I have built a guitar tube amp based on this board here. This is a tweed champ 5f1,
https://tubedepot.com/products/5e1-5f1-champ-printed-circuit-board
It powers on, and I have measured the plate voltages, plus cathode voltages. I checked that everything is grounded correctly.
When I play guitar through the amp it distorts horribly when I hit a string a bit harder. That is when I have the volume pot set low, say about 1. If I turn the volume up it distorts horribly all the time and is definitely not like the youtube videos I have seen of this amp.
I swapped out the tubes with a mixture of old and new but no difference.
I wonder about my plate voltage on the 6v6. It measured 373v and the screen grid was at 326v. The voltage across the cathode resister (476 ohms) was 21 v. I have read some things which say this is OK, and others which say this is too high. But will this cause the distortion I am getting?
How can I diagnose this issue?
https://tubedepot.com/products/5e1-5f1-champ-printed-circuit-board
It powers on, and I have measured the plate voltages, plus cathode voltages. I checked that everything is grounded correctly.
When I play guitar through the amp it distorts horribly when I hit a string a bit harder. That is when I have the volume pot set low, say about 1. If I turn the volume up it distorts horribly all the time and is definitely not like the youtube videos I have seen of this amp.
I swapped out the tubes with a mixture of old and new but no difference.
I wonder about my plate voltage on the 6v6. It measured 373v and the screen grid was at 326v. The voltage across the cathode resister (476 ohms) was 21 v. I have read some things which say this is OK, and others which say this is too high. But will this cause the distortion I am getting?
How can I diagnose this issue?
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Moved to the I & A forum since this is a guitar amp thread.
What specifically is the loudspeaker driver you have connected to this amp?
Recall that the driver choice should be of the PA, guitar type, high (100DB+) efficiency.
Wiring up our common HiFi driver will get you very little volume here. So much so it might seem to be malfunctioned in this scenario.
--------------------------------------------------------------Rick...........
Recall that the driver choice should be of the PA, guitar type, high (100DB+) efficiency.
Wiring up our common HiFi driver will get you very little volume here. So much so it might seem to be malfunctioned in this scenario.
--------------------------------------------------------------Rick...........
I am using the speaker from a Fender Vibra Champ XD, so it should be fine. The speaker works fine in the Vibra Champ XD.
I also built another guitar amp based on a single EL84 and the same speaker works OK on that amp.
I also may have caused confusion with my comment of the volume being low. I mean I have the volume pot set low, so about 1, and if I take it any higher the harsh distortion gets worse.
The volume out of the speaker is not low.
I also built another guitar amp based on a single EL84 and the same speaker works OK on that amp.
I also may have caused confusion with my comment of the volume being low. I mean I have the volume pot set low, so about 1, and if I take it any higher the harsh distortion gets worse.
The volume out of the speaker is not low.
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That's what I was thinking too...after perusing the schemo' ..the power tube slamming full...
---------------------------------------------------------------Rick......
I have disconnected the negative feedback, and used my multimeter to double check resister values. Will check the pot though.
I did do something different than the circuit though, I didn't have switchable input jacks so I just wired one of the jack inputs with the 68k resister in series for the input, and put a 1M resister between the input jack pins, i.e. between the signal and the ground. I don't really understand the double switchable circuit though so I assume this will be OK? I based this on my other tube amp I built and it had the same setup.
I initially didn't have the 1M resister between signal and ground on the input jack and I though 'ah, that's it'. But when I added it, it didn't seem to make any difference.
Any other resisters I can tweak to reduce the gain and see what happens?
I did do something different than the circuit though, I didn't have switchable input jacks so I just wired one of the jack inputs with the 68k resister in series for the input, and put a 1M resister between the input jack pins, i.e. between the signal and the ground. I don't really understand the double switchable circuit though so I assume this will be OK? I based this on my other tube amp I built and it had the same setup.
I initially didn't have the 1M resister between signal and ground on the input jack and I though 'ah, that's it'. But when I added it, it didn't seem to make any difference.
Any other resisters I can tweak to reduce the gain and see what happens?
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I found the issue, and I posted about it around 6 hours ago. For some reason the post never came through.
The wire which takes the signal into the second half of the preamp tube from the pot was too close to one of the wires from the primary of the output transformer. As soon as I re positioned the wires the problem went away.
If you look at the 5F1 layout you would be correct in noting that the wires from the pot should go nowhere near the primary of the output transformer. I decided to change the layout so that the tubes would be at the front of the chassis. It turned out to be OK in the end when I looked carefully at the layout and thought carefully where interference may be occurring. Lesson learnt.
I went through the circuit over and over with my multimeter and eventually decided that a simple circuit like this, that there was nothing wrong with it. That is when I really started googling and searching for another cause.
Amp sounds fantastic.
The wire which takes the signal into the second half of the preamp tube from the pot was too close to one of the wires from the primary of the output transformer. As soon as I re positioned the wires the problem went away.
If you look at the 5F1 layout you would be correct in noting that the wires from the pot should go nowhere near the primary of the output transformer. I decided to change the layout so that the tubes would be at the front of the chassis. It turned out to be OK in the end when I looked carefully at the layout and thought carefully where interference may be occurring. Lesson learnt.
I went through the circuit over and over with my multimeter and eventually decided that a simple circuit like this, that there was nothing wrong with it. That is when I really started googling and searching for another cause.
Amp sounds fantastic.
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