Direct Coupled driver risks and dangers

[Rod Coleman]

> I meant your Shunt Cascode boards.

The Shunt Cascode boards will be compatible with fixed-bias, and direct-coupling to the power tube grid. They have the low-capacitance followers tightly-coupled to the output stage, on the same board. It's more than just a convenience - with recent low-C FETs, wiring reactance causes more problems for a high-gain circuit than the active components!

 

[Trajan]

Running A2 with a 300B direct coupled seems like a recipe for disaster. Pick a transmitting tube like an 809. There are tons of amazing transmitting tubes.

 

[audiowize]

It's not exactly running A2...

 

[force of 1/2]

Running A2 with a 300B direct coupled seems like a recipe for disaster. Pick a transmitting tube like an 809. There are tons of amazing transmitting tubes.

Hi Trajan,

Thanks for your response.

Could you elaborate on your statement? What would the disaster be, exactly, and what would the conditions be to cause it?

I have read knowledgeable people state that people state that the 300b doesn't work well in A2, and other knowledgeable people state that it does (to a certain point) so long as one is not asking it to draw excessive or constant grid current.

Regards,
John

 

[force of 1/2]

It's not exactly running A2...

Hi Audiowize,

Thanks for your response.

My understanding is that this circuit will only go positive on the grid for short periods during signal peaks. So, not A2 technically, but there is potential to swing into A2 territory on occasion.
Is this correct?

Thanks.

Regards,
John

 

[force of 1/2]

Is it better if the 12bh7 comes up to operating voltage before the 300b?

Sorry that this is all sinking in so slowly.

As I have the amps set up now, I have two power switches:
1--the first powers up the 300b filament and bias
2--the second powers up the B+ and 6.3v filaments.
(the switches are connected in series so that the B+ and 6.3v cannot be powered without the 300b filament and bias being powered first)

When I have put a meter on the bias voltage, it starts out at -200V and slowly drops to -96 as the 12bh7 warms up.

I could easily change this around if it is better for the 12bh7 to power up before the 300B.

 

[6A3sUMMER]

force of 1/2,

When you power the filaments first,
And then power B+, do you get a big bang or snap in your loudspeakers?
The answer to that question is a 'partial answer' to your question.

What is the exact and complete schematic of your amplifier?
That may help to get the rest of the answer to your question.

 

[Paul Joppa]

Rod Coleman is right. I believe this originally came up with the Bottlehead Paramount (no longer in production), which was a 300B amp with the option of converting to a direct-coupled 2A3 version. On startup, the driver plate went to ca. +300 volts before the 2A3 cathode was warm, and there was some arcing (I think grid-cathode arcs) which damaged the cathode's emission. This did not happen with old-stock 2A3s or the Sovtek 2A3, but it did happen with the EMLs. We modified the design with a soft-start driver plate load; I have since learned that Blencowe advises keeping the bias under +100v to prevent this arcing.

Incidentally, Jac was able to re-form the cathodes of the damaged 2A3s, but felt that their lifetime had been significantly shortened.

... I think Jac is referring to the kind of DC-coupled circuits that use indirectly-heated drivers whose anode is dc-coupled to the directly-heated triode end-stage.

In this case the DHT's grid is fed a large positive voltage, if all power supplies come up together (not unusual in recent commercial amps, with one PT and solid-state rectifiers).

EML tubes are very well-made - better than other modern DHTs. But with that comes high cost; this means that taking risks with the circuit could lead to expensive failures...