Bogen CHB100 / Ampeg B15... maybe.

[soundmasterg]

I still don't understand why the 5U4 rectifier is in there. I understand why you added it, but there is really no point to delay the screen voltages until the plates are getting their supply. If you think about it.....the plates will always get their supply first since an output transformer has less resistance than through the power supply dropping resistor to the screens. Having that 5U4 in there makes your screens a lot lower than they could be, which limits the power of the amp quite a bit. The phase inverter voltages being lower also limits the power because the phase inverter determines how much drive the power amp gets. The cathodyne inverter needs a higher voltage supply to equal the paraphase or LTP phase inverters in drive too.

The 7868's are really cool tubes, but probably not best for a bass amp. They would sound good but not give as much pwoer as the 6L6. Changing to a 6L6 will require more gain and drive from your preamp. EH makes some very good, modern 7868's that put out as much power as the old ones, and sound just as good. 6L6's should be a better match for your output transformer though. A slightly higher filament current, but at only .4ma difference the power transformer shouldn't complain.

The stock Bogen preamp in these wasn't really a great design IMHO for bass or guitar. It was a PA amp and was sort of a compromise. I completely gutted mine and changed it up....a parallel 12AX7 into a FMV tone stack, a volume control, a 5879 pentode, a volume control, into a LTP phase inverter, and a fixed bias 7868 quad with no NFB or switched to have NFB. Then I added power scaling to get overdriven tones at lower volumes. For your purposes as a bass amp, you still need good gain levels, but higher voltages in the preamp will give more headroom and allow a larger input signal. In concert with that, the bias of the preamp and phase inverter stages should be set to allow the largest input signal. I think you will get a lot closer doing this to your optimum sound rather than sticking closely to the Bogen design....and that 5u4 is limiting your power, introducing sag, and limiting your voltages. In my mind, that should go.....

Greg

[dcgillespie]

Soundmaster -- your points are all well taken. The idea to use a second B+ supply was to free up as much available current from the main supply for output stage plate current as possible. Since the OP was trying to maximize power output, and the original power transformer was deemed rather small for the task at hand, the OP decided to augment the original supply rather than replace it.

The additional supply does not need to use a rectifier tube, but it is mandatory that the output tube plate voltage be in place either before, or no later than when the screen voltage is applied, so as not to permanently ruin the output tubes -- this due to excessive screen dissipation if the screen voltage is applied to warm tubes even momentarily before the plate voltage is applied. In any event, I think the idea of using a rectifier tube was as much to make sure the plate voltage was there first, as it was to possibly limit some excess voltage he was getting from the new screen supply. In any event, even with a 6L6 output stage, the current draw from the screens alone will go from about 16 ma under quiescent conditions, to about 40-45 ma under full power conditions. With the current 5U4 based screen supply, this level of increased current flow is not going to produce nearly the sag the screens would be subjected to, were they supplied by the main B+ supply as is traditionally done. Its a different approach for sure, but one that should let the output stage develop all the power that the original power transformer (now plate transformer) will allow the output stage to produce.

On a separate note, I have not forgotten about looking for the article you wanted to see. It has just been a very busy time for me lately, and I haven't had much time for my hobby.

Dave

[soundmasterg]

Soundmaster -- your points are all well taken. The idea to use a second B+ supply was to free up as much available current from the main supply for output stage plate current as possible. Since the OP was trying to maximize power output, and the original power transformer was deemed rather small for the task at hand, the OP decided to augment the original supply rather than replace it.

The additional supply does not need to use a rectifier tube, but it is mandatory that the output tube plate voltage be in place either before, or no later than when the screen voltage is applied, so as not to permanently ruin the output tubes -- this due to excessive screen dissipation if the screen voltage is applied to warm tubes even momentarily before the plate voltage is applied. In any event, I think the idea of using a rectifier tube was as much to make sure the plate voltage was there first, as it was to possibly limit some excess voltage he was getting from the new screen supply. In any event, even with a 6L6 output stage, the current draw from the screens alone will go from about 16 ma under quiescent conditions, to about 40-45 ma under full power conditions. With the current 5U4 based screen supply, this level of increased current flow is not going to produce nearly the sag the screens would be subjected to, were they supplied by the main B+ supply as is traditionally done. Its a different approach for sure, but one that should let the output stage develop all the power that the original power transformer (now plate transformer) will allow the output stage to produce.

On a separate note, I have not forgotten about looking for the article you wanted to see. It has just been a very busy time for me lately, and I haven't had much time for my hobby.

Dave

Hey Dave,

Thanks for elaborating on the thoughts behind the design process. It is an interesting approach and I understand better now what the goal was. Not the way I would have gone about it but this is a DIY forum after all, so kudos to the inventive nature of the design! :)

I understand about getting busy Dave....I've been that myself lately and haven't had any time to revisit my power scaling issue with my Bogen. Get to finding the article when you have time. There is no hurry on my end.

