I dunno, that schematic doesn't look right to me...
Have a squiz at the HV caps to make sure none are punched through and leaky to earth.
Have a squiz at the HV caps to make sure none are punched through and leaky to earth.
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JB-150 Tube heating issue help!
- Thread starter 443
- Start date
They actually look pretty good, though at some point I will probably replace them and a bunch of other stuff in there too. Someone tried to wire a radio shack 120VAC lightbulb into this amp as an indicator light to show the amp is "on" and they hooked it to the HV side, it smoked bad, and made a little bit of a mess inside the front part of the amp, so I will be replacing a lot of stuff in that area, wires with scorched insulation, the rectifier diodes, filter caps, etc.
Good Catch!
That schematic was probably drawn for the benefit of the FCC at that time.
Looking at the schematic, if you simply change the "T" and the "R" connections mentally, you can see the amp working in transmit.
.
Yeh I figured that.
I'm sure that would have fooled them !!! (not).
Although if the FCC is anything like the ACMA down here in Oz, they are office monkeys who wouldn't know a diode from a dog****.
I'm sure that would have fooled them !!! (not).
Although if the FCC is anything like the ACMA down here in Oz, they are office monkeys who wouldn't know a diode from a dog****.
So I forgot to get my 5W zener diode order in on time to get them today, will have them next week. Question, will 1W Zeners work? I don't suppose I could parallel a couple? Isn't the thing that the first 1 starts to conduct and the others just sit there or will they actually work in parallel? Reason I ask is because I have 1000 1W zener diodes here at my shop, but 0 5W zeners...
Okay, looking at the schematic now, please since you said I have to visualize it backwards because of the FCC thing, should the tube have:
Receive Bias: -32VDC
Transmit Bias: -16VDC
Or should it be the other way around like the schematic shows?
Thanks again for the help. Greatly appreciated, for some reason I really like this little amp, want to use it as my baby driver here!
Toll_Free
Well-Known Member
Chris,
Sorry, just got home from So Cal... Been working through the thread.
LOTS bigger amp than ya really need
LOL
The MORE NEGATIVE voltage is the cut-off voltage. Meaning, you want the tube CUT OFF in receive.
And yes, you can parallel the zeners. When I did a lot of Pride DX300 work, I started paralleling the zeners because I had a shitload of them, and the Pride is known to launch the zener for bias. Once I paralleled them, no mo problems.
--Toll_Free
Sorry, just got home from So Cal... Been working through the thread.
LOTS bigger amp than ya really need
LOL The MORE NEGATIVE voltage is the cut-off voltage. Meaning, you want the tube CUT OFF in receive.
And yes, you can parallel the zeners. When I did a lot of Pride DX300 work, I started paralleling the zeners because I had a shitload of them, and the Pride is known to launch the zener for bias. Once I paralleled them, no mo problems.
--Toll_Free
Someone else had said I could just run -47VDC to it all the time and not worry about switching it from TX to RX, just always -47VDC? Is that right?
Thanks for the help guys, I may try some zener paralleling, just till I get the bigger part in or do something else to make the bias voltage.
Thanks for the help guys, I may try some zener paralleling, just till I get the bigger part in or do something else to make the bias voltage.
Toll_Free
Well-Known Member
The only prob with -47 all the time is it will increase carrier levels needed to get the "modulator' effect.
All a "modulator' is is an amplifier biased to where no current flows during carrier conditions... Or zsac at most.....
--Toll_Free
EDIT: If you don't run enough carrier, with -47 on the grid all the time, you'll end up with that 'sandy' super-swing sound. To get a 'modulator' to SOUND good, you have to monitor input and output carrier levels. When they are MATCHED (ie, 4 watts carrier in produces 4 watts carrier out), THEN you have the modulator effect (an Intermodulator, is more like it)
All a "modulator' is is an amplifier biased to where no current flows during carrier conditions... Or zsac at most.....
