pompeiisneaks wrote: ↑Thu Aug 31, 2017 11:12 pm
You'd need to check if the transformer itself can handle the current requirements. See if you can get datasheets on the transformer. Look up the code on it and see if you can find anything.
~Phil
No way to measure that?
Not easily, it's based upon the construction of the transformer, size of the gap, size/gauge of the windings etc.
Sent from my LG-H872 using Tapatalk
That said, what would one check if putting el34s in this amp?
If the screen resistors were changed to 1k and bias adjusted for el34s, what would be the
measure that would quantify a happy amp?
Not easily, it's based upon the construction of the transformer, size of the gap, size/gauge of the windings etc.
Sent from my LG-H872 using Tapatalk
That said, what would one check if putting el34s in this amp?
If the screen resistors were changed to 1k and bias adjusted for el34s, what would be the
measure that would quantify a happy amp?
Not sure I understand. If you go over the rating, you'll just burn up the transformer. You could try biasing them to lower current to try and be a bit more cautious, but that's all just a Gamble
pompeiisneaks wrote: ↑Fri Sep 01, 2017 1:20 am
Not easily, it's based upon the construction of the transformer, size of the gap, size/gauge of the windings etc.
Sent from my LG-H872 using Tapatalk
That said, what would one check if putting el34s in this amp?
If the screen resistors were changed to 1k and bias adjusted for el34s, what would be the
measure that would quantify a happy amp?
Not sure I understand. If you go over the rating, you'll just burn up the transformer. You could try biasing them to lower current to try and be a bit more cautious, but that's all just a Gamble
Sent from my LG-H872 using Tapatalk
Sorry for the ignorance. I honestly know just enough to make myself dangerous. I'm just trying to understand, without the PT spec, what would be the point of measurement in the circuit that would elude to El34's being able to run fine or cause problems.
In most cases, the power transformer would run just fine with any tubes in it. The problem is how long. If the wire in the winding can't handle more than say 140mA and the EL34's pull 150mA, it will work until the windings overheat and melt and short dead, then things get gnarly. So just 'measuring it in circuit' only tells you the transformer is within voltage ratings that you want, but not that it can handle higher current over longer periods of time. If you're really adventurous, you could try them, and keep an eye on the power transformer by touching it periodically (carful it can get really hot if over it's current rating) and see if it gets too hot. Knowing what is 'too' hot is a guess too, so that's the danger. If it stays mostly just warm, you 'may' be okay, but you may still be only 10mA over the rating and have 50 hours life left before it blows. It's just a gamble at any point, but if it's worth it to you, go for it.
Basically you could be running 200mA through a transformer designed for 100mA and that's really ugly. This amp had 6L6's so that's usually about 116mA if I recall (plus add in whatever the preamp tubes use, but that's usually not tons, maybe 20mA or so per dual triode if I recall). but the EL34 pulls 100mA per tube if I'm looking at the right spot, so they can pull a lot more. (Depends on tube and datasheet, though, some seemed pretty similar to 6L6's so you'd have to see what kind you'd want too.)
Not only did that answer my question, but I actually learned something from all that. I definitely have a better understanding of what those numbers mean in a spec now.
Seeing as to how important that mA number is to an amps design I'm surprised there isn't a better way to reference that spec outside of a data sheet.
Even I still feel a bit overwhelmed with Power supply and OT design, but that's the basic concept, make sure the tubes have enough current to run and aren't over voltages for their maximum rated requirements etc. It's a big balancing act. You can go overkill, but it costs a ton, or you can just make sure it hits the right minimums and such. Some of the classic fender tone was that they were marginally under spec but available in bulk so they didn't burn up, but were taxed quite a bit, causing more sag and overall 'tonal differences' that defined those amps. (I may not be explaining that very well, but the concept makes sense to me)
Here is some information I picked up along the way a few years ago. I can't take any credit for it and I can't vouch for its accuracy. There are several methods listed, each from a different source. I didn't cite the sources because I'm not sure attribution is called for nor am I certain if the providers of the information want their names in circulation. It might allow you to approximate the information you are looking for.
mA Rating of Unknown Transformer
From the Primary Winding
For a 120 volt AC supply the VA rating and primary resistance is as
follows.
