GM70 SE bias

[claudiomas]

on the table I have a 6E5P-E in triode with a 2A3 in DRD mode with choke 200H 10uF transformer 10K: 10K bifiliar transformer
basically a DRD in a parafeed way to attack a GM70.
It's a quick and dirty montage
I study the phenomenon that the last watts degrade due to the grid which becomes increasingly negative, especially at high frequencies
in practice at a high volume 18.5V on 8ohm at 24Khz the milliampeter of the GM70 drops and the distortion is high
to solve this phenomena a small xenon valve to the grid of the GM70, in this case a SG5B-V for a bias of 73V for an anodic supply of 900V
and here the maximum power threshold reaches 20Volts on 8 ohms
always at 24Khz without distortion of the sinusoid
a little push in A2 doesn't hurt
the current passes from 100 to 120mA and the game is done

 

[huggygood]

there's a way to do some cool stuff with your SG5B-V!
would you have an equivalent model that could regulate in 250 / 300V, always 5 to 10mA?

 

[claudiomas]

two in series

 

[huggygood]

I love your simple and pragmatic answers.
ciao e grazie Claudio

 

[huggygood]

on the other hand, does it pose a noise problem to put two in series?

 

[claudiomas]

no noise on this application, for psu reference 0.1uF // each glow tube
shorts video with and without xenon glow tube:
Microsoft OneDrive - Access files anywhere. Create docs with free Office Online.

 

[huggygood]

I like the light produced by these little tubes,
I will try to buy some to make screen control tests

 

[claudiomas]

Bouyer ST75 with OB2
and servo bias

 

[huggygood]

thanks .
do you still have your two Bouyer for sale ?
would you be interested in an exchange ?

 

[claudiomas]

my Bouyer are gone, output transformers are for PA not HiFi !
and splitter transformer to be checked for quality...
power supply its very good with primary multi taps
and octal sockets first class triple fourch
for the GM70 bias trick i was happy to quick,
in lower frequency xenon tubes ignition limit max power
so to follow better way
i go try cathode follower for little A2 mode

 

[claudiomas]

these test are with 126C, 10mA IT
without the xenon tube 21Watts 1% THD 1Khz
very good linearity
of course the best way its very powerfull IT like Sowter and 300B driver

 

[huggygood]

Okay, don't worry .
as I told you, I'm learning and these amps are good for learning the operation of an inter-stage transformer and then it's tanks.
I would later find a pair of ST30 for fun.
I will continue to follow with interest your work with these regulators.

 

[claudiomas]

i now four kinds of IT and grid choke
1)gapped IT for SE to SE or SE to PP
differents ratio alloweds
windings series or parallel
2) bifiliar gapped 1:1 no phase inversion max 250/300V between primari and secondary, low current
3) no gap for balanced drive PP to PP
4) no gap SE to SE or line or PP
no DC capacitor drive
5) grid choke no DC can do elevation attenuation or splitter
only one winding whit taps
come to my workshop i can show you how they work
experts can develop i'm just user no winder

 

[deafbykhorns]

these test are with 126C, 10mA IT
without the xenon tube 21Watts 1% THD 1Khz
very good linearity
of course the best way its very powerfull IT like Sowter and 300B driver

Might of mentioned this once or twice but im using Monolith IT 6e5p-300b-gm70
Easily exceeds A2 with low distortion figures and great sound

 

[claudiomas]

LL2756 much cheaper of Monolith and perfect
but i try this time with 35 euros IT
to follow

 

[claudiomas]

so schematics for low cost amplifier
DRD choke loaded driving bifiliar IT
845/GM70 etc
the 126C its perfectly linear in this way can drive GM70 A2
its cheap way but very nice and last watts A2
bias supply should be low source avoid doublers etc
everyone now how to do a 2A3 DRD, i like 6E5P-E
in triode its perfect
choke 126C choke 157G IT 126C
PIO 20UF rated 500V

 

[claudiomas]

sorry bias total resistor shoul be low,
not 1Kohm error in schematics
but 1Kohm 3W pot +2K7 6W in series
if bias supply 100V

 

[claudiomas]

symple mosfet shunt bias for A2
just need to find resistors value for limited thereshold
but work perfect like this, its a quick and dirty test

 

[deafbykhorns]

The hammond 126c will work fine too, i did the same thing prior to monolith.
I have done cap, dowdy choke, hammond it, silk it and finally monolith.
The hammond was limited due to max current but running the 300b(JJ2A3) at 30ma had some low distortion figures and sounded better than i expected.

 

[claudiomas]

yes defbykorn bifiliar IT its cheap to build primari and secondary are wind toghether same time but between windings voltage DC should not be more of 250/300V and current should be low
so this way to drive GM70 , post 16, give very good result only small GM70 grid bias current
my lasts GM70 SE i use Sowter 8424S and Lundhal medium size IT
neverthless need very low source negative psu if fixed bias
833A need muscle bias
A Ciuffoli use resistors for 845/GM70 but way not shunt regulation
any idea welcome