Question: Is a 30-watt amp head going into a 2x12 cab good enough for medium-sized gigs?

[drtechno]

Copied and pasted marshall support emails. yeah help me find the info..Over the years I got about ten guys contacting me if I ever figured it out. They wanted to do it too. go bottom to top-

I see how very helpful they were. So, it looks like this is going to be another backwards engineer so that a correct transformer could be used.

Must have been a marketing thing to build something like this in a 1U rack, because any tube engineer in their right mind would never use such a small chassis.

Ok, since we are 6bq5/el34 I would have to say its a transformer with specs around 400V b+ with 30V bias and 6.3V heaters. Now I would have to get you to measure the physical size of it (diameter and height). I would have to guess its probably one of the European odd sizes of core that were not efficient and most have discontinued those core sizes in their main offerings. But never the less, there are some that would be close enough in the current standard sizes. Like a 72VA or a 70VA core would be close in size to the outdated 67VA core for example.

 

[Old Music Guy]

Don't bash yourself in the head over it. Unless you have dealt with the particularities of these devices for years you wouldn't know some of the back shop practices. I've been a tech for many years in a lot of different sectors and there is always learning curves and practices. When I served in the USAF I was an ECS/electrics specialist and had to learn how to fix everything that had a wire or bleed air connected to it across the whole air frame inventory. Before that I repaired Consumer stereo equipment, restore antique radios and TVs and current consumer electronics until society changed and made everything disposable. I also repaired the tube amps from the local music store that subcontracted the work to me. But if you want me to component level repair a variable speed drive for a 3 phase motor I would have stumbling blocks and learning curves because I don't fix those things on a regular basis and don't know the common practices behind it. So I would be lost just like the motor control repair tech would be, if working on a bias oscillator circuit for a tape deck. Does that make me less or incomplete as a tech? No, and the same does apply to you in this aspect.

Don't bash yourself in the head over it. Unless you have dealt with the particularities of these devices for years you wouldn't know some of the back shop practices. I've been a tech for many years in a lot of different sectors and there is always learning curves and practices. When I served in the USAF I was an ECS/electrics specialist and had to learn how to fix everything that had a wire or bleed air connected to it across the whole air frame inventory. Before that I repaired Consumer stereo equipment, restore antique radios and TVs and current consumer electronics until society changed and made everything disposable. I also repaired the tube amps from the local music store that subcontracted the work to me. But if you want me to component level repair a variable speed drive for a 3 phase motor I would have stumbling blocks and learning curves because I don't fix those things on a regular basis and don't know the common practices behind it. So I would be lost just like the motor control repair tech would be, if working on a bias oscillator circuit for a tape deck. Does that make me less or incomplete as a tech? No, and the same does apply to you in this aspect.

No troubles. We all know what to do given any certain device. No offense taken.
And, thank you for your service.

 

[ecc83]

To confuse you even more, both traditional transformers and torroids are nothing but inductors.
In a traditional transformer, a voltage is applied to the "primary" winding. The primary winding is isolated from the "secondary" winding by an iron core. The voltage applied to the primary winding creates a magnetic field which "induces" a voltage on the secondary winding. The voltage on the secondary winding is a function of the ratio of the primary winding to the secondary winding, (This can further be explained with regard to "step up/step down transformers).

Torriods differ insofar as that the round iron core, in which the winding is wound, does not induce a voltage to a secondary. Thus, the voltage out is proportionate to the number of windings, etc, That is why there are no "taps". This is what makes torroids more efficient, there is no wasted energy (conservation).

I am sorry OMG but how then do you account for this please? https://cpc.farnell.com/multicomp/m...1398&s_kwcid=AL!5616!3!490691434445!!!network}!370085955250!&gad_source=1&gclid=Cj0KCQjwxeyxBhC7ARIsAC7dS3_R_sk5LEqX0T4Pfh_6uWczJNGTXO2zlyWfphBblzm2W2m1jePmzF0aAmIEEALw_wcB

Torroids are indeed more efficient than traditional 'E-I' transformers but only when they are working at almost their flux limit, i.e. close to saturation. This does make them ideal for valve amplifiers, especially cathode biased OP stages that run at high anode currents and in (close to) class A. Class B and class D transistor amps will gain little in efficiency as their load current is low and programme dependent. It may seem strange then that virtually no valve amps use torroids whereas a great many transistor types do!

