Steve Farris ODS #075

[dogears]

Andy, it was 100s not 170s if I recall......

Also, I think it was a .05 midcap and 100K slope... You sure it was Skyline? It had a .01uf coupler on CL2 I think, as well....

Dogears may remember more than I do.

- series number? (170's ?)
- silverface or blackface? (blackface)
- full size presence control or small presence control or accent switch? (full size pot)
- combo or head? (head)
- What kind of wood was used for the cabinet? (pine perhaps)
- tolex or suede? (tolex)
- colour? Black
- grill cloth? Black
- ratio control? No
- if a combo: what kind of speaker? (NO)
- 4 ohms or 8 ohms OT or switchable? (rotary switch)
- Rock/Jazz with a PB middle position? (2-positions)
- PB footswitch? (two way OD gain)
- 100/40W or 50 Watt? (100-W)
- export PT or 117/120V? (don't recall it being export style)
- any updates? (skyline EQ, fat-city, etc.) skyline
- on what records (some examples would be fine) ? (don't know)
- pictures? Maybe, have to check some old CD's I may have )

Wish I could recall more, but it was close to ten years ago !
I may have notes, but cannot promise...I do recall 220-150-K plate values...

[FUCHSAUDIO]

Been a long time....I'll see if I have pics.....4 computers ago, but perhaps on a disc someplace...

[Max]

Been a long time....I'll see if I have pics.....4 computers ago, but perhaps on a disc someplace...

Andy,

thanks for looking.

Btw.: Yesterday two Berklee brainiacs unveiled their revolutionary "IBS" (Instant Backup System) solution on CNN:

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http://www.polaroid.com/category/0/2669 ... %238482%3B

http://images.sabob.com/products/images ... e_box.jpeg

Cheers,

Max

[Max]

Andy, it was 100s not 170s if I recall......

Also, I think it was a .05 midcap and 100K slope... You sure it was Skyline? It had a .01uf coupler on CL2 I think, as well....

Scott,

do you, like Andy, also remember a rotary switch to set the output impedance?

I ask, because I never saw one of these rotary switches upon the back of one of the "4th generation" amps, not in real live and not on a picture.

If it indeed should have one of these rotary switches, it could - from all I know - most probably not be an early eighties amp?

Cheers,

Max

[talbany]

Max
oops!!
Thought I had 1 but wrong generation.. Ignore

Tony

[Max]

Another example for a 4th generation "classic" amp:

"Old Larry Carlton guitar rig video"

https://tubeamparchive.com/viewtopic.php?t=11970

Andy, Scott,

did Andy Brauer's amp look like this one of Larry Carlton?

Cheers,

Max

[Max]

Max, I noticed that you posted some great clips with stratocasters and marshall 4x12's. Those sounded great! Were these done with an amp with a classic tonestack? I personally like that stack with stratocasters.

The clips 1 and 3 https://tubeamparchive.com/viewtopic.ph ... 218#120218 have been done with a "pre-classic" 2nd generation ODS (2nd clip is a 50 bucks flea market find). Cabinet was a basketwave Marshall 4x12 still with the original 25W pre-rolas, but wired for a total load of 4 ohms. FET input, Volume and OD both full up.

Yes, I like this tone, too, cuts through a dense mix and is not too "stylish" for my taste. But I must admit, that I liked some of the "classics" (not all!) even better (#075 best) for such a kind of strat tone: A bit more of a fat body in the upper registers of the plain strings to "carry" the tone, still enough of the natural ("clean") character of the guitar in the OD, and a bit more elegance in the OD harmonics (if the OD circuits, especially the snubbers, are to my personal taste).

I like a fat and bright strat tone with the stock bridge pickup. That is what #075 does brilliantly. Other (and better) players had the same impression.

BTW:
I offen read here, that members like amp A better with singlecoils and amp B better with humbuckers. My personal taste in regard to all the original Dumble ODS amps I have played is a bit different. If I liked one very much with singlecoils, I always liked it with humbuckers, too, and if I didn't like it that much with singlecoils, I didn't like it that much with humbuckers, too.

A great difference in regard to the OD tone of the "pre-classic" and "classic" amps is the OD circuit of course. The "classic" amps not only have a different tone stack but a different OD circuit, too. E. g.: trigger, ratio and snubbers you find in the "classic" amps (starting with the first "transition generation" amp), but not yet in the "pre-classic" amps of the first three generations.

