[Lengy]

I'm looking to put a separate regulated heater supply on my Cornet2, I am correct in thinking that the first 12ax7 should get 12v and does it matters if it's AC or DC?

Thanks

[hagtech]

All the tubes in a C2 are wired for 6.3V.  However, if you are doing an octal version, the choice is up to you.  And yes, the input tube should have DC heater.  Maybe second tube too.

jh

[Lengy]

I have a lab power supply with adjustable B+ and a fixed 6.3v AC output that I wanted to use, so I'm correct in thinking that the 6.3v AC will not power the heaters?  Once I get the correct heater supply can you tell me where I should input it on the board?

Thanks

JL

[poty]

I'm confused with your questions. Could you clarify?
"I'm looking to put a separate regulated heater supply on my Cornet 2."
vs.
"I have a lab power supply with adjustable B+ and a fixed 6.3v AC output that I wanted to use."
In my opinion - those 2 messages are opposite each other.
If the second statement is correct then you can use your existing power supply to power the Cornet fully (including heaters). You can connect the output of the regulated B+ in place (in the holes) of C103 (probably replacing R220 and R221 with wires) and fixed AC 6.3V to existing GRN-GRN connectors. The PCB board has its own rectifier and smoothing filter for heaters and while I'm for regulated 6.3V, the device will work as is too.

[Lengy]

Thanks,  I've got myself confused!  I've done exactly as you mentioned with the B+, but I did want to regulate the heater power supply and input DC directly from the power supply to the amp.  To get the amp working again I will do as you have mentioned, but if I wanted to input a regulated dc heater supply from a lab/external supply how would I do it making the fewest changes to the amp?

Thanks

John

[poty]

John, there are two possibilities:
1. Add a simple circuit to existing heaters power supply. Then all you should do - just connect to the same grn-grn connectors the AC voltage with enough voltage and power to ensure regulation and replace R223 with a low-drop regulator. If you know your AC 6.3V properties - you can even do that with the existing power supply.
2. Connect fully external regulated DC power supply. Then you can connect it directly to the points 2 and 4 of the D101-D104 bridge (and probably short out the R223).

[Lengy]

I've connected the supply to grn and grn, but the red light will not go green, I'm going to check the voltages, try the original transformer. 

[poty]

The fact that you have red light on means that you have heater voltage. Green light will be on if B+ is enough, so you should check not green-green, but B+ circuit.

[Lengy]

Could it be that I'm waiting for the light to go green, then switching on the B+? I was under the impression that I waited for the heaters to get to temp then switch on the B+.

[poty]

In original power supply for Cornet 2 the circuit works in the following way: after applying power three different "processes" begin at the same tiime:
1. the heating of the V100.
2. rectifying of the AC 6.3V and charging the C106+C209-211.
3. the voltage applies to the anodes of V100.
6.3V circuit begins to supply power after around 0.5 seconds and allows the red light to go on. the heating of the V100 take from 1 to 10 seconds (depending on the tube) to prepare V100 after which the V100 begins to conduct (rectify), so the delay for B+ realised in such way. The B+ allows the green light on. So if you only apply 6.3V you can't see the green light.

[Lengy]

Does your explanation of the switch on process mean that the H+ and B+ need to be switched on at the same time, as I have switched on the B+ 15sec after the H+ and the LED did not change colour.  As there is now no delay in the B+ as it is coming straight from the lab supply, if I were to switch them both on simultaneously would the B+ damage the valves as they are not at the correct temperature?

 

[Lengy]

It's up and running, I found a bad joint on r107.  It's a bit early to comment on the sound, but thanks for your help.   I'll post about the improvements in a few days.

[poty]

Does your explanation of the switch on process mean that the H+ and B+ need to be switched on at the same time...

No, it doesn't.

... as I have switched on the B+ 15sec after the H+ and the LED did not change colour.

You have already found the problem! You should know that there should be no less than 230V at the top wire of R107 to close the Q101 and open the Q100.

As there is now no delay in the B+ as it is coming straight from the lab supply, if I were to switch them both on simultaneously would the B+ damage the valves as they are not at the correct temperature?

You are right. There should be some way of delaying B+ for several seconds. I solved the problem by a simple circuit using a SS relay (while I use the original power supply schematic I nevertheless not very glad of its very small delay time and made some changes). You can use everything from 555 timer to manual switch.
As you can see - the initial delay of several seconds works well too in many DIY and assembled versions of Cornet, so it seems not very big problem for the tubes to have both B+ and H+ applied at the same time.

[Lengy]

The cornet 2 is now sounding much quieter and all the buzzing disappeared, which was the main aim.  I'm interested in anyone's thoughts about putting a regulated H+ supply and the audible benefits this would bring

[poty]

In my case regulated H+ was necessary, because I use my own PCB and tubes with different heater currents. The result was also promising to the sound! I've got noticeable drop in noise floor, especially hum, with several of my high-output MCs, overall sound also changes slightly to the better side, but not so dramatically. For myself I decided that the mod pay for itself (cost only a few $$).