No bass with new O/T's

[Mik1066]

Yes mistake
Missed the ground connection on the OT voltage now halved

 

[Francois G]

Could you elaborate, please? I’m not understanding your statement.

 

[TheGimp]

With the ground connection missing from the output side of the transformer, there is no return path for negative feedback.

Once the ground connection is made, there is negative feedback and the output voltage drops.

 

[Mik1066]

DMM was connected to to the amplifier ground so it was referencing to this rather than the negative side of the output winding as there was no link between them.

Unlike the OT still in the chassis which read true as it had that connection

 

[Mik1066]

If the amplifier does not have Schade, Global, or Cathode negative feedback from an output transformer winding, then . . .
You can Triode Wire the Pentode or Triode Wire the Beam Power tube, to get its plate impedance, rp to be lower (1.5k for example).
1.5k rp will drive the 5k and 10 Henry primary a lot better at 40Hz, than no negative feedback and a pentode/beam power plate impedance, rp of 20k (just an example).

Thanks for the suggestion 6A3sUMMER

I rewired tonight for Triode mode.

Big improvement, better bass response and has tamed the top end as well so
now quite listenable.

I'll have a crack at the NFB over the weekend to find out what additional improvements that will make.

Only downside of wiring for Triode mode is that I dont have a pair of glowing VR valves which I used to regulate the screens.

 

[6A3sUMMER]

Just use dropping resistors to drop B+ voltage to get the VR tubes to glow (even though you will not be connecting the VR voltage to screens).

Just for an example of what you may do with an OD3:
Other VR tubes will only change the voltage, but the principle is the same.
Example B+ is 300V
0D3: 160V to ignite, 150V operating
0D3 Minimum operating current is 5 mA
300V B+ - 150V VR = 150V drop in the resistor
Use 6mA or 7mA current, it will be sure to start and keep the 0D3 glowing nicely, but not waste lots of power.
150V/0.007A = 21.4k
150V x 0.007A = 1.05 Watt
Use either a 20k 3 Watt resistor, or a 25k 3 Watt resistor in series from 300V to the 0D3.
. . . Glow, Glow, Glow

The OD3 does not have to be 'functional', no need to connect it to the screens.
Its function in your case is to look good.

Happy listening, happy looking!

 

[Mik1066]

Thanks

2.2K is all I needed as I have an OC3 in series with an OD3