Thanks.

Greg

[ioginy]

This design has developed into a bit of an oddity, that's for sure. I also ened up gutting the whole thing, replaced the input and preamp with a Ampeg B15 design for ideal bass response. Also included a mid boost switch because the fellow who will be playing the amp is using an acoustic bass. Should be pretty nice sounding. I have already swapped the 7868's to 6L6's, so there's no going beck on that one, and the only remaining original circuit from the bogen is the PI. I was thinking of kicking that out to but it does seem to work very nicely. Right now I just have to get my voltages in check and lose some distortion. Sadly I'm in the mountains for the next week so it'll be a bit before I can get to it.

Soundmaster: do you have a schematic of the mods you did to the bogen to you tore down?

[soundmasterg]

This design has developed into a bit of an oddity, that's for sure. I also ened up gutting the whole thing, replaced the input and preamp with a Ampeg B15 design for ideal bass response. Also included a mid boost switch because the fellow who will be playing the amp is using an acoustic bass. Should be pretty nice sounding. I have already swapped the 7868's to 6L6's, so there's no going beck on that one, and the only remaining original circuit from the bogen is the PI. I was thinking of kicking that out to but it does seem to work very nicely. Right now I just have to get my voltages in check and lose some distortion. Sadly I'm in the mountains for the next week so it'll be a bit before I can get to it.

Soundmaster: do you have a schematic of the mods you did to the bogen to you tore down?

What were you going to do with those 7868's? I like those tubes but am dirt poor at the moment living off loans going to school.

I didn't like the stock PI in there myself and experimented with quite a few before I settled on a LTP.

I do have a schematic for my mods but its on paper. Once I finalize it then I will be putting it into a computer program. I'd be happy to share when I get to that point, though mine is for a guitar amp and would probably need the preamp voltages upped, caps made larger, and the gain reduced to be nice for bass.


Greg

[ioginy]

I hadn't really thought about what to do with the 7868's. You got anything you wanna trade for them?

I was thinking about throwing a bassman PI. It's stable, tested and true.

Before I do that though, I'm going to test all the voltages and take out that 470K plate resistor in the first stage and replace it with a 220K. I think could help my gain issues a bit. I wonder why Ampeg did that

[ioginy]

OK, i swapped the 470K plate resistor to a 220K and it definitely made it a little more manageable. I can still get some screaming distortion out of it, but now I can also get some decent clean headroom. I'd like to clean it up even more if possible.

I also took idle voltages.

b+ to OPT: 481VDC
to 6L6 plates: 409VDC
to first stage: 330VDC
to second and PI: 310VDC

In my second power supply I put the 6L6 plate tap after the 1K smoothing resistor. The voltage before it is 422VDC. Other than making the rectifier solid state, would there be a way to get the B+ voltage up a bit?

Hare are the plate readings directly from the socket points.
First stage (12AT7) - a: 177VDC
b: 118VDC
Second stage (12AX7) - a: 310VDC (the cathode follower)
b: 110VDC
PI (6C4) - 226 VDC

The 6L6 current readings are better now, though the one still reads 10mA higher than all the others. I'm going to have to do a little digging to see what I can find there.

[soundmasterg]

OK, i swapped the 470K plate resistor to a 220K and it definitely made it a little more manageable. I can still get some screaming distortion out of it, but now I can also get some decent clean headroom. I'd like to clean it up even more if possible.

I also took idle voltages.

b+ to OPT: 481VDC
to 6L6 plates: 409VDC
to first stage: 330VDC
to second and PI: 310VDC

In my second power supply I put the 6L6 plate tap after the 1K smoothing resistor. The voltage before it is 422VDC. Other than making the rectifier solid state, would there be a way to get the B+ voltage up a bit?

Hare are the plate readings directly from the socket points.
First stage (12AT7) - a: 177VDC
b: 118VDC
Second stage (12AX7) - a: 310VDC (the cathode follower)
b: 110VDC
PI (6C4) - 226 VDC

The 6L6 current readings are better now, though the one still reads 10mA higher than all the others. I'm going to have to do a little digging to see what I can find there.

I think your plate voltages are as low as they are because you put the tap after that 1k resistor....there is a lot of current through that resistor so there is a large voltage drop there. Put the tap on the other side of that and your voltage should be a lot higher. The plate tap is usually right after the first filter and before any resistors or chokes because you get the most power that way. Mine is like 487v or something like that without the power scaling installed and I have it hooked up right after the first caps after the doubler.

