Bogen CBH 100 , Make good guitar amp ?

Can I use Russian output tubes in it ? 6p41s number. Has 524 volts on plate and negative 27 bias voltage after rebuild with no output tubes in it. Had a hard time with this amp, a capacitor was marked wrong at 30 volts that was actually 300 volts , then the new capacitor was shorted out from a small dent in it , I finally figured out the two capacitor problems with it and it works now. The wiring diagram made me question the capacitor that was marked wrong and experience led me to the shorted capacitor problem , I'm lucky I had two extra capacitors on hand of that value. I understand that the Russian tubes are rated lower voltage and have a few watts less output. It is not really clear what the actual limit's are with the Russian tubes ? , The tube data doesn't give you that information , only the recommendation for the tubes, no one building guitar amps really listens to them anyway from what I have seen. Actual meltdown limit's are what I need to know. Plus a 4 tube load will lower the 524 voltage down considerably. I'm not sure how to figure out what the voltage drop would be ? I was thinking it would drop to 390-400 volts most likely but I am not sure about that. I rewired and put a 12ax7 in stage 1. Put in a 1/4 plug on mic 1 input and was thinking of jumping 1 and 2 mic inputs for more gain like is done with a Plexi. A guy named Daveorin on DIY said that the Russian tubes can handle excess of 500 volts with no problem and sound great with low distortion. Not a member there so I cannot download his test results. He had about 10 downloads of test results. Seems to me like I could save a hundred dollars on tubes not only with this amp but also with my Ami jukeboxes as well. I have , some 7868 tubes but they are all used tubes and in working Ami Jukebox amplifiers, and I have a few single tubes also that I can use for repairs . I use cheaper subs all the time in Whirlitzer 6973 amplifiers with almost no problems at all. Once in awhile a tube may red plate because it is a weak tube to start with, but it only happened twice out of a lot of tubes. I just like using affordable NOS output tubes rather than used if I can find them reasonably priced. 120 dollars for the EH new are not reasonably priced. USA NOS are even more expensive. It is a good tube just way too expensive for new EH and NOS USA. I think that the substitute change is worth it in this case. The whole idea was to make a cheap hi wattage guitar amp that sounds good. 120 dollars for output tubes ruin's the whole project from my perspective. If it can be done with the Russian tubes it makes the project actually affordable for others. I know that USA bias is a must for some people but I see that they have foreign car's and trucks and other stuff ! , It is a little late for saying that we cannot buy foreign tubes isn't it ? , Especially since All new tubes are foreign made. Plus NOS Blackburn Mullard EL 34 is about the best sounding tube ever made and it is foreign. I finally broke down and bought a used Toyota pickup and I don't think GM or Ford ever made a better running or more efficient truck. Not as luxurious as American made but I don't buy a pick up truck for luxury. I still have my big GMC 4wd for towing but it is a total gas hog. Money is harder to come by today than it used to be, gas is expensive, I live in a small town and people here are not spending it very freely. I try to support my hobbies by selling off stuff I repaired or replaced with better stuff. I can't really put too much money in my projects if I ever plan on selling it. People here just will not pay 120 dollars for output tubes. My plan is to use it for a while and then I will sell it for a small profit. That's what I do with most of my higher power guitar amps. Why wait for the thing to wear out or for the OT to go bad before selling it. Use it for awhile and then let it go. That way I always have a fresh rebuilt amp to use. Plus it still sounds great when I sell it. One day I may decide to keep it forever but that has not happened yet. I'm always thinking of finding a NOS replacement pre amp or output tube that is affordable . I will never give up on that ! . These are direct pin and that is a big plus. Any data that will help me with this amp and changing the tubes is appreciated . I'm not a tech or professional, I'm just a self taught layman who wanted to learn who read a WW2 soldiers training course, I seem to do pretty good with this stuff.
 

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See Pic , Is the black wire NFB ? And should I use common for the speaker negative instead of ground ? , There's two marks of paint on the chassis over common and 4 ohm. The black wire was just hanging loose . The black wire connects to a green that goes into the output transformer. If I run 8 ohm speakers what do I do with the black wire ? Put it on 16 ohm or leave it off ? I don't quite understand what they are saying with the writing there on the chassis ?
 

