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

[Roger (K8RI)]

On 1/20/2015 7:31 AM, Peter Voelpel wrote:
> No to all.

There are many things we can do, and in the time of the commercial multi 
tube amps things like IM products were ignored.  Many thing worked in 
spite of ignoring good practice.  I ran grid drive with a resistive load 
to provide driving voltage to a   6C21 on 75 and 40 in 62 or 63.  It was 
only years later that I found that when I was on 40, the band was 
useless for anyone else in town. At the time, no one ever said anything 
including the ham who lived just 5 houses down on the other side of the 
street. If I was on, they'd just go to another band.   These amps may 
even sound good on frequency, but the odds are that without careful 
balancing, or selection, the IM products will be pretty bad and quite 
noticeable off frequency. Been there, done that, and have the 
testimonials (which are not complementary)  to prove it<LOL>  At the 
time I thought the amp worked quite well.  It was only after changing to 
a pair of 813s and learned a bit more about amp design that I found out 
how bad the 6C21 had been.

Fixed resistors will provide negative feedback and indirectly reduce the 
problems of mis-matched tubes, but did you have the IM checked? As I 
understand, the sweep tubes were not that great for IM to begin with, 
but Carl and others are likely to know more about that. Often, as in my 
case, no one would complain back in those days unless you got on a net 
frequency, often when the net was not on the air.  The 3.895 group had 
their protectors even back then<:-))

6KD6s and other sweep tubes are getting scarce and expensive. E-Bay  
shows NOS 6KD6s for $20 to ridiculous, but the average appeared to be 
around $30.  I need 6L6s for my guitar amp, but the audio phools have 
driven the price up to the point where repairing the Super Twin Reverb 
(195 W of music power with 6, 6L6s) is no longer worth the price of the 
tubes.

I can purchase a good, new SS concert amp for only a little more. 
However, until I get my left hand and arm working again, I'm not even 
considering that.  Right now I've only reached the point where I can 
hold the neck of the electric, let alone the acoustic.  I'm unable to 
fret the strings on the electric, so I know the acoustic would be a lost 
cause.

6146s (A & B) are pretty much the same.  Guess I shouldn't have given 
the batch of 6146As away<:-))  But they went to local hams who could use 
them.

73

Roger (K8RI)

>
> I used cathode resistors 10 Ohm with all my multi parallel sweep tubes in
> the past like 14x PL36, 6x PL519, 6x 6DK6 etc. and just a single amp meter
> and bias pot.
> Also had 2x12 PL519 push pull working.
>
> Never selected tubes either, they all where tv set pulls, except for the
> 6KD6 amp which I used mobile.
>
> 73
> Peter
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Thomson
>
> 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.
>
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