Trying to figure out why I have ABSOLUTELY no output. I rebuilt a friends SF Bassman. Just a few minor mods to the first channel, second channel is stock. Both channels mix at one triode like the AB165 did. I used the NFB of the AA864.
1. Would that cause a problem for some reason.?
2. With no power tubes I have 392 VDC on the plates. With power tubes installed, the VDC drops to 305. Have tried 2 other sets of power tubes with the same results. The fuse does not blow under any of these situations. The bias circuit seems to function normally.
3. What can cause the VDC to drop like this if the tubes are good.? The amp has a new Heyboer OT that was working before this rebuild. I have checked all my wiring a few times, and all seems to be good. But I cannot help thinking I have something put together wrong some where.
Any suggestions for me.?
Thank You
No output
Moderators: pompeiisneaks, Colossal
Re: No output
check to see if you have negative bais to the output tubes.
Re: No output
Have you done the pop test at each stage's grid? (working from back-to-front)
He who dies with the most tubes... wins
Re: No output
Pop test.?
I am afraid I do not know what that is. How do I do that.?
Thanks
I am afraid I do not know what that is. How do I do that.?
Thanks
Re: No output
with the amp switched on, carefully touch a Vmeter probe on each stage's (signal) grid connection. There should be an audible pop if its working properly. The pops get louder the closer to the input stage you get.
He who dies with the most tubes... wins
Re: No output
OK....thanks.
I will give it a try........
I will give it a try........
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guitarmike2107
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Re: No output
It would be normal for the voltage to drop a bit due to the resistance in the power transformer and output transformer, not sure you should be dropping that much voltage unless there is something wrong with the OT. Does this amp have a valve rectifier?
Even with the low voltage you should have some output, so something else is wrong too, is there any hiss/background noise at the speaker.
Even with the low voltage you should have some output, so something else is wrong too, is there any hiss/background noise at the speaker.
billyz wrote:The Pop test is a quick simple test to help you determine the part of the circuit which is not passing the signal. Basically you you begin by using a Volt meter( set to measure DC Volts) to probe the circuit AND LISTEN for the POP noise it makes( through the amps Speaker) when touching the circuit . For example, you would start at the power tubes, you will hear a small pop through the speaker, as you progress to the Phase Invertor stage the pop will be louder. Again the preamp stages will will be louder still. If at one stage you fail to hear a POP , that will most likely be the part of the circuit that has failed.
So you might want to start at your first preamp tube and test the pins 1 (plate v1a) pin 2 (input grid v1a) and pin 6 (v1b plate) and pin 7 (v1b grid).
If you can hear the POP , most likely your problem is before these test points, like the input jack. If no POP it is after or right there. This does not work on every part of the circuit. The more you poke around the better you'll get at it.
This assumes you have already verified the later parts of the circuit. I believe you said only channel one is dead.
Re: No output
Like I said......there is absolutely no output.
No hiss.
No noise.
No hum.
Nothing.
I do not have anywhere near the experience most of you guys have.....but I do not think I have ever heard (if i can use that word) an amp that made NO noise through the speaker.
I have another Bassman with an OT that I KNOW is good.
Maybe I should jumper it into this amp just to see if I can get some noise out of it.
It really bothers me that the power tube plates drop almost 100 volts when the power tubes are installed.
Thank You
No hiss.
No noise.
No hum.
Nothing.
I do not have anywhere near the experience most of you guys have.....but I do not think I have ever heard (if i can use that word) an amp that made NO noise through the speaker.
I have another Bassman with an OT that I KNOW is good.
Maybe I should jumper it into this amp just to see if I can get some noise out of it.
It really bothers me that the power tube plates drop almost 100 volts when the power tubes are installed.
Thank You
Re: No output
Check the speaker and the output jack wiring. If you changed any of the jack connections, you may have accidentally shorted out the speaker connection.
What model Bassman was this? Did you do anything to the output tube socket wiring? How much current are the output tubes pulling? A 100V drop with tubes in implies a lot of current. They should certainly hum audibly.
What model Bassman was this? Did you do anything to the output tube socket wiring? How much current are the output tubes pulling? A 100V drop with tubes in implies a lot of current. They should certainly hum audibly.
