Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: acheld on October 30, 2021, 10:27:48 am
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Reading through recent posts by tschwarz, I was reminded me that I have a similar question, though perhaps stated slightly differently. I want to spec my power transformers with some insight as to how they might perform in an amp design -- focussing only on the HT circuit. (Let's forget about heater requirements for the moment.)
The voltage and current specs of the transformers are well documented, at least by Hammond and Edcor.
But when I go to the tube datashteets, it gets confusing. I normally look at max plate current in the push-pull configuration. But . . . class AB1 or AB2, or another?? At which voltage? Each data sheet has different operating modes, different voltages etc, making it difficult to know what numbers to pay attention to.
Examples: https://frank.pocnet.net/sheets/127/6/6L6GC.pdf or https://frank.pocnet.net/sheets/191/6/6V6.pdf
My amps have been EL-84, 6V6, 6L6 in push-pull, all taken from Fender designs, and all have worked well. But I'd like to venture off the beaten path now. It seems very easy to "over-spec" a power transformer; but OMG they are heavy and big, and I'm wanting "slimmer" designs. Hence the desire to have some clue about how a PT might perform. I'm OK with using a marginal PT for a certain design, but I want to know what the margin actually is.
So, I'm asking how best to use the tube data sheets in order to understand how a particular transformer is likely to behave . . .
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how the PT preforms is directly related to how you, the operator, uses it.
if it's not loaded 50% of the time
if it's run full throttle til you reach the melting point of solder
Class A, B, C, D......... operation
at the end of day it's a coil of wire, find the diameter, the length, hunt up a cheat sheet for max current for a piece of wire at a given voltage, do the AC wiz-kid math, subtract the thermal, add the thermal.........
we may wear the same size shoe, but our application dictates the longevity
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....But . . . class AB1 or AB2, or another?? At which voltage? Each data sheet has different operating modes, different voltages etc, making it difficult to know what numbers to pay attention to. . . .
99% of guitar amps run in A1 or AB1. Class 2 is grid current, which can't work with capacitor-coupled drivers.
The early SVT, the Fender 300, and an odd little Fender student bass amp, use buffered or transformer drive and potentially can go to grid current.
What numbers? Most of them are important in some way.
There is no shame in copying known-good plans.
There is totally no shame in using available transformer SETS, where the PT and OT worked together in some popular product.
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... I want to spec my power transformers with some insight as to how they might perform in an amp design ...
There is no shame in copying known-good plans.
There is totally no shame in using available transformer SETS, where the PT and OT worked together in some popular product.
You will find most commercially-available transformers align with commercially-popular amp products. You might not wish to "copy" but you will find yourself recreating a wheel Fender or Marshall or Vox or Dr Z already made.
The design always starts with the output tubes & output transformer, with a goal of specific power output to the speaker(s). But the process is recursive & iterative, and after you pick voltages & impedances & currents out of the air you wind up having to adjust to commercially-available parts ("I planned for 5420Ω at the output transformer, but 4k, 5k, 6.6k are the available options. I planned for 268mA, but the available power transformers only do 240mA or 300mA.")
It would be better to copy known-good plans as PRR suggests. Then learn in parallel how to do the many steps of the design process, using the details of a classic known-good plan to check your work/understanding.
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I don't know if this will help?
Reading the bottom half of this page - under the heading Design Example - was a sort of Aha! moment for me, especially the simple method of determining the current requirements for tubes:
"The EL34s are rated for 25 watts maximum, so they will probably be biased around 0.7 x 25W = 17.5W. However, we should work with the maximum average figures, i.e. full drive when the average dissipation increases to 25W. Thus if the raw DC supply voltage is 400V then they will each consume about 25W / 400V = 62.5mA, or 125mA for the pair."
And, the diagram at the very bottom made it even clearer. Then, I started to notice that current production 50W Marshall replacement PT's often have a 150ma rating. :think1:
http://www.valvewizard.co.uk/smoothing.html
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All of the above comments are helpful, and appreciated.
There is a part of me that wants to "design for tinkering." Design an amp that can be changed around, a la different output tubes (not just the 6L6 family), and others. HPB stated what should have been obvious to me, but I had been trying to ignore -- your design starts at the end, the output tubes and OT.
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HPB stated what should have been obvious to me
:laugh:
He's consistent! when I went down my early rabbit hole roads, I asked the same basic questions and to this day, we get the same basic answers.
all of my "proto-types" that went well started at the PA. Once I settled on a PA I liked, it's off to the schematic junkyard to salvage a million variations of Pre-amp stages I could stitch til the solder ran dry
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I don't know if this will help?
http://www.valvewizard.co.uk/smoothing.html
Yes it does.
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...."....if the raw DC supply voltage is 400V then they will each consume about 25W / 400V = 62.5mA, or 125mA for the pair."....
This must be a finger-slip. It ignores power to the load. (And plate heat usually peaks at some output less than maximum.)
No data-sheets have survived?
Experiments not possible?