[Amps] Multi-tube vs Single or Dual-tube amps?

[Jim Thomson]

Date: Mon, 19 Jan 2015 19:27:31 -0800
From: Bill Turner <dezrat at outlook.com>
To: Amps group <amps at contesting.com>
Subject: Re: [Amps] Multi-tube vs Single or Dual-tube amps?

------------ ORIGINAL MESSAGE ------------(may be snipped)

On Mon, 19 Jan 2015 20:48:26 -0600, you wrote:

> ( and eight 1646s might not hit 15 meters )
>
>Don W4DNR

REPLY:

That problem of excessive output capacitance was solved long ago.
Consult any recent ARRL Handbook in the amplifiers section. Look for
info about an L-network in the plate circuit. 

Bill W6WRT

##  look at the old galaxy 2 kw PEP INPUT sweep tube amp.  It used
10 sweep tubes in parallel.   With 1 kv + 2 amps of plate current, the
plate load Z is through the floor.   Like  295 ohms !!!    With a plate load Z
that low, the required tune and load C is sky high....and the required 
tank coil for the PI net is extremely low.  That was  a 80-10m amp. 

##  You have another issue with 2 amps of plate current.  The circulating
current with a tank Q of 10 means the circulating current is aprx 20 A,
which is a HUGE amount..which will fry the band switch contacts. 

##  The tube C... X 10 ...is the last of your problems.   As you parallel tubes,
the tube C increases..so what.  At the same time the plate load Z is dropping,
which will require MORE tune and load C.....and LESS tank coil. 

##  In this  extreme case of 10 x sweep tubes in parallel, they actually used 
a unique V divider  plate load Z ... STEP UP circuit.    To INCREASE the p295 ohm
plate load Z..up to higher value, like 1200 ohms, such that you could then
build a normal PI net around the higher plate load Z. 

##  Like Bill mentions, just by installing a small amount of uh, typ .3 to .7 uh
BEFORE the main PI net, will transform the plate load Z  DOWN in value, to a 
level where a normal PI net can be built.     In fact, that is the only way you can get
tubes with huge amounts of anode to grid C..to even work on the upper bands.
My 3CX-3000A7 has a measured  24 pf  from anode to grid.... sitting on the wooden 
bench. measured with my B+K  875B  lcr meter.    Once plugged into its socket, the
stray C now increases  to 33 pf.    The MIN C on my 12-500 pf vac cap is 12 pf. 

##  Ok, now we have 33+12 = 45 pf ..with the tune cap completely UN meshed. 
This will result in high tank Q on 15-12-10m.   And that’s running 1.6  Amps of 
plate current at 5 kv under load.   Plate load Z is aprx  1838 ohms.  By inserting
.66 uh between plate block caps and main C1  tune cap, things happen.  The 33 pf
of tube C  plus the .66 uh coil  will form a step down L network.   This  will  transform
the 1838 ohm plate load Z...down to just under 1100 ohms  on 15m.    A lower plate
load Z means more  C1 + C2 is required..and less main tank coil.   Good, now I can
also use a lower loaded Q  on 15M.    In my case, since the amp only covers 160-15m,
this worked out  good.  Loaded Q on 15m  is now just 8.    Since the 15m position is re-used
on 17m, the tank Q on 17m increases  to 12. 

##  The issue gets worse if lower plate current is used, with the same high 5 kv B+ voltage. 
With only .8 A of plate current, plate load Z will increase from 1838 ohms to  3676 ohms..or
double.   For lower power applications, you are advised to reduce the B+ value..to lower
the plate load Z. 

####  A buddy just completed a YC-156  80-10m amp.   That tube has a whopping
36 pf of anode to grid C...which rises to 51 pf when bolted to the chassis.   The 
vac tune cap only has a 10 pf min C.   So out starting point is 61 pf.   The value of the
coil B4 the main pi net had to be selected very carefully.  That amp has 6 bandswitch
positions, 80-40-30-20-15-10m.   It works good on all 6 bands, but we had to use the 
GM3SEK  PI software to design it. 

##   Multiple sweep tubes or 6146s is the other extreme of the equation. 
4-6  813s is a non issue...even on 10m.  Ditto with 4 x 4-125As.  or 
even 4 x 3-500Zs.     The problem with multiple tube amps is trying to
balance the plate current between them all.  They really need to be matched.
They also need to have individual, adjustable bias controls, so that the individual
idle currents  can  be matched.  They also require individual plate current metering. 
One of the old arrl hand book 4 x sweep tube amps did just that.  It used 4 x edge
wide plate current meters, with 4 x bias pots.   Then the drive has to divide equally. 
Then u have another issue with airflow dividing equally.  The INPUT Z will  also 
drop to a lower value when multiple tubes used.  IF anything goes wrong, multiple
tube amps are harder to trbl shoot. 

## A single tube in GG is much easier to deal with.   You don’t put multiple engines under the
hood of your car, you use a bigger engine.   A 4 x tube amp requires 4 tubes, 4 x chimneys,
4 bias circuits, 4 sets of metering,  4 x anode connectors,  and 4 x sockets..and 4 x parasitic  suppressors.  

##  A  YC-156 or YC-243  or 3CX3000F7  requires no socket, no suppressor, no special anode connector.

Jim   VE7RF