314V PT no center tap... building a 18w ... drop B+ How?

[Chubsman]

I have a PacTran PT that has no CT on the HV...which I measure 314VAC from lead to lead. This thing has no 5V tap either, so has to be SS recto...

314 X 1.4 = 440DCV unloaded.

I want to drop the B+... zener or droping 10W resistor?

[leslie]

Good voltages for 2XEl34
or 4x6v6(or two 6v6 20w)
Of course, if HT can carry enough current.

[Chubsman]

I think I'll use it in my 2XEL84 20W Lead/Bass (2061X) and put a 10W dropping resistor in the B+ line... anything I should worry about doing this...? Zener Diodes better for this?

[zaphod_phil]

Using a resistor to drop the volts will make it sag like crazy. So I would recommend zeners instead.

[mcrracer]

I too would like to know...how to add zeners to accomplish this. I have seen circuits but would like a definitive answer please?

[zaphod_phil]

Simple. Just stick one or more zener diodes in series with the B+ line, more or less anywhere between the first filter cap and the OT connection, with the stripe end(s) pointing back towards the rectifier. Check that the power dissipated by each zener (zener voltage X max current) is under half of what the zener power rating is. For large voltage drops you can add zeners in series, and their zener voltages add together, so you get a voltage drop approximately equal to the sum of their voltages. This also helps divide up the power dissipation between the zeners. Make sure the zeners are mounted with free air around them, so they don't overheat.

[Chubsman]

Would I be better building this with 6v6 tubes?

Reason I say this is:
They often run in amps with B+ in excess of 400V with no difficulty. If the PT sags more you are of course safe, but if it is above 400V, no worries for 6V6s. They'll draw similar current from the HT as EL-84s would too, so safe there also. In the heater department they draw less current than EL-84s, .45 amps vs .76 amps, so no worries there either.

What does an 18watt sound like with 6v6?

[mcrracer]

Thanks ZP!

[Chubsman]

Would 5W zeners work in a 18w amp?

[zaphod_phil]

It depends on how many volts you're dropping in the zener(s) multiplied by the maximum B+ current. Them make sure the zener is rated for at least double that figure. So for example, I recently installed an 18V zener in an amp with a pair of 6V6s, idling at 20mA, to get the voltage on the 6V6s down to around 440V. At peak the 6V6s will be taking about 30mA each max. Then allowing a further 10mA draw for the PI and preamp in the case of this amp, the power dissipation in the zener is 18V X 70mA = 1.26W, which is comfortable below 2.5W for a 5W zener.

If you use 6V6s, it may still be a good idea to use a larger resistor in the rail to drop the preamp voltages down to what they would be in a regular 18W.

[Chubsman]

Ya I decided to just go with 2XEl84 and drop the voltage to 330V

This is a no CT 314V PT... so I am guessing I need to drop around 100V !...

I am trying to figure out what my options are... it's a 20W lead/Bass (2-12ax7 and 2-el84)

Wondering if 100V is too much... perhaps a combination of zener and large resistor...

Any advice would be appreciated...

Here is what I have:
NO CT PT with 314v lead-lead (No 5V tap... so no tube recto)
20W lead and bass kit
5W zeners
10W ceramic resistors...


What would you do... in order to not smoke this baby...

thanks
chubs

[ajkimba]

Simple. Just stick one or more zener diodes in series with the B+ line, more or less anywhere between the first filter cap and the OT connection, with the stripe end(s) pointing back towards the rectifier. Check that the power dissipated by each zener (zener voltage X max current) is under half of what the zener power rating is. For large voltage drops you can add zeners in series, and their zener voltages add together, so you get a voltage drop approximately equal to the sum of their voltages. This also helps divide up the power dissipation between the zeners. Make sure the zeners are mounted with free air around them, so they don't overheat.

Wouldn't installing the zeners between the first filter and the usually red OT lead just lower the voltage to the OT and then plates of the power tubes? What about lowering the rest of the amps voltage, would you in that case install the zeners between stand-by and first filter?

I use to install zeners off the HV ctr tap but you suggested this is not a good idea?

[zaphod_phil]

Wouldn't installing the zeners between the first filter and the usually red OT lead just lower the voltage to the OT and then plates of the power tubes?

Not in an 18W (as well as many other amps), since the screen grid, PI and preamp power rails are all fed from the B+ line between the first filter cap and the OT centre-tap connection, via suitable voltage-dropping resistors. So everything gets dropped by the zener voltage.

I use to install zeners off the HV ctr tap but you suggested this is not a good idea?

Personally, I don't like it, but it's not like it's a total no no.

Ya I decided to just go with 2XEl84 and drop the voltage to 330V. I am trying to figure out what my options are... it's a 20W lead/Bass (2-12ax7 and 2-el84)

In which case you need to have the EL84s running at around 400V, not 330V, or you'll have an 18W not a 20W. Just make sure to use rugged EL84s, such as EL84Ms, 7189s or preferably 7189As. If your B+ voltage turns out to be much above 400V, you can drop the remaining volts using zeners. To limit the power dissipated in individual zeners, you can connect two or more in series - eg three 9V zeners will give you 27V drop. You don't want to use a resistor here, as the amp will get saggy, and Marshall 20W amps are all about punch.

[Chubsman]

Thanks ZP...

To be honnest what is really confusing me is the fact that I do not have a CT on the PT....

So If I use a bridge recto... will I still calculate the B+ with 1.4 X AC the PT is giving out stand alone... in this case 314V X 1.4 = 440V B+ ?

So then I would use a couple of zeners at the start of the B+ to lower around 40V... Am I making sense?

[GibsonGM]

Just to jump in here..
Is this the proper way to apply the Zener mod? I need to drop say 20V from my B+, want to make sure i'm doing this the right way. If the diagram is correct, it could help others doing the same...thanks!

[57chevy]

So If I use a bridge recto... will I still calculate the B+ with 1.4 X AC the PT is giving out stand alone... in this case 314V X 1.4 = 440V B+ ?

Yes. However, that's the UNLOADED B+ voltage level. When you plug in your tubes that voltage will drop depending on the current capacity of your PT even if you use ss diodes. It may even drop down to an acceptable level for you.

Is this the proper way to apply the Zener mod?

Yes

[zaphod_phil]

Is this the proper way to apply the Zener mod? I need to drop say 20V from my B+, want to make sure i'm doing this the right way.

It's correct.... well almost ..... except that your zener diodes are the wrong way round. They need to look like they're pointing back toward the rectifiers and filter cap. Zeners are meant to operate reverse-biased.

[GibsonGM]

AAAHHH, This way, then...I thoughts so but wasn't sure.
Thanks!

***EDIT - correct diagram inserted per 57's comment below, cathodes towards standby (+) side ***

[57chevy]

You had it right the first time.