Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: stevehoover on November 27, 2011, 05:22:51 pm
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Hi Folks,
I just built a point to point (low watt) mini 5E3 lite, from bits left over from other projects.
I'm running one a OT5PP 22K output transformer at 4 ohms into an 8 ohm speaker
and a pair of EF86's cathode biased at .9 watts plate dissipation.
I'm using two 12EYA's in the preamp.
Because of the output tubes used, my B+ string is down by a good bit from traditional values.
I'm at 168V after the third filter cap / preamp supply node.
My question is:
What is the best way to compensate for the lower B+ supply voltages in the preamp and PI.
and stay as close to the tonality of the original 5E3 preamp circuit.
It actually sounds pretty tweedy the way it is, (I really like the sound of shared cathodes) but I'm trying to educate myself and get it as close as I can.
If anyone can point me in the right direction I would appreciate it.
Thanks for your input,
Steve
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> I'm using two 12EYA's in the preamp
I am not familiar with that tube type.
> I'm at 168V
If not using the high-gain 12AX7 or similar, and using the low-voltage finals, there's no need for high supply voltage.
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Oops that was 12AY7's
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> small signal pentodes ... 271V on the plates and 153V on the screens... drop in voltage necessary for the screens my 3rd B+ node is lower 167V than the traditional
With big final tubes running high G2 voltage at significant current it is convenient to put G2 and preamp on the same B+ "highway".
However when you want low Vg2 for small-finals yet still have high preamp voltages, you turn to a split system.
Take preamp off the main +270V through a decoupling and ripple network. Two stages probably pull 2mA. 22K against 20uFd may be enough filtering. 2mA in 22K is 44V drop. You have 225V at preamp, which is far less than the later amps, but not too low for the older Fenders. It would be good to use a little less gain such as 12AY7, and maybe not max-up a hot guitar.
To get G2 voltage down where small pentodes need: simple dropper resistor is foiled by the change of G2 current from idle to loud. Use a voltage divider wasting as much current as it delivers to G2 at idle. The idle G2 current may be 1.5mA. We send 3mA into the divider and tap so 1.5mA flows to G2, the excess to ground.
See attached sketch and calculations.
> My concern is that because my preamp voltages are lower the character of the amp will change.
It HAS changed. Final tubes more sensitive and less powerful. Different output iron. Very different acoustic level in the room (which I assume is your intent). Some people run mini-amps with mini-speakers and there's no way it won't be a mini-sound; with a proper full-size (8"-12") cone it's big enuff but not loud enough to "feel" right.
However I think it can be tweaked to a "good" sound. Balance gain and B+ so it loafs on easy pickings, strains on hard picks, and overloads when pushed. Adjust bass mid high screech for good spectral balance.
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Thank you so much!!!
I thought of a split system early on, but wasn't sure if it was a good idea.
For sure it will not sound like the real thing...this one is for my sanity and my family while we are all in close quarters.
I didn't think about the current difference to the screen.
What would be a good value for the cap at the bottom of the divider?
I will post my results when I get this one finished.
Thanks again,
Steve