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

[Roger (K8RI)]

On 1/19/2015 5:25 PM, Peter Voelpel wrote:

The design needs to take into consideration the characteristics of the 
tube(s) and allow for such,  In the case of the Emtron DX-2so ans DX-3sp 
the bias is set for each tube individually.  The closer the tubes are 
matched, the lower the IM products.  That something most of the multi 
tube amps using sweep tube did not address. Many of those old designes 
had pretty poor IM products

So, you have the question of how close to each other are the basic tubes 
matched and will adjusting the bias individually be good enough?    The 
4CX1500B and the Chinese FU728F have excellent IM characteristics and 
differences can be taken care of with the individual bias adjustments. 
BUT they are high gain Tetrodes and require little drive, which means 
dialing back the output of the exciter.  If this is done in the exciter 
using its ALC each transmission, or even each transmission "may" have a 
leading edge, full power pulse while the ALC is being developed to limit 
the power. These can lead to the pulses over driving the amp creating 
splatter. Already mentioned are the low impedance of the input and out 
put circuits when using multiple tubes and matching multiple tubes along 
with the additional power to light the fire.  Lower voltage, but higher 
current takes a slightly larger transformer.

Another downside is the additional area required for the tubes as well 
as enough and equal cooling to each tube.
I would no longer call 6146s cheap if purchasing new.<:-))

Multiple tubes "usually" means smaller tubes, which means a more compact 
amp, but the power supply needs to be capable of delivering the desired 
voltage and current, but the PS can be remote.  Then its size may be a 
non issue.

The problem with legal limit (and beyond) single tube amps is that most 
of the tubes are BIG!  Too bad the 3CX3000 series can not be mounted 
horizontally.  Allowing for the tube height plus clearance above and 
below makes for a cabinet of substantial size.

73

Roger (K8RI)

> I guess a 813 or 4-125 is cheaper
>
> 73
> Peter
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of qrv at kd4e.com
> Sent: Montag, 19. Januar 2015 22:28
> To: amps at contesting.com
> Subject: [Amps] Multi-tube vs Single or Dual-tube amps?
>
> >From time to time a multi-tube amp shows up, such designs were
> common with sweep tubes, I think some with 4x150's, and 6146's.
>
> Other than the added sockets and wiring complexity are there
> additional down-sides to such a design?
>
> e.g. Eight 6146's should generate 600w 160-10m and are cheap.
>
> Just pondering ...
>
>


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