Bogen CHB100 / Ampeg B15... maybe.

[ioginy]

I'm going to take some voltage readings tomorrow, maybe they will clear up some of the confusion.

[dhuebert]

Good discussion here, guys. One thing I would like to add---A tube rectifier is a poor choice for a bass amp. It makes for a high impedance power supply. This translates to lots of sag as the power tubes make current demands on the power supply. This has the effect of increasing distortion and softening your attack. Again- great for guitar but a severe compromise for bass.

Don

[dcgillespie]

Hi Don -- Thanks for chiming in. I know you've been down this road with some of the beasts you've built!

Your comment on the use of a rectifier tube in a bass amp is well taken.

The suggestion for the tube rectifier was in relation to building a separate additional supply to take care of all the heaters, and the B+ needs for everything EXCEPT the plates (only) of the output tubes -- This in an effort to still use the original power transformer, but get more out of it for increased power output with an anticipated new OPT. The use of a slow warm rectifier in this case would primarily ensure that the screen voltage was not there before the plate voltage, while the SS main supply would take care of the high current needs of the output stage. The screen current does increase with the application of power, but nothing like it does with the plates!

Dave

[soundmasterg]

Thanks for the update Soundmaster. I must admit however that I am a little confused. Consider that:

1. With a 2500 ohm load (maybe a slight bit low, but still more appropriate than 5K), the power output should easily be up in the 70 watt range, at least in mid-band power output anyway. If it is not, then either the transformer cannot pass the power (which I doubt -- it should easily be able to handle that power level at 1 kHz), or there is significant power supply droop from quiescent to full power output.

2. If the transformer is in fact a 2500 ohm unit, the screens should not be glowing on full power/overload surges as the OP reports they are. With the reduced power output you say you are getting, and the glowing screens the OP says he is getting, something is fishy here. If the power supply is in fact holding up well enough under full power conditions, and the transformer is in fact a 2500 ohm unit, then the screens should not glow, and the power should be there. What is your source for the primary impedance information regarding this transformer?

Dave

I measured the OPT myself here Dave. I put 1/2v AC across the 8 ohm tap of the secondary side and read what the voltage was on the primary, and then divided by 1/2. I then squared my answer and multiplied by 8, which gave me around 2500. Of course I'm assuming that based on my one example that all of the CHB100's have the same OPT with the same primary impedance. Remember that the stock power supply is a doubler, and is what I am using, so it won't regulate that well under normal use, even with the two 220uf caps I have in mine. The OPT looked small to me for four 7868's, so it wouldn't surprise me if that was restricting the power output some compared to what it could theoretically do. I am using new Electro Harmonix 7868's, but I also used some used OS 7868's in mine and did not get glowing screens. I am using a 470 ohm screen grid resistor at each screen on mine. When I checked for output power on mine I was using a 1KHz signal too.

Greg

[ioginy]

470 ohm screen grid resistors sound like a very good idea. I'm using 10 ohm on one per side and nothing on the other per side. That is definitely not a good thing. Not sure why I did that in the first place. Must have had a good reason, but can't for the life of me think of what it was.

[dcgillespie]

Thanks Soundmaster -- So to be clear, when you drove the connections that you clearly know to be the 8 ohm tap with 0.5 vac, then 8.84 vac was produced between the plate-to-plate connections on the primary winding. This would represent a turns ratio of 17.68:1 to the full primary, meaning that the 8 ohm tap would reflect 2500 ohms, correct?

If this is the case, then there is something amiss. 3000 ohms would be about idea for a quad of 7868 tubes, where a pair of these with 450 on the plate, 400 on the screens driving a 6000 ohm load will put out 35 watts RMS all day long. To get only 50 watts or so out of a quad means that one of the above conditions has not been met. The variation of 500 ohms in loading just won't loose 20 watts.

One possibility would be the tubes, where the above information is based on good NOS tubes. JJ 7591s for example could not produce this level of power, but only about 55 watts in an ideal quad scenario -- along with glowing screens to boot. They are just a very poor copy of the original device. EH tubes on the otherhand will produce virtually 90% of the original (NOS) device's power output capabilities.

Finally, voltage doubler supplies are the very best supplies when it comes to voltage regulation in the face of changing current conditions. This because the winding resistances in the transformer are reduced to a minimum. Add to it big cap storage reserves and low drop silicon rectifiers, making the full wave voltage doubler unmatched in its voltage regulation capabilities.

