Bogen CHB100 / Ampeg B15... maybe.

[soundmasterg]

Lots going on now!

Greg -- One of your previous posts indicates you were getting over 90 watts RMS out of the unit before you modified yours with the power scaling circuit. You also provided some power supply readings at various power levels. This answers many questions!

1. It shows that the original OPT as a matter of size can in fact pass the power level the tubes and power supply are capable of -- even if only at mid-band frequencies.

2. It also shows that the proper load (OPT impedance) is there to sink the current that the tubes and power supply are capable of sinking/supplying themselves.

3. It shows that the power transformer is rather weak, as a 60 volt drop on the main output from a 480 vdc quiescent condition indicates a regulation level of 12.5%.

This is all much closer to what the "evidence"indicates should be produced, although frankly, your power output reading seems somewhat high. With 420 vdc on the plates, and only 318 vdc on the screens, the tubes simply cannot pull enough current across a 2500 ohm load to develop the kind of power you are indicating across an external load. A 90+ watt power level WOULD be developed at the plates of the output tubes in this scenario, but after typical OPT losses, the power actually delivered into an external load would be around 75 watts RMS maximum. A number of things can come into play to create power reading errors, being the accuracy of the load used at operating temperature, connecting lead losses versus where the measurement is taken, and meter accuracy, both with respect to calibration and frequency response.

As for the delayed screens, the OP may have used a tube due to my warning to never have screen power applied to warm output tubes without the plate power applied also. To do so would ruin the tubes rather immediately.

Regarding the article, I'm glad you found it! I believe I also have another article comparing a traditional full wave design with SS diodes, versus a full wave SS voltage doubler. I will look to see if I can find that one for you as well. Again however, due to the considerable winding resistance difference in the power transformers of these two designs, the SS doubler does win out.

As an example, my primary system is of my own design, employing PP 6550s in each channel, with each channel capable of 75 watts RMS from 20 Hz to 20 kHz at this power level. The worst case scenario is 75 watts RMS at 20 Hz, where each output stage is drawing ~ 375 ma, or .75 amp with both channels driven. These amplifiers are powered by a common power supply, with common active regulation, which supply the output stages a steady 485 vdc regardless of signal level in either channel.

This regulator is supplied from a raw supply consisting of a full wave voltage doubler configuration, with two 5A high speed diodes, two 400 mfd 350 v doubler caps, and a power transformer rated at 1 KW. With a 203 vac secondary, it is capable of delivering nearly 5A RMS current flow. Against this, the complete amplifer under worst case conditions requires .8 amp to produce full power in both channels. But to provide this current level from the power transformer, this requirement must be doubled due to the doubler's action, and, the current level must be increased even further because of the cap input nature of any doubler design. In this case then, the effective draw from the power transformer is just over 2.25 amps at full power under the conditions described above.

At quiescent conditions, this supply is delivering 535 vdc to the regulator. Under worst case full power conditions, power to the regulator drops to 517 vdc. With only an 18 volt drop, this means the regulator itself is supplied with very well regulated voltage, with regulation better than 3.5%. With this kind of performance, the regulator's ability is enhanced, as the drop across it is minimized, as is dissipation, and performance is ensured at line voltage variations of +/- 5%. This kind of performance could not be delivered with a traditional full-wave design, as the winding resistances for the power transformer of such a supply -- while low -- would still be significant compared to that of the transformer I am using, where total winding resistance in ohms can be counted on one hand.

While my example may be extreme (requiring soft start circuits, etc.), it still illustrates the capability that the full wave voltage doubler is capable of. When that is considered along with the ease of transformer design it creates, it is no wonder that virtually all of the high power amplifiers converted over to this type of design when dependable SS rectifiers became available for this type of circuit.

Dave

Dave,

Forgive me for a typo that I went back and corrected. The 90+ watts were peak and not RMS. Those were taken with the amp full up, lots of distortion, etc. The RMS power on that revision was 51 watts RMS. That revision had a cathodyne inverter (12AV7) and a 6BR8 in the preamp, whereas the current revision is using a LTP (12BZ7) and a parallel 12AX7 and a 5879 in the preamp and gets 58 watts RMS before the power scaling was added. It sounds better too with the current revision. I haven't checked the power output since adding power scaling because I know I have a problem with the power scaling circuit. It functions fine and reduces power as it is supposed to, with the bias tracking the power reductions, etc., but the B+ MOSFET is dropping around 40v and should not drop so much. Kevin O' Connor (power scaling circuit inventor) recently emailed me regarding some info I supplied him about it and said I have a couple things hooked up incorrectly, so I'll be delving into that after finals (next week) and an out of town trip, hopefully, although the next term of school will be starting up too. I will have to find some free time because I want to finish the Bogen project and get back to my Vox AC100 project which is 3/4 of the way done.

I would suspect that part of the previous sag was due to a problem I had with the first dropping network in the amp that I have since fixed. Part of it is likely due to a lower quality power transformer too I would guess. It isn't all that large for a 50+ watt amp. Once I get this amp completely functioning correctly I will take all the readings again and post the results and then we should be able to tell about the transformer spec. I plan to add a power scale disconnect switch to the amp so I can get readings without it in the circuit too.

