Bogen CHB100 / Ampeg B15... maybe.

[soundmasterg]

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.

Adding the cap to the NFB network basically feedbacks only low frequencies below the cutoff point of that cap. That could have caused your motorboat problem. Disconnect the cap to see of course. It is a fallacy to believe that since you are amplifying bass, you only need bass frequencies so don't go too overboard on trying to just amplify bass. The more highs you have, the more present the bass will sound to a point.

I forget what your cathode resistor value was on the first stage. Keep in mind that the cap's effect on the sound changes with the size of the resistor. Fender used a 25uf in conjunction with a 1k5 resistor. Go up in the size of the resistor and you don't need as large of a cap to have the same effect. In bass amps, Sunn used a 250uf with a 1k5, and some Marshalls used a 330uf with a 1k5. So lets assume you have a 1k5 and a 22uf on the first stage with around 220v on the plates. The next stage you may have 230v on the plate, and a 4k7 with a 10uf on the cathode. A coupling cap between the two might be a .022uf with the coupling cap sizes getting progressively larger as you go through the amp to where the PI might have a pair of .22uf or .1uf leading to the power tubes. Making the second stage have a 4k7 will allow it to have a larger input signal before it distorts and having a higher voltage (within reason) on the plates will allow that stage to be able to swing larger signals, which it needs for bass.

The 330k resistors instead of 1M aren't a choice I would make for a guitar amp. For bass it might work ok. The lower value makes it harder for the driving stage to supply current and reduces gain. Having a high voltage on the cathodyne PI will allow it to swing more voltage and to get more power out of the amp, and stay clean longer up the volume range. Putting the 330k resistors in there makes the PI work harder.

Biasing with a scope is very inaccurate as your result will change every time you do it. That method also tends to bias cold. The scope is more useful for troubleshooting and for setting the gain structure throughout the amp. You can see what the signal does as you adjust parts basically. Very useful for that. The 1 ohm resistor in the cathode is a great way to bias. It includes the screen current as well as the plate current, but thats ok. It ends up biasing a little on the safe side as a result, and with that method of monitoring the voltages, it allows you to set the bias where you want it within the typical 60% to 70% dissipation range.

Greg

[ioginy]

Hmm, excellent points.

I did some mods..



It sounds great, however it isn't very loud. I've checked my values and everything is as it should be. It still distorts a little too early, but I know how to handle that. Just a little tweaking, thanks to soundmanters advice. The lack of volume on the other hand is very worrysome.

-edit-
i pulled the cathode follower out as an experiment, but i can't see that killing the volume that much. I'll put it back in now though, as it didn't seem to improve anything when it was removed.

-edit-
scratch that, I put the CF back in a all it did was create noise. With it gone there was almost no hum, with it in, it came back with a vengeance.

-edit-
I also messed up on how the pot was wired on the second stage cathode. I also removed the 100K resistor after the master as I thought that might have something to do with the loss of volume... it didn't.
None the less, here is the corrected schematic.

[dcgillespie]

A couple of thoughts to consider if I may?

1. I may not have understood your comment correctly, but adding a small cap across the feedback resistor actually increases feedback at the higher frequencies -- this to help keep the feedback signal at supersonic frequencies from reach 180 degrees of phase shift before circuit response tapers off OLG appropriately. Adding an appropriate value can stabilize an unstable amplifier, but adding too much can unstabilize an otherwise stable amplifier. The value is best determined empirically for each particular OPT with square wave performance analysis.

2. The value of the input grid resistor in a Fender style PI is actually only one element that determines the working input impedance of the stage. The true input impedance will be found to be twice the value of the input resistor, multiplied by the feedback factor. Therefore, in a classic Fender circuit -- even one using 330K grid resistors -- the input impedance of the stage is 2 X 330K X 1.85 (the latter figure being a typical FB factor) = 1.221 meg input impedance. As can be seen then, the actual operating input impedance is very much higher than the simple input resistor would imply, due to the bootstrap effect of the LTP PI stage, coupled with the way Fender chose to insert the FB signal.

