??????
Sorry i dont understand that either. I've never built amp before i dont even know what is a b-.
But the info you got there look like to set a good individual bias for each driver tube but dont talk about EL34. Is that bias gone to work whit those.
By the way the link i sent you also have your amp on ebay just do a search for bogen.
Sorry i dont understand that either. I've never built amp before i dont even know what is a b-.
But the info you got there look like to set a good individual bias for each driver tube but dont talk about EL34. Is that bias gone to work whit those.
By the way the link i sent you also have your amp on ebay just do a search for bogen.
Okay, I was looking at some other circuits and got to thinking, why can't I just change the cap and resistor values to change the voltage? This is what's reccomended on the Triode Electronics site for the Quiksilver 8417 amps, why not the Bogens? I think it would be much easier then cramming in more stuff.
I also found a thread on here about a guy having identical problems as me, low volume with what sounds like preamp distortion. It turned out to be a problem with the tone controls, he recapped them and it fixed the problem. I might order some replacement caps for my tone stack too.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=13721&highlight=
Also, an MX-60A popped up on ebay and it just looks like an M60A with extra inputs.
I also found a thread on here about a guy having identical problems as me, low volume with what sounds like preamp distortion. It turned out to be a problem with the tone controls, he recapped them and it fixed the problem. I might order some replacement caps for my tone stack too.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=13721&highlight=
Also, an MX-60A popped up on ebay and it just looks like an M60A with extra inputs.
Although I'm not getting much input here anymore, I'll at least say that I got the tubes tested. The 8417s check out good with similar results. The 7247 is in incredible shape, from how it looked on the tester I think it will outlive me. 
Strangely, the 6EU7s tested completely dead, but this can't be the case because they light up and I was getting sound so they obviously can't be dead.
Strangely, the 6EU7s tested completely dead, but this can't be the case because they light up and I was getting sound so they obviously can't be dead.
Hi Darren,
I agree. If the filament lights you will get some kind of reading on at least one tube. Test them in circuit by measuring the plate voltage and also the DC voltage across the cathode resistor (to figure out the current draw).
I really wouldn't worry about it unless you have changed the coupling caps and have no/low signal.
-Chris
I agree. If the filament lights you will get some kind of reading on at least one tube. Test them in circuit by measuring the plate voltage and also the DC voltage across the cathode resistor (to figure out the current draw).
I really wouldn't worry about it unless you have changed the coupling caps and have no/low signal.
-Chris
The 6eu7's of the m60a amp I just sold had a bad set of 6eu7's that I replaced. A bad tube may still light up. The cause of a bad tube may be the result of leakage. That is why one of my tubes was dead. That may also be the cause of why you had that loud noise when using the amp. On another amp I had the 7247 had leakage and it caused a lot of noise and half the tube was dead and the tube filament was working just fine. I also have some 8417's that were used alot and are almost dead and still fire up. Also did you make sure to test both plates of the tube?
ethel188 said:
I'm not sure about both plates, the shop guy was running the tester, not me.
The needle was as far the the left (bad) side as it could go for both tubes, due to the strange pinout of the 6EU7 it could have just been a matter of his tester not being set up for them. The "loud noise" I heard sounded awfully like an explosion of some sort, could that be a result of leakage?
On another note, I got adventurous (or stupid, take your pick
) and fired up the amp without tubes. No hum, and and all the voltages were normal. Then I put the tubes back in (stupid, I know) and fired it up again. It's acting exactly like it did before, barely audible hum but low volume. I didn't keep it on long and I don't intend to fire it up again without new caps, but it seems to be alright. What puzzles me is that if there's no hum, and all the voltages are fine, what could have exploded?
-Darren
I thought we were waiting for your new parts to arrive and get installed
.I'd rewire the socket for a 12ax7 and use one of your 12ax7 spares.
I've had tubes that light up but are dead. Or at least one triode section. Since you know it is dead, don't use it.
I haven't looked at your schematic but when I was debugging my HK PA amp (with help from this forum) I was able to bypass the tone stack for initial testing.
Once you get your new parts installed, hookup a load (old speaker), short the inputs and measure every pin on every tube.
Post voltages. I like Va for plate voltages, Vk for cathode voltages, Vg1, Vg2, ... for grids. Don't forget to check heater voltages. My HK used the heaters for the driver tubes to bias the output tubes. I pulled the mic driver tubes so the output tubes were not getting biased. This threw me off until more wiser members straightened me out.
We can start looking at suspect voltages (input grids should be zero or less), cathode biased sections should fall within spec, ...
rick
I'm holding off on ordering parts until I get this stuff figured out. I like to have things planned out before I start. I'm on the verge of getting the bias supply design right, and once I get that finished it's time to order the parts (finally).
Not everyone has spare 12AX7s lying around. Besides, the uniqueness of the 6EU7 makes me want to keep them in there. Ethel, I don't suppose you have a pair of working but weak 6EU7s and a 7247/12DW7 lying around that you could let me have for a few bucks? Something that's not completely dead, but something that I could throw in there to find out if my tubes really are shot.
The distortion I got really sounds like preamp distortion, because I can duplicate that sound with other amps and I could tell when the output tubes start distorting in the 7-8 volume range. How would I get that kind of distortion if the tubes are bad?
What is the purpose of shorting the inputs by the way? I always test by plugging my MiniDisc player through the RCA jacks and playing music through it.