If the transformer's numbers are as they are represented to be, then further investigation is needed.

Dave

[soundmasterg]

Thanks Soundmaster -- So to be clear, when you drove the connections that you clearly know to be the 8 ohm tap with 0.5 vac, then 8.84 vac was produced between the plate-to-plate connections on the primary winding. This would represent a turns ratio of 17.68:1 to the full primary, meaning that the 8 ohm tap would reflect 2500 ohms, correct?

If this is the case, then there is something amiss. 3000 ohms would be about idea for a quad of 7868 tubes, where a pair of these with 450 on the plate, 400 on the screens driving a 6000 ohm load will put out 35 watts RMS all day long. To get only 50 watts or so out of a quad means that one of the above conditions has not been met. The variation of 500 ohms in loading just won't loose 20 watts.

One possibility would be the tubes, where the above information is based on good NOS tubes. JJ 7591s for example could not produce this level of power, but only about 55 watts in an ideal quad scenario -- along with glowing screens to boot. They are just a very poor copy of the original device. EH tubes on the otherhand will produce virtually 90% of the original (NOS) device's power output capabilities.

Finally, voltage doubler supplies are the very best supplies when it comes to voltage regulation in the face of changing current conditions. This because the winding resistances in the transformer are reduced to a minimum. Add to it big cap storage reserves and low drop silicon rectifiers, making the full wave voltage doubler unmatched in its voltage regulation capabilities.

If the transformer's numbers are as they are represented to be, then further investigation is needed.

Dave

Dave, your numbers were correct. I think it was like 2460 ohms or something but 2500 is close enough and probably what was intended.

You don't think the smaller size of the OPT in the Bogen contributes to the lower power than expected? My brother's Silvertone 1484 put out 25 watts stock with two 6L6GC's at 475v plate and 350v screen, and when I moved the screen up to the plate supply and filtered it, the amp only made 30 watts, so the very small OPT on that amp limits the power and bandwidth quite a bit I'd say. If a Super Reverb OPT was put in it, it would probably make over 50 watts I would guess. The OPT in the CHB100 is smaller than a Super Reverb 50 watt OPT.

The EH7868's have been great so far. They sound just as good as old stock 7868's and seem to put out as much power. I'd guess their 7591's are very close, though I wish they had them in a smaller bottle like the old 7591's. At least JJ used the correct bottle size.

I'm surprised what you said about the voltage doubler being a good supply for regulation as everywhere else I've read that it does well in continuous service but suffers during peaks and doesn't regulate as well as the more common full wave supplies.

Greg

[dcgillespie]

Greg --

I've got in my files various articles from the late 50s to early 60s where extensive tests were done on otherwise identical high power amplifiers, except the B+ supply was changed from a convention full wave SS supply to a full wave SS voltage doubler of identical voltage and current ratings. In all cases, power rose, distortion dropped, and regulation improved notably with the voltage doubler circuit. Note too that many of the most respected high power high fidelity amplifiers of the day adopted this format as well -- Heath W6M & W7M, Mac 275, Eico HF87 & HF89, etc. to name a few. Theory backs the results up as well, as the typical winding resistances of transformers used in doubler applications are extremely low compared to those used in conventional circuits. Therefore, there is less voltage drop for a given current flow than in conventional applications. Also, with the additional capacitance available, the peak current capabilities are higher as well. Therefore, with lower internal supply impedance and greater peak reserve available, all else being equal, convention supply configurations cannot hope to match the performance of an equivalent voltage doubler supply.

I am obviously at a disadvantage not having a CHB100 to actually observe on the bench. Therefore, I don't know how good the regulation is of the stock supply (even as a V-D design, it can still be undersized), how effectively the screen supply is regulated relative to the plate supply, how effective the PI stage is, and finally, how well the OPT can deal with the power developed. From my chair, I'm operating on theory only, where in there typically should and would be more power produced than is for the conditions stated. However, as you point out, if the OPT is undersized, and the regulation supply is not where it needs to be, then power will suffer for sure.

It would be interesting to know how well the power supply does perform regarding its ability to regulate from quiescent to full power conditions, and if the output transformers are identical (if even by part#) between yours and the OPs.

Thanks for supplying your data to the discussion. It is obviously most helpful!

