Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on June 12, 2017, 07:03:08 pm
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Hi guys, Guidance please, in relation to this http://el34world.com/Forum/index.php?topic=21998.0 (http://el34world.com/Forum/index.php?topic=21998.0)
I bought this as a chassis only and no info is available to guide me to the speakers that were used.
I have a head/cab of about the same vintage that has Plessey 12ux50/16 ohm speakers.
So at this stage I'm confident that these speakers were probably used.
I can get 12u50/16 ohm but the OT used in the above post has two secondary windings when in parallel gives 3.7ohm and in series 15ohm. OT wired for 15ohm.
From what I know 8ohm speakers were a bit rare and a single speaker is only rated at 50w.
Plessey speakers https://www.google.com.au/search?q=Plessey+12u50+picture&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwi6stbCvrnUAhUCxrwKHT5-AxMQsAQIIQ&biw=1920&bih=963#imgrc=vqMM6piTlja97M:&spf=1497311110408 (https://www.google.com.au/search?q=Plessey+12u50+picture&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwi6stbCvrnUAhUCxrwKHT5-AxMQsAQIIQ&biw=1920&bih=963#imgrc=vqMM6piTlja97M:&spf=1497311110408)
This is about as close as I can find for the speakers used.
http://www.grouseguitars.com.au/sold/fisonicbigcombo.htm (http://www.grouseguitars.com.au/sold/fisonicbigcombo.htm)
So using the two 16ohm speakers in parallel for 8ohm on the 3.7 ohm tap.....
OR
Two 16ohm speakers in series for 32ohm on the 15ohm tap.....
Would either be a good way to go????
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either way seems fine, IF you're trying to double the primary impedance
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Ditto. 2:1 or 1:2 is usually OK. The other option suggested by the google images site is 4 speakers in series-parallel for 16 Ohms.
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Thanks guys.
jjasilli, I am making the amp as a 2x12 combo.
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Hi Timbo,
You are always best off to get as close to the optimimum pri z as possible as everyone should imagine but presented with either of the two other choices I find that going higher is preferred over going too low. Going higher will tend to give a more normal tone response (likely a bit less high end resonse) at a little less power output. Going lower often times will cause a premature and harsher breakup character that's not all that pleasing. Please give us feedback when you finish especially if you experiment with both?
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Thanks mate, These old amp are usually CLEAN all the way and I think that this would be in the 50-60w output range.
So going a higher load sounds great to me, even with a reduced output.
The Plessey I'm looking at I haven't bought yet, so for the moment I have a pair of 8ohm Eminence I'll use.
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After more thought. Theoretically it's better to put speakers in parallel than in series. In series, if 1 speaker blows, the amp has no load.
Also putting them is series forces them to share the output, cutting the amp's effective power. E.g., Amp with 16 Ohm, 50W output > into 2X 8 Ohm speakers in series. W = V2/Ohms. 50 = V2/16; V2 = 800; V = 28. In series, ea speaker gets only 14V. In parallel, ea speaker gets 28V.
Your effective power will be cut in half again by the load mismatch to the OT secondary. BUT this could work to your advantage if you want to drive the amp hard at lower volume. Tough it may be hard on the amp.
Other ideas: Build a matching 2X12 extn cab for more Plessey's. "Suffer" with non-Plessey's.
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After more thought. Theoretically it's better to put speakers in parallel than in series. In series, if 1 speaker blows, the amp has no load.
Also putting them is series forces them to share the output, cutting the amp's effective power. E.g., Amp with 16 Ohm, 50W output > into 2X 8 Ohm speakers in series. W = V2/Ohms. 50 = V2/16; V2 = 800; V = 28. In series, ea speaker gets only 14V. In parallel, ea speaker gets 28V.
Your effective power will be cut in half again by the load mismatch to the OT secondary. BUT this could work to your advantage if you want to drive the amp hard at lower volume. Tough it may be hard on the amp.
I've never experienced or at least noticed my amp dropping to half output power simply by going up or down on output z - something seems amiss with the calcs/theory?
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Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.
A: It is awfully expensive, and ridiculous that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.
As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.
A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.
The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.
The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.
From this site: http://www.londonpower.com/tube-amps-faq (http://www.londonpower.com/tube-amps-faq)
With respect, Tubenit
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And some more relevant info:
Q: An "expert" suggested that I change my speakers to ones that match the highest impedance tap on my amp. How do I do this and still have the option of using a second cabinet when I play out? I think I would need three cabinets to achieve this.