--Toll_Free
EDIT: If you don't run enough carrier, with -47 on the grid all the time, you'll end up with that 'sandy' super-swing sound. To get a 'modulator' to SOUND good, you have to monitor input and output carrier levels. When they are MATCHED (ie, 4 watts carrier in produces 4 watts carrier out), THEN you have the modulator effect (an Intermodulator, is more like it)
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Update:
Finally got the bias straightened out on recieve. The wires were not in the right places on the relay for switching from -36VDC to -16VDC from receive to transmit. Tube no longer heats up during receive, bias stays at 35.9-36VDC. On transmit it's a little high, about 16.5-18VDC (can't remember exactly). The amp "works" but it's most all carrier and has hardly any swing, and most I can get is about 15W out of it with 30 watts going in... So, I am a little stumped about this, being a tube amp noob, but I am thinking the transmit bias being a little high is the problem. I have gone through every circuit in the amp and tested every part, and it's all as it should be. Someone had the wires backward on the relay, and the voltage divider resistors that were supposed to be there for dropping the -32VDC(actually -36VDC in this particular amp) to 16VDC had been changed. They had a 100K and a 220K resistor in there, where the schematic calls for 2 100K's. So I put 2 100K's in there, and the bias voltage sits a little higher than -16VDC. So I guess since it's not too hard to do, I am going to add a resistor after the 2 100K's to lower the transmit bias voltage a little more, to see if that will make the amp work right. I could be on the complete wrong path, but hey, worst I could do is it wouldn't work any better way I see it. Gotta learn somehow. Maybe I should open some of the 50's-60's textbooks on electronics and tubes I have here... Nah, will do that later....
Finally got the bias straightened out on recieve. The wires were not in the right places on the relay for switching from -36VDC to -16VDC from receive to transmit. Tube no longer heats up during receive, bias stays at 35.9-36VDC. On transmit it's a little high, about 16.5-18VDC (can't remember exactly). The amp "works" but it's most all carrier and has hardly any swing, and most I can get is about 15W out of it with 30 watts going in... So, I am a little stumped about this, being a tube amp noob, but I am thinking the transmit bias being a little high is the problem. I have gone through every circuit in the amp and tested every part, and it's all as it should be. Someone had the wires backward on the relay, and the voltage divider resistors that were supposed to be there for dropping the -32VDC(actually -36VDC in this particular amp) to 16VDC had been changed. They had a 100K and a 220K resistor in there, where the schematic calls for 2 100K's. So I put 2 100K's in there, and the bias voltage sits a little higher than -16VDC. So I guess since it's not too hard to do, I am going to add a resistor after the 2 100K's to lower the transmit bias voltage a little more, to see if that will make the amp work right. I could be on the complete wrong path, but hey, worst I could do is it wouldn't work any better way I see it. Gotta learn somehow. Maybe I should open some of the 50's-60's textbooks on electronics and tubes I have here... Nah, will do that later....
Finally got it working...
I guess the tube is a little on the dead side, with the proper bias (-16V) after I fixed the voltage divider circuit, amp was more of a power absorber than an amplifier.
Finally gave up and changed the bias to -36VDC on TX and RX, and amp works fine. Apply a little carrier to it, and background noise, hashy sound etc all goes away. When the tube is cold, amp only does about 40-60 watts, LOL, but once I talk on it a while it heads up to about 100-125 watts. It was a used tube from a JB200 and I have no idea what it has already been through.
Pulled the other tube out of the JB200, it made a spark when turned the amp on, a little blue spark inside, then as I talked on it a little it made more sparks in there, and I took it out. The output seemed good from the tube, but there are some little flakes (like a grey color) inside the tube, not sure what those are, look like ash or something, I was wondering could those have been causing the sparking? Should I toss the tube or will it eventually quit the sparking inside?
Wondering if I had a "new" tube in the amp, with the bias set back to -16VDC would it actually work? How much power is a JB150 supposed to put out with a good tube anyway?
What is it about converting to a EL34B tube? Are they that much cheaper than an NOS 8417? I seen them from $10-$100 for EL34B's. Some of them were some special Tug-Sol tube with higher than normal impedance. What's that all about? Is it just a drop in replacement, or are there some other changes to run the EL34B in place of the 8417?
Being a tube amp noob sucks, but learning a little as I go, when I get time with read up a little in the textbooks. I have a couple old tube amplifiers that I am going to scrap for parts and build a decent unit out of to use on my base. They have already been hacked up and are multi-colored and what not already so I figure what I have got to lose, the stuff is paid for, and some of it works, some of it don't.... Been fun playing with the Blackcat, amazed at how simple it is....
I guess the tube is a little on the dead side, with the proper bias (-16V) after I fixed the voltage divider circuit, amp was more of a power absorber than an amplifier.
Finally gave up and changed the bias to -36VDC on TX and RX, and amp works fine. Apply a little carrier to it, and background noise, hashy sound etc all goes away. When the tube is cold, amp only does about 40-60 watts, LOL, but once I talk on it a while it heads up to about 100-125 watts. It was a used tube from a JB200 and I have no idea what it has already been through.
