Yamamoto A08S clone

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svante
Posts: 11
Joined: Wed Feb 24, 2021 6:17 pm
Location: Sweden

Yamamoto A08S clone

Post by svante »

Hello

I have now finalized (at least first version of the amp) my YamamotoA08 clone.

I found a nice industry looking case on action site that I have used, Bought the output transformers on E-bay from Ukraine, hand wounded on old military irons.

First I planned to use DC filament, but the main transformer I bought Hammond 302AX had to low voltage to allow rectifying, so I switched to ac filament and hum-balance pots on the 45 tubes. Can’t her any 50Hz hum, so it works fine.

After building exact as the Yamamoto schedule that can be find on line, I had high THD, >4% at one watt 1000 hz. An almost al from the driver 717A tube. After a lot of testing, I ended up with around 2,5% by using an blue led as voltage lift for driver tube cathode. That was the solution that worked best in terms of THD, maybe I will try to change to an 6SN7 tube or something else to lower the THD.

I have Tektone perfect SET 15 speakers, stated being 98 dB efficiency (in reality maybe a bit less is my guess).
Playing jazz to medium levels, it sounds really nice, actually a bitt better than my 300 b, it’s easier to separate things in the high midst and top end, like brushes on the cymbals, or metal string sliding sound. But if I try to play Led Zeppelin on high volume, the amp goes out of steam, so for rock it needs higher efficiency speakers.

Br Svante
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Tables_and_Tubes
Posts: 38
Joined: Thu Feb 25, 2021 2:25 am

Re: Yamamoto A08S clone

Post by Tables_and_Tubes »

Awesome repurposing of the case,and I really like your use of the vertical space for the layout! I hope that you can get the distortion issue worked out. Keep us posted.

Dan
svante
Posts: 11
Joined: Wed Feb 24, 2021 6:17 pm
Location: Sweden

Re: Yamamoto A08S clone

Post by svante »

Hi Dan, thanks
I will play around with some more variations of connections for the 717A tub, but this might be the level this tube produce, the sound is wery good at medium level (maybe 1/2 W or so), and then THD is <1%.
Br svante
svante
Posts: 11
Joined: Wed Feb 24, 2021 6:17 pm
Location: Sweden

Re: Yamamoto A08S clone

Post by svante »

After some more hours, I managed to adjust one side to 1,4% THD (1w). that was with an used WE 717A tube, on the other side, it stayed at 2,3%, that with an Tung solo NOS tube. So I will by some more WE tubes and se how they behaves. And maybe some C3G tubes, that I have seen used as 45 driver in single stage setup.

Br Svante
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