I like to have things planned out before I start. I'm on the verge of getting the bias supply design right, and once I get that finished it's time to order the parts (finally).Not everyone has spare 12AX7s lying around
. Besides, the uniqueness of the 6EU7 makes me want to keep them in there. Ethel, I don't suppose you have a pair of working but weak 6EU7s and a 7247/12DW7 lying around that you could let me have for a few bucks? Something that's not completely dead, but something that I could throw in there to find out if my tubes really are shot.The distortion I got really sounds like preamp distortion, because I can duplicate that sound with other amps and I could tell when the output tubes start distorting in the 7-8 volume range. How would I get that kind of distortion if the tubes are bad?
What is the purpose of shorting the inputs by the way? I always test by plugging my MiniDisc player through the RCA jacks and playing music through it.
Hi Darren,
There is a very easy way to test if the 6EU7 (excellent tube BTW) is working. Plug them in and see if they are drawing current. If the current is in the normal range, chances are the tubes are good. Might be noisy but they will pass signal.
The "normal range" requires a schematic. I would just compare to the drops a 12AX7 gives in a similar circuit.
-Chris
There is a very easy way to test if the 6EU7 (excellent tube BTW) is working. Plug them in and see if they are drawing current. If the current is in the normal range, chances are the tubes are good. Might be noisy but they will pass signal.
The "normal range" requires a schematic. I would just compare to the drops a 12AX7 gives in a similar circuit.
-Chris
Don't think I'll be needing any new tubes anymore
I've been prodding around testing voltages most of the day and just for the heck of it I fired it up and ran some music through it. I don't know what happened but it was loud when I turned it up. Of course I had to see what it would do with a guitar input. I plugged it in and immediately notice two things: First, it shook the walls with the input and master volumes turned to "5". Second, it sounded awful - no high frequency sound at all. Then I remembered that, being a PA amp, it had a treble cut circuit on the RCA jack inputs. I plugged it into the "aux" input to bypass the first gain stage. It sounded great, and even though it only went through one preamp stage it was still pretty loud. I didn't turn it up enough to get distortion, I can't wait to get my new caps and the stuff for the bias supply mods so I can start turning it into a real guitar amp.
It's amazing how often things work out like that - all you have to do is take it apart without changing anything and it ends up working.
Thanks for the help guys!
-Darren
I've been prodding around testing voltages most of the day and just for the heck of it I fired it up and ran some music through it. I don't know what happened but it was loud when I turned it up. Of course I had to see what it would do with a guitar input
. I plugged it in and immediately notice two things: First, it shook the walls with the input and master volumes turned to "5". Second, it sounded awful - no high frequency sound at all. Then I remembered that, being a PA amp, it had a treble cut circuit on the RCA jack inputs. I plugged it into the "aux" input to bypass the first gain stage. It sounded great, and even though it only went through one preamp stage it was still pretty loud. I didn't turn it up enough to get distortion, I can't wait to get my new caps and the stuff for the bias supply mods so I can start turning it into a real guitar amp.It's amazing how often things work out like that - all you have to do is take it apart without changing anything and it ends up working
.Thanks for the help guys!
-Darren
Well, I realize now that starting this project was one of the dumbest things I've ever done. It's not possible to desolder anything in this amp's bias supply. I generally don't have trouble soldering anything but desoldering is a different story, especially with my crappy desoldering pump (one of those cheap spring-loaded things - useless). There's one pin with five wires going into it, holding an iron to it starts to melt everything (insulation, resistor leads, etc.) except the solder.
I guess my question now is would it work alright to just cut the wires off and redrill the terminal strip holes out? I swear the only thing that could melt that solder is a blowtorch.
-Darren
I guess my question now is would it work alright to just cut the wires off and redrill the terminal strip holes out? I swear the only thing that could melt that solder is a blowtorch.
-Darren
Okay, I gave up on trying the bias supply mods. My tubes are fine so it's not neccesary for now. I'm still waiting for my capacitors, I should have know better than to buy from Canada but the prices were good.
Anyway, I'm probably going to play through it clean for a while, the guy I met on Ampage that's helping me suggested that I keep the circuit relatively stock, but I want to at least try cascading the inputs to see what kind of distortion I can get out of this thing. So the article that gave me the idea for the PA conversion suggests a load attenuator circuit between each stage because of "too much gain". Is there anything to this? I was hoping to connect the output of each pot straight into the grid of the next stage. I can't inagine there could be too much gain simply because it'd adjustable with the volume pots between each stage. Also, there was a guy on here a while back that simply cascaded the inputs in an MX60A and loved it.
By the way, the guy I bought the MX60A manual from was an idiot. I never got my manual, and he said they were backed up several weeks. At least I got my money back.
-Darren
Anyway, I'm probably going to play through it clean for a while, the guy I met on Ampage that's helping me suggested that I keep the circuit relatively stock, but I want to at least try cascading the inputs to see what kind of distortion I can get out of this thing. So the article that gave me the idea for the PA conversion suggests a load attenuator circuit between each stage because of "too much gain". Is there anything to this? I was hoping to connect the output of each pot straight into the grid of the next stage. I can't inagine there could be too much gain simply because it'd adjustable with the volume pots between each stage. Also, there was a guy on here a while back that simply cascaded the inputs in an MX60A and loved it.
By the way, the guy I bought the MX60A manual from was an idiot. I never got my manual, and he said they were backed up several weeks. At least I got my money back.
-Darren
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