2W output transformer in Champ?

tobyk

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Hello, I was wondering if a 2W OT would work in a 5F1 Champ? I'm after some nice compression at higher levels, like the original had.
Also, what would the original have, watt-wise? Thanks!
 
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Jon Snell

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It will work but not for long. A transformer rated at 2W is not as large as the original 5W transformer.
If you want compression, either use a pedal or reduce the power available. Place a 470R 5W resistor between the 5Y3 cathode and the main smoothing capacitor.
 

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2L man

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On SE power amp the OT max bias current define how much power it can produce so find out what your OT is capable. If it is less than 40mA 6V6 must be biased colder and sound suffer. There are lower power tubes which run hotter using lower bias.
 

tobyk

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It will work but not for long. A transformer rated at 2W is not as large as the original 5W transformer.
If you want compression, either use a pedal or reduce the power available. Place a 470R 5W resistor between the 5Y3 cathode and the main smoothing capacitor.
Thank you Jon, I tried using a light bulb between the OT and the speaker, that also did the trick. But I’d rather have the compression built in. Would you say the effect of this resistor will be the same as an overwhelmed OT?
 

tobyk

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On SE power amp the OT max bias current define how much power it can produce so find out what your OT is capable. If it is less than 40mA 6V6 must be biased colder and sound suffer. There are lower power tubes which run hotter using lower bias.
Thanks, I’ll check that.
 

dan40

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You could take Jon's idea for reducing B+ voltage and power output even further by replacing the power transformer with a lower voltage unit. Running with a lower voltage PT will drop the B+ throughout the amp which creates a drop in total output power and volume. The 470 ohm resistor is much easier though and easily reversible if you ever want to go back to the original power levels.
 

tobyk

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You could take Jon's idea for reducing B+ voltage and power output even further by replacing the power transformer with a lower voltage unit. Running with a lower voltage PT will drop the B+ throughout the amp which creates a drop in total output power and volume. The 470 ohm resistor is much easier though and easily reversible if you ever want to go back to the original power levels.
Thanks. It seems when people are talking about power supply shortage, they are using the term ’sag’. Is that the same thing as compression, which I before thought happened mostly in the output transformer? Maybe topic for another thread.. ’Sag’ to me indicates a slower response when playing.
What I’d like to know: Will the effect of power supply shortage be the same as an undersized output transformer?
 
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VintageSG

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Single ended amps operating in Class A do not exhibit sag as the power draw is constant. Class AB has a variable current draw. Solid state rectifiers are essentially 'instant', so no or little sag there. It's vastly overrated, requires the amp to be cranked to quite high levels to get that starvation, and most listeners wouldn't notice anyway. It is not the same as an undersized output transformer.
OT compression begins when the core is saturated and in the immortal words of Scotty, 'She cannae tek annea moar Cap'n'
Imagine the peak and trough of your signal getting smooshed each time the core saturates, then releasing that pent up stored energy as the wave returns to the zero line. OT compression has nothing to do with the power supply. Sag is a shortfall, compression ( in the OT ) is a surfeit. Single ended OT compression, while sounding rather glorious, comes with a caveat. If the core is saturated, excess current is converted into heat. Single ended transformers have a hard enough time as it is.

Creating a shortage from the power supply will limit the current, therefore power, therefore compression/saturation, therefore heat.

You could convert the output to triode operation. That'll knock the power down from a 6V6 to ~3 Watts. A triode-strapped 6p1p-EV is around 2 Watts.
 

tobyk

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Single ended amps operating in Class A do not exhibit sag as the power draw is constant. Class AB has a variable current draw. Solid state rectifiers are essentially 'instant', so no or little sag there. It's vastly overrated, requires the amp to be cranked to quite high levels to get that starvation, and most listeners wouldn't notice anyway. It is not the same as an undersized output transformer.
OT compression begins when the core is saturated and in the immortal words of Scotty, 'She cannae tek annea moar Cap'n'
Imagine the peak and trough of your signal getting smooshed each time the core saturates, then releasing that pent up stored energy as the wave returns to the zero line. OT compression has nothing to do with the power supply. Sag is a shortfall, compression ( in the OT ) is a surfeit. Single ended OT compression, while sounding rather glorious, comes with a caveat. If the core is saturated, excess current is converted into heat. Single ended transformers have a hard enough time as it is.

Creating a shortage from the power supply will limit the current, therefore power, therefore compression/saturation, therefore heat.

You could convert the output to triode operation. That'll knock the power down from a 6V6 to ~3 Watts. A triode-strapped 6p1p-EV is around 2 Watts.
Thanks, I don’t want to knock the power down, though. I understand OT compression and power supply compression are different things, but how do they differ, if at all? – that is the question. Both generating compression. Does OT compression vs power supply compression (with a 470R resistor as mentioned above) have different tonal characteristics?
 

2L man

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On Single Ended OT the output power begins to drop on low frequencys first. You can think that when there is positive wave phase the current flow longer exceeding the bias and it has longer time to "turn" core magnetism and finally core saturate and begin limit the power.

