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I'm having trouble with an Electar Tube 10 amp I'm rebuilding.
First off I should tell you that I'm a newbie and I know just enough to be dangerous. I've read about how tube amps work, but I've never actually sat down and looked at tube charts and drew load lines to determine what component values I should use, etc. Instead, I just followed other people's suggestions and looked at other circuits like Fender Champ circuits for ideas.
The amp is an Electar Tube 10 (see schematic: http://www.freeinfosociety.com/elect...iew.php?id=375.
Here are some changes I made based on other people's mods.
- changed cathode resistors R3 and R1 to 1.5K
- changed coupling cap C2 to .022
- removed grid resistor R6
- removed grid-to-ground resistor (that's what Aiken calls it, he says "grid leak resistor" is the wrong terminology in this case) R5.
- Removed negative feedback loop by removing R14 and C11.
- I think I changed coupling cap C5 between 2nd gain stage and tone stack to .022 also.
- Changed the output transformer to something bigger.
Basically I made the preamp a lot more like a Fender Champ. These mods all worked. However, after modding my amp so much, the copper traces on the printed circuit board started coming up in a few places. So I decided to rebuild the modified amp from scratch on a turret board. Along the way, I destroyed the original power transformer, so I replaced it with one of Webers transformers used for Champs and Princetons. But when I was finished, the amp wasn't very loud and it would get a nasty squeal and buzz as I turned up the gain and the 120HZ hum was pretty bad.
One problem I had was a wrong resistor in the power amp.
But it appears the main trouble I'm having with my amp is in the coupling between preamp stages 1 and 2. If I run preamp stage 1 directly into the power amp (C2 into master volume input), it works fine. If I run preamp stage 2 diretly into the power amp (R2 into pin 7, C5 to master volume input) it works fine. If I use both preamp stages (C5 into master volume input), and set the master volume at 10, it squeals as I increase the gain, it gets buzzy and nasty when it's way up, and the 120 Hz hum is too loud. If I mess with the master volume too, it does weird stuff, but I don't remember the details.
This all got me to thinking about the purpose of grid resistor R6 and grid-to-ground resistor R5. I've read a little about them, but I was wondering if one of you can explain their purpose and why a Fender Champ 5F1 doesn't use them.
I added grid resistor R6 back in but used a value of 220K as suggested by one of the websites on Electar Tube 10 mods. This worked a little better. With the master on 10, when I turn up the gain, it starts with a little noise, almost like a scratchy pot from 1 to 2. Then from 2 to 4, its OK. At 4 the volume dies way down, at 6 a squeal starts coming in and gets louder as the gain is increased. At 9 and 10, it is sounds really nasty. I think the 120 Hz hum is still pretty loud at this point, I don't remember for sure.
Then I added grid-to-ground resistor R5 back in. I didn't have a 330K resistor, so I used a 470K resistor, I read that it just has to be a large value. I didn't think I needed this because I figured the gain knob provide the grid-to-ground as mentioned in other ampage posts (but what if the gain is at 10, zero resistance?). Also, some of the Electar mod websites didn't say it was needed. Well, when I added it back in, the aforementioned noise problems disappeared, but the amp was way too quiet. My amp was louder when I was only using preamp gain stage 1 into the power amp. If I add the tonestack back in, it will probably be even way more quiet.
So does anyone have an explanation of what's happening?
Thanks.
- Clint
First off I should tell you that I'm a newbie and I know just enough to be dangerous. I've read about how tube amps work, but I've never actually sat down and looked at tube charts and drew load lines to determine what component values I should use, etc. Instead, I just followed other people's suggestions and looked at other circuits like Fender Champ circuits for ideas.
The amp is an Electar Tube 10 (see schematic: http://www.freeinfosociety.com/elect...iew.php?id=375.
Here are some changes I made based on other people's mods.
- changed cathode resistors R3 and R1 to 1.5K
- changed coupling cap C2 to .022
- removed grid resistor R6
- removed grid-to-ground resistor (that's what Aiken calls it, he says "grid leak resistor" is the wrong terminology in this case) R5.
- Removed negative feedback loop by removing R14 and C11.
- I think I changed coupling cap C5 between 2nd gain stage and tone stack to .022 also.
- Changed the output transformer to something bigger.
Basically I made the preamp a lot more like a Fender Champ. These mods all worked. However, after modding my amp so much, the copper traces on the printed circuit board started coming up in a few places. So I decided to rebuild the modified amp from scratch on a turret board. Along the way, I destroyed the original power transformer, so I replaced it with one of Webers transformers used for Champs and Princetons. But when I was finished, the amp wasn't very loud and it would get a nasty squeal and buzz as I turned up the gain and the 120HZ hum was pretty bad.
One problem I had was a wrong resistor in the power amp.
But it appears the main trouble I'm having with my amp is in the coupling between preamp stages 1 and 2. If I run preamp stage 1 directly into the power amp (C2 into master volume input), it works fine. If I run preamp stage 2 diretly into the power amp (R2 into pin 7, C5 to master volume input) it works fine. If I use both preamp stages (C5 into master volume input), and set the master volume at 10, it squeals as I increase the gain, it gets buzzy and nasty when it's way up, and the 120 Hz hum is too loud. If I mess with the master volume too, it does weird stuff, but I don't remember the details.
This all got me to thinking about the purpose of grid resistor R6 and grid-to-ground resistor R5. I've read a little about them, but I was wondering if one of you can explain their purpose and why a Fender Champ 5F1 doesn't use them.
I added grid resistor R6 back in but used a value of 220K as suggested by one of the websites on Electar Tube 10 mods. This worked a little better. With the master on 10, when I turn up the gain, it starts with a little noise, almost like a scratchy pot from 1 to 2. Then from 2 to 4, its OK. At 4 the volume dies way down, at 6 a squeal starts coming in and gets louder as the gain is increased. At 9 and 10, it is sounds really nasty. I think the 120 Hz hum is still pretty loud at this point, I don't remember for sure.
Then I added grid-to-ground resistor R5 back in. I didn't have a 330K resistor, so I used a 470K resistor, I read that it just has to be a large value. I didn't think I needed this because I figured the gain knob provide the grid-to-ground as mentioned in other ampage posts (but what if the gain is at 10, zero resistance?). Also, some of the Electar mod websites didn't say it was needed. Well, when I added it back in, the aforementioned noise problems disappeared, but the amp was way too quiet. My amp was louder when I was only using preamp gain stage 1 into the power amp. If I add the tonestack back in, it will probably be even way more quiet.
So does anyone have an explanation of what's happening?
Thanks.
- Clint
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