Any high-end 6BM8 push pull tube amp schematic ? - diyAudio
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Old 21st October 2010, 07:23 AM   #1
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Default Any high-end 6BM8 push pull tube amp schematic ?

Hello

I have a Pioneer SX-34 with a defect FM tuner section, since I did not find the problem of the FM tuner section, I would use the transfos and output tube to made a good tube amp...

As it is the SX-34 amp sound good but maby I can find a better schematic than the SX-34 amp section.

Any better push pull tube amp schematic than the SX-34 amp ?

I dont think it would need a 12AX7 input tube since cd player give arround 1 volt output and this voltage can drive the driver section of the 6BM8 tubes.

Here's a link to the schematic of the Pioneer SX-34;

AK Database PDF Gallery - Pioneer Receivers/Pioneer SX34 Schematic

Bye

Gaetan

Last edited by gaetan8888; 21st October 2010 at 07:28 AM.
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Old 21st October 2010, 07:55 AM   #2
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The ECL92/6BM8 has been described as 1/2 an ECC81/12AT7 + a derated EL84. I'd consider this an invitaion to build an El Cheapo variant. Our Scott-donor ECC81/EL84 triode class A El Cheapo is a real stunner.

Should be good for 2-3W in Class A, or 8-10W in AB.

dave
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Old 21st October 2010, 08:08 AM   #3
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The ECL82(7W Pa, comparably low S) does unfortunately not remind anything about EL84. If we instead check the ECL86 this one is a derated EL84 from 12W to 9W Pa. The triode is 1/2 ECC83. Rewiring the Pioneer?
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Old 21st October 2010, 10:49 AM   #4
45 is offline 45  Italy
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Quote:
Originally Posted by revintage View Post
The ECL82(7W Pa, comparably low S) does unfortunately not remind anything about EL84. If we instead check the ECL86 this one is a derated EL84 from 12W to 9W Pa. The triode is 1/2 ECC83. Rewiring the Pioneer?
Why unfortunately? Sonically speaking it can give at least the same results. Actually, I think it is easier to get better results with ECL/PCL82's...
It has a lower gm but still better than other tubes like 6v6, 6l6 etc...and the sensitivity is still very good.
It has got a pretty lower plate resistance (12K to 20K in penthode mode and 1K2 to 1K6 in triode mode, depending on the operative conditions).
Finally the triode section is much better than the ECL86's one and better than most ECC83's. It's simply more linear. I would compare it to 1/2 5751 or 6SL7 in terms of average specs.

You get 4-5W from a triode connected PP and 8-10W in penthode connection.
In SE you get 1.5W at 5% THD (triode connected) with only 5.5W plate dissipation.
The output section is more efficient than those of EL84 and ECL86 and, once again, it sound's nice!

45
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Old 21st October 2010, 11:59 AM   #5
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I've never used this tube but it does look interesting. Using good iron, it should make a sweet little amplifier.
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Old 21st October 2010, 12:44 PM   #6
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I've got 4 NOS Mullard 6bm8's and good iron so I'm also looking for a good schematic.


Check out this thread

ECL82 (6BM8) PP Stereo
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Old 21st October 2010, 02:49 PM   #7
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Quote:
Rewiring the Pioneer?
Think I'm with Revintage on this one.

Using the tape recorder playback input seems a straight route to the volume pot bypassing the 12ax7 and the tone controls without need for any rewiring.

Would upgrade caps as a starter



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Old 21st October 2010, 08:03 PM   #8
rmyauck is offline rmyauck  Canada
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I also vote to keep the design as is. I think it's it point to point wiring which is easy to rebuild upgrade caps etc. That amp design is and was used in many high end designs out there so just check resistors and replace caps etc. Also check, clean, and maybe replace tube sockets, connectors, pots, etc. Will be worth something in the end if restored and kept together. Maybe you can figure out the Tuner problem down the road. Plus it should sound really nice fixed up!


Randy

Last edited by rmyauck; 21st October 2010 at 08:16 PM.
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Old 21st October 2010, 08:46 PM   #9
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Hello

I think the amp of the Pioneer SX-34 look a bit like a Mullard PP amp schematic that I've seen somewhere.

Yes I can bypass the 12AX7 tube, it will made the amp less noisy.

I have recap a part of it but since it seem to worth it I will recap all the power supply and amp section.

In another thread few guy try to help me to find out why the FM tuner are not working, the local oscillator tube are ok but the local oscillator are not working, so I will not try anymore to fix the FM tuner.

The amp by itself do have a nice sound and good soundstage, I did put new Sylvania 6BM8 tubes sometimes ago.

Btw, does it would sound better if I take out the negative feedback resistor and cap, so the amp would be without negative feedback ?

Thank

Bye

Gaetan

Last edited by gaetan8888; 21st October 2010 at 08:57 PM.
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Old 24th October 2010, 03:03 AM   #10
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Hello

I've recap my SX-34, replaced some cracked mylar cap in the amp section, and put a zobel network at the output transformer secondary.

Now bypassing the preamp and tuners sections, the sound now are much better, wide soundstage and lot of air arround the musical instruments, nice definition, nice decay of the music, this amp sound excellent.

The Sylvania 6BM8 power tubes are quite good indeed.

Bye

Gaetan

Last edited by gaetan8888; 24th October 2010 at 03:09 AM.
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