I put this down to cost and design inertia. The design of valve amp traffs is very, very mature and getting a torroids worked up with at least two secondaries, HT and heater then testing it to ensure it stayed inside design limits for load/line voltage variations would be an expensive, protracted task with several re-builds. Might as well stay with the tried and trust square jobby! The gain in efficiency is not that great in any case.


Torroids have a lower radiated hum field than E-I traffs but it is not zero! Care must be taken over orientation of the lead out wires.

Much valuable information about power transformers can be found in Douglas Self's book on transistor power amplfiers.

Dave.

 

[LazerBeakShiek]

Torroids are indeed more efficient than traditional 'E-I' transformers but only when they are working at almost their flux limit, i.e. close to saturation. This does make them ideal for valve amplifiers, especially cathode biased OP stages that run at high anode currents and in (close to) class A. Class B and class D transistor amps will gain little in efficiency as their load current is low and programme dependent. It may seem strange then that virtually no valve amps use torroids whereas a great many transistor types do!

Most of my guitar amps coincidently have torroid transformers..

If you dudes want the best in amplification...Go Van Nuys CA..those guys in Cali made the best shit. ADA was the brainchild of todd Langner. Of Star Wars skywalker sound ranch in the mid 70's. ADA , 'Custom' as in CAE. NOT 'Kustom' . Kustom are crap. CAE is Suhr/Bradshaw if those names arent familiar you dont really play guitar.. Those Star Wars sound people went on to make the best amps in history, and furthered Dolby and THX sound over time...The Star Wars people even brought us the pig nose portable amp...

ADA and CAE products kick the shit out of the competition.. they go places the others don't.

edit- CAE, Bradshaw, and Suhr have nothing to do with star wars that Im aware of ...they just happened to be out of Van Nuys as well.

 

[drtechno]

Most of my guitar amps coincidently have torroid transformers..

BTW, I have that ecc83 dude on ignore.
But that is interesting that your amps you pick has toroids. Was that deliberate?

Torroids can be efficient when chosen and installed correctly. The drawback to them is they can operate hot if they are not spaced off the chassis correctly. But there is always a temperature rise in a unit with a transformer. That is why they are typically mounted above a sub chassis.


Btw, I still need physical dimensions of the transformer in that Marshall unit so I can source who and how much they want to make one. Even though Marshall should have never made it in a 1U chassis because of the lack of dissipation. I've been reading up on the failures people have with it and I can tell its related to it being crammed into a 1U chassis. There are a lot of transformers I see that are for stereo 6BQ5 amps but they don't have the fixed bias winding because most el84/6bq5 amps are self bias.

 

[ecc83]

You can ignore me all you like but that does not make you right. Torroids will not only run hot if mounted so as to form a shorted turn they will destroy themselves!

Their main advantage in modern gear is that they allow a much lower profile chassis e.g. 1U rack gear. Valve power amps are SO inefficient that the gains made by re designing an expensive transformer for them is hardly worthwhile. Might as well go the whole male pig and develop a SMPSU.

Dave.

 

[drtechno]

According to the schematic provided by drtechno, the transformer is a torroid. Hence, unlike "traditional" transformers, there is no "secondary" winding from which to tap off of.
Marshall did this because torroids are way more efficient than a traditional transformers and run very much cooler thus not wasting power in the form of heat.

There is plenty of information on torroids if you want to check it out.

They are still called secondaries regardless of core type. A transformer w/o secondaries is called an auto-transformer regardless of core type.
Torroids run cooler (normally if sized correctly and the primary voltage is not greater than the listed voltage for the coil) than EI types normally because there is so much surface area the windings can dissipate the heat. But there will be always temperature rise and this is a known rating listed on the transformer. The design issue with the Marshall is the 1U space chassis. I can size a torroid to go in it, but its not going to fix this design flaw. If I ended up with a unit like this, I would just mount a normal transformers on top and put a 2U vent cover above and a 1U vent plate below in a rack.