3. As for biasing an output stage, everyone will have their own favorite approach, but for MI amplifiers, a scope is very useful in allowing the bias to be set at the minimum quiescent current possible that will keep the output stage out of crossover distortion territory. For high fidelity work, a distortion analyzer is usually the most accurate approach to use when maximum performance is the primary consideration.

Dave

[ioginy]

1. I used the ampbooks calculator to determine my NFB values. It seems to be working well enough, though when the amp is working properly I would like to fiddle with the values a bit to get them dialed in.

2. Phase inverter values are something that I haven't had a chance to get my head around yet. I figured I'd stick with the original tried, tested and true fender LTP and output and I should be fine. Ideally I want the amp to be as clean as possible, and fender is known for their clean amps.

3. It's quite funny how many opinions there are out there for properly biasing output tubes. I had a funny thought while working on the amp today. I pulled one of the bias supplies out, so there was only one. Then dialed it in to -40VDC. Slapped it in and all of a sudden the amps output is void of hum.... odd. Amazing how following the old design perimeters still works like a charm.

That all being said, I'm not getting the volume out of the amp that I should be, so I am obviously doing something wrong.

[soundmasterg]

Dave,

You said what I meant to say but didn't quite say correctly about the feedback and the cap. What I meant was like you said, that the cap feedbacks the high frequencies, causing a high frequency rolloff in any stages inside the feedback loop. Thanks for saying it clearer...haha.

Cory,

You're losing a bit of signal at the input with the 120k and 100k setup instead of the usual 1M and 68k setup. You may also increase the second stage coupling cap to a .047uf, getting progressively larger as the signal goes through the amp. I don't see anything obvious that is causing your low power however. You might try subbing in some solid state diodes for the 5U4 temporarily to see if your power level goes up. You should have at least as much volume as a Twin Reverb or a Dual Showman or something like that. What are your voltages supplying the various stages?

You may consider adding another stage of gain in there instead of using the cathode follower. Maybe have your first gain stage, then the tone stack, then another gain stage, and then another gain stage, before the PI. Just voice the 3rd gain stage with a large cathode resistor to take a large signal without distorting....and maybe use a 220k plate resistor on the second stage for a different flavor to the sound. Moving the tone stack closer to the front of the amp will give you more tonal variation than towards the back of the amp...basically like Fender's blackface circuits as compared to the tweed circuits.....though you're using a different tone stack than the blackface....the principle is the same.

Just throwing ideas out there if you feel like trying them out.

Check out the aiken amps site for some interesting reading on biasing.

Greg

http://www.aikenamps.com/Biasing.html

[ioginy]

I was looking at the 100w fender amps last night to see what they were doing. "The Twin" has a good layout. I had one of those amps a few years ago and I loved it. Super loud and impossible to get into distortion unless i switched channels. The 120K input is left over from the ampeg design, but I've altered it so much that I think I will go with the 68K design.

With the CF taken out I have en entire socket empty, so I was also thinking of putting another gain stage in front of the PI. I'll do some more research and see if I can get this sorted. I'll also do a full voltage reading on the amp when I get home from work tonight.

[soundmasterg]

I was looking at the 100w fender amps last night to see what they were doing. "The Twin" has a good layout. I had one of those amps a few years ago and I loved it. Super loud and impossible to get into distortion unless i switched channels. The 120K input is left over from the ampeg design, but I've altered it so much that I think I will go with the 68K design.

With the CF taken out I have en entire socket empty, so I was also thinking of putting another gain stage in front of the PI. I'll do some more research and see if I can get this sorted. I'll also do a full voltage reading on the amp when I get home from work tonight.

That particular Fender amp SUX for guitar...haha. I do like some of the vintage Fender amps for bass, but probably something like a Dual Showman or Bassman head. I like the vintage Sunn amps myself the best for bass, which are basically based on the Dynaco hifi amps. I have a 2000S head and cabinet. I also have a vintage SVT which I like, and like the Portaflex sound a lot but have never had one. They are all quite different designs too from each other.

