Trace Elliot VA400, VR350, & Hexa Valve

[Wasnex]

Would you pm me the protection circuit? I've problems with the VA350 in this area.

Done

 

[Malletta]

When switched on, the amplifier warms up for about 20 seconds.
At this stage the amplifier remains in stand-by (green LED on).
When the warm up LED goes off, the amplifier is activated by pressing the ON (reset) switch.
The coils of the two relays are powered and the contacts pass the Vac power to the medium and high voltage rectifier bridges.
On the front panel there is also a protection activated indicator LED (red LED).
The protection checks whether the 6550 control grids are powered.
There are six 22 ohm 3 watt resistors that perform this function.
The rear switch, for the three available output powers, connects the cathodes of the 6550s in increasing pairs.
So, if you use it systematically, over time, you will have pairs of endings with different efficiency.
Therefore I recommend always leaving the amplifier with the switch for maximum power.
The 6550s will be less stressed and will last longer.
If you need less power, use another amplifier!
Many switching functions are controlled by the two CMOS 4093BEs.
The power supply for this part of the circuit is reconstructed starting from the filament voltage of 6.3Vac.
I hope this is helpful enough for everyone.

 

[cfortner]

Do you know the best BIAS current for 6550 and KT88?

My VA350 shows 730V anode voltage, which is more than normal 6550 resist. I want to install a sextett of 6550EH, Electro Harmonix does rate them at 800V. Alternatively I would install KT88.

 

[Wasnex]

Do you know the best BIAS current for 6550 and KT88?

My VA350 shows 730V anode voltage, which is more than normal 6550 resist. I want to install a sextett of 6550EH, Electro Harmonix does rate them at 800V. Alternatively I would install KT88.

AFAIK spec is 685V plate. 730V sounds too high for me. Check the primary taps on the power transformer.

As I posted in the other thread, I recommend staying away from EH 6550 and KT88, especially with circuits that push the voltage.

Factory bias is -42V, but I assume that was with graded tubes.

AFAIK the VA, Valve, and V-Type output sections are all pretty much the same circuit with minor variation. Factory bias for V-Type, V6, and V8 is 30mA pre tube. It's necessary to use bias probes on the V-Type and V6 because they tie the tube cathodes to ground. The V8 has 10 ohm cathode resistors.

Spec for

• V-Type and V6 (30mA +/- 2mA)
• V8 (300mV +/- 30mV)

At 685V this is about 58% dissipation.

I have the plate voltage of my V-Type documented at 698V. At this voltage 30mA is almost 60%.

58% at 730V is 27.8mA

 

[cfortner]

Hi Wasnex,

thanks a lot, I just wondered if the primary is wired on 220V (I'm from Germany). If so, 685V may rise to appr. 720V, here in Germany the wall power voltage is 230V (228-235V).

 

[Wasnex]

Hi Wasnex,

thanks a lot, I just wondered if the primary is wired on 220V (I'm from Germany). If so, 685V may rise to appr. 720V, here in Germany the wall power voltage is 230V (228-235V).

Yes, perhaps you can change the taps and get the plate voltage to a more reasonable range.

 

[cfortner]

Hi,

I wired the primary on 230V (was 220V) and now I have 675V anode voltage!!

And 340W at 4 ohms at 9% THD.

Thanks again!

 

[cfortner]

Last work:

There are 1 ohm 3W 1% resistors only in the kathode of the middle pair of valves. I'll install these resistors at the other valves also for controlling if BIAS current.

 

[Wasnex]

Last work:

There are 1 ohm 3W 1% resistors only in the kathode of the middle pair of valves. I'll install these resistors at the other valves also for controlling if BIAS current.

IMHO, might as well do all six cathodes, so you can check how well the tubes are sharing current.