Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: acheld on June 29, 2022, 10:23:02 am
-
It's choke day for basic questions.
I've been using chokes in my power supplies for quite awhile, but always basing them on known amp designs -- eg, the copy and paste approach (which has worked out just fine until now).
While perusing Hammond's offerings: https://www.hammfg.com/electronics/transformers/choke (https://www.hammfg.com/electronics/transformers/choke), it occurred to me that I really didn't know what the specifications really meant.
Of course, the inductance is obvious (most Fender amps are 4H), but what is the DC mA rating mean? Is it 4H @ 50mA DC for the 194A/125C3A? Is there a max rated current?
Reason I'm asking is that I'd like to re-design some of the power supplies to a couple of my amps -- and tailor the choke and PT to better fit what I'm doing with the amp. Hard to do when you're not sure what the specs actually mean.
-
AFAIK the dc rating is the max DC current the choke is rated for. I know you probably want to hear from our experts here like Sluckey PRR and HPB and so do I .
But often times I refer to Aiken for info http://aikenamps.com/index.php/chokes-explained
-
Additional to the Aiken info, I recall reading that a very overrated (in regard of max DC current) can be counterproductive, as with real world chokes, inductance drops somewhat with current.
But underrating is a bad idea too, as saturating the magnetic circuit will cause the inductance to collapse, and obviously there’s the risk that the coil may overheat and damage its insulation.
-
tailor the choke and PT to better fit what I'm doing with the amp
read the paper mresistor posted, you want to answer the ?'s on what you intend;
stabilizing G2, overall PS filtering, or voltage dropping.
-
It's worth noting that your expectation of choke performance will be subject to a large technical tolerance due to many aspects. For starters, Hammond put a +/- 15% tolerance on a part's inductance and DCR. Any e-cap used before and after the choke will also have a similarly large tolerance on the capacitance value that interacts with the choke's inductance. The other main aspect is that Hammond specify the inductance value at a 30Vrms 60Hz applied test condition, which may be significantly different (eg. higher) than the operating condition in your amp, and that can mean that the actual inductance imposed by the choke is lower than expected (eg. if the operating ripple voltage is lower). Another subtle difference is that the inductance measurement is at 60Hz, whereas the operating condition in an amp is twice that frequency with substantial higher harmonic voltages.
You may base your re-design on simulation tools such as PSUD2 or LTSpice - and it's worth noting that those tools base their choke part on a simple ideal inductance model (with DCR), so if you put in a particular value of inductance then any result is prone to all the accumulated tolerances that you may or may not be aware of.
But you may just base any 're-design' on swapping in a different choke and just listening for any difference - that then avoids all the technical uncertainty.
-
OK, thanks much. I re-read the Aiken article, and the thing that popped out and made sense was the difference between capacitor input and choke input power supplies.
All the Fender (and most guitar amp) power supplies are, of course, capacitor input supplies where the filter choke is mainly dealing with the screen current (as well as the small amount of HV current to the preamp tubes). These chokes therefore have much lower max and operating current requirements than "choke input" circuits.
Merlin's new power supply book is lying on my coffee table gathering dust -- I'm going to have to read it now, as I was thinking of creating a new beast with choke input. (That is probably not practical).
-
I have built a couple choke input PS amps.
They were mostly experimental (unusually) prototype circuits.
I want to build a bass amp with a choke input PS. Talk about not practical.
-
There are two common power supply configurations: capacitor input and choke input
From Aiken’s documents, I just noticed the above.
It seems a bit strange to describe choke input HT supplies as common, as I can’t remember ever having seen an audio valve amp :w2:
-
Here's one... http://sluckeyamps.com/hammond_2/AO-63.pdf
-
I want to build a bass amp with a choke input PS. Talk about not practical.
I can tell you two things with certainty.. it will be expensive and it will weigh a ton. I converted a Baldwin organ amp that had a huge choke in the power suppy to a choke input 5F6A and it's a pretty nice amp that weighs a ton. The choke up front was mainly used to drop a lot of HV to an acceptable level. The choke is larger than the OT. If you can find an old Baldwin organ with that choke it would save you some moolah.
Of course anyone that's picked up an Ampeg SVT knows how heavy bass tube amps can get.