If you have an entire socket empty, that is two stages if you use a 12AX7 or similar. You may think about trying a 12DW7. They are half an AX and half an AU in one bottle. You can have the high gain from the ax side and have a high current setup with the au side right before the PI to help drive things along. A 12AU7 will not distort nearly as easily as an AX and "sounds" really nice and rich. You could also just use an AU anyway since you already have a bit of gain with the two AX stages. The AU has a gain of 17. An AY might be a good choice too. They also "sound" better than an AX with a rich harmonic spectrum and have a gain of 44.

Voltages would certainly help, and you may also see what kind of power you are getting out of it. Set it up for when it sounds clean completely, and then measure the ac voltage on the speaker tap and square it, then divide by the impedance. That will tell you the power output in watts....though without a scope it will probably be a little optimistic since we often ignore a bit of distortion the way we hear and so the amp is distorting a bit and we think it isn't. Its also better to use a resistor load when doing this instead of a speaker, but you can get close.

Greg

[ioginy]

Hey now, I loved that amp... but that is a beauty of tube amps, people can argue about what is better until their blue in the face.

Those are some good suggestions. I haven't had a chance to get and work done on it in the past week or so, but I was able to get the voltages just now. The PI voltage is definitely off kilter and the preamp voltages seem a little high, but everything else looks to be close to spec for the most part.

B+ voltages
OPT CT: 497v
Screen supply: 433v
PI plate: 425v
Preamp plate: 386v

12AX7 Voltages (at socket)
Plate (both): 387v
Cathode (both): 2v

12AT7 Phase inverter (at socket)
Pin 1 plate: 297v
Pin 6 plate: 424v
Pin 3 cathode: 177v
pin 8 cathode: 160v

6L6 voltages (at socket)
Plates (all): 494v
Screens (all): 431v

Any thoughts?

[soundmasterg]

Did you like that amp for bass, or for guitar? :) I've found that something appropriate for one is often inaapropriate for the other, unless clean is your desired sound for both, or distorted is your desired sound for both. Personally I thought that particular Fender amp to be hard and harsh for guitar, though I'm sure it's cleans are probably ok.

As far as your voltages....

Your B+ is about right for that amp with a little less overall current draw. It depends on how it is biased of course, and the stock Bogen bias setup doesn't allow for enough voltage to run the tubes in a nice bias range, which means generally that they run hot. I would think that would especially be the case with your 6L6's in there? But again, it depends on how you have them biased.

Your screen supply is low because of the tube rectifier in there lowering the voltages down about 50v. That right there limits your power a bit as the screen actually has more control over the overall power out in a pentode or beam power tetrode.

Your preamp supply does seem quite a bit on the high side....a usual Fender blackface amp ranges from 170v to 230v right at the tube itself, and probably 100v higher on the supply. That is with 100k plate resistors. Bass amps may go up to 250v-270v typically, but the Fender blackface higher ranges are perfectly fine too. The 12A*7 types usually have somewhere around a 330v max so keep that in mind.

Those are the thoughts off the top of my head....

I haven't had a chance to work on mine either lately. School has me buried. I need to finish up getting the power scaling working correctly and get the light up nameplate working correctly to where the light from behind doesn't shine around the edges.....easier said than done. Other than that, and making a cover for the amp, mine is mostly done. It will work reliably now and sound good if I don't use the power scaling, and if I do, I get about an hour of use before I can hear any hint of oscillation.

Greg

[ioginy]

I ran into a bit of a problem the other day. I went to test the amp after a couple of days (didn't alter anything before testing) and no sound came out. I checked my voltages and found no issue, then did a point test with a 9V battery and found that the signal was dying around the first or second stage of the preamp. The power amp is dandy, as well as the phase inverter.

I checked everything out, checked for shorts, did the changes I intended to do hopeing that would fix a possible stray wire that I had missed. After the mods I tested again and still no sound. Did more checking, then did a voltage check and noticed that the last B+ supply was slowly dropping. I kept an eye on it and after it went from 300V to 179V I noticed that dropping resistor that fed that supply was turning brown. I turned everything off and started to do a little research. Sadly I can find nothing other than there is way too much current being drawn through that resistor. My only thought is that the filter cap may have gone bad. It wouldn't explain the dead silence though. A little signal should at least slip through right? I'm going to try replacing the cap and resistor and check for shorts down the line, but I am a little baffled.

