Repurposing a Bogan CHB-100, too many triodes

[AmberB]

So, I've been working on this CHB-100 for a few weeks, off and on.  I had originally planned to replace the 7868 power tubes with EL-34s, but after it was pointed out to me that the heater current of the EL-34 is about twice as much as a 7868, I went with 7591 power tubes, basically 7868s in an octal bottle.  I used the Ampeg values for the tone controls for better guitar frequency response.  I pulled the 6EU7 out of the amp and replaced the 9 pin socket with another new one.  It's easier to wire it up if you don't have to first pull the old parts of the socket.
Anyway, I wired up V1 for a 12AX7 and ran one triode into the other and then on to the tone circuit tube.  I used a volume control between the first and second triodes of V1, and then the master volume between the second triode of V1 and the input of V2, which drives the tone controls.  That setup proved to be way more gain than I wanted, not much clean sound before it distorts.
I know that there's various ways to lower the gain between stages, but it's a lot of messing around to get to somewhere near what you want, and I decided to go a different route with V1.

I rewired V1 so that both triodes are in parallel.  I ran the signal output from that to the input of V2, the triode that drives the tone circuit, using the gain control there.  The output of the tone circuit goes back to the other triode in V2 and gets amplified and sent to the 6C4 triode, which is the split load inverter for the power tubes.  I put the master volume on the input of the 6C4.

So, having explained all of that, are there preferred plate resistor and cathode resistor values for a 12AX7 in parallel for V1?  I just used a 100k plate resistor and a 1.5k with 22uf cap for the cathode.

[tubenit]

Try 56K plate, 1.5k cathode, 5-10uf cathode cap and see if you like that?   

I usually aim for plate voltage somewhere between 140 to 180v on 12AX7 plates.   This is simply a personal preference.

With respect, Tubenit

[sluckey]

The MARSHALL 1974 18 WATT and the Matchless Lightning both have parallel triodes (12AX7) for preamps. The Marshall uses 100K and 820Ωn and the Matchless uses 220K and 1.5K. I've built both. I really like the Marshall but hated the Matchless. Still have the Marshall but the Matchless is long gone. I also used the Marshall preamp in my dual lite amp, which is my most favorite of all my projects.

I'm just using those two amps as an example to show that there are many different ways to set up parallel triodes. Most people consider 100K and 1.5K to be ideal values to use with a single 12AX7 triode. So technically, if you parallel two 12AX7 triodes you should halve the values of the plate/cathode resistors to maintain correct operating point.

I would like to see how you connected the MV to the input of the split load inverter. Because of the special biasing for the cathodyne it would be easy to get it wrong. Schematic please.

[tubeswell]

...are there preferred plate resistor and cathode resistor values for a 12AX7 in parallel for V1?  I just used a 100k plate resistor and a 1.5k with 22uf cap for the cathode.

That depends on one's 'preference'.


2 parallel 12AX7 triodes with a 100k plate resistor and a 1k5 cathode resistor is 'almost like' running a single 12AX7 with a 200k plate resistor and a 3k cathode resistor - assuming all other things are equal. As sluckey says, schematic please.

[AmberB]

I definitely have too much gain in the preamp on this amp.  It starts to distort before it gets very loud.
I also found out another thing.  I've been testing the amp with just 2 power tubes in it.  When I put the other 2 power tubes in it, I get some strange noises when I play the guitar through it.  No idea what that's about...

I'll have to draw up a schematic for how I wired the master volume into the amp, but looking at it, I think I did mess up the bias on the 6C4 split load tube.  I'm going to have to remove the grid resistor and put the 1 meg volume control in it's place.  The grid resistor is a 1 meg resistor.  The grid resistor is tied to the point between the 820 ohm resistor off the cathode, and the 22kohm cathode load resistor to ground.  I think I need to run the ground of the master volume pot to that point instead of ground.  I probably should run the other end of the pot to the grid of the 6C4, and then put the signal from the previous stage in through the wiper.  That way the pot acts as the grid bias resistor.   I could be totally off on this idea, but it's something to try.

[AmberB]

After doing some more thinking about this amp, I decided that putting the master volume before the 6C4 is probably not a good idea no matter how it's wired. 
My next idea is to use one triode to make a cathode follower to feed the split load tube, and put the master volume before the cathode follower.  That way the bias of the split load triode is not affected.  I thought about another gain stage, but I really don't need any more gain.  If I was to use the triode as a gain stage instead of a cathode follower, I would have to knock down the gain of it, and the gain of the tube feeding it.
I don't know if it will work, but it's worth trying.

[AmberB]

This is the original Bogan schematic for the CHB-100

[tubeswell]

...another thing.  I've been testing the amp with just 2 power tubes in it.  When I put the other 2 power tubes in it, I get some strange noises when I play the guitar through it.  No idea what that's about...

If you sub in just the 'other' pair of output tubes, do you get the weird noises?

[AmberB]

"If you sub in just the 'other' pair of output tubes, do you get the weird noises?"

No, it works the same with either pair of tubes.  It's only when all 4 are in that the problem happens.  I think it may have something to do with me messing up the bias of the split phase tube, but I could be wrong about that.

[tubeswell]

So does the noise only happen when you play loud? Or?

[AmberB]

It seems that I have too much gain in the preamp, probably in the first couple of stages.  Removing the master volume does nothing to help the problem with distortion.  I have too much signal somewhere that's distorting the preamp.  Gotta find that first, I guess.

[AmberB]

"So does the noise only happen when you play loud? Or?"

No, it happens at fairly low volumes also.

[tubeswell]

Have you tried a fresh set of tubes?

[AmberB]

The set of power tubes are a new set of JJ 7591s  The preamp tubes are old 12AX7s, but they test within normal parameters.  The 6C4 is original to the amp.  I have a NOS Channel Master 6C4 to try.