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The words are talking about the plugs below the terminals. Specifically, The Black wire with the forked terminal is the "speaker impedance selector" as marked in the schematic. The other end should be attached (soldered" to the two round three pin plugs for the speaker outputs just below the terminal strip in your last picture.
You can leave it disconnected, remove it entirely or add a 1/4 " output jack and tie it to the + tip, then use the terminal strip to change the impedance of the jack to match the speaker cab. For the Terminals you have G (Chassis Ground) C - Common, 2 Ohms, 4 Ohms, 8 Ohms, 16 Ohms, and Remote 1 and Remote 2 listed. ( although the schematic doesn't list 2 ohms as an option)

Hope this helps, but I would double check that the wires match the schematic ( and 2 ohms seems to disagree as an option per the schematic)
 
The speaker is either 4 ohm 2/12's each 75 Watt Celeste's in parallel = 150 Watt's , or 8 ohm all four in series = 300 Watt's , 1960 A Marshall speaker cabinet, JCM 600 series. I have had it a long time, I normally use the 8 ohm 300 Watt series switch position. I understand G is chassis ground and C is speaker ground but the nfb tap from the OT is the black wire and those wires are not always put in the same place by amp builder's. Fisher puts it on the 16 ohm tap and leaves it there if 16 ohm speakers are used , seems to me that Bogan thought differently than Fisher about nfb making it adjustable. I am not used to having a option for nfb on the OT speakers. I don't usually use nfb on my amplifier scratch building , my understanding is that it lowers the power output to stabilize the pre amp section some and thus it lowers distortion slightly. If my understanding is correct. I use cap's usually to stabilize my pre amp design, if it needs it , instead of using nfb. I never thought about nfb much to be honest, max output was always the goal usually. I guess in high power situations it becomes much more useful than in robbing power from already low power amplifiers. I know that I have a 1959 6V6 PP stereo Bell Pacemaker that has less power output than two scratch built Hammond 6V6 PP stereo amp's with the nfb option disconnected. The Hammond scratch built amp's sound clearer than the 59 Bell Pacemaker also. It seems to me that using too much nfb can be a problem with the sound. I am waiting for the Russian substitute tubes and I decided to use them in my jukebox amplifier and to use the jukebox USA tubes in this amplifier. I do not think that the Ami jukebox's use really high voltage in them like this amp. And the tone may be a little bit better with the USA tubes in it , after all this is a 100 Watt guitar amplifier and tone is everything , the jukebox amplifier with the Russian substitute tubes will be OK. I have a 75 Watt el34 linear amp but it needs a new OT and that is why I am building this one instead. I just hope that these tubes can sound as good as el34 or 6l6 tubes. I am not much on sterile sounding tubes in a guitar amplifier. I put 2 NOS RCA long black plate's in the pre amp section. If they don't work out I will get 2 vintage Mullard tubes or Ei tubes. Not a lot of choices with 7868 tubes out there ! . Seems like Sylvania and Westinghouse made most of the vintage one's and it is not any better today ! . But if the Russian substitute tubes work in my jukebox amplifier I can take those tubes out for this amp. I have to see a quad work before I can take the USA tube's out for this amp. I'm not going to ruin my jukebox amplifier just to get tubes for this one. Both have to work good.
 
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Been looking at the MM5 Rowe/Ami tube amp diagram and it appears that the amp runs at 360/370 vdc plates and screen and neg 15 bias, that is much lower than the Bogan at 450 vdc but the design is not the same, the OT primary has center taps going to screen , I don't know what current it will flow at ? No winding spec's are given, I guess I will have to just try it to find out if the Russian substitute tubes 6p41s will work in there ? Not knowing the Russian substitute tubes actual limit's and not knowing what the Ami CT winding are either kind of put's it on the tubes to tell me what they want , I can always put some resistance on the center taps to screen if the current run's away on them. I may be able to increase the negative bias some also ? . Just do not know what the tubes can take until I try them.
 
Screen taps make it ultralinear. Fixed voltage is standard pentode operation.

