally produce 3rd order harmonics.. 3rd order generally sound thicker more complex but can have an adverse effect especially when chording since 3rd's play a crucial role in western style music..

Where=Where you are in the Trans curve has to do with many factors..The mu of that 1 tube/current cathode draw(or where it will bias) and voltage(all being equal)=every tube is different.. and most important plate impedance as this effects all of the above it also effects frequency response through Miller effect.. You can hear it mainly in the top end and noticeably less bright..This gives it the smoothing effect everyone refers to here..

add all this mentioned and you get the effects odourboy posted..I am sure there are other ways of explaining this..Best I can do after pulling an all nighter..Anyone care to add revise please feel free..
Non Technical version!!
Tony

[jelle]

One can tell by the type of compression that's happening. Of course this effects gets masked when a HRM is used. But overall, a low plate breaths a bit more ands is less compressed. The additional compression of the high plate circuits and the resulting lack of 'air' in those sounds, IMHO, led to the development of the HRM. Just my theory.

Max, if you are interested in learning how different plate loads affect the amount of harmonics and the kind of harmonics generated, *yes, they do affect this too* then please look at RDH4. The tube circuit is not exactly new technology, we have about 100 years of R&D that is well documented on the topic. This is well reviewed in a textbook manner in RDH4. When you are at it, try to understand the 'Miller effect' too as it is relevant in this situation. On a side note, the loading of the tube, or Zout of the plate of the triodes changes too and this has effects on how filters operate.

http://www.ax84.com/rdh4.html

Have fun!