[SoundmasterG]

It sounds like you need to reduce the gain and change some things up. I think you could come out more ahead on this by using a different phase inverter tube and design. the 6C4 isn't a great tube to use IMHO. It does have good current gain but it can be susceptible to high frequency oscillation, at least in my experience. It doesn't have much voltage gain on it's own either...I think around 17.5 or something like that. Try taking one gain stage out entirely and see if you can get the amp to behave with just two stages before the phase inverter. If you can then reduce gain of your third stage and add it in, and maybe lower the gain of the second stage if you choose the run the third gain stage too. A schematic of what you have right now would help.

It's not often that people mess with these Bogen CHB 100's seems like. I haven't seen many posts on them anyway. I messed around with one about 7 years ago and it has become my go-to amp to use. I gutted it though and started over with my own circuit. I kept Bogen's basic layout, though I should have changed it as the layout has a couple issues, chief among them is the transformers are right next to each other and both being Z mounted kind of interfere with each other and cause more hum than I'd like. I optimized grounding in the amp and added tricks like raising the center tap of the heaters for example. I also went with a parallel 12AX7 on the input, though I put switches in so I could deactivate either side and voiced them for less and more gain, and of course together has the most gain. I don't remember right now what values I picked but I think it was a 100k plate resistor and an 820 ohm cathode resistor when both sides of the triode are going at the same time. I can't sure though without seeing my schematic, but I moved in June and am still unpacking so it will be awhile until I can find it. I used a tone network after that stage and then into a 5879 pentode, but I used an adjustable mid control that I think I got from Merlin where it shifts the slope, then into a 12BZ7 LTP phase inverter so I can really drive the power amp. I could use a 12AX7 there also, but it sounds so good with the 12BZ7 I haven't tried that yet. The 12BZ7 has the same voltage gain as a 12AX7 but more current gain (half the impedance) so it works really well to drive power tubes in comparison to a 12AX7. They are often microphonic though. I stuck with the 7868's. I added power scaling to the power section and have a master volume after the gain stages but before the phase inverter. It doesn't put out as much power as I thought it would but it is around 55 watts RMS...could be a little more as I'm being conservative when I measure the power output. I could probably get a little more distortion on the sine wave and still call it clean as I think the traditional measurement is 5% distortion? The amp has decent cleans but not as nice as a Fender or Vox, but at full tilt it is like AC/DC type Marshall distortion, but with a thicker sound. Here are a couple pics.

Greg

[PRR]

The Bogen (with an 'e') is a very fine sweet power amp. Nothing wrong with that split-load for this chore. I ran five of the 150W version, goosed to 165W, in a PA rig.

But you do not want excess gain in any high-power amp... don't go using-up all the sockets of a multiple-channel amp on one channel without careful math and planning.

Here's a mosh of two well-known amps which adds-up about OK.

[AmberB]

PRR, that does look like a good combination.  I may have to go that way.  I have another CHB-100 that I'm planning to rebuild with a pair of EL-34s instead of a quad of power tubes.  The PT has enough heater power for 2 EL34s, and I can wire the output transformer secondaries to compensate for the impedance mismatch.  With the amp I'm working on right now, I really don't want to do that, I already have the 4 octal sockets for the 7591s, and I'd like to get it to work with them.
I'm working on lowering the output of each triode, one at a time, until I get the amp cleaned up.  I'd like to be able to turn the gain up at least half way, with the master all the way up, before I get any real distortion.  I can't do that yet...

[SoundmasterG]

PRR, that does look like a good combination.  I may have to go that way.  I have another CHB-100 that I'm planning to rebuild with a pair of EL-34s instead of a quad of power tubes.  The PT has enough heater power for 2 EL34s, and I can wire the output transformer secondaries to compensate for the impedance mismatch.  With the amp I'm working on right now, I really don't want to do that, I already have the 4 octal sockets for the 7591s, and I'd like to get it to work with them.
I'm working on lowering the output of each triode, one at a time, until I get the amp cleaned up.  I'd like to be able to turn the gain up at least half way, with the master all the way up, before I get any real distortion.  I can't do that yet...

It sounds like you definitely have too much gain if you can't get up on the gain control half way before it starts to distort, so you're on the right track to lose some. How about just taking one triode stage out of the circuit and see how it behaves? That will give you an idea of how much gain you need to lose so you can tweak the other stages to work better together? It should be pretty easy to bypass the signal around a gain stage and see what happens. Do you have an oscilloscope?  You can follow the signal through the amp and see where your distortion is first occurring. To keep the amp touch sensitive you want the output stage to distort before anything preceding it does.


The OT impedance on these CHB 100's is 2500 ohms for the 4 power tubes btw. One other thing to keep in mind about the original 7868's or the 7591's you are using now is that for a power tube they have a lot of gain, so they don't need much gain in front of them to get to full power. Any additional gain you have past that turns into distortion. The one I built has just the parallel 12AX7 stage on the input, and a tone stack which loses some gain, and then a 5879 pentode into the phase inverter. So two gain stages only, and it is plenty.

Greg

[AmberB]

Yeah, I'm probably going to have to break out the o-scope to figure out what distorts first. 
Thanks for the info about the impedance of the OT.  I never tested it, I was guessing that it was about 3k-3.3k judging by what a pair of 7868s normally have.

[SoundmasterG]

Yeah, I'm probably going to have to break out the o-scope to figure out what distorts first. 
Thanks for the info about the impedance of the OT.  I never tested it, I was guessing that it was about 3k-3.3k judging by what a pair of 7868s normally have.

You're welcome. I tested mine to see if that was why I wasn't getting more power out of the amp than I was.


Would be interested to see what you find out if you pull out the scope and find out what is going on.


Greg