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Re: need to see a schematic


Wow. This schematic looks similar to alot of the China-clone
845 amps. ;) It's certainly compact and attainable.

Some comments:
1. You always want to make sure 6.3V filament power is there.
Without it, if the tubes arent conducting any current then there
is no voltage drop across the plate load resistors which means
the coupling caps are seeing full killer B+ which is not a
health situation.

2. Im guessing operating points are as follows:
EL34 @ 290V and 30mA, 6N7 at 6ma per triode section at 200V.

3. Make sure the secondary of the 125ESE is pathed to ground. Why?
Were a dielectric breakdown to occur, the speaker wires and parts
of the speakers could elevate to 1kV and create a lethal shock hazard. Dont leave the output transformer floating like the schematic implies. I highly doubt the 125ESE is wound with the
proper insulation level to take anything north of 500V. Be

4. Be VERY careful with your clipleads. I doubt they are rated
for 1kV. Clipleads can be prone to cracked insulation from regular
use. Fine cracks in the insulation is OK at a few hundred volts. At
1 kV, its a supreme liability.

5. The 10K pot followed by the 5K fixed resistor in the bias
circuit is very smart. The 5K resistor prevents you from
accidentally biasing down to 0V. To be extra safe, you could
bracket with potentiometer between two resistors and choose values
such that the range of bias voltage is narrowed even further.

6. Try a 6W6GT in place of the EL34. :) They are $3.00. The ones
I bought from AES (several brands) sound spectacular. Another tube
I really like is the Valve Art KT66. Guitar Center carries them
under the "Groove Tube" brand. $35/pair where I got them.

7. I used a similar circuit with similar tubes but went with
a direct coupled approach. You could try that too later on after
you work the kinks out and listen to the amp for awhile. In the
direct coupled approach, you'd choose a new bias point for the
6N7s at around 150V on the plate. The plate of the 6N7 would
then be tied directly too the EL34s grid. The EL34 cathode
would now need to be elevated to a voltage of 6N7Plate + 30V to
set its bias. Assuming 30mA bias for the EL34, then, Rk =
150+30 / 30mA = 6Kohms 5.4W. Another big resistor. The
Direct Coupled approach removes a capacitor and a 90 degree
phase shift from the picture. The risk of a direct coupled approach
is that the input tube must ALWAYS have filament power and must
never be removed from its socket. Why? Because the EL34 is depending
on the 6N7's operation to set its fixed bias point. Without the 6N7,
the EL34 would lose fixed bias and would conduct like mad likely
harming the EL34 but not the amp.

In my case, I chose to use D.C. to remove a cap coupling stage.

You have ALOT of options for colouring in the tone of the amp.
Vary the input tube and the driver tube. I played around with this.
Trying the space of [6C5, 6P5GT, 6SN7, #76] x [6V6, 6W6, EL34, KT66, KT88, 5881].

I found the 6P5GT and #76 input tube to sound the best. I also
found that beam power tubes (KT88, KT66, 6W6GT) sounded very good.
I was not impressed with the Sovtek EL34, the E-H 6V6GT or the 5881.
The Sovtek KT88 sounded better than the EL34 or 6V6GT. The
Ei Fat Bottle EL34s sounded very good, better than the KT88. The
Valve Art KT66 and various NOS 6W6GT sound exceptionally good
I think. Something about beam tubes over pentodes. :) Grover
seems to like the Ei KT90's if I remember.

-- Jim

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  • Re: need to see a schematic - Jim Doyle 10:48:52 12/26/04 (0)

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