push-pull OPT 6P14P-ev or EL84m

[crazyfrog]

I guess Eli's arguments can be discussed but since a bit of headroom is necessary, OPTs will be a bit bigger than shorter. These days I check for some vintage stuff and push-pull tube amps with 7189 giving 20w x2 have no small irons.

 

[kodabmx]

I use Energy RC70 speakers. My apartment is 380 sqft. 15W is plenty loud at 30Hz in this set up, and the cones hardly move. When powered at 200W the excursion of the driver is still only about 1 cm, but all the dishes in the kitchen cupboards jingle and rattle. Small drivers in a large well tuned enclosure can make great bass. And besides synthetic bass how often do you need full power at rated output? I have a pair of Hammond 1650N (not NA) trafos rated for 60W that are fine to 80W on most normal music. The schematic I posted uses $35 toroids for OPT.

Here's a picture of another one I built. 6P3S using VPT18-5560 (100VA) for OPT.

It provides 18W in class A, and uses a voltage doubler from an isolation toroid for power for the output (300V) and an 12V SMPS for heaters and a boost converter for the front end (420V)

 

[artosalo]

I quickly simulated the output stage of Sherwood S5000. Some details of the OPT have been assumed (Lpri,Rpri etc.).
It puts out some 22 W without exceeding the Pa-max specs. The bass end frequency response drops at 19 Hz (to -3 dB) with 40H primary inductance.

 

[kodabmx]

How much does power drop by using 250R on each cathode bypassed with 330uf instead of fixed bias?

 

[artosalo]

With 420V +Ub and 250 ohms cathode resistors, the anode current is excessive, 57 mA, which creates 23 W anode dissipation.
With 430 ohms the anode dissipation is still out of the specs, 15.3 W, but I simulated that.
Maximum Pout = 19.5 W, cathode voltage at idle = 16.7 V and at full drive 18.6 V.

 

[crazyfrog]

Ouch! fixed bias is a must! If you don't have the tap, a little tiny transformer makes the job. In the case of el84 or 7189, just need around 15VDC.

 

[crazyfrog]

BTW thanks for the simulation on post 23 I could get a similar vintage iron maybe. Just need a good opportunity.

 

[kodabmx]

With 420V +Ub and 250 ohms cathode resistors, the anode current is excessive, 57 mA, which creates 23 W anode dissipation.
With 430 ohms the anode dissipation is still out of the specs, 15.3 W, but I simulated that.
Maximum Pout = 19.5 W, cathode voltage at idle = 16.7 V and at full drive 18.6 V.

So use 6P3S instead and all is well... Really I would use 250R with B+ 300V or so. Thanks for simulating it.

Presumably the fixed bias output was biased further toward class B than the cathode biased version.

 

[matejsirk]

I quickly simulated the output stage of Sherwood S5000. Some details of the OPT have been assumed (Lpri,Rpri etc.).
It puts out some 22 W without exceeding the Pa-max specs. The bass end frequency response drops at 19 Hz (to -3 dB) with 40H primary inductance.

Hi,
my calculation for OT is different: (3,96+1,76)*4 = 22,88H (Lpri) - so the transformer is 4.5K:8?

 

[artosalo]

Your calculation is wrong. You can not add directly 3.96H + 1.76H.
CT to UL is 40 % of turns of half primary and UL to A is 60 %.
But this transformer has also small error. The half primary is 11H and total primary 44H.
Correct primary values for 40H (40% UL) total inductances are:

- CT to UL 1.6 H
- UL to A 3.6 H

 

[artosalo]

This is 40% UL OPT with 40 H total primary inductance.