You get cleaner sounds in the preamp with a higher voltage on the tubes, and large cathode resistors so the tube can take a large input signal without distorting. You have to be careful with that though because that makes the tube run cold, and it can sound cold too if you do that everywhere. It is useful to have a hot biased stage, then a cold biased stage, then a hot biased stage etc., to get a nice and well rounded harmonically rich tone. I'd probably start with a 100k plate load on the first stage and a voltage on the plate itself there of 220v or so. Make the cathode a 1k5 with maybe a 220uf filter cap there. This gives good gain and good sound. Fender was pretty close to the ideal values for that first stage. Then in the second stage, you can have a 220k for a different sound, and maybe a 4k7 on the cathode with a 25uf cap, and maybe around 240v on the plate. You can move the values around, and see how the sound changes, but everything affects everything else...parts choices and voltages etc. Aim for high plate voltages in the preamp though for a bass amp and that will usually get you where you want to be.

Greg

[soundmasterg]

Oh I just noticed that you are using a 12AT7 (gain of 70) in the first stage....so the recommendations I gave you will be a bit off. Why use that tube? They don't sound very good....they are used in audio often but are not really an audio tube and have a harsh sound. When they are used it is usually as a phase inverter or reverb driver where it doesn't impact the sound as much. If you want a gain level similar to a 12AT7 but something better suited for audio, try a 5751. (gain of 60) The NOS GE 5 stars are fabulous tubes.

You may try a 12AU7 if you are looking for a different flavor, but you would want to go with maybe a 15k - 47k plate resistor and maybe a 4k7 - 10k cathode resistor for that one. Those are much lower gain (17) though but that may be what you are looking for. You could also try a 12AY7 (gain of 44) which I really like the sound of though I haven't tried it in a bass amp before. Using the cathode cap helps to add gain and done right it will sound good. It also allows you to adjust the frequency response of that stage it is on by the size of the cap.

As far as the 7868's...I'm broke and in school, and don't know if you would want to trade anything for them or not....but you could always sell them on ebay too.

Greg

[ioginy]

I'll try fiddling with the plate resistors a bit more. I think I'll leave out the caps though, as I am trying to reduce gain, but maintain volume. I used a 12AT7 in the first stage as well to try and keep the gain a little lower, however I intend on putting a 12AX7 in now that I have the gain a little more under control.

I was also thinking of replacing the 1K - 10W reistor with a 100ohm - 10W. I figure that will bring all the b+ voltages up a bit as well. I'll move the b+ taps over so the 6L6 plate tap is before the large filter resistor (though I didn't do that originally because I thought there would be too much ripple). That should increase the b+ all the way down the line. I'll just basically remove the last tap and move everything up.

As far as the 7868's...I'm broke and in school, and don't know if you would want to trade anything for them or not....but you could always sell them on ebay too.

I'd be down with trading something. I'm in no rush to get rid of them and it sounds like you have an affinity for those tubes. They'll be safe and sound for you

[soundmasterg]

I'll try fiddling with the plate resistors a bit more. I think I'll leave out the caps though, as I am trying to reduce gain, but maintain volume. I used a 12AT7 in the first stage as well to try and keep the gain a little lower, however I intend on putting a 12AX7 in now that I have the gain a little more under control.

I was also thinking of replacing the 1K - 10W reistor with a 100ohm - 10W. I figure that will bring all the b+ voltages up a bit as well. I'll move the b+ taps over so the 6L6 plate tap is before the large filter resistor (though I didn't do that originally because I thought there would be too much ripple). That should increase the b+ all the way down the line. I'll just basically remove the last tap and move everything up.

As far as the 7868's...I'm broke and in school, and don't know if you would want to trade anything for them or not....but you could always sell them on ebay too.

I'd be down with trading something. I'm in no rush to get rid of them and it sounds like you have an affinity for those tubes. They'll be safe and sound for you

The cathode caps add gain, but its not like they add distortion. You can voice the stage to get good gain without distortion, and adding the caps can create steps in the frequency response that are useful to sculpt your tone. Put a cathode cap on the first stage, then make the cathode resistor on the second stage large so it can accept a large input signal before distorting and add a cap there too, and your gain levels will be good, your volume will be good, yet as long as you use higher voltages in your preamp, the tubes will take higher input signals without distorting. Its just a matter of voicing things. Try it...you may like it...and its easy to disconnect it if you don't.

You may also try LED bias on the cathodes...look at Merlin's Valvewizard site for that one....I think he has it there. Its in his book for sure. If you can't find it on the site though, do a search at AX84 for it. Lots of threads about it.

