High B+ voltage on Extended FPIII

[Ritchie]

I am getting a B+ voltage of 240v after the rectifier and 230v feeding the regulator. My line voltage is a little high at 125Vac.
I noticed the raw B+ voltage on the schematic as 180v. Is it ok to feed the regulator with 230v or is it causing it to dissipate to much wattage and the parts may not last as long due to stress. I was considering increasing my power supply dropping resistors to get a voltage closer to the 180v. My FP uses 6CG7's, what is the minimum voltage the regulator input has to see?
Also, each channel draws approx. 20 mA, is this correct?

Thanks,
Ritchie

 

[Paul Joppa]

Which output transistors do you have? Originally we used the MJE350, but I think the current kits are shipping with MJE5731As - they are larger, TO-220 types and can handle more dissipation. If you have the MJE350s, we can send you some clip-on heat sinks.

The circuit diagram is in error - my fault, I'm afraid. I drew it when I designed the parts, and I guessed wrong about the voltage. Should have modified the diagram as soon as the prototype was running. The Extended manual voltages show 220v at T10 and 230v at T7. (The voltage would be closer if the circuit were drawing 70mA per channel!)

The diagram is correct in showing 16mA to the shunt regulator; that does not include the 3mA or so through the bias resistor R2 on the C4S board, so a total of about 19mA per channel. You are very close to that.

 

[Ritchie]

Paul,

I have the MJE5731A's on the regulator board and MJE350's on the ccs.
What is the minimum B+ voltage the regulator must see?
I am thinking of trying chokes in the p.s., so I can choose a DCR that gets my B+ down a little while I'm at it.
What values do you recommend for choke and resistor if using CLCRC,  C=220u,220u,100u, hoping to avoid ringing?
I also put a regulator supply on the filaments and like what I am hearing so far? Still need more time listening though.

Thanks Paul,
Ritchie

 

[Paul Joppa]

For eXtended, you want the voltage to exceed 150v by a minimum of 10v, at the lowest operational power line voltage (usually 10% below 120v). Close enough is to add another 10%, for 176v total at 120v power line.

To avoid ringing in the power supply, the best tool is PSU Designer II - freeware downloaded from Duncan Amps. To use it you'll need to know the open-circuit voltage (174v) and the effective resistance of each winding (133 ohms). Then just model the supply - you already know the current draw (20mA). It's a bit of a learning curve but well worth it. The inductance and the resistance of any choke will both play a part, so there's no "optimum" - it's more like a variety of pretty good choices. Hammond 155J is 15 henries, 30mA, and about 1000 ohms, looks like it would suit your purposes. 154G is a bit smaller and might work, too.

In most cases, the 6.3v winding won't make enough voltage to keep a regulator from dropping out of regulation. Just FYI.

 

[Ritchie]

Thanks Paul,

I'll give that a try.
I used an 8v, 3A separate transformer for the filament regulator for the reasons you stated.

Ritchie

 

[Paul Joppa]

Cool! 8v is the winding in the Eros transformer, too!

If you stay with 12AU7s you could probably make a voltage doubler from the 6.3v winding to power a regulator - now that I think about it...  :^)

 

[Ritchie]

Thanks,
I might give that a try, I'm assuming the 12au7 would be a drop in replacement for the 6cg7 other than the heater supply, or would I have to change any circuit values? I find the separate transformer and heater regulator circuit for the 6cg7 does introduce alot of heat in the chassis. If I go to the voltage doubler w/12au7 my current would be half which would lower the heat dissipated by the regulators heat sink and would alow me to get rid of the extra transformer. Just not sure how the 12au7 will sound vs. the 6cg7?

Ritchie

 

[Ritchie]

Hey Paul,

I tried your suggestion of using the 6.3v winding on the FP transformer into a voltage doubler feeding the regulator circuit.
It worked like a charm and I have considerably less heat, I couldn't even touch the regulator heatsink before with three 6cg7's
at 1.8A. Now that I'm set up for 12v tubes I was wondering if I can use the 12BH7 in the circuit and in the B+ regulator?

Thanks,
Ritchie

 

[Paul Joppa]

The 12BH7 has a lower plate resistance and would require changes in operating conditions in order to get the right voltages. You could probably do it with increased current everywhere, but the C4S boards might then overheat.

You can look up the curves, at a bias of 1.57v (the HLMP-6000 voltage) and plate voltage of 75v. Increase the regulator current by the same ratio, and calculate the C4S dissipations - should give you an idea of what would be involved.

 

[Ritchie]

Since going to the 12v heater I have been using 12au7's and I'm very pleased with the sound.
Not worth the trouble to modify for the 12bh7 which will no doubt be different, but the au's are sounding so good in my system
I'm content for now. I think I'm hearing why you chose to design around this tube.

Thanks,
Ritchie

 

[Ritchie]

Wow! this is a crazy hobby, after living with the 12au7's for the weekend I'm switching back to the 6cg7's.
At first the 12au7's seemed more detailed and I felt I was hearing more into the recording but over time they became fatiguing compared to the 6cg7's, which to me are more natural and musical sounding. Good learning experience and no doubt results are system dependent. I'll probably outboard the heater regulator to deal with the extra heat generated by the cg7 filaments.

Ritchie