Now some of the "pre-classic" amps could have a bit more "fat body" for my personal taste and some of the "classic" (and skyliners) do have an OD tone, that sounds a bit too "filtered", "styled", "artificial" and "synthesized" for my personal taste. And #075 for my ears has just a perfect combination of the open and still very "natural" character of the "pre-classic" amps and the fat, sparkling and singing elegance of the classics and skyliners. It is a bit as with a violin: the violin, the violin bow and the amount of bow resin and the playing can lead to tones that are a bit too noisy (or "scratchy"), and to tones that are a bit too dull and polished for my personal taste. I like the violin tones in the middle of these two extremes best, like this one:

http://www.youtube.com/watch?v=D4uvXc00PsY

This Massenet again with some noise but live pictures:

http://www.youtube.com/watch?v=zhFcBGQLehw

And Mozart (probably trading licks with Jimi now):

http://www.youtube.com/watch?v=hPT-aXdpzuc

(good headphones are perhaps a worthwhile investment for listening to "net-music")

I think btw, that her "Dunn-Raven" is a bit better suited for her current more matured (IMHO) playing than her "Emiliani" from the Karajan days. The "Lord Dunn-Raven" for my ears has a bit more sparkle, brightness and body, and is perhaps a bit better suited to go from a very soft to a hard attack. Not "better" than sweet "Emiliani" in an intrinsic sense, but perhaps a bit "better" suited for what she is up to currently.

Back to classics, pre-classics, strats and #075:

I like an OD lead sound without any gap between the fundamentals and natural harmonics of the guitar and the OD-generated harmonics. Otherwise I perceive the OD-generated harmonics as "noise" that does not mingle with the natural harmonics and adds to their "sing", but stays separated from them - as noise.

Hard to explain it with words, but I thought it would perhaps be worth a try at least, even if I should fail to find the right words.

So far this lenghty blah-blah about my personal taste in regard to "pre-classic" and "classic" amps and how I like these in combination with a strat.

Cheers,

Max

[Max]

BTW, the OT mismatch that you mentioned can work for some people as it 'cleans' up the sound a bit. On the other hand, I have killed some OT's this way in 50W amps as some of the current production OT's cannot handle the voltage spikes that occur at these settings.

All the best,

jelle

Hi Jelle,

I have always wondered, why all the silverface, transition generation and at least some of the 4th generation combos are equipped with a 4 ohms OT, as all of them have an 8 ohms speaker, just as #075 ( pictures of the back with speaker: http://www.audities.org/audities/collec ... ive_a.html )

I wondered because of this:

- a Fender 8 ohms OT of a similar kind would have been available too in the late seventies
- most of theses combos have been custom built. By this it was known to Alexander, if the customer planned to use this combo with an extension cabinet or not.

I tried to find someone in the EV company to talk to about this topic. And indeed I got contact to one of the older EV engineers, who was still familiar with the specs of the EVM 12L series I and series II speakers and these older Fender transformers.

I asked him what he would recommend in the end, to use such a combo with an 8 ohms extension cabinet to get proper matching or use it "mismatched" just as it is. He did not hesitate for a second and said to my surprise: "Use them as they are, don’t use an extension cabinet."

As his answer was surprising (for me), I asked why:

His answers:

- with an extension cabinet there could occur phase cancellation problems
- these old Fender transformers have been made in a way, that they can handle a "mismatch" of at least one step without any problem anyway
- the real impedance of the EVM 12L in important parts of the frequency range of such a kind of guitar amplifier is more near to 6 ohms than to 8 ohms anyway
- because of the small "mismatch" the power amp will produce less THD
- the drop in volume because of the small "mismatch" will hardly be audible at all
- if you switch the power amp from 100 to 40 W, the impedance of this EVM 12L and the power amp impedance will be matched a bit better with a 4 Ohms OT than with an 8 ohms OT.

Resulting recommendation from the view of this EV engineer:

Use these "mismatched" Dumble combos just as they are if you like their tone and performance this way. This "mismatch" even has some advantages. No need to use an extension cabinet from a technical point of view. But if you prefer their tone and performance in combination with an 8 ohms extension cabinet, this will not be a technical problem, too.

