how to drop heater voltage?

[HTH]

I've got a homebrew here that I'm tweaking, the PT was originally running a pair of EL34s and three ECC83s - I'm using a pair of 6V6s instead and the heater voltage is a bit on the high side (7.65vac).

How can I drop the voltage down without introducing any 'sag' into the amp?

Can I use a zener diode between the heater tap CT and ground? I'd only be needing a 1v zener at ???watts right?

[Tom Phillips]

You could add regular rectifier dides as shown in the attached figure.
Note: You won't see the voltage drop if all the load is removed.

[HTH]

thanks for that Tom,

can I use 1N4007s - we can't get stuff from Mouser very easily here in the Uk and I have a bag full of 1N4007s anyway.

... Ian

[Tom Phillips]

thanks for that Tom,can I use 1N4007s - we can't get stuff from Mouser very easily here in the Uk and I have a bag full of 1N4007s anyway.... Ian

Ian,
You cannot use a 1N4007 because it is only rated for 1 Amp current. I figure that you will be pulling around 3.6A so I suggested a diode rated at 6A. You can use any other rectifier diode of similar rating that you can find locally. You only need a low voltage diode. The diode I picked was convenient because it had hookup lead form factor. Above 6A they may be stud mount or push on spade leads.
Tom

[Amp Kat]

How can I drop the voltage down without introducing any 'sag' into the amp?

Can I use a zener diode between the heater tap CT and ground? I'd only be needing a 1v zener at ???watts right?

Dropping the heater voltage won't cause any sag whatsover so your cool there. You can use the 1N4007's or even as low as a 1N4004 power permitting.

If you use a zener you'll need a 6 volt zener. A 1 volt zener will only give you 1 volt. Take the total current draw from the tubes and multiply it by your 6 volts or whatever to get the wattage and then maybe tack on a hair for safety. You can use a power resitor as long as you use the same method for power but I like Toms idea better or the zener.

[Matt T.]

You can use a power resitor as long as you use the same method for power but I like Toms idea better or the zener.

The problem with using a power resistor (or two) is that the voltage drop is dependant on the current...and the current is dependant on the tube type. EL34s are 1.5A per tube and 6V6s are only .45A per tube...so for two tubes you have over 3x the heater current with EL34s (not counting preamp heaters).

[HTH]

ok, thanks for all the replies - much appreciated.

I'm busy tweaking the amp now anyway so I'll get those 1N4007s in the heater supply and give the valves an easier time (heh).

(EDIT): argh, I have two contradicting replies here - one saying 1N4007s are ok to use and another saying no way - HELP!!!

[Tom Phillips]

ok, thanks for all the replies - much appreciated.

I'm busy tweaking the amp now anyway so I'll get those 1N4007s in the heater supply and give the valves an easier time (heh).

(EDIT): argh, I have two contradicting replies here - one saying 1N4007s are ok to use and another saying no way - HELP!!!

Ian,
I think you missed something above. The 1N4007s will burn up under the current load of your heater supply.
Tom

[HTH]

Ian,
I think you missed something above. The 1N4007s will burn up under the current load of your heater supply.
Tom

ok Tom, I'll have to pick up some 6A diodes tomorrow. I should be ok as long as they're higher than the heater voltage, right? (e.g. anything above 10v)

[Tom Phillips]

ok Tom, I'll have to pick up some 6A diodes tomorrow. I should be ok as long as they're higher than the heater voltage, right? (e.g. anything above 10v)

You have around 7.65 volts RMS out of the transformer. That's 10.8V peak. So if it were me I'd use a diode rated at 20V or more. You will probably find that the supplier will stock units rated 50V or greater. For general purpose use it's just not worth the trouble to stock much lower voltages.

FYI, with the circuit I suggested, the total heater load current flows through the diodes. I figured 0.9A for each 6V6 and 0.6A for each ECC83 (0.3A per half) for a total heater current of 3.6A. If you have any other questions about the circuit configuration or methodology, just ask.

Have Fun,
Tom

[HTH]

Cheers Tom, if I have any more questions I'll be sure to ask.

(again), many thanks - always appreciated.

[HTH]

I picked up some 6A, 50v rectifier diodes today - they're brutes (REALLY big)

I'll report back on how they work out.

[Gregg]

Or you can simply use couple of resistors instead of diodes.

[HTH]

ok, got those diodes in there and they've worked a treat - spot on 6.3vac

so for my second question, who's gonna explain what's going on here - how does this mod work? is it simply the voltage drop over the diodes?

[Tom Phillips]

Ian,
Yes it's just the voltage drop of the diodes which is approximately 0.6V through a forward biased junction. There are two 0.6V drops in the circuit at all times to give the 1.2V drop you needed. There are 4 diodes making up two back-to-back pairs because we are dealing with an AC circuit. Only one diode in each pair is conducting at a given time. One of the pair for half of the sine wave and the other one conducts during the other half. There is one pair on each transformer lead in order to maintain balance with respect to the center tap.

One reason this exchange was successful was because you asked a clear question and provided accurate voltage reading information.

Glad it worked for you,
Tom

[HTH]

Tom,

thanks for the explanation, it even made sense to MY theory-challenged brain

I get a bit of a thump when I switch the amp back on standby now though - any suggestions to solve this? (cap across the switch?)