A: Yes, and what a waste of your money.
Not too surprisingly, this is the same expert as in the tube-pulling/power reduction question. He really should stop talking about transformers.
Rest assured, the impedance taps on your amp are there for your convenience, to use as you will. Connecting the rated cabinet impedance to the identical rated tap selection will get you the rated power bandwidth of your amp into that load. As stated above, any "mismatch" reduces power and bandwidth, and that is all.
If you are using your 4-ohm cabinet and the 4-ohm tap, does it matter if the 16-ohm tap is being unused? Of course not. This subject is explained in detail in TUT3, as the "Myth of Encompassment" - a myth created purely to sell speakers and transformers. To unsuspecting players and readers of the "expert's" column, it is no more than a scare tactic.
Transformer designers take into account the loads to be connected to the device. There is limited space in the winding window for each lamination size, and the designer wants the space to be fully utilized. The percentage of space used is the "build". Ideally, all windings are used all the time, to keep parasitic effects to a minimum. When there is a tapped secondary, some of the secondary may not be loaded under certain conditions, so those "free" parts of the winding can potentially upset the parasitic balance. The amount of upset is usually so small as to be insignificant, even in hi-fi where such a thing might matter. In MI, there is no concern whatsoever.
In most amps, you can set the impedance selector to whatever sounds best. The one caveat is: NOT in English amps. Having replaced more Marshall OTs than anything else, I would advise that the impedance selector always be set to the rated load, or less.
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. . . Oh & I forgot to mention the option of an impedance matching transformer, as sold by Weber.
@jojokeo: "I've never experienced or at least noticed my amp dropping to half output power simply by going up or down on output z - something seems amiss with the calcs/theory?"
Good point. Halving the amp's power output (in watts) drops the SPL 3dB, which is barely noticeable, as you say. Halving it again (E.g., 4 Ohm secondary > 8 Ohm speaker load) drops another 3dB for a total of 6dB. This is noticeable.
1 50W 8Ω amp > 8Ω speaker = full power
[1 50W 8Ω amp > 8Ω speaker] + [1 50W 8Ω amp > 8Ω speaker] = +3dB
1 50W 8Ω amp > 2X 8Ω speakers in parallel = +6dB -- 1/2 load = 2X power for a 3dB gain; 2nd speaker @ 2X power for another 3dB gain
1 50W 8Ω amp > 2X 8Ω speakers in series = -3dB (I think, will have to look up) -- -3dB for 1/2 power; -3dB for voltage sharing; +3dB for second speaker surface area(???)
@Tubenit. Speaker loads are very small, only a few Ohms. As the speaker load approaches -0- the amp is looking more and more rapidly at a short circuit, which will attempt to draw infinite current and may burn-out something: tube, tranny whatever. Tranny's are often robust and can be expected to handle a 1:2 mismatch as stated. However, overdriving the power amp (2X power) in conjunction with 1/2 load (another 2X power) may draw a total of 4X the rated current through the tubes, OT and speakers. It's not just the winding ratio of the OT, but also the current handling capacity of the wire used in the winding.
In this case there's a larger load so excessive current is not at issue.
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Regarding the speaker selection, if the referenced link is correct and you are using a A&R 4008-15, OT, The dash 15 indicates 15 ohm load. As you stated, you can wire the secondaries in parallel giving you a 4 ohm tap. If you wire those 16 ohm speakers in parallel, then then wire the OT in parallel, if you wire the those 16 ohm speakers in series, then wire the OT in series.
Can you get away with an 8 ohm load on a 16 ohm tap, probably, but don't crank the amp up, or it might be bar-b-que time.
A while back, I was reviewing OT specs, and something different, The suggested nominal speaker load varied by the "effective load resistance plate to plate" RLa-a. The spec sheet had a matrix of RLa-a and the suggested speaker load by tap. I wish a had made a copy of the specs.
The way I read that spec sheet, it pays to match the reflected impedance on an O/T to the effective load resistance of the tubes. Match wrong, and it might be like plugging that 16 ohm load in the 4 ohm or 8 ohm tap. It works, but, at reduced power.
(it is my understanding using the link, the A&R transformer is an Australian product. ) Let us know what power tubes you are using.
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Thanks for the info guys.
I took some voltages to find Zout.
Wired for 15ohm=332
Wired for 3.7ohm=1332
KT88/6550
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Based on a number of Replies, I looked further into the question of operating issues when using a higher than rated speaker ohms; i.e a lighter load.
