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Output transformer sizing...again...


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Greetings one and all,
Just want to make sure I am on correct by saying if I use let's say a 30w transformer in a 100w application, even if the impedance is correct, I will lose low end at high power.


You'll probably saturate the transformer long before you get anywhere close to your target power level, particularly at frequencies below 100Hz or so. Not a particularly good design approach if you are looking for anything approaching decent linearity. It all depends on how the core handles the higher flux density, and what sort of other losses are present. (copper, etc) This might be ok in a guitar amplifier though.. :eek:
Wont you ruin your output tubes when the transformer saturates ?

This is an often overlooked possibility. I have seen red plateing in my red board when cranking too much power through too small of an OPT. Swapping out the OPT solved the problem. Usualy the sound quality is poor at this point so you won't keep cranking the volume. The results of using a small OPT in a guitar amp can be disastrous.

Way back in the 60's I played in a band. The old wives tale of the day was "don't play bass (guitar) through a Bandmaster, you will blow it up". One of the guitar players had a Bandmaster (and parents with too much money) so he tried it. It did sound rather cool for a while, then the tale proved to be true. I didn't understand the reasons at the time, but now I know that the tiny OPT was severely saturated and the 6L6GC's had to eat all of the power, until one blew up.

If your speakers have a high impedance below 80 Hz or so you can often get away with pushing the ratings a bit. If you use a subwoofer and an active crossover in front of the amp, so that the amp sees no low bass, then the ratings can often be abused....a bunch, like twice the rated power.
my order will be 10 weeks from time of placing it to shipment.

Dead normal amount of time. Had you ordered from me it would have been 8 weeks for a custom part. I would be two weeks quicker because I run a smaller operation. Only my distributors have any finished stock and then only for the SLO clone market of 100 watt OPT's.

You must understand that it takes as much time to run one part as it does to run 100 parts, except for the actual labor of making the thing. And even there a competent crew can make 5 to 7 parts of a known product compared to making one of an unknown product.
Hopefully this isn't considered a hijack....

I found a Bogen 830340-000 locally today for $5 and couldn't pass it up. It was at a local electronics store on the consignment shelf and they tried to tell me it was a power transformer. Color code was all wrong.

I looked up the pn and found it was in the Bogen CHB100 (W PA) amp that was rated from 20Hz to 20KHz +/-1 a yardarm (2dB).

The iron weighs in at 4# and seems pretty solid. Nice thin lams. Good wire lengths and no sign of over-temp in it's previous usage, so I figure it is ok to use.

That said, I find it difficult to believe I'm going to get 100W at 20HZ out of anything driving this without massive gnfb, and even then I find it suspect.

It was originally driven by a quad of 7868s so I figure around 3K3 to 3K6 p-p.

What is a reasonable number for it in terms of power out at low frequency (20Hz)?

My thoughts are to use it for a tube center (base) channel. Probably not the best idea as SS is reputed to be a better soln for low frequencies. However, that is what I'd like to do. I would prefer to switch to a PP pair rather than PPP.

Other than dc resistance to confirm what it probably was, I have made no measurements of it.

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Well, the numbers add up to 100 watts at 1khz, of music power, from a PA amp, so distortion might be a problem at that point. 40 watts AC rms down to about 50 Hz equates pretty well with a 4 pound transformer. It will run pretty warm at that point, but that's just the nature of the beast.

Certainly 3.3k is the optimal operating point for four of those tubes. You will need a 6.6 kz primary reflected Z for two tubes and 20 watts will be all they can write.
Thanks BudP.

That confirms my suspission that it is probably a 20-25W transformer. I was thinking of putting an 8 Ohm speaker on the 4 OHm tap to reflect 6.6Kz if I ran two tubes. I may switch to 6L6s since I have them and don't have 7868s, those were just what Bogen used.

I went back and read the Bogen spec again. They don't say at what frequency they get 100W, but do spec 5% distortion. I guess if they tested at 1KHz they might squeeze 100W (IHFM, wasn't that a peak reading?) out of it.

Frequency Response does not spec at what power level, so 20Hz to 20Khz could have been at 1W.

I'll probably play around with it a little to see how it sounds when I breadboard my 6L6 (6P3S) PP amp. As a center channel it would primarirly be providing base in watching TV, so not a lot of continuous power and it wouldn't run very warm.
40 watts AC rms down to about 50 Hz equates pretty well with a 4 pound transformer.

YEP, I have a bunch of surplus "guitar amp" OPT's that weigh about 4 pounds each. The original spec sheet said "80 VA from 80 Hz to 4KHz." My testing reveals no problem cranking 150+ watts through them at 1KHz. 40 watts at 50 Hz sounds about right. I measured 50 watts at 70 Hz very cleanly. Saturation begins at about 20 to 25 watts at 25 Hz depending on what is driving the transformer, and how the DC balance is adjusted. These transformers work best with a slight DC imbalance.