Pushing Hammond 269EX power rating

[casey73]

I worked for CE Distribution for 12 years and purchased two MOD102+ kits before I retired in 2019. I built one while still working there but made some changes to it. I installed an IEC socket, used better 22 AWG PVC wire instead of the 20 gauge cloth covered wire that comes with it, put large rubber feed on the bottom and added a Hammond cage on top as well as a nice handle. I also replaced replaced the 3 way off-stby-on power toggle switch with a simple Carling SPST switch.

I recently finished the second amp kit and made significant changes to it. I did the same changes as I did with the first build, but put in a Hammond 125CSE OPT, a small Fender choke, installed an octal socket and run a 5881 in it. I also added a second output jack so it can go into 8 or 4 ohm loads. And I installed a 10W 300 ohm wire wound adjustable resistor for the output tube bias set at 226 ohms. And I replaced the Marshall style knobs with min-Davies knobs. I used the same 269EX PT that comes with the kit on both amps. The 269EX is rated 71ma and the original MOD102+ with an EL84 runs cool at 35ma. The second MOD102+ with the 5881 runs 55ma. The transformer gets warm, but it's still running under rated spec. There is a little residual hum that I attribute to a larger OPT close to the PT and to drawing more current causing the OPT to pick up some AC hum from the PT. You can't hear the hum while playing.

I have a spare 269EX power transformer and a large batch of NOS 6AQ5/6005W power tubes, all tested with several matched pairs. The math tells me I can run a pair of 6AQ5s in parallel at about 70ma. Conventional wisdom says you should have a little headroom between rated maximums and actual current draw, but I'm thinking of throwing caution to the wind and putting together dual output tube class A amp in the name of science. I've heard that Hammonds are conservatively rated and can take some abuse. I'd be interested in hearing of other experiences with Hammonds or pushing other PTs to their limits.

[LOUDthud]

Those Hammonds are tough. Don't worry about them unless they "leave a mark".

[Stevem]

Do you have the funds and the room for a separate filament transformer?

That would drop the temperature of the PT a nice amount and extend its life if your pushing its limits.

[Helmholtz]

Measure the input current, calculate VA and make sure the VA rating of the PT is not eceeded by more than maybe 5%.

[casey73]

Do you have the funds and the room for a separate filament transformer?

That would drop the temperature of the PT a nice amount and extend its life if your pushing its limits.

I have several small NOS 1.2A 6.3 transformers, however, the 269EX filament winding is rated 2.5 amps. Something to consider if I have the space depending on what size chassis I use.

[xtian]

Pull the power tubes out and listen for hum. If none, it's not the PT/OT arrangement.

[casey73]

Pull the power tubes out and listen for hum. If none, it's not the PT/OT arrangement.

Good point. I'll try that

[rangdipkin]

If you can hold your hand on it without screaming, its ok!

[casey73]

Pull the power tubes out and listen for hum. If none, it's not the PT/OT arrangement.

Good point. I'll try that

Pulled the output tube and hum disappeared so it's just the inherent hum SE class A amps are prone to, not inductive hum.

[Guy77]

Pull the power tubes out and listen for hum. If none, it's not the PT/OT arrangement.

Good point. I'll try that

Pulled the output tube and hum disappeared so it's just the inherent hum SE class A amps are prone to, not inductive hum.

That's great to hear that your transformers are quiet!
A method that might get rid of that bit of hum is to elevate your Heaters with a little bit of DC current. Its done wonders for me and others in the past.

Here is the link that explains it.
https://ampgarage.com/forum/viewtopic.p ... 5&start=30

Cheers
Guy

[casey73]

I may try that. I designed the MOD102 that AES/CE Distribution sells as a kit. The first version tapped DC off the output tube cathode applied to the filaments instead of the split 100 ohm resistors to ground on the second version. I never took the time to compare the two.

[LOUDthud]

It's simple to test the affect of heater elevation if you have a dedicated Voltage divider and cap. Simply ground the plus side of the cap with a clip lead to ground. Power the amp up and listen to the hum. While the amp is on, carefully remove one end of the clip lead and allow the Voltage to rise up. The hum should get quieter. In one test I did, it went down 9dB.

[didit]

You might find some useful insights in this thread ::

https://ampgarage.com/forum/viewtopic.php?t=31659

Best ..

[GlideOn]

I was pulling 81ma out of it for a 6550 prior to upgrading to an AnTek Toroidal. Used maybe about 6 months.

It was admittedly hard to tell if it was sagging under demand as my output transformer was also a 5k 125SE Hammond OT which is only 15w and bass wasn't the greatest.

PT never gave any gripes though. It stayed cool the touch.

[casey73]

I was pulling 81ma out of it for a 6550 prior to upgrading to an AnTek Toroidal. Used maybe about 6 months.

It was admittedly hard to tell if it was sagging under demand as my output transformer was also a 5k 125SE Hammond OT which is only 15w and bass wasn't the greatest.

PT never gave any gripes though. It stayed cool the touch.

81ma. That's impressive.