Easiest way to figure out what current it runs at is to break the cathode connection to ground and put a 10 ohm resistor in that spot. Measure the voltage across the resistor and do the math. 0.5v across 10 ohms is 50ma.

Most bias supplies have a couple of resistors to set the voltage. One will come from the bias filter cap, the other goes to ground, the mid point usually goes to the tubes. Changing the values of those resistors will change the bias voltage. I usually like to put a pot in the side going to ground to get an adjustment. Can't throw out specifics without a schematic to look at, but say the resistor going to ground is 15k. I'd change that to a 10k resistor and a 10k pot. Wire one end of the pot to the wiper, depending which end you tie changes which direction you turn the pot to increase current through the tubes. With the pot at half travel, its at the stock setting, and I get adjustment either direction. If thats too touchy, you can use a 12k resistor and a 5k pot to get the same effect with less adjustment range. Usually I pick my pots by what is on hand, and I have 5's and 10's around. The reason I put it in the ground side, if the pot fails, the grid goes to full negative voltage instead of losing the bias supply entirely.

Some designs do connect the pot wiper out to the grids, but I've had pots go flaky and lose contact before. I nearly melted down an EL34 in my Pilot from that.
 
I have some 5 and 10k bias pot's I can use on the Bogan and Ami, About the UL tap's on the Ami Rowe amplifier ? Is this going to be a problem with the 6p41s tubes in the Ami amp ? Or will the lower voltages make it work OK ? I understand it will have some extra grid resistance through the UL windings but will that be enough resistance ? I think you are saying to control it on the cathode instead of slowing it down some on the screen ? Maybe I don't know exactly how UL windings act as grid resistance. I kind of thought that the grid resistance was used instead of cathode resistance to keep the tubes power output up higher and that UL used this method for the maximum power output. But looking at the 24 Watt maximum output of the Ami amp that is no where near the maximum output. It is typical AB output at about 1/2. I kinda thought that current may not even be a issue with 360 volts and 15 negative bias. I could be wrong about that and the tubes could still red plate on me. My Fisher 660 had a issue with the original output tubes not matching up properly, I switched the tubes into different pairs and it is OK now. Even Fisher did not match the tubes very well. I can hear the mis match during startup when the tubes are still warming up. The left channel comes on way before the right channel does, When it gets hot it evens out. Maybe I am over thinking it all and I should just try it first and see if it sounds OK and it looks OK on the plate's and the bottle temperatures. One guy said UL was no good on 6p41s tubes but I think he was talking about it running at 450 volts also. Poor tube matching seems to cause me more problems than anything else does . I guess that a pot on each tube is needed with the crummy tube suppliers. Would it be any advantage to put a pot on G2 also ? Or would it be a problem ? I know on big amp's we use flameproof resistor's but the vintage one's came with carbon resistor's on them and they seemed to sound better to my ears. I don't know how they sound better than flameproof resistor's but I think that they do ! , It is just power supply voltage but it still seems to make a difference. Like a RCA tube compared to a Westinghouse tube. I think a carbon cathode resistor sounds better than a pot also ! . My second choice would be a adjustable ceramic on the cathode.
 
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The cathode resistor is just there as an easy way to monitor current flow through the tube. Its not to slow anything down. 10 ohms won't reduce the power enough to notice.

Adding a 100 ohm resistor between the screen and UL tap isn't a bad idea though. No idea what those Soviet tubes are going to do honestly, you're in unknown territory here. Do not put a pot on the screen. Don't put one on the grid either, just in the bias supply circuit to allow adjustment.

25w from a push-pull pair of 7868 is actually about right.

I honestly dislike carbon resistors. They're noisy and don't stay on value. Nothing to love about them, other than the "tink" sound they make hitting the bottom of the trash can. I will admit the color stripes were often better and easier to read though.
 
Put the 7868 tubes in the Bogan and checked the voltages, 510 vdc plates, 495 vdc screen and negative 26 G1 , amplifier working now but new E caps are 450 volts and amplifier is running at 510 volts !
 