Re: No output
Just a "normal" SF Bassman. Looks like it was the AB165 circuit.Firestorm wrote:Check the speaker and the output jack wiring. If you changed any of the jack connections, you may have accidentally shorted out the speaker connection.
What model Bassman was this? Did you do anything to the output tube socket wiring? How much current are the output tubes pulling? A 100V drop with tubes in implies a lot of current. They should certainly hum audibly.
I gutted the amp and rebuilt it.
I have checked all the wiring twice.
I have checked for shorts to ground with a DMM.
I have compared my speaker wiring to a working Bassman amp.
With power tubes in, B+ drops to 305, with 34 mA through 1 ohm cat resistors.
Either the OT is dead (from CT to either side is about 45 ohms and 48 ohms I believe) or I have SOMETHING wired wrong somewhere. I just cannot see anything.
Believe me.....I can easily admit a mistake. I rewired/rebuilt the entire amp. It certainly would not be surprising to find I did it wrong somewhere. I just cannot see anything yet. I wish I could.
Maybe I should throw up some pics. Who knows.....you guys might see an obvious mistake that I have been staring at for the last 48 hours.......
Thank You
Re: No output
Happy to say I found THIS problem.
There was a single strand of stranded wire jumping from the Tip lug to the Switch lug on the speaker jack. It was down low, so it was easy to miss when looking at the amp from the back, like we usually do when we work on amps.
Obviously, when I wrapped the wire around the lug, I let one of the strands get away from me, and it ended making contact with the switch lug.
I "knew" it had to be some simple mistake I had made.
Sorry to drag you guys through this....I really am. I am not sure I would have ever seen this if I had not turned the amp around to get a "different" view.
Thank You EVERYBODY For Your Help
There was a single strand of stranded wire jumping from the Tip lug to the Switch lug on the speaker jack. It was down low, so it was easy to miss when looking at the amp from the back, like we usually do when we work on amps.
Obviously, when I wrapped the wire around the lug, I let one of the strands get away from me, and it ended making contact with the switch lug.
I "knew" it had to be some simple mistake I had made.
Sorry to drag you guys through this....I really am. I am not sure I would have ever seen this if I had not turned the amp around to get a "different" view.
Thank You EVERYBODY For Your Help
Re: No output
Always pre-tin the stranded wires before attaching less chance of this occurring.
Glad you got it going.
Mark
Glad you got it going.
Mark
Re: No output
That is advice I will remember.M Fowler wrote:Always pre-tin the stranded wires before attaching less chance of this occurring.
Glad you got it going.
Mark
Of course now I can hear a bad hum that increases with the volume knob on the Normal channel.
The hum is not effected by the removal of any of the preamp tubes.
And, I am still dropping 90 volts when the power tubes are installed. Obviously, I still have a problem somewhere. At least I can hear it now.
Guess I will start with power tubes and see if that helps.....
Re: No output
You mentioned the NFB. If you moved it from one side of a triode to another, you may have to reverse the output wires of the OT. (Actually, I think the symptom is squealing, not hum.)
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solderstain
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Re: No output
Since we haven't seen pictures of what you did, I'll just share some comments about YOUR comments about using the AA864 NFB.
To follow up on xtian's post:
As someone who has done the hybrid AA864/AB165 conversion on two AB165 circuits, and converted a Pro Reverb to a "Bassman Combo" using the same hybrid circuit and a real '66 Bassman OT, I can tell you that when using an AB165 OT and converting (fully or partially) to the earlier AA864-style output stage, you need to swap the OT SECONDARY leads - meaning, the lead that was grounded at the output jack should now be hot and vice verse.
The AB165 OT secondary phase is the opposite of an AA864 circuit (don't ask why they did that... no one knows...).
You may have known that already, but in case you didn't, make the change.
To follow up on xtian's post:
As someone who has done the hybrid AA864/AB165 conversion on two AB165 circuits, and converted a Pro Reverb to a "Bassman Combo" using the same hybrid circuit and a real '66 Bassman OT, I can tell you that when using an AB165 OT and converting (fully or partially) to the earlier AA864-style output stage, you need to swap the OT SECONDARY leads - meaning, the lead that was grounded at the output jack should now be hot and vice verse.
The AB165 OT secondary phase is the opposite of an AA864 circuit (don't ask why they did that... no one knows...).
You may have known that already, but in case you didn't, make the change.