Pulled the other tube out of the JB200, it made a spark when turned the amp on, a little blue spark inside, then as I talked on it a little it made more sparks in there, and I took it out. The output seemed good from the tube, but there are some little flakes (like a grey color) inside the tube, not sure what those are, look like ash or something, I was wondering could those have been causing the sparking? Should I toss the tube or will it eventually quit the sparking inside?
Wondering if I had a "new" tube in the amp, with the bias set back to -16VDC would it actually work? How much power is a JB150 supposed to put out with a good tube anyway?
What is it about converting to a EL34B tube? Are they that much cheaper than an NOS 8417? I seen them from $10-$100 for EL34B's. Some of them were some special Tug-Sol tube with higher than normal impedance. What's that all about? Is it just a drop in replacement, or are there some other changes to run the EL34B in place of the 8417?
Being a tube amp noob sucks, but learning a little as I go, when I get time with read up a little in the textbooks. I have a couple old tube amplifiers that I am going to scrap for parts and build a decent unit out of to use on my base. They have already been hacked up and are multi-colored and what not already so I figure what I have got to lose, the stuff is paid for, and some of it works, some of it don't.... Been fun playing with the Blackcat, amazed at how simple it is....
That's funny.
I am not sure about the 220pf grid load cap and the 3.3K ohm resistor. I don't think mine has that. Must be for the JB12, not the 150. The relay thing is different in mine also, the bias is switched on the relay contacts in the middle (and lower left side of relay).
The tube is good. After getting a few bugs worked out with my microphone, I get 100-125 watts all the time, mic was cutting out, didn't know it, and sometimes I'd only get 40-60 watts on peaks, and then other times it would jump up to 100+ watts. Wierd, pot in bottom of mic going bad or having a bad spot in it I guess, moved it around a little, works fine now.
Question though for those who know. I want to put a NEW EL34 or 6CA7 tube in this amp, and I have a JB200 also, it had 2 8417's I had to pull 1 for the JB150, the other I tested, it works but arcs inside everynow and then, some kind of flakes inside there...
So I want to get new tubes for both. What is needed to change from 8417 tubes to EL34/6CA7 tubes? Just bias voltage? if that?
Is there a difference in the EL34/6CA7 tubes advertised as for "audio" (AF) amplifiers and the standard tubes? I wouldn't really think so, more of a marketing gimmic it would seem, but maybe not?
Thanks again for all the help with this antique, I have gotten really good reports on it, had to turn my mic gain down to half, but once I did that it sounds really good, which that's all that matters, and 100 watts gets out quite good just by itself, really impressed at how far I can make contacts just on the JB150.
I am not sure about the 220pf grid load cap and the 3.3K ohm resistor. I don't think mine has that. Must be for the JB12, not the 150. The relay thing is different in mine also, the bias is switched on the relay contacts in the middle (and lower left side of relay).
The tube is good. After getting a few bugs worked out with my microphone, I get 100-125 watts all the time, mic was cutting out, didn't know it, and sometimes I'd only get 40-60 watts on peaks, and then other times it would jump up to 100+ watts. Wierd, pot in bottom of mic going bad or having a bad spot in it I guess, moved it around a little, works fine now.
Question though for those who know. I want to put a NEW EL34 or 6CA7 tube in this amp, and I have a JB200 also, it had 2 8417's I had to pull 1 for the JB150, the other I tested, it works but arcs inside everynow and then, some kind of flakes inside there...
So I want to get new tubes for both. What is needed to change from 8417 tubes to EL34/6CA7 tubes? Just bias voltage? if that?
Is there a difference in the EL34/6CA7 tubes advertised as for "audio" (AF) amplifiers and the standard tubes? I wouldn't really think so, more of a marketing gimmic it would seem, but maybe not?
Thanks again for all the help with this antique, I have gotten really good reports on it, had to turn my mic gain down to half, but once I did that it sounds really good, which that's all that matters, and 100 watts gets out quite good just by itself, really impressed at how far I can make contacts just on the JB150.
443 -
The cap and the resistor deal is for the early 12 and 150. Also the bias voltage was hooked up direct in the early models, and was relay-switched in later versions. I would stick with the 8417, as the amount of wattage gain with the EL34/6CA7 isn't really worth the effort. 73s.
- 399
The cap and the resistor deal is for the early 12 and 150. Also the bias voltage was hooked up direct in the early models, and was relay-switched in later versions. I would stick with the 8417, as the amount of wattage gain with the EL34/6CA7 isn't really worth the effort. 73s.
- 399