One of the flaws in the Marshall was the use of 115V primary and not accounting for the temperature rise when the voltage is above 115V. That is why only the 230V is available because their power is 220 or 230V overseas and those units wouldn't have issues compared to the 115V models that 120V(or more in some areas) is applied to it.

There are always plus and minuses with different transformers and core types. Torroids still can create the eddy current effect. Its just a different magnetic field pattern and using a chassis too small can multiply these effects and compound the heat build up issue inside them.

 

[Old Music Guy]

Without going into the design choices that Marshall chose, I will just refer back to the schematic that was posted It could not be any more obvious that Marshall chose a torroid.

Marshall Exemplar Input Transformer
I've dropped in a link FOR REFERENCE showing that 115V transformers are available. THIS IS JUST EXAMPLE, and NOT meant to be construed as a direct replacement.

 

[Old Music Guy]

One of the flaws in the Marshall was the use of 115V primary and not accounting for the temperature rise when the voltage is above 115V. That is why only the 230V is available because their power is 220 or 230V overseas and those units wouldn't have issues compared to the 115V models that 120V(or more in some areas) is applied to it.

Sorry if I don't see it as a flaw. I would be more inclined to think that Marshall made amps for the European market and amps for export.
Seeing as that most of the world uses 220V, and the American market is way smaller, it would make economic sense to use torroids for cost reduction. (Based on domestic/export volume).
Whereas other manufactures might use either/or. While it may cost them a few dollars to use an E/I transformer, it's way cheaper to do that in the long run rather than built voltage specific.
IMHO that is

 

[ecc83]

Voltage tapppings of equipment such as radios and TV s were mandatory post WWll (at least in UK) because we had such a range of mains voltages around the country and even in different parts of towns and cities! (that's "private enterprise" for you!) Lowest was I think 200V and highest 250V. The old valve driven B&W TVs were especially voltage sensitive. Too low and the CRT was dim. Too high and you risked burning out the CRT filament and that was expensive! So we had taps on dropper resistors every 10V or so.
Radios had usually 3 voltage ranges from 200 to 250 using taps on the transformer, so all this was good, necessary engineering.

Quite where the idea came from that you needed to make your kit usable in the USA as well as Blighty I do not know?
What were people doing? 6 months touring UK then over the Pond* for the Winter? Rinse and repeat next year?
A dual winding transformer is always going to be more expensive to make, a little larger and probably marginally less efficient. Then, it is quite difficult enough to get a traff right at one mains voltage and frequency, allowing as you must for at least a 10% tolerance. Why confound the felony?
Then there is the issue of the voltage selector, it adds more cost of course but do you make it "user operated"? If so you run the risk of a wrong setting and they will NEVER fit the right 'king fuse! So, put the adjustment inside, a solder job even? Reviewers will moan about that.

Best then to just make the fekkin thing for its designated country...PROPERLY!

*And if they are THAT rich, just buy two sets of gear and save all those shipping fees.

Dave.

 

[Old Music Guy]

Best then to just make the fekkin thing for its designated country...PROPERLY!

I don't disagree, further, that was exactly my point. Good, efficient design will always suffer to make way for saving a penny.

 

[drtechno]

Sorry if I don't see it as a flaw. I would be more inclined to think that Marshall made amps for the European market and amps for export.

it is a small oversight, that in some cases they would be over voltage. That is why they don't make it anymore.
Since their European model is 230V and they use 220 power Then the B+ would be less than what was intentionally built.
But even then, they are running the el84 tubes way out of their range for no reason. There is no advantages but to the person that sell them replacement tubes. Even if it doesn't nuke the output transformer, which these things do at times. The tube is a 5W tube that they are trying to get 10W out of it, that is why its a bad design. There is no magic guitar fairy dust statement that can't detract from that.