Dave

[dhuebert]

I've got in my files various articles from the late 50s to early 60s where extensive tests were done on otherwise identical high power amplifiers, except the B+ supply was changed from a convention full wave SS supply to a full wave SS voltage doubler of identical voltage and current ratings. In all cases, power rose, distortion dropped, and regulation improved notably with the voltage doubler circuit.

Most interesting! I wonder if any of the conventional supplies tested used solid state rectifiers. With fast recovery diodes and the huge capacitors now available it would be interesting to redo some of these tests.

Don

[dcgillespie]

Yep -- while some also included a V-D test against a conventional tube rectifier circuit, also included was conventional SS full wave circuits as well.

Dave

[soundmasterg]

Greg --

I've got in my files various articles from the late 50s to early 60s where extensive tests were done on otherwise identical high power amplifiers, except the B+ supply was changed from a convention full wave SS supply to a full wave SS voltage doubler of identical voltage and current ratings. In all cases, power rose, distortion dropped, and regulation improved notably with the voltage doubler circuit. Note too that many of the most respected high power high fidelity amplifiers of the day adopted this format as well -- Heath W6M & W7M, Mac 275, Eico HF87 & HF89, etc. to name a few. Theory backs the results up as well, as the typical winding resistances of transformers used in doubler applications are extremely low compared to those used in conventional circuits. Therefore, there is less voltage drop for a given current flow than in conventional applications. Also, with the additional capacitance available, the peak current capabilities are higher as well. Therefore, with lower internal supply impedance and greater peak reserve available, all else being equal, convention supply configurations cannot hope to match the performance of an equivalent voltage doubler supply.

I am obviously at a disadvantage not having a CHB100 to actually observe on the bench. Therefore, I don't know how good the regulation is of the stock supply (even as a V-D design, it can still be undersized), how effectively the screen supply is regulated relative to the plate supply, how effective the PI stage is, and finally, how well the OPT can deal with the power developed. From my chair, I'm operating on theory only, where in there typically should and would be more power produced than is for the conditions stated. However, as you point out, if the OPT is undersized, and the regulation supply is not where it needs to be, then power will suffer for sure.

It would be interesting to know how well the power supply does perform regarding its ability to regulate from quiescent to full power conditions, and if the output transformers are identical (if even by part#) between yours and the OPs.

Thanks for supplying your data to the discussion. It is obviously most helpful!

Dave

Dave,

My pleasure to help if I can! I would love to get my hands on those file sof yours...I'm sure it would be interesting reading.

My CHB100 has been completely changed from stock...I'm using the chassis and tube sockets and transformers and that is about it. So a straight up comparison between mine and the OP's amp isn't going to be the best. That said, I would expect pwoer levels to be somewhat similar. Mine is using a parallel 12AX7 on the input, into a tone stack and volume control, then into a 5879, then another volume control, then into a LTP phase inverter, then the 7868 power section. The power section is fixed bias and can use NFB or not. I think the original amp was using a cathodyne phase inverter if I remember right? I'd have to look at the schematic again to be sure, but that would change the performance of the amp for sure. Mine also has the added 470 ohm 5 watt screen grid resistors, and I think the OP's amp would greatly benefit from some stiffer screen protection there than stock.

To check the power regulation, you take the voltage readings at RMS power levels and then take them again full up and see how they compare to get your regulation percentage, is that right? I can check that if so. My power level is a bit reduced since I added the pwoer scaling to it as the power scaling mosfets suck up some of the B+. I haven't checked the pwoer output yet since adding that circuit but I think it went down about 15v from where it was before I added that circuitry, and even though its a trivial amount when you're talking about 480v, I can hear the difference in how quickly the amp overdrives as compared to before. I suspect the preamp voltages are reduced more than I had before and if I bump that up, I might get it closer to where it was before. Anyway, if that is the proper method to check for regulation, let me know and I can check and post results. If not, then let me know and I can do the same. I can get the transformer numbers also.

Greg

[dcgillespie]

Greg -- I will try to dig up an article or two, and either point you to a source, or try to scan it and send it to you.