30 VA = 30 to 40 ohms
50 VA = 13 to 16 ohms
80 VA = 7 to 9 ohms
120 VA = 5 to 6 ohms
160 VA = 2.5 to 3.5 ohms
225 VA = 1.8 to 2.2 ohms
300 VA = 1.0 to 1.3 ohms
500 VA = 0.45 to 0.55 ohms
Simply multiply all ohmage values by four (4) for a 230 / 240 volt supply.
Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings.
From the Secondary Winding
One very general way, based on a copper loss of say 4%. The ht winding rating is probably the most important. Find the secondary voltage, e.g. 300Vac. Take 4%, giving 12V. Measure the winding resistance; one half if 300-0-300V (i.e. a 300V winding), if bridge then the whole winding. The current will now be that which causes a 12V drop across the winding d.c. resistance. Thus dividing 300V by the resistance would give a ball-park figure for current (Ohms Law). Heaters more difficult; same method, but low
voltage winding resistance is usually impossible to measure accurately. The heater current could be expected to be in line for the output stage that would require the previously calculated anode current.
One degree further would entail loading of the transformer. Sometimes this is possible with mains globes. Such a load across the ht winding for >1 hour should cause the transformer to get only slightly warm, as you still have no heater load, unless you simulate that too. Very generally heater and h.t. load can be assumed to have similar power figures.
Temperature Method
The operating temperature should not exceed the boiling point of water, 212F. Just load it up until you get close to that on a sustained basis. Remember to consider the VA rating applies to the whole transformer and allow something for the filament windings.
Radiotron Designer's Handbook
A good rule of thumb for the current limit of a transformer can be found by measuring the cross-sectional area of its core in inches-squared, mutliplying that value by 5.58 and squaring the result. (Refer to the Radiotron Designers Handbook 4th ed. page 235.) This formula gives a reasonable approximation of the total VA rating of the transformer. You may have to remove the transformer bell-ends to obtain these measurements.
For example, for a transformer that uses E-I laminations to form the core, you would measure the width of the middle leg of the E part across the face of the laminations. If you were looking at the E as if it were a letter, this would the height of the middle leg. Next measure the total thickness of the stack. Multiply those two values to obtain the cross-sectional area. Suppose you have a transformer that uses an EI-76.2 standard core, where the width of the middle leg of the E is 1 inch and suppose the stack is 1.25 inches thick (yes this is a Fender Vibrochamp transformer, part numbers 125P1B or 022772). This gives a cross-sectional area of 1.25 (1.00*1.25) and an approximate VA rating of 48.65 [(1.25*5.58)^2]. That rather handily corresponds to Hammond's rating for their replacement transformer of this type whose secondary windings are: 325-0-325 @ 81 mA, 6.3 @ 2 A and 5 @ 2 A:
(325*.081)+(6.3*2)+(5*2) = 48.92
I should mention that this gives a decent approximation of maximum total current you can expect the core to support. Other factors such as the gauge of wire used in the windings, number of turns, number of windings, insulation thickness (which may drive up core size), etc. place additional limits on the maximum current a given transformer winding can deliver.
I burned in a set of 6l6s and messed with the amp for a couple hours.
I ended up changing (from the modified schematic)
V1a to 1k/100uf
220pf across r23
The amp sounds good, like a pushed bassman. It takes pedals great and is loud as my 100 watter.
It's a pretty dry and tight amp, but good for what it does.
I have a bit up fizzle and clipping when the gain gets pushed too far, so that's my next debugging. I think I will like the amp more with 5881s. It feels like it's fighting the bloom of the 6l6s and the tightness of the circuit.
All in all, I'm thrilled every time I can get one to work and sound acceptable.
Old hat, but thank you all for your help and patience with a newb.
