Hammond 300BX for a Dynaco modified Mullard 5-20 circuit

[wongkk]

I have this hammond 300BX power trans, 1650P OPT and 193J choke (10H). To build a stereo EL34 UL P-P under 1 chasis, do you think the voltage and current will be overkilled? Any changes need ?

Here's the link which the amp I intend to build :-

http://www.triodeel.com/dynamull.gif


Thanks and regards !!

Adam

[gingertube]

No I don't think its overkill

With a 5U4 rectifier you should get about 480V DC across the first capacitor (the 30uF) and perhaps 465V DC across the 20uF on the other side of the choke. I would increase the 20uF to say 100uF the choke will prevent this stressing the rectifier too much. Watch the capacitor voltage ratings - you WILL have to use series electrolytics with voltage share resistors across them.
I would also add 1K 5W resistors in series with the EL34 Screen connections to the ultralinear taps.

Do NOT be tempted to use 5AR4 rectifier instead - it would push the DC supply (across the 30uF) to about 520V - Too High.

Hope this is some help,
Cheers,
Ian

[cogsncogs]

@ wongkk

No overkill here! The Hammond 300BX would be about perfect in this case. Two 47uF 350WV caps in series with 100k -220k 2W resistors across each cap of the first capacitor, two 100uF-220uF 350WV caps in series with 100k-220k 2W across each for the second should do the trick. For the 12pf cap from the screen grid of the bottom EL34, I would use a high quality (500V min.) NPO ceramic or a silver mica @1kV, if you can find one of that value. 15-10 pF would probably be a good sub. For the 20uF cap after the 18k 1W (6CG7/6FQ7, better yet a 6SN7!), a 47uF 500WV or two 100uF 350WV in series would be a good choice here.
The gain of this amp is a little on the high side.

Good Luck
Wayne

[cogsncogs]

You could use a 5R4, more voltage drop across the rectifier.
67V @ 250mA, 63V for a 5R4-GYB. That is if you like the cool look of a 5R4! Downside; somewhat poorer voltage regulation and a little less power.

Wayne

[wongkk]

Regarding the 12pF cap from the screen grid of the bottom EL34, can I eliminate it ?

Any idea what does it do to the overall circuit?

[cogsncogs]

Regarding the 12pF cap from the screen grid of the bottom EL34, can I eliminate it ?

Any idea what does it do to the overall circuit?

It is a HF compensation cap. It's there to stabilize the amp at high frequencies, as it is used in the original circuit utilizing the specified/original components.
If you plan on building this amp let's say with a different OPT or even a clone/replacement transformer it may or may not be needed. It may well require some other HF compensation scheme. It's very similar to: (the same really, diff tubes)

EL34 Push Pull weirdness

Except I like the 6CG7/6FQ7 LTP much better! I would either connect the EF86 as a triode or use a lo-med mu triode such as a 5687.
You could go 6CG7/6FQ7 all the way! Might be a little dark sounding tho. Might be best to use a bright sounding tube as the first (AF amp) stage.

Wayne