The classic definition of power supply regulation is to express the change in output voltage as a percent of the quiescent voltage. As an example, suppose that under quiescent conditions, the output of the raw supply is 485 vdc. Then, under full power conditions, the voltage drops to 455 vdc. This is a difference of 30 vdc. As a percent of the quiescent voltage then, this represents a power supply regulation of 6.19%. This would be typical performance for the power supply of say an Eico HF-60, where the output stage power is taken straight off the rectifier tube. Regulation is improved, but at the expense of filtering.

If a section of filtering is used between the rectifier and the output stage (such as a choke as used in a Dynaco MK III for example), the regulation will then suffer more, but filtering is improved. In the case of the Dynaco, now the drop is more on the order of ~ 40 vdc, producing 8.25% regulation. If the power transformer is properly sized, the CHB100 should regulate better than either of these examples (in the 5% range), with filtering performance at the output of the rectifier that is at least equal to, or superior to that available at the output of the choke in the stock Dyna supply.

Dave

[soundmasterg]

Greg -- I will try to dig up an article or two, and either point you to a source, or try to scan it and send it to you.

The classic definition of power supply regulation is to express the change in output voltage as a percent of the quiescent voltage. As an example, suppose that under quiescent conditions, the output of the raw supply is 485 vdc. Then, under full power conditions, the voltage drops to 455 vdc. This is a difference of 30 vdc. As a percent of the quiescent voltage then, this represents a power supply regulation of 6.19%. This would be typical performance for the power supply of say an Eico HF-60, where the output stage power is taken straight off the rectifier tube. Regulation is improved, but at the expense of filtering.

If a section of filtering is used between the rectifier and the output stage (such as a choke as used in a Dynaco MK III for example), the regulation will then suffer more, but filtering is improved. In the case of the Dynaco, now the drop is more on the order of ~ 40 vdc, producing 8.25% regulation. If the power transformer is properly sized, the CHB100 should regulate better than either of these examples (in the 5% range), with filtering performance at the output of the rectifier that is at least equal to, or superior to that available at the output of the choke in the stock Dyna supply.

Dave

Dave,

That would be awesome to get some info on those articles, thanks a bunch!

I took some voltage readings on my CHB100 tonight, but these are with the power scaling in circuit. It drops from a 480v supply to 439v at the plates currently at idle. It was quite a bit higher before, so I'm going to try some things to minimize the loss through the power scale circuitry. I'm not sure that I would be able to get a similar idea of regulation as compared to stock right now with this circuitry in there. I have to do a little more research and will probably be adding a lift switch and associated circuitry for the power scaling so I can get a full power option if I choose without the power scaling involved.

I have a Conn organ power amp chassis that I modified that has 3 power amps in it, all using a common PT that has a full wave doubler, and each output section uses it's own phase inverter and OPT. If I get a chance I can check that one and see how well it regulates, though of course it won't be the same as the Bogen. The Conn is using Foster transformers for the most part (one small Schumacher in there for the 7868 pair) and according to Andy Marshall at THD Electronics, Foster made great stuff. The PT on this one is MUCH larger than the Bogen. It will give me an idea of the regulation capability of a doubler though. Once I get some more of the power scaling circuitry figured out, then I can get a test done on my Bogen too, and hopefully by then, the OP will have figured out some more about the issues on his amp. Thanks for the info Dave!

Greg

[ioginy]

The chb100 that I am using has also been rather gutted, original sockets, chassis and transformers are all that is left. If you would like some comparison readings I would be more than happy to to check for you. If anything, for cusiosities sake.

The Part # on my OPT is 83-340-000 with 606801 below it.

The doubler I used is altered however because I am using cans and have common negative leads. There is a schematic of the new design up there somewhere. I need to re-do it to acomodate for the new screen resistors.

I also just recieved a new output transformer I ordered. It is rated for 100w with a 2000 ohm primary and a 4 ohm secondary. I'm going to draw up a new schematic for all of your scrutiny, that also being said, I now have a very nicely functioning chb100 OPT I'm not really in need of anymore.

Greg, you ever thought of having a chb100 stereo ????

[ioginy]

470 ohm screen grid resistors sound like a very good idea. I'm using 10 ohm on one per side and nothing on the other per side. That is definitely not a good thing. Not sure why I did that in the first place. Must have had a good reason, but can't for the life of me think of what it was.

Nope, I'm a moron. I'm using 100 ohm screen resistors. The 10 ohm resistors off the plates of one tube per side are left over from the original bogen design. Could anyone shed some light on why they did this, and if it is adversely effecting my amp?