What I glean is that the issue is inductive reactance in the OT primary. The OT is a step-down transformer with a big winding of thin wire (low current) on the primary side, and a smaller winding of thicker wire (higher current) on the secondary side. If the speaker ohms are increased, this reflects back to the primary giving it a commensurately higher impedance. The effect may be to raise B+ voltage. IOW, the higher primary impedance will drop less "raise" voltage as supplied by the PT. (Ohm's Law)
This extra voltage must be dropped across the PT primary. Within the OT's operating parameters the flow of electric energy (watts) is largely dissipated by the primary coil's generation, by induction, of a magnetic field. At some point saturation will be reached -- no matter how much more el. energy is supplied, no more magnetic field can be generated. So the extra energy must be dissipated as excess heat which might harm the OT. Going further. . .
If voltage is increased beyond saturation, at some point the electricity cannot be contained by the insulation covering the wire of the winding. This is the point of inductive breakdown. The winding effectively ceases to be a coil; it's just a lump of metal. So it fails to generate any magnetic field. As a result all of the el. energy passing through it produces a catastrophic over heat condition.
This is not to say this will happen in any particular amp. It is an exploration of the effect upon an OT as a lighter and lighter speaker load approaches the condition of an open circuit.
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I can get 12u50/16 ohm but the OT used in the above post has two secondary windings when in parallel gives 3.7ohm and in series 15ohm. OT wired for 15ohm.
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I will make a bet, that if you check the DCR of the two secondary windings, you will find that they are not the same. I am guessing that the lower resistance winding has larger wiring, therefore, it can be used for a 4 ohm load only. Could you run a 4 ohm load only on the other secondary winding, the answer is yes, but I do not recommend doing that as it does not have the capacity of the lower resistance winding.
Basically, you can wire the transformer to provide two outputs.
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My understanding is that, typically, one winding is wound over the other one, so the same number of turns uses a different length of the same gauge of wire. Hence a different static ohm's reading. More expensive hi-fi trannies equalize the windings.
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I have seen some notes, on OT design, I wish I could quote them, but, many times the 4 ohm windings are a larger wire because they need to to provide the power. I would expect the 4 ohm windiings to be approximately double the mils of the rest of the secondary windings.
Using data from Hammonds 1760E, a 4,8, 16 PP OT, the data sheet shows primary voltage is 346v, and the secondary voltages are 7.7. 11. 15.5 v respectively. Since the OT is rated at 15 watts, using the 4 ohm tap only, the current is roughly double that of the 16 ohm tap. Hence the comments in my previous post.
Drgonzo
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Suppose we have an output transformer with two secondary windings each with the same number of turns and the same gauge wire. The transformer is designed so that either of the two windings matches to a 4 ohm load.
q1: What load will be matched by the two windings in parallel?
a1: 4 ohms. These don't add like resistors or capacitors. They add like batteries. Two 1.5V batteries in parallel is still 1.5V
q2: How much current can the two windings in parallel carry?
a2: Like batteries, twice as much as a single winding.
q3: What load will be matched by the two windings in series?
a3: 4x the parallel combination, so 16 ohm.
q4: How much current in series?
a4: Half as much as in parallel.
q5: What does the last answer imply about power handling?
a5: If we call the current at full power through the parallel combination i, then the max current through the series combination is i/2. For the first case
P = i2x4
and in the second case we get
P = (i/2)2x16 = (i2)/4 x 16 = i2x4
It's the same in both cases. The lesson is to use both identical windings in parallel. Don't use just one of them with other flapping in the wind.
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TB,
I do not disagree with your analysis.
Drgonzo
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Holy shit this is like drinking from a firehose.
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Holy shit this is like drinking from a firehose.
:thumbsup:
Just hang around, pretty soon your brain will go though it's 1st meltdown :laugh:
But you'll come out with a re-wire, complete with understanding, sorta
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Found some very limited data on A&R 4008-15 audio transformer (search string "Australian audio transformer 4008") from "Australian Transformers 2.pdf" output high fidelity
pri. inductance 5,000 ct; 43% screen taps, 3,7 or 15 50w;m 40 c/s primary inductance at 10v 50Hz Henrys 150, Leakage inductance milli henrys 6; typical applications KT88s in UL circuit; 8 lbs,
Looking at the circuit, and a GE KT88 spec sheet, the KT88s, are more than capable of warming up the transformer.