 

[ecc83]

A good internet friend of mine who is an 'all knowing' retired guitar amp has told us many times that it was common for US amps to come here with 220,230 &240V tappings on their transformers. Taking our "normalized" 230V as gospel they set the amps for that and biased them accordingly. Shame then that we mostly get 240V+ and the amplifiers ran the valves hot and were unreliable. The advice is to seek out that 240 (245?) wire and swap it for the 230V one.

Just a bit of FYI for accurate data? The EL84 has a max anode dissipation of 12W and it capable of 5.7W at 10% THD from a single valve cathode biased. Two in AB push-pull will safely give you about 15W cathode biased and perhaps 20W fixed biased but few amp makers seem to do that? I do agree that there is no good reason, i.e. it does NOT sound better, to run the valves over the 12W limit as indeed some of the early Vox AC30s did.

See? I do not ignore people because I am NOT twelve.

Dave.

 

[LazerBeakShiek]

Just a bit of FYI for accurate data? The EL84 has a max anode dissipation of 12W and it capable of 5.7W at 10% THD from a single valve cathode biased. Two in AB push-pull will safely give you about 15W cathode biased and perhaps 20W fixed biased but few amp makers seem to do that? I do agree that there is no good reason, i.e. it does NOT sound better, to run the valves over the 12W limit as indeed some of the early Vox AC30s did.

Is it different in some way, but not better OR no better, no different...because it could be someone elses better..does that read right?

 

[ecc83]

Is it different in some way, but not better OR no better, no different...because it could be someone elses better..does that read right?

Better...definitely better maybe.

Dave.

 

[Old Music Guy]

Since their European model is 230V and they use 220 power Then the B+ would be less than what was intentionally built.

I may be taking some liberties here, but here's a thought. In America, (and I can't speak for Europe) the only thing that is truly reliable is the frequency, i.e. 60Hz. We, as a rule, always designed (and tested) power supplies to perform reliably at input voltages of 120V +/- 10%. We all know low line/high line conditions occur during the course of the day depending on the load. We all remember how our old TV set's CRTs would crap out over time due to low line, or "brown outs".
This is not to start any arguments, but just something that came to my mind. Especially when choosing torroids or E/Is.

 

[ecc83]

I may be taking some liberties here, but here's a thought. In America, (and I can't speak for Europe) the only thing that is truly reliable is the frequency, i.e. 60Hz. We, as a rule, always designed (and tested) power supplies to perform reliably at input voltages of 120V +/- 10%. We all know low line/high line conditions occur during the course of the day depending on the load. We all remember how our old TV set's CRTs would crap out over time due to low line, or "brown outs".
This is not to start any arguments, but just something that came to my mind. Especially when choosing torroids or E/Is.

I have always wondered what the design centre voltage actually was in the USA? Here I have read glib comments of anything from 110V to 120V with, I had assumed a 10% tolerance. Or is mains voltage set state by state?

Frequency is held very precisely over here as well, a throwback to a time when there were lots of mans powered electric clocks. We also have a forensic data base where scientists can check the exact mains frequency at any time or date. This can be used to validate recordings.

Dave.

 

[TalismanRich]

US power for single phase is supposed to 120v at 60 cycles. Typically the power comes in on two phases. Using a split phase center tapped line, you can run 120 or 240v. Two phase equipment like electric driers, electric stoves and A/C units will use split phase, but is commonly referred to as 220V wiring.

In the past, the voltage could be as low as 110. Still a lot of people refer to US as 110v or 220v. I guess old habits die hard.

 

[drtechno]

This is not to start any arguments, but just something that came to my mind. Especially when choosing torroids or E/Is.

If you use a transformer that is below the wall voltage (115V transformer on 120V wall voltage) it will heat up proportionally (like 125F instead of it normally running at 90F.
That is why I choose 120V and 125V transformers for new builds.

But you still haven't measured the Marshall transformer physically.

 

[Old Music Guy]

But you still haven't measured the Marshall transformer physically.

I really don't want to start an argument, I really don't.
Firstly, what do you mean by "physical description" ? Fit, form, and function?
Secondly, how would someone go about measuring the "inductance" of a transformer i.e. inductor?