Congrats for figuring out some ways to polish this piece of stuff without ending up with a smelly mess. I generally opt for making my own sort of mess, though I can understand the gratification of fixing up someone else's, esp if the amp was cheap to begin with.
If anyone is unsure: I can highly recommend Stephe's modifications. I changed everything except the preamp tube circuitry. With the plate to plate feedback I'm still trying around. I listen with a Fostex 126 horn and the modification made the bass much louder and more accurate. The amp runs very long at a stretch, the power transformer gets warmer than before, but is still easy to touch - I can live with that.
Thank you, Stephe, for sharing your knowledge.
Thank you, Stephe, for sharing your knowledge.
Could you comment on this please? "Having that said, if you know how to work on tube amps, you can try changing the 500ohm cathode bias resistor of A10/A12. Just raising the bias current by 10% the sound gets amazingly clean "
thx
bob
"You can safely replace that 500R with a 250R, which really clean up the sound and it then produces the power it's rated at. I show all of this in my video series on modifying this amp."
I watched that video but it's a little above my comfort zone, but changing out 1 resistor is fine if I knew where it was and what part to get?
thx
bob
I watched that video but it's a little above my comfort zone, but changing out 1 resistor is fine if I knew where it was and what part to get?
thx
bob
You really need to do more than just change 1 resistor to fix this amp, sorry.
I recently got an A12 from Amazon. Unmodified it was a mess! It sounded pretty good playing simple stuff like a string quartet at unrealistically low volume levels but it fell apart as soon as I tried to push it in any way.
Just to see I turned it up a bit and put on Van Halen. It was at about 78dB and sounded terrible! I turned it up a couple of notches and it sounded worse but was still running 78dB. It was clipping so badly that I don't think that it would get my Klipsch LaScalas to 80dB. One clean watt runs them at over 100dB so I was only getting fractions of a watt.
I did Skunkie's upgrades and the amp is transformed! I installed some good tubes and now I have a amp that is worth listening to.
I do have a comment about the videos: at the beginning of the second video of actually doing the upgrades when you install the cathode resistor. You never say what resistor that you are installing and I just know enough to be dangerous, so I had to pause the video, take a screenshot and zoom in to read the bands to verify that it is 100k.
It was only when I got to the final step and had to install the last 100k resistor to the input that I realized that I probably should have used the 2 watt resistor for the cathode instead of the 1 watt that I used there.
I swapped the resistors on the second channel and will probably go back and switch them on the first channel.
The circuit diagram that you provided doesn't show what power ratings each resistor is spec'ed at, which would help.
Also, the 430k 2 watt B+ bleeder resistor isn't listed on the parts list.
All in all, I am very happy with the results!
Just to see I turned it up a bit and put on Van Halen. It was at about 78dB and sounded terrible! I turned it up a couple of notches and it sounded worse but was still running 78dB. It was clipping so badly that I don't think that it would get my Klipsch LaScalas to 80dB. One clean watt runs them at over 100dB so I was only getting fractions of a watt.
I did Skunkie's upgrades and the amp is transformed! I installed some good tubes and now I have a amp that is worth listening to.
I do have a comment about the videos: at the beginning of the second video of actually doing the upgrades when you install the cathode resistor. You never say what resistor that you are installing and I just know enough to be dangerous, so I had to pause the video, take a screenshot and zoom in to read the bands to verify that it is 100k.
It was only when I got to the final step and had to install the last 100k resistor to the input that I realized that I probably should have used the 2 watt resistor for the cathode instead of the 1 watt that I used there.
I swapped the resistors on the second channel and will probably go back and switch them on the first channel.
The circuit diagram that you provided doesn't show what power ratings each resistor is spec'ed at, which would help.
Also, the 430k 2 watt B+ bleeder resistor isn't listed on the parts list.
All in all, I am very happy with the results!
Dear Stephe:
I did not clearly express how much I appreciate you and your YouTube channel and all that you do. I know just enough about this stuff to be dangerous and so I could never undertake a project such as this one without the help of someone who truly knows what she is talking about.
After watching your series on the A12 I wanted to see for myself and I have to say that I'm VERY impressed. Your mods have transformed it from being basically useless into a pretty impressive little amp. I don't understand what the people who rave about it are hearing that they think it sounded good out of the box. I can understand someone who has never heard a good tube amp being impressed, but Steve Gutenberg?!? I thought that he knew what he's talking about. Oh well.
Again, I just want to make sure that I express how much I enjoyed the process and how much I appreciate your efforts.
Best,
Pete
I did not clearly express how much I appreciate you and your YouTube channel and all that you do. I know just enough about this stuff to be dangerous and so I could never undertake a project such as this one without the help of someone who truly knows what she is talking about.
After watching your series on the A12 I wanted to see for myself and I have to say that I'm VERY impressed. Your mods have transformed it from being basically useless into a pretty impressive little amp. I don't understand what the people who rave about it are hearing that they think it sounded good out of the box. I can understand someone who has never heard a good tube amp being impressed, but Steve Gutenberg?!? I thought that he knew what he's talking about. Oh well.
Again, I just want to make sure that I express how much I enjoyed the process and how much I appreciate your efforts.
Best,
Pete
Steve Gutenberg is always over optimistic, I don`t remember any critical opinion, about anything from his videos.
Thanks for the note. That A12, after those simple mods, is probably one of the best bargains in tube audio.
I had assumed that he only reviewed things that he likes and so never had anything very negative to say.
Pete
That's the way most online reviewers are.
some have even said it on record.
if they don't like something, they send it back..............
some have even said it on record.
if they don't like something, they send it back..............
Yes, but that doesn't explain why Steve Gutenberg gave such a glowing review to such a flawed product.
Pete
Steve presented this thing as a great new bargain that he had found and he wanted to introduce to people. It is hardly a mass-market item in wide distribution.
Pete