That is what this engineer has told me. As I am no engineer, I don’t be able to tell, if it is right or wrong, what he told me. But I can tell, that I don’t now anyone who ever had any problem with an OT or power amp failure with one of these "mismatched" Dumble ODS combos.

And I personally don’t know a player, who has ever used one of these original "mismatched" combos with an additional extension cabinet.

But as you know there are players, who play one of the original heads with an 4 ohm OT in combination with an 4 ohms 2x12” cabinet.

So my personal impression is: Just a matter of personal taste, at least in regard to the originals I am familiar with. If there are problems with some current production transformers, I don’t know, as I am not a tech or engineer.

But based on my own practical experiences with original ODS amps with such a Fender 4 ohms OT I can say:

No problem, just use your own ears to decide, if you like an 8 ohms speaker/cabinet better or a 4 ohms cabinet. 16 ohms I did not "risk" up to know. But I don’t know, if this really would be a risk. As I think this question is rather academic (all 16 ohms cabinets I know of could be soldered for a total load of 4 ohms, too), it did not bother me that much from a practical point of view, and because of this I did not ask this EV engineer concerning this question.

What do other members here know and think concerning this topic?

Cheers,

Max

[Structo]

Well I'll take a stab at it and say that the Fender Twin OT was probably one of the few 100 watt OT's that were robust enough to handle the power.
And since the Twin had two 8 ohm speakers in parallel, that is why they are 4 ohm.
So that is what HAD used.

My problem will be that my 2 EVM's are 16 ohm and in parallel so I need the 8 ohm tap on an OT.

[Max]

Well I'll take a stab at it and say that the Fender Twin OT was probably one of the few 100 watt OT's that were robust enough to handle the power.
And since the Twin had two 8 ohm speakers in parallel, that is why they are 4 ohm.
So that is what HAD used.

My problem will be that my 2 EVM's are 16 ohm and in parallel so I need the 8 ohm tap on an OT.

Showman had 8 ohms, or am I wrong?

Max

[Structo]

Could be but there are 13 different Showman circuits.
Are you thinking of the Dual Showman? Or single Showman?

12529A OT was used on the Dual and it's a 4 ohm.

The single Showman used a 12530A OT.

What I was saying is that HAD probably had experience with the Twin iron and felt it was reliable enough for his amps.
And since 8 ohm speakers were prevalent then as well as 4 ohm, he just used what was going to stand up on tour.

But, lets face it, we don't know what he thought, or why he did a lot of things.

I'm just happy to be privileged enough to have these schematics and be able to build a sweet sounding amp for myself.

[Max]

The single Showman used a 12530A OT

.

Yes, and because this one was principally available too (just as the 125A7A 8 ohm for 50W amps) at this time, I wondered for some time, why he did not use these two 8 ohm transformers for the combos.

But obviously, perhaps because of some of the reasons the EV engineer has told me, perhaps because of some other unknown (does anyone here know some?) reasons, he preferred the combination of a 4 ohm OT and an 8 ohm speaker in his combo ODS amps - though similar 8 ohm transformers have been principally available at this time, too.

All the best,

Max

[Structo]

All I can think of Max, is that he liked how it sounded being one step off.

Hmmm, I'm running a 8 ohm load (2 x 16 ohm) , maybe I should flip my amp to 4 ohm and see what happens.

[talbany]

Some Tech Stuff!!

Pro's and cons..
different taps

The number of turns is determined primarily by the required primary inductance for the desired low frequency response point, but the high frequency response and transient response may suffer using a higher tap
The lower impedance tap or less winds used creates less capacitance across the wind effecting frequency response (top end)
Therefore yielding a more open airy (Chime if you will) tone/feel..

Higher impedance taps..Unfortunately, when you increase the number of turns, you also increase the capacitance and leakage inductance, which then limits the high frequency performance. The leakage inductance is proportional to the square of the number of turns, so you must decrease the number of turns to reduce the leakage inductance, but this is at odds with the need to increase the number of turns (higher tap) for good low frequency response. Different winding techniques, such as interleaving, can help reduce the amount of leakage inductance, and improve the high frequency response. However you also increase the harmonic complexity/overtones excited by the added turns.
Therefore yielding a slightly darker thicker/dense tone..