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Working Good now ! , Tubes were the problem not the 510 volts on the plate's. Had a run away tube I think because of mis matching ? Or that is what it seemed like was happening. Put everything back to stock and disconnected the 30 ohm balance pot between the cathode's and installed four 10 ohm 1% resistor's to check the mA on each tube. Moved two tubes and replaced one and got it up and running again, then I tested all 7 tubes I had and marked down the mA on each tube. What I ended up with is 20/24 on one side of the amplifier and 35/35 on the other side and a 39/45/50 mA left over. Not really sure if I should use the 45/50 pair in place of the 20/24 mA pair or not ? . Great sounding amplifier ! , I think that it May be too good for guitar use , would make a nice monoblock amplifier with another one rebuilt just like it. I had a vintage quad from a jukebox amplifier and they are not matched very well at-all ! , I'm not a big fan of 20% matching , they need to be much closer than that in high power amplifiers. Basically I have 5 tubes that are 20/24/30/45/50 that really sucks ! , I can use the 20/24 and the 45/50 pairs but neither one is a good match. Mine are all vintage USA Sylvania and RCAs tubes I think , though one may be a Westinghouse. Seems like it is a bit expensive of a tube venture for 510 volt 7868 vintage tubes or even with new tubes with these amplifiers, just lucky I had a few extras that were laying around. I learned a great lesson on this amplifier ! Tube spec's are not the answer to the problem with higher voltages on the plate's , good tubes are the real answer to higher voltages on the plate's ! . Just sucks that tube Maker's sold such sorry matched Quad's to people. But at lower voltages they sometimes still work anyway ! . When Bogan says that 20% higher voltages are possible, He really meant it ! . The amplifier is quiet at idle and sounds great. I think it is too good for guitar use and should be a monoblock.
 
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The Russian subs have lower transconductance, so they will likely need more negative bias or you could add a cathode resistor per pair to make up the difference. 50 mA at 500V is 25 watts, WELL over the max for a 7868. Only way to use tubes that far apart is with separate bias controls per tube.
 
Tom Bavis said:
The Russian subs have lower transconductance, so they will likely need more negative bias or you could add a cathode resistor per pair to make up the difference. 50 mA at 500V is 25 watts, WELL over the max for a 7868. Only way to use tubes that far apart is with separate bias controls per tube.
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I changed my plan and Was planning on using the 6p41s when they get here in my MM5 Ami jukebox amplifier at much lower voltages 360 vdc. I am using the Ami vintage Quad of 7868 tubes in The Bogan CHB 100 and I have 3 others . These results I tested are from vintage 7868 tubes in The Bogan CHB 100 @ 513 vdc. The tube data from Sylvania says 90 and 100 mA current max on cathodes if I am reading it correctly ? 70% of that is pretty high up at around 65/70 mA . The data is at 425 vdc fixed bias. -20 vdc on grid voltages. What I am trying to figure out is what is best for 100 Watt's with four 7868 tubes . Where to put cathode current, low down in the 20's, or 30's or 50's or 60's. Either 20's or 30's mA both sound about the same to me. 20/24 mA on one pair and 35/35 mA on the other pair. I know 35 mA is good usually with EL34 and 6L6 amp's. I have a 45/50 mA pair of 7868 tubes also, I was wondering if they would be better than the 20/24 mA pair in the amp now ? . I have no experience with these 7868 tubes other than in Jukebox's with much less voltages. I have four 10 ohm resistors on the cathodes and I have disconnected the 30 ohm balance pot. The amp is quiet at idle without using the balance pot. I feel safer with four 10 ohm resistors. Plus I can read the voltage flow on each tube. Match tubes or adjustable cathode resistor's , which way to go ? , The amplifier is working good right now like it is. Just found two more vintage 7868 tubes in my tube caddy. Got two 7868 vintage quad's now , 4 matched pairs and one tube leftover. 10% or less on 3 pairs and 20% on 1 pair. I Did not know that I had that many vintage 7868 tubes. The tube pairs all performed well at 515 vdc , screen 495 and fixed bias with negative 26 bias on G1 and 10 ohm cathode resistor's. 20% tube matching is good enough for this amplifier to work. Anything past 20% will red plate or start running the mA up past 60.
 
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Cathode resistor blew and took out a tube ! , Matched the tubes within 4 mA and put it back to stock with the balance pot between the cathodes.
 

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