need advice on an output tranny.

[vavlehead]

Picked a cool amp from a local builder. Bilco.
Its 2 6v6s cathode baised. About 12 watts. One channel.
The preamp section is nice: hi/lo inputs , gain and volume, t/m/b with presence.
Half power switch on the back panel.

Has a 5y3 rec tube.
I have been testing out different tubes. I stuck a set of 6L6s in there and really like the tone!! I used a copper cap gz34 in the rec tube section and it tightened up nice.

The PT is a mojo PT-290; 3 on the heaters and 120 mains. There is 3 preamp tubes so I think thats a good fit.

The output tranny however is the M25 mojo: 18 watts. 4/8/16 taps on a switch. 8k on the primary.

I'm thinking I need to replace the OT.
Something that is good for 25 watts or more with a 5K primary.

Thoughts/Advice??

thanks!!

[kg]

what makes you think you need to replace the OT?

[vavlehead]

what makes you think you need to replace the OT?

Not really sure I need to.
But using the 6L6s does not the impedance change?
And I'm not sure of how many watts the 6L6s will push. At idle I have 350VDC on the plates drawing 55ma each. But I think with cathode biased set up when signal is applied current drops.
So not sure.

[MWJB]

Lots of old, cathode biased, low voltage, tube amps ran 6L6s at primary Z's about 7-8K...Put an 8ohm speaker on the 16ohm tap to see of you prefer 4K?

Your power tubes are biased relatively cool (but not so much as to be an audible issue), so current may still have room to rise somewhat if the power supply allows, however nothing like the kind of rise you'll see from a fixed bias amp, if it sounds good I'd stick with the current OT..."if" it burns (doubtful, unless you always play it dimed) then think about replacing it.

I'd be more concerned with making sure I had a 10W cathode resistor & a 50v minimum rated bypass cap as cathode voltages will have risen with the bigger tubes.

[vavlehead]

Lots of old, cathode biased, low voltage, tube amps ran 6L6s at primary Z's about 7-8K...Put an 8ohm speaker on the 16ohm tap to see of you prefer 4K?

Your power tubes are biased relatively cool (but not so much as to be an audible issue), so current may still have room to rise somewhat if the power supply allows, however nothing like the kind of rise you'll see from a fixed bias amp, if it sounds good I'd stick with the current OT..."if" it burns (doubtful, unless you always play it dimed) then think about replacing it.

I'd be more concerned with making sure I had a 10W cathode resistor & a 50v minimum rated bypass cap as cathode voltages will have risen with the bigger tubes.

Thanks MWJB...

The amp does sound great. I will try the 16 ohm tap. I do have an 8 ohm speaker.

He's got 100uf /100v on the cathodes. Each cathode has its own resistor/ cap. But he has 2 resistors in series on each ( 4 total ) . a 190ohm + 270 ohm.
The 270 is the heavy duty Dale 6.5 watter. the 190 is a carbon comp is big... I'd say at least 2 watts, it 3/8' in diameter.

[ThermionicScott]

Not really sure I need to.
But using the 6L6s does not the impedance change?
And I'm not sure of how many watts the 6L6s will push. At idle I have 350VDC on the plates drawing 55ma each. But I think with cathode biased set up when signal is applied current drops.
So not sure.

The way to find out, of course, is to hook the amp up to a variety of dummy loads, put a 'scope on it, and see which load produces the most power. From that you can figure out the ideal primary impedance. Then listen to that and find out if it sounds good.

- Scott

[kg]

personally i'd just skip to the last step, and just run whatever sounds best!

[deluxe5D3]

I agree with kg...there is no one "magic Z" number. You can go with a slightly higher pri Z for less distortion, or go lower for more power...

[vavlehead]

update..

I wound up getting a Mercury Mags 40 watt tranny for a Vibroverb. FBFVO-40. Single 8ohm tap. Installed a SS rectifier.
I now have 375VDC on the plates and drawing 55ma on each tube.
Serious improvement in tone!!!