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Repurposed Bogen CHB100
I took a Bogen CHB100, gutted it, added my own circuit to it, and then added power scaling to it. It is the same size as a 5E3 Deluxe but a bit taller and heavier. It has a single 12, is about 50-60 watts, and has a breakup like a Vox in that it is smooth with lots of harmonics and touch responsiveness, but it is thicker sounding than a Vox and gives close to but not quite JCM800 Marshall distortion when up loud. Its very versatile and every guitar I have tried through it sounds good. The power scaling circuit made it so I can use it for any gig, though it did take awhile to dial it in just right. It is PTP and looks pretty messy but it is wired up pretty well given that I stuck with the Bogen layout. If I did it again I would build it with a better layout and probably on a PCB or a turret board at least. Would be easier to work on it!

With 480V B+ before I added power scaling, I would have thought it should have higher power. A 7868 with those voltages should be around 44 watts per pair, but this was only giving me 55 watts with a true RMS meter and four 7868's, though I was measuring that on the scope with the signal completely clean. Since RMS is supposed to be measured with 5% distortion, then it is likely a bit higher power than that 55W since I measured it with no distortion, but I don't think it is close to the theoretical 88 watts that the RCA manual says I should have.


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(09-12-2018, 11:52 PM)soundmasterg Wrote: ... A 7868 with those voltages should be around 44 watts per pair, but this was only giving me 55 watts with a true RMS meter, though I was measuring that on the scope with the signal completely clean. Since RMS is supposed to be with 5% distortion, then it is likely a bit higher power than that...Greg
Hi Greg,
I assume there is a typo in your post since the "only giving me 55 watts" is higher than you expected rather than lower. Regarding measuring RMS power, the manufacturer can choose any distortion level at which to specify the RMS output power of their product. A high end HiFi  amp may be measured at a very low distortion level such as 0.01%THD or less. It is pretty common for musical instrument amp manufacturers to use 5% or more and there is a lot of fudging the specs by many companies. Your True RMS meter will even correctly measure the RMS power of a fully clipped square waveform.
Hope my explanation makes sense.
Hi Tom,

I edited my post to make it a little more clear what I was talking about. According to the RCA tube manual I should have been getting somewhere around 88 watts RMS since there are four 7868's in use and a pair should be 44 watts, and I only got 55 watts, but as I said, I measured that with the signal on the scope completely clean. If I measured it at the 5% distortion that MI amps are often measured and quoted at, then I am sure it would be higher. I'm not sure how to tell that it is 5% distortion as opposed to say 7% or 3% though without a distortion meter of some sort, which I don't have.

Hi Guys

Greg, you might be misinterpreting the tube data sheet. The applications they list are exactly that: applications. Your transformer set may use four tubes and have the listed voltage, but if the OT impedance is different , the PT is not capable of the current needed, and/or the drive is less in the amp than on the data sheet, then you get less output.

A pair of KT-88s can be used to get 150W but they only produce 25W in my Studio amp. That is because the amp is only designed to deliver 25W regardless of the tube type plugged in. Similarly, Bogen designed a lot of PA amps that would be mounted in quite variable locations and may not have a constant or recommended load, so they tended to be conservative with power output from a given tube set.

Fifty-five watts through the combo speaker will be a bit blaring, I suspect, but would be much fuller and as loud as you think it should be through larger cabinets and/or through multiple speakers positioned as TUTs and our Speaker Book show, focusing the sound where you are playing.
Hi Kevin,

When I was determining the power level of this amp, and another I built based on a Vox AC100, I was trying to figure out why the RMS power was lower than I expected it to be with both. With the Vox, they claimed 100 watts RMS, but I was only getting about 85 watts RMS going directly in the phase inverter with a sig gen and around the same with the whole amp going and a sig gen going into the input of the amp. I built that out of a Sovtek Mig 100U, but I replaced the output transformer and choke with some clones of an AC100 one from Heyboer. It doesn't make the 100 watts I thought it should but it sounds WAY better than the stock Sovtek did.

With this amp I was figuring that with four 7868's and the voltages that are in the amp (480 B+ b4 power scaling was added) and the output transformer having a 2500 ohms primary impedance that the power should be higher. I was looking for the 88 watts the RCA manual insinuated it might be. I figured it must be the transformer set that was bogging it down, but some others on some forums who have been around a long time and are very experienced said that Bogen didn't often understate their power ratings and that a CHB100 should be 100 watts. The power transformer is smaller than I might expect, but it is also using a full wave voltage doubler setup. I plan to experiment further with this setup as I was able to find another working CHB100 to play with. I might get the transformers analyzed down the road from that one too.

Its not that I want more power out of the amp necessarily. It is loud enough for most of what I might want to do and with the single speaker it works perfectly at local blues jams and what not, getting breakup at just the right volume levels. It would certainly be louder through a 4x12 or something. I was mainly wondering why it wasn't making the power I thought it should and want to make sure I am using the correct methods so that if I design an amp to do something and put out a certain power level that it does so.

If the transformer set can produce the power and the voltages are sufficient then if you input the phase inverter with a sig gen that can supply the current, you should be able to drive the power amp to max power and get the 88 watts or whatever the manual says. Since I'm not getting the power I think it should and I have tried inputting the sig gen directly into the phase inverter, then I would guess it is the transformer set that is causing the lower than expected power level?

Hi Guys

You have to get the amp up to clipping to clearly see the real limit of voltage swing on the scope, and clipping should be symmetrical. Just a little flattening. Read the voltage on the scope and back off to just before clipping and you can use a meter to take the RMS reading.

Going by the data sheet one would expect to use 3k3-aa for 88W output. 2k5 is too low and you'll get less power. What is Va and Vs under full load?

Have fun
Hi Kevin,

I've brought it up to clipping and then backed off slightly to just before clipping and that was when I got 55 watts or so, before power scaling was added to the amp. That was at 480V B+. Power scaling steals a little bit of voltage so if I remember correctly it was around 470V with power scaling added, and I never did check the power output with power scaling on it, but it might be a hair less.

In the RCA manual for a pair of 7868's, they list 44 watts for a 6k6 load, so the 3k3 sounds about right. Its been several years since I did this, but I think I measured the OT at a 2k5 load, but I could be wrong on that. For Va and Vs under full load....I'll have to get back to you. It might be awhile until I get to this as I've been working on a long term car repair for my former daily driver and once I find a couple hoses I can put it back together finally. Working on it outside and trying to beat the winter weather, and will be out of the country for a couple weeks in October so I'm running out of time to get that done.

I am curious why the power is lower than I would expect on this amp though so I will update with that info when I have it. I was thinking it was the transformer set and the capabilities of them/size of them. I have another CHB100 that I was going to experiment with down the road and was possibly going to get the transformers analyzed too depending on what I find out.

Thanks for the assistance!


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