Your example amp sounds interesting! I've only done guitar and bass amp projects for myself and fixed a lot of amps for others at this point, but I do have some hi-fi amps to work on. I have a Dynaco Stereo 70 and a Scott 299B to overhaul among others. I also have a 1964 RCA console stereo to overhaul, but it works perfectly (except for FM) still on original tubes and caps so I haven't bothered and use it often. I would be very interested in seeing the other article that you mentioned regarding the SS voltage doubler vs the traditional full wave SS setup.

Greg

[ioginy]

I think I may have found the short problem, however I need to double check with you guys because I have never run into this before and I am a little confused.

I was checking with a 9V battery to see where I was getting leakage. I connected the neg of the battery to the my DMM and the positive of the battery to the positive of the first filter cap and dropping resistor (rectifier tube is not installed). Touching the DMM to the chassis I got 8v, after disconnecting the negative leads of the filter caps, the voltage went away. I tried this with the chassis both floating and with it connected to an earth ground source. Both with the same result. I'm going to continue testing to see if I'm missing something, but I'm currently rather confused by the whole scenario.

Also, I was wondering if the voltage doubler could be causing this. I did a little reading and discovered that a voltage doubler actually puts out equal positive and negative voltages that just create a larger difference in voltage, creating literally double the voltage. However, the negative voltage is going to ground. I was thinking this could be fiddling with my readings.

I have also decided to change the voltage doubler to this design:

It seems like a more stable design than what I am using right now.

[ioginy]

I just had a thought, and changed the earth ground source (wall outlet) and tested again.
Same location for the positive from the 9V battery. No voltage in the chassis until I attached the negative leads of the filter caps to ground. It went up to 8V and slowly dropped, .1v per second or so. I checked to make sure it wasn't just draining the battery and the battery is all good. It is sitting at a constant 8.1v while the chassis is slowly dropping bit by bit.

[ioginy]

Aaaaaaand how! I just realized i was using the heater CT as the Hi-Voltage secondary. That might have a little something to do with something or other.

[ioginy]

Right then.

I changed the doubler back to the original bogen
http://makearadio.com/schematics/images/bogen-chb100b-6.jpg

Double checked all my grounds, fired it up and blew the fuze.

*shakes head* I am at a loss here. It's like the negative voltage from the doubler is totally screwing with the second power supply.

I think I'll call it a day for now, let my head clear.

[Mike2]

Hello there, just cleaned up my old bogen chb100, plug it in ...1 cap literally poped and smoked .... 100uf cap.... it is the rectifiers that are short i think any one can suggest replacement , i was thinking 1n4007....thanks all

[ioginy]

That's what I've been using and they are doing wonderfully.

[ioginy]

SUCCESS!!!

Turns out I had one of the diodes in the doubler reversed. All this time, a frikken backwards diode.

That being said, I now have 501VDC to the plates and 527VDC to the screens. I think I'll replace the 1K dropping resistor with a 5.6K wirewound that I have laying around. That should drop it enough to keep it pretty even.

[ioginy]

Got the amp all fired up and it sort of worked.

I turned off the stand by after it had warmed up, then turned it off, then back on again. When I turned it on the second time there was a blue flash in the power tubes and the fuse blew.

I put a new fuse in, turned it on again but left the standby on to allow the screens to warm up after the plates. I plugged in the guitar to try it out, but when I turned it up past half there was a blue flash in one of the power tubes and a puff of smoke from inside the chassis. One of the 10 ohm that connected the plates had completely burnt through.

I double checked my voltages and recorded them on the revised schematic. Some how I am getting no voltage drop through then entire second power supply filter stages.

Also, the plate voltage is 20 volts below the screen voltage, which is a huge problem at the moment. I was hoping replacing the 1K dropping resistor with the 4.6K would bring it down, but it looks like I am going to have to put an even bigger resistor in.

Also, the standby blowing fuse problem. I have no idea where to start with that one.

Anyones input would be very much appreciated.

[dcgillespie]

Ioginy -- A few points to consider:

1. You are starting to accumulate so much screen resistance, that while the source regulation might be very good, the regulation at the screens may in fact end up very poor with so much resistance needed to drop the voltage to a proper level. Screen current under quiescent conditions will typically be on the order of 4 or 5 ma per tube, while at full power, it can be upwards of 15 to 20 ma per tube in your case. This is about a 4X change in current draw per tube, meaning that whatever drop you are producing across your filter network under quiescent conditions to produce a proper voltage level, will increase 4 times at full power. This cannot be good regulation, and will produce problems for sure. At the heart of the problem is the fact that your second power transformer is one of too high a voltage. There are solutions, but using a proper transformer to minimize overall screen supply impedance is the best one.

2. Regarding your standby switch, you really do not need to switch the plate B+ at all in your standby circuit. Simply turning the screen voltage supply on and off will either cause the tubes to conduct, or go into cut-off as a large negative voltage on any grid would do.