Moving the taps over to where the plate tap is right after the first cap will increase your voltages and power a lot. The ripple pretty much gets rejected in the push pull nature of the output stage, so don't worry about that. The screen and beyond is where you need good filtering to get rid of ripple. Large filtering in the first stage will tighten up the bass though and make the amp quicker to respond....all good things if you are intending it for bass. I would shoot for a minimum of 110uf on the first filter. You could go up higher still...200uf is fine. With the screen you are limited in your choice since you went with the tube rectifier there, but I think 40uf should be fine. Take a look at other amp schematics and with the exception of Sunn bass amps, they almost always have the plate connected before any RC filters or resistors and right after the first cap. The Sunn uses a choke on the whole amp like Dynaco did on their hi-fi stuff and the amp is UL so that is a completely different beast there.

I think I fixed my power scale issue on my amp after emailing with Kevin O' Connor a bit. I'll find out over time as in my case I had a divider supplying the screen to get the screen voltage below the 440v max for the 7868 and that was causing the voltage drop in the plate mosfet, and also causing the 1k 5w resistor in the divider to blow after about a month of use. I got rid of the divider and just went with the 1k there, and the screen is running about 470v now. I will probably change to individual 1k 5w screen grid resistors to make sure they are happy at the higher voltage. I am using the EH 7868's and they are great for a guitar amp and sound just as good as vintage ones. They probably don't last as long however.

I would like to trade for your 7868's, but have no idea what you would like to trade for?

Greg

[ioginy]

I'll try some cathode caps and see how it sounds. A little more gain would definitely be good.

I'll look into LED biasing. I've never really paddled around in those waters, but I've heard of it before and it sounds useful.

I would like to trade for your 7868's, but have no idea what you would like to trade for?

I'll send you the four tubes and their sockets. I'm moving some time soon and need to turn a lot of my parts into completed amps. I'm always short on tube sockets, tone caps and filter caps. My local electro store carries most things but I mention tubes to them and they kind of glaze over and twitch.

Stuff like that would be right up my alley, or throw some suggestions out. I'm pretty easily pleased.

[dhuebert]

Twitching, eh? I get that when I try to buy a battery for my beetle or my TR6.

"Triumph; is that a GM?"
No
"Triumph; is that a Ford?"
No
"Triumph; is that a Chrysler?"
No
"Triumph; is that a GM?"
No
Repeat as necessary.

Don

[soundmasterg]

I'll try some cathode caps and see how it sounds. A little more gain would definitely be good.

I'll look into LED biasing. I've never really paddled around in those waters, but I've heard of it before and it sounds useful.

I'll send you the four tubes and their sockets. I'm moving some time soon and need to turn a lot of my parts into completed amps. I'm always short on tube sockets, tone caps and filter caps. My local electro store carries most things but I mention tubes to them and they kind of glaze over and twitch.

Stuff like that would be right up my alley, or throw some suggestions out. I'm pretty easily pleased.

Remember that gain scheduling is the key. If you don't have a scope, then it is much harder to do, but if you have a stage with a fair bit of gain and you don't want distortion or overdrive, then sizing the components around the next stage and the voltages for that enxt stage appropriately will allow that large signal from the first stage to go into the second stage and get amplified without distorting. So if you add a cap to the first stage, and you have more distortion than you would like, adjust the voltage supply and the cathode cap and resistor for the second stage up and it will be able to take a larger signal.

I haven't done LED biasing myself yet either....one of these days I plan to.

I appreciate you offering to send me the tubes and their sockets! I will pm you my details. What kind of tone caps or filter caps would you be interested in? I don't have much but I have some. I don't have any sockets at the moment however.

On another note, my amp is working well, except for when on power scaling. If I leave it on long enough in power scaling mode, it starts sounding like some oscillations are going on. Frying sounds and static....It functions fine without blowing anything, and the power scaling works....it is just very noisy. Probably due to the small chassis and layout compromises that entailed, and maybe the MOSFETS in the power scaling need to be throttled down some...I have read that this can happen sometimes. I will get to mine as time allows in between school.

How are you coming with yours?

Greg

[ioginy]

I did some quick drop in's a little bit ago. Threw in a 22uf cap with the first stage cathode resistor and put a 100pf cap on the NFB resistor. Not sure how I did it but now the amp motorboats and doesn't have the volume it used to. With the info you've given me, I'm going to set the first stage gain and then move the cathode resistor switch to the second stage. I think that will be a more effective adjustment. Sadly it's necessary or I would take it right out.
I also noticed, looking at the bandmaster 135 and 100 schematics that the PI has different values from the PI in single PP amps. The 1M resistors are 330K. I am also going to re-do my bias control, as I think I am just barely getting -40vdc and I would like to have a bit more adjustment.
I sadly don't have a scope, though getting one is on my list of things to do. Properly biasing (though I am sure someone will shoot this down) is easier with a scope. The 1 ohm cathode resistor is handy, though not very accurate.