Cause and effects of mismatching..(Under extreme conditions)

When you use a load that is lower than the intended load, the output has to drive the load (speaker) with more current because it is a lower impedance than is expected....
Two inherent problems associated with transformers are flux leakage and regulation..... Flux leakage is also referred to as leakage inductance. It is related to the current in the secondary... these problems increase as the current increases... As the current draw in the secondary increases, the primary has a more difficult time transferring the signal to the secondary, so the secondary signal to the load gets squashed, or 'soft-clipped' (Which some actually prefer w/mild clipping)
This soft clipping is called regulation. When you put a heavier load on the output than was intended, it will pull the output voltage down, hence regulation.... The leakage inductance problem arises because the current from the heavier load causing the regulation to occur reduces the efficiency of the transformer by not allowing the output to follow the input... Transformer designers simulate or view this problem as having extra inductance in series with the primary. The extension of this idea then, is that with the heavier load, you could affect the efficiency of the transformer, alter the frequency response (due to the extra leakage inductance in series with the primary), and cause other distortions to occur...

Mismatching the other way. A speaker is a current operated device in that it responds to the current through it to generate a magnetic field that works against the magnetic field of the speaker magnet to make the cone move in and out. Thinking in very short amounts of time, when the output charges up the voice coil with current, then the signal goes away or gets reduced, the cone system moves the voice coil back to its home or resting position.... As it is moving back, it generates a voltage that is fed back up the line into the transformer and appears in the output circuit of the amp. This generated voltage is often referred to as flyback voltage of back EMF because we are charging up an inductor, then when we disconnect or stop charging the inductor, the magnetic field in the inductor collapses and induces this big voltage into itself. This big voltage then 'flies back' to the source of the charging current. There is a mathematical formula to determine how big the voltage is and it is related to the inductance of the voice coil,(Impedance) the amount of time it was fed current, and how much current it was charged with....

The bottom line is that the voltage fed back to the output circuit is oftentimes much higher than the voltage that was used to drive or charge up the voice coil initially. This voltage gets transformed up by the turns ratio of the output transformer, and in many cases can be over 1,000 volts. What happens then is that arcing can occur between the pins on the output tube socket.... Once this has occured, a carbon path forms on the tube socket between the pins. The carbon path allows a steady current to flow between the pins and eventually burns up the socket due to the heat that is generated. For example, it wouldn't be too uncommon to see a transformer turns ratio of 30:1. If we had a voltage fed back from the voice coil that was around 50 volts, 30 times 50 would be a 1,500 volt spike at the plate of the output tube. This is why you often see builders connect diodes in a string between the output tube plates and ground. They are trying to suppress these spikes and dissipate the energy in the diodes rather than allowing an arc to occur at the tube socket. So, when you use a higher impedance load on a lower impedance tap, the turns ratio is higher and resulting fed-back (flyback) voltage gets multiplied up higher than what it would have been with the correct impedance load.
It's just about impossible for me to answer how long an amp would last under these conditions. It all depends on how the designer took these potential problems into account in his or her design with regards to the quality of the tube sockets, the use of stringed diodes, the output circuit operating voltages, how hard you push the amp etc....

For speakers connected in series, there appears to be less control, and more 'back EMF' from the speakers fed back into the output circuit. While that seems rather chaotic, many players prefer the series connection, as it gives them a more textured tone, enhanced breakup, and overall a more desireable tone for guitar work. It's totally subjective, of course, and many factors affect the end result, such as voice coil size, gap energy, closed back/open back, output circuit damping, etc. The best thing to do, in my opinion, is try both arrangements since you have the luxury of impedance tap selection, and go with the configuration you like the best.

Tony

[Structo]

Thanks for the Great explanation Tony.

You mentioned the carbon tracks on the sockets, also something else that can happen is the high voltage can punch holes in the wire insulation inside the transformer.
Resulting in an output transformer that seems to work OK at lower volumes (voltages) but when turned up, starts having problems due to the internal shorting of the winding.

I think many of use forget how much a guitar tube amp uses ball park figuring when designing the output transformers since the impedance dances all over the place.

It is sort of a crude interface between the amp and our ears.
Since the impedance changes with frequency, we can't really say it is strictly an 8 ohm device, it just happens to center around that when operating.

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.

Anyway, fun to play with this stuff, that's for sure.