Search HiFi and guirat SE OT datasheets and HiFi output powers are rated usually to 20Hz but guitar OT to 70Hz or 100Hz and look OT size and weight. Rule of thumb is that to half the low frequency power response the core cross section must about double! This rule about follow the PP OTs as well.
 

tobyk

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It will work but not for long. A transformer rated at 2W is not as large as the original 5W transformer.
If you want compression, either use a pedal or reduce the power available. Place a 470R 5W resistor between the 5Y3 cathode and the main smoothing capacitor.
Jon, would you say putting a 10K screen resistor on the 6V6 tube would yield a similar compression result? Or is a 470R resistor directly after the 5Y3 better for compression?
 

King Fan

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@tobyk, you seem to feel your questions haven't been answered, but I feel like 'the truth is up there' in the answers above. For example, you want to know the difference between PT sag and OT saturation? VintageSG tells us: it's the difference between empty shelves in the kitchen and an overloaded table in the dining room. Right off the bat, they answered the question: can you use a 2W OT in a 5W amp? Not safely; an overloaded PT gets hot. (Recognize the 5W OT is already overloaded in a Champ; the charming sound of a Champ above 6 is the 5W OT already getting saturated; the nasty sound above 10 is that saturation compressing all the bass frequencies into flub and flatulence.)

If it helps, Mark Baier of Victoria Amps wrote a great discussion of undersized '50s Fender OTs and saturation:

https://victoriaamp.com/understanding-your-output-transformer-part-one-by-mark-baier/

What is your sonic goal exactly? to get even more of that OT saturation? to get it earlier on the dial at lower volumes? You say you don't want to drop power, but power supply sag and OT compression may work together sonically. The 'sag resistor' or smaller PT (+/– maybe a brown box 'vintage voltage adapter') might help do that.

You might even look into adding a "Herzog box" to your Champ which lets you plug it into another amp as a super-compressor/singing fuzz. Some folks will up the filtering (especially the reservoir, say 32uF) to decrease hum when they do this.
 

tobyk

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@tobyk, you seem to feel your questions haven't been answered, but I feel like 'the truth is up there' in the answers above. For example, you want to know the difference between PT sag and OT saturation? VintageSG tells us: it's the difference between empty shelves in the kitchen and an overloaded table in the dining room. Right off the bat, they answered the question: can you use a 2W OT in a 5W amp? Not safely; an overloaded PT gets hot. (Recognize the 5W OT is already overloaded in a Champ; the charming sound of a Champ above 6 is the 5W OT already getting saturated; the nasty sound above 10 is that saturation compressing all the bass frequencies into flub and flatulence.)

If it helps, Mark Baier of Victoria Amps wrote a great discussion of undersized '50s Fender OTs and saturation:

https://victoriaamp.com/understanding-your-output-transformer-part-one-by-mark-baier/

What is your sonic goal exactly? to get even more of that OT saturation? to get it earlier on the dial at lower volumes? You say you don't want to drop power, but power supply sag and OT compression may work together sonically. The 'sag resistor' or smaller PT (+/– maybe a brown box 'vintage voltage adapter') might help do that.

You might even look into adding a "Herzog box" to your Champ which lets you plug it into another amp as a super-compressor/singing fuzz. Some folks will up the filtering (especially the reservoir, say 32uF) to decrease hum when they do this.
Thanks, it was actually that article that got me started thinking about this stuff. My sonic goals – I guess I wanna make the best sounding 8” SE amp I can. The one I have is not a Champ really, it’s my own design. All octal, 6SJ7+6SL7 preamp tubes (gain padded down). The OT right now is rated 25W, which of course us too big and gives the little speaker bass amounts it sometimes can’t handle. So I thought about trying a small OT to get the bass down a bit and introduce some compression into the mix. Also thought of experimenting with putting a pot on the pentode tube’s screen bypass to get some compression there. I guess I’ll start with buying a 5W rated, 50’s spec OT. I’ll also try the easy mod Jon suggested above.
 

2L man

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I have used Hammond 1750C (50mA max bias) and 1760C (40mA max bias) in Champ and Princeton and both have sound great. They are same size and I don't understand where comes the 10mA bias current difference? I recall I did bias both to about 45mA.

I am now building a Champ where I use 8 ohm speaker on 16 ohm output of the 1760C to get 4000 ohm OT impedance for 6V6 and B+1 is going to be about 280VDC.
 

tobyk

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I have used Hammond 1750C (50mA max bias) and 1760C (40mA max bias) in Champ and Princeton and both have sound great. They are same size and I don't understand where comes the 10mA bias current difference? I recall I did bias both to about 45mA.

I am now building a Champ where I use 8 ohm speaker on 16 ohm output of the 1760C to get 4000 ohm OT impedance for 6V6 and B+1 is going to be about 280VDC.
Thanks, I’ll check them out. They look similar to Mojotone’s BF Champ OT. (which I believe is Heyboer made)
 




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