5 Volt center tap on Hammond PT/New AC30/2 build

[randalp3000]

So I started a new AC30 build with some Hammond transformers and the power trans(270HX) had a center tap for the 5v rectifier so I tied it to ground with the other center taps. I wanted to check my heater voltages under load because the power trans also has taps for 115 and 125 volts so I popped in a 5ar4 and el84, hooked my meter up to the high voltage side of the 5ar4 and fired it up. The high voltage fluctuated for a minute while the 5ar4 heated up then my variable power supply made some noise, blew the fuse in my amp, and took out my meter.

Long story short, I guess you should not use the center tap on the 5v for the recto???? disconnected, shrink wrapped and all is well. Very glad I figured this out before finishing the amp. Now I can't decide between the 115 and 125 primary, the power at my house is about 121.

[M Fowler]

Yeah I learned that lesson with my first use of a Hammond PT.

When wiring the tube rectifier wrap the unused CT and only use the two 5v tap leads.

Mark

[rdjones]

So I started a new AC30 build with some Hammond transformers and the power trans(270HX) had a center tap for the 5v rectifier so I tied it to ground with the other center taps. I wanted to check my heater voltages under load because the power trans also has taps for 115 and 125 volts so I popped in a 5ar4 and el84, hooked my meter up to the high voltage side of the 5ar4 and fired it up. The high voltage fluctuated for a minute while the 5ar4 heated up then my variable power supply made some noise, blew the fuse in my amp, and took out my meter.

Long story short, I guess you should not use the center tap on the 5v for the recto???? disconnected, shrink wrapped and all is well. Very glad I figured this out before finishing the amp. Now I can't decide between the 115 and 125 primary, the power at my house is about 121.

Use of the centertapped 5V winding dates back to a time when virtually all 5V rectifiers were directly heated, the B+ was then taken from the centertap.
Connecting the centertap to ground is a direct short of the B+.
The more modern design indirectly heated cathode 5AR4 has it's cathode connected to pin 8.
Drawing B+ from pin 8 eliminates the flow of B+ current through either the filaments or the 5V winding.
The centertap on the 5V winding is a "vestigial" connection for compatibility with very old radios and amps.
It should not be used on any design that might have an indirectly heated rectifier plugged into it (virtually all tube rectified guitar amps from the '50s on).

If you don't want to make the primary voltage switchable just wire it for the higher voltage.
AC line voltages will probably continue to creep upwards, mine here is always well in excess of 125V.

rd

[randalp3000]

Thanks RD and Mark,
Is it ok to run the filaments slightly low? I was getting 6.2 volts with the 125v primary, 6.7v with the 115 tap.

[martin manning]

Interesting bit of history there rd, thanks. I think either of those filament voltages will be just fine.

[Structo]

Having the heater voltage at or a bit below 6.3 vac will increase tube longevity substantially.

"Getting the Most Out of Vacuum Tubes", Robert B. Tomer 1960

[rp]

Been there and RD and TAG save my butt on this one too. I was going to either order a new PT or gut the thing in the morning! Hammond can really suck in some ways, both the 115/125 primaries that get nothing right or their lack of mention of the 5V CT in their documentation.

http://ampgarage.com/forum/viewtopic.ph ... c1&start=0

[rdjones]

Having the heater voltage at or a bit below 6.3 vac will increase tube longevity substantially.

"Getting the Most Out of Vacuum Tubes", Robert B. Tomer 1960

Most tubes will be perfectly happy at reduced voltages.

Based on my observations on a tube tester, tubes in good condition can withstand lower filament voltage before emission drops.
This is actually how many testers will measure "life" by using lowered filament voltage.

rd

[rhinson]

for tubes with 2.5v filaments as well----old 2a__ type triodes, 300b, etc. rh

[rp]

Just note that it's not just the filaments with the 200 series Hammonds. The given HT sec rating is from the 115V tap - which in NYC gave me 7.1V on the heaters, that's just too much. The 125V tap fixed that but instead of the 290V I expected and wanted on the EL84 plate I then got 255V. That's a big difference and I was pissed and had to order resistors to drop the heaters. The 300 series on the other hand always work out right. Just be warned with this series of Hammond PTs, the OP got lucky he got an acceptable heater range with his wall voltage.

[randalp3000]

Hope to get it running tonight and will let you know what voltages I get when it's running under full load.

[randalp3000]

Well after 3 sets of output tubes and another fuse my rectifier tube is bad. It would work fine with just the preamp tubes in but with the power tubes it would make a crackling sound and draw tons of current from my power supply when the filaments heated up.

So with a SS plug in and the 125v tap I'm getting 335v on the screens and 6.7 on the filaments, bias with current tubes is 96mA per pair. Power trans is 275-0-275 230mA and wall voltage is 122 volts. Secondary voltages seem pretty low but the filaments are a bit high.

Anyho, amps sounds very nice so far and very quiet. Got lucky again. Now for that switch hole on ch1, I'm thinking series coupling caps ala brilliant channel.

Thanks for the info on the center tap and hopefully this will save someone else some time if they encounter a 5v center tap.