[ioginy]

I ran into a bit of a problem the other day. I went to test the amp after a couple of days (didn't alter anything before testing) and no sound came out. I checked my voltages and found no issue, then did a point test with a 9V battery and found that the signal was dying around the first or second stage of the preamp. The power amp is dandy, as well as the phase inverter.

I checked everything out, checked for shorts, did the changes I intended to do hopeing that would fix a possible stray wire that I had missed. After the mods I tested again and still no sound. Did more checking, then did a voltage check and noticed that the last B+ supply was slowly dropping. I kept an eye on it and after it went from 300V to 179V I noticed that dropping resistor that fed that supply was turning brown. I turned everything off and started to do a little research. Sadly I can find nothing other than there is way too much current being drawn through that resistor. My only thought is that the filter cap may have gone bad. It wouldn't explain the dead silence though. A little signal should at least slip through right? I'm going to try replacing the cap and resistor and check for shorts down the line, but I am a little baffled.

[ioginy]

This is also the schematic of the changes I made.

In trying to find a signal path problem I took out the master volume and the first triode bias adjust.

[ioginy]

Ok, first off i'm an idiot and put in 220 ohm plate resistors instead of 220K. With that fixed there is volume a plenty, also the 12at7 I was using when I head volume problems was a bum tube. I replaced that with a new one and all that volume just came rushing back.

Now... that distortion problem is back with a vengeance. I am going to pull that 12AU7 CF out and just leave it as a driver and raise the cathode resistor to a 3.3. I am also going to raise the second gain stage cathode resistor to a 2.2 and see how that shapes up...

[soundmasterg]

It always helps to check parts values before you put them in...haha. I think everyone has made that mistake once or twice.

So are you using a 12AT7 for the third gain stage and CF or are you using a 12AU7 like the schematic indicates? A 12AU7 would be much happier with a 22k to 47k plate resistor, (increase the wattage to 1 watt) and it's corresponding cathode resistor should be like a 4k7 to a 10k. As you go up with the Ck it will allow that stage to take in more voltage before it distorts. The CF doesn't add any gain and actually loses just a tad by itself. Having it directly coupled to the previous stage allows the previous stage to realize it's max gain, but with a 12AU7 there, it isn't loaded down as much by what follows it anyway as a 12AX7 would be, or a 12AT7 for that matter.

Looking at your schematic again, it looks like you have a 1+2 arrangement before the PI, using all 12AX7 stages, and then you have a 12AU7 stage for a CF? Is that what you still have? That basically puts you in the same territory as a blackface bassman for gain...and then you have some 220k plate resistors there which add more gain. Try replacing the 3rd gain stage with a 12AU7 gain stage instead. For bass you don't need so much gain because bass pickups are hotter, and the strings have more energy. Reduce gain by either replacing stages with lower gain tubes, or take stages out entirely.

If you wanted to experiment with something, you could connect the 12AU7 plates in parallel (use a 47k plate resistor), connect the grids in parallel too, and use seperate cathode resistors for some tonal coloration. Pick some moderately high values though...4k7 to 15k. I did this on the input of mine and it is a very interesting sound...thicker and harmonically richer. Of course I'm using my amp for guitar though.

It would be very helpful for you at this point to have a signal generator and a scope. With those you can put a 200mV - 500mV signal into the amp and see what happens with the stages as you change controls and turn volumes up etc. You can see which stages start to distort and where and adjust parts accordingly. Without a scope you're sort of shooting in the dark on this at this point....

Greg

[ioginy]

Excellent advice, thank you. I am still wrapping my head around how the cathode and plate resistors effect them, and what the requirements of each tube is.

None the less, this is the updated schematic of the alterations I made.



Sadly I must have done something wrong because I am suddenly getting absolutely no sound. I'm going to check all my values and maker sure there are no stray shorts.

Sadly my signal generator is my guitar and my scope is my ears at the moment. Some day soon I'd like to get some proper equipment, but for now I get to take shots at the dark.