3. On a separate note, the stock bias configuration is really quite poor, allowing the bias voltage to likely increase as power output is increased. I would strongly considering using an extra winding or source from your (or a) second transformer to stabilize that voltage as well.

Dave

[ioginy]

1. You are starting to accumulate so much screen resistance, that while the source regulation might be very good, the regulation at the screens may in fact end up very poor with so much resistance needed to drop the voltage to a proper level. Screen current under quiescent conditions will typically be on the order of 4 or 5 ma per tube, while at full power, it can be upwards of 15 to 20 ma per tube in your case. This is about a 4X change in current draw per tube, meaning that whatever drop you are producing across your filter network under quiescent conditions to produce a proper voltage level, will increase 4 times at full power. This cannot be good regulation, and will produce problems for sure. At the heart of the problem is the fact that your second power transformer is one of too high a voltage. There are solutions, but using a proper transformer to minimize overall screen supply impedance is the best one.

Well... I do have another power transformer that would do the job nicely. If that is the best course of action, then it is what I will do. Thanks.

2. Regarding your standby switch, you really do not need to switch the plate B+ at all in your standby circuit. Simply turning the screen voltage supply on and off will either cause the tubes to conduct, or go into cut-off as a large negative voltage on any grid would do.

Well that's simple enough. I'll pull that side of the standby switch off.

3. On a separate note, the stock bias configuration is really quite poor, allowing the bias voltage to likely increase as power output is increased. I would strongly considering using an extra winding or source from your (or a) second transformer to stabilize that voltage as well.

The bias I have put in isn't the original bogen circuit. That one was not adjustable and seemed a little shady so I changed it up to a dual Marshall style. It looked solid and seems to be tried and true.
That being said, when you say use use an extra winding or source from the second transformer, do you mean just change over the source voltage from the original transformer to the secondary of the screen supply transformer? Or do you mean combine them? Swapping the source would be no big deal, though I don't really understand how that would stabilize it. If there is a better bias circuit you can suggest I would be all over that!

Thanks for the input and your help.
Cory.

[ioginy]

I swapped out the second power transformer ( 330-0-330 ) and am not getting a wonderful 464VDC idle screen voltage. I'm a little concerned though with the dropping resistors. Both the second and third b+ supplies are reading 463VDC respectively, both with 10K dropping resistors between them and the previous filter cap.

Is this because I am getting more current from the transformer because I am not supplying the plate voltage as well?

That all being said, I tried it out and it works lovely. There is a little hum but when I touch the chassis it goes away. I have a feeling it's a bad ground on the input, which i'll sort out later.
It also has a lot more distortion than I would like. I tried variations of 12AX7's, 12AT7's and 12AU7's on both the input tube and the tone stack tube, obviously the lower the gain, the less distortion, but there is still far too much for a bass amp.
Is there any further way of reducing distortion?

One more question while I'm thinking about it. The cathode follower before the tone stack... I used a 220K cathode resistor for the follower, but I don't actually know what I SHOULD be using. I always see cathode followers with the cathode resistor equal to the "leader" triodes plate resistor. I figured it would be a safe bet until I could do some testing. Is there a formula for this, or should I "play it by ear"?

[dcgillespie]

Cory -- By rough estimates, the preamp and driver stages are pulling about 7 ma in your design. That means that the first 10K B+ dropping resistor should be dropping around 70 vdc across it, and the second one about 25 volts for the values you have listed. The phase inverter is the primary current user in this scenario, with an estimated current draw of around 4.5 ma. If you are not seeing these kinds of voltage drops, or virtually none at all, then something is wrong with the wiring or biasing of these earlier stages.

The cathode follower should draw at least 1 ma of current, so the cathode resistor for this stage should likely be about 100K, but this will also depend on the plate voltage of the stage preceding it.

The bias circuit would be greatly improved if it were sourced from the screen supply transformer -- same topology, but derive the voltage from the HV winding of the screen supply. The values in the bias circuit will likely require adjustment to bring your target voltage back in line however.

The original bias supply is basically operating off of the ripple current from the doubler. As the load on the doubler increases, the ripple current does as well, meaning that the bias increases as demand for current from the output stage increases. Sourcing the bias from the screen supply removes this issue, and will stabilize the bias supply regardless of power developed.

What OPT are you using at this point?

Dave

[ioginy]

I swapped the bias to the screen supply secondaries, good advice, thanks.

I am using the Weber 100W transformer. 2000K primary with a 4ohm secondary.

I'll take a look tomorrow to see if I mis-wired anything. As long as the schematic is correct, I will go by that.

Here is the updated schematic.


I need to lower the value of the first resistor in the bias supply, as I am getting a max of -24VDC. However, one of the fellows at ax84 suggested I might be getting the wrong draw from my power tubes. I don't have any 1ohm resistors on hand so I'll have to grab some to test.

--edit--

I also just remembered that I took these readings with no tubes installed other than the rectifier.

[dcgillespie]

Your standby switch is shown incorrectly on your schematic. It needs to be in the supply line for the screens, not the plate. I hope you did not wire it that way! In the scenario you show it, turning the unit on standby only allows the screens to operate, and would destroy them quite quickly!

Dave