Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on July 24, 2010, 03:44:17 pm
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I own an old British Hacker amp. It has been manufactured by a British company called Hacker Radio LTD. Since there have been made several modifications during the years I would like to find the original schematic for the amp. The tubes are 2 x ECL86 and 1 x EZ81. If there are not schematic available I could change the circuit by using the existing tubes so I'm very interested to find a schematic using 2 x ECL86. If you know any please reply.
/Leevi
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Hi Leevi, is your google broken :grin:
Here is a schematic to one Hacker amp with 2 x ECL86 and 1 x EZ80:
http://sites.google.com/site/projectdevoid/file-cabinet (http://sites.google.com/site/projectdevoid/file-cabinet)
I also found couple of interesting Mullard circuits that use ECL86 tubes:
http://www.r-type.org/static/2-3.htm (http://www.r-type.org/static/2-3.htm)
http://www.r-type.org/static/4-10.htm (http://www.r-type.org/static/4-10.htm)
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Hi Leevi! I have built one amp with a pair of ECL86 tubes. It is Bluetone Express Baby with Trainwreck Express preamp with my own power amp design. If I remember right, its schematics can be found in my web pages www.bluetone.fi and there Bluetone Express Baby. I like the sound of that tube very much, but they start to be pretty expensive. That is why I do not use them in other projects. Anyhow I still have a couple of them (NOS) if you need. Harry
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Thanks for the replies guys!
VMS: My Google is working well and now even better when I downloaded the Google's own browser named Chrome. Maybe I should be more patient when searching things. The schematic under the link you send is pretty close to circuit of my amp. The Mullard circuits are interesting too.
Harry: I found the circuit from your great pages but it seems to be meant for a power amp only. My idea was to build both the preamp and power amp by using two ECL86s.
Leevi
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> both the preamp and power amp by using two ECL86s
Doubtful.
You have two power pentodes. Each needs about 10V peak or 7V RMS drive. You probably want to work push-pull. You should use a push-pull driver. With two triodes the best you can do is a high-gain volt-amp follewed by a cathodyne. Gain to each grid will be 70 at best. The input sensitivity is about 7V/70 or 0.1V or 100mV RMS. Guitar amps typically have input sensitivity of 50mV to 5mV. You need another gain of 2 to 20.
Maybe the most direct "cheat" is to use a small MOSFET as the cathodyne. Then you have two triodes' worth of gain which is more than ample.
Alternatively you could sneak a JFET in as the first stage. Well-done it has less hiss and hum than a typical tube. And the distortion can be "similar" to a tube.
The only push-pull commercial amp with just two triodes as pream/driver is the small Fender bass amp. It used an interstage transformer to get phase split. It has some collector appeal, mostly I think because it was so mediocre that it didn't sell well when new and is now slightly scarce. The interstage transformer was custom and not easily emulated. And transformer-driven guitar amps have rarely been as popular as cap-coupled drivers.
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Yes, it's challenging to get it working as a p/p amp. I thought another idea:
I could use the rectifier socket for a 12AX7 and replace the EZ81 with an SS rectifier.
Then I have more choices for the preamp and I could use the circuit Harryk presented
in his reply.
Thanks
Leevi