Re: Can a 211 amp use an 845



jd76duke@xxxxxxxxxxxxxx wrote:

> Thanks Patrick. I've been building amps for a few years but generally
> I build from existing circuits or cut and paste different stages
> together into something a bit different. I've never gotten into the
> curves. All I know is that the 211 and 845 use the same plate voltages
> and have the same heater voltages.
>
> I love the 211 but I was just wondering if the amp can do both. If the
> flexibility is there, I'd like to try it.
>
> Thanks,
>
> John

The 211 has about the same plate dissipation
as the 845, and both will have Ea = around 1,200v and Ia = 55 mA.
Therefore RL could be the same.
heater voltage is the same.

Where Ea is at 1,200v for the 211, there is no need to
push the tube into grid current, and the 15k load V swing
will be about 530vrms for 18.7 watts with idle dissipation at 65 watts.
About 50vrms of grid drive is needed, gain is about 10.2,
with Ra at around 3k and µ = 12.
So the damping factor for RL = 15k is a healthy 5.

In practice the load value will be ever changing, since speakers vary,
and so if 15k is right nominally, then expect
RL to drop to say 7.5k and up to 50k.

The 845 µ = 5, Ra = 1.7k and with Ea = 1,200v, RL = 15k,
gain = 4.4.
The Ra line at Vg = 0V is steeper than with 211, so Vswing into the load
can be slightly more than 211, and there is no need at all to use class A2.

So the Vg1 = 136vrms, nearly 3 times the figure for the 211.
But this is quite doable using a 2A3 or 6V6 or EL84 or EL34 etc,
if the driver Ea = about 320V and the plate current is from a good
high inductance choke, so that the main load seen by the driver is
only the cap coupled grid bias R for the 845.
The power output will be slightly higher than the 211, if loading is the
same,
and damping factor will be much better because the ratio of Ra : RL is
better than the 211.


Judging by the curves of the 845, it could perhaps just as well be set up
at Ea = 900v,
Ia = 72 mA, and RL can be considerably lower at 13k, and po = 16 watts,
gain is still around 4.3, and Vg = 106vrms.

This latter recipe wouldn't suit the 211.

Its all doable.

If I was doing it, I'd choose the 845, since its quite linear,
and no need at all to go near grid current, and
its not too hard to make 150vrms drive voltage using choke loading
or IST, although the IST option demands that the tranny have wide
bandwidth.

Ongaku uses a follower to drive the grid of the 211, but why should we
slavishly
follow the maker of an expensive amp?

The HD of any driver one uses will be nearly all 2H like the
output triode, and methinks there will be considerable 2H natural
cancellation
between the driver and output stages, without having to deliberately
load the driver low to force it to make more 2H than normal and get the
most
cancellation.


Looking at the curves, the 845 can make a naturally low Ro amp
at the expense of lower gain, ie, needing over twice the drive voltage
as for the 211.
The 211 is biased rather like a 300B, but the 845 at 1,200v needs
about -180v bias.

A 300B is also a nice driver with IST, or choke + CR coupling to the grid,
but if you need say 130vrms to the 845 grid, then about 32 vrms is needed
at the 300B grid, so the input tube also needs to be gainy to make the amp
sensitive enough.
If 6SN7 with gain of 16 is used, the amp then needs 2vrms from a preamp.

A bootstrapped follower, or µ follower 6SN7 can give a gain of about
18, and far better linearity than resistance loaded 6SN7.
The HD of the input should be as low as possible because it *adds* to the
HD of the output triode.

One could always use a µ follower based on a 12AT7 bottom paralleled gain
tube
and also top follower tube, and get less than 0.4% 2H at 32 vrms output.
It would have about 3 times the HD of the 6SN7, although gain will be about
45.

Now since the nominal max power output of these tubes is about the same at
say 21 watts, and thd = about 3% if we were very lucky with no loop FB,
then at 2 watts where we do our listening we may find the thd = about 1%,
since thd falls about proportionately with output load voltage.
The higher the RL, the lower the thd, and lowest thd
is where RL is not present, and the load is a horizontal line on the plate
curves.
but thd will still be about 1% at say 600vrms at the output anode,
since the plate curves are not exactly spaced apart.

Now I realise you have never bothered with tube curves,
and just hooked things up and listened, but had you
investigated the mysteries of plate resistance curvature at different
values of grid bias voltage then you may have enjoyed a bigger sonic treat
than you have using the cut and try method coupled to
educated guestimates.

Other tubes worthy of consideration are the GM70 and 805....

Patrick Turner.














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