36 to 18 switch--possible?

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coug64
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36 to 18 switch--possible?

Post by coug64 »

I have a friend who has a custom built, (4) EL84 output and he wants to easily be able to drop it to 18 watts. Very nice build, the tone blew me away. I'm an old Fender knock-off builder (78 years old) and this amp has got me thinking more about sophistication in amp building. But, for now, is it possible to easily cut-out two of those output tubes without changing the impedance of the output transformer?
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crgfrench
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Re: 36 to 18 switch--possible?

Post by crgfrench »

Yes. You can pull 2 of the EL84s. Or you can install a switch, see my thread "36W turretboard layout", there is a half power switch in the schem and layout diagram you could install.
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Re: 36 to 18 switch--possible?

Post by Daviedawg »

I have an Orange TH30 in my care just now, with 4xEL84s. It has two paired cathode bypass circuits to ground. It switches to half power by switching in a 100k/3w resistor on one pair between the bypass and ground. Other poles on the switch also switch the OT output impedance from 16 to 8 ohms.

This works quietly and any change in the tone is really hard to detect.
The switch is a three pole, two for the speaker pair and one for the cathode ground.

Dd
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Re: 36 to 18 switch--possible?

Post by mlannoo »

crgfrench wrote:
Fri 10/02/20 8:30 pm
Yes. You can pull 2 of the EL84s. Or you can install a switch, see my thread "36W turretboard layout", there is a half power switch in the schem and layout diagram you could install.
I built my 36W TMB back in 2007. A few years later I wanted to add a half power switch using the method you describe above. I took cues from Ceriatone at the time and ended up installing it successfully (I think). Problem is, I really don't hear a difference between the two. I was expecting it to be a bit more significant volume-wise. I wonder now if perhaps my "mod" does not work?
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TriodeLuvr
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Re: 36 to 18 switch--possible?

Post by TriodeLuvr »

Half-power is only a 3dB change. That's considered to be just perceptible by most people. For a substantial difference, it would need to be attenuated by 10dB (1/10) or more.

If you need less speaker volume, my suggestion is to build an outboard resistive divider. It's more effective and a lot less complex than modifying the innards of the amp.

Jack
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Re: 36 to 18 switch--possible?

Post by Bieworm »

I have a 1/4 power switch on my amp. It's pretty obvious from 20 to 5W. But the downside is loss of clarity and loss of balls..
It's the method with the load resistors on the speaker output. I wouldn't do it again and never use it
IMHO the exciting sensation you experience from an amp is the movement of the baffle. How can you expect the same when the baffle isn't moving that way? For great sounds you have to move some air... the alternative is only a compromise...not the real deal, no way
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Re: 36 to 18 switch--possible?

Post by TriodeLuvr »

Bieworm wrote:
Thu 02/04/21 1:50 pm
I have a 1/4 power switch on my amp. It's pretty obvious from 20 to 5W. But the downside is loss of clarity and loss of balls..
It's the method with the load resistors on the speaker output. I wouldn't do it again and never use it
IMHO the exciting sensation you experience from an amp is the movement of the baffle. How can you expect the same when the baffle isn't moving that way? For great sounds you have to move some air... the alternative is only a compromise...not the real deal, no way
Clarity (transient response) degrades because output system Q and damping factor are reduced by the resistive network. There's no perfect solution to this situation. You have to either accept the compromises or build a smaller amplifier. I'm already thinking I'll want a 4W or 5W amp when my current project is finished. Maybe a SE 6V6 or something similar spliced to the Marshall high-gain preamp.

Jack
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Re: 36 to 18 switch--possible?

Post by Bieworm »

Same here. But rather like 1 watt or something.. ofcourse with trem and verb :D
But first I'm going to do some guitar builds I've been thinking of. Right now I'm hotrodding my mustang build. Also resoldering the whole thing. I've learned a lot with the recent amp building .. so I will translate those into my guitars..
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Re: 36 to 18 switch--possible?

Post by JMPGuitars »

Bieworm wrote:
Thu 02/04/21 3:21 pm
Same here. But rather like 1 watt or something.. ofcourse with trem and verb :D
But first I'm going to do some guitar builds I've been thinking of. Right now I'm hotrodding my mustang build. Also resoldering the whole thing. I've learned a lot with the recent amp building .. so I will translate those into my guitars..
I'm likely to eventually make a low watt SE power section version that can be used on the Superlite TMB or Trem TMB.
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Re: 36 to 18 switch--possible?

Post by TriodeLuvr »

Bieworm wrote:
Thu 02/04/21 3:21 pm
Same here. But rather like 1 watt or something.. ofcourse with trem and verb :D
I remember seeing a now-gone Website using a 6BL7 or 6BX7 for this. Not sure if it was push-pull or SE, but I think one of these tubes would be a good match for a 5K OPT with both sections in parallel. Output power would probably be in the range of 1 or 2 watts. Then there's the 1626 for output below 1W. If you want a SE pentode output stage, the little 5686 Noval 9-pin is the ticket. About 8W plate dissipation and only 350mA heater current. It's a decent match for any OPT from roughly 5K to 10K.

Jack
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Re: 36 to 18 switch--possible?

Post by crgfrench »

The perfect SE output section for an 18 watter would be wire the EL84s in parallel instead of push-pull and use an Allen TO-15 output transformer.
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Re: 36 to 18 switch--possible?

Post by Bieworm »

Ok the mustang is hotrodded now. I like it better with the humbucker at the bridge !!! It's cheap Artec hot rails alnico 5 pickup.. but it sounds really amazing. I'm not a fan of duncans or dimarzios with those ceramic magnets. I like the weaker vintage vibe of alnico
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Re: 36 to 18 switch--possible?

Post by TriodeLuvr »

My second guitar was a sunburst Jaguar, a '67 I think. Bought it secondhand when I was 19. Really a beautiful instrument, never should have let it go.

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Re: 36 to 18 switch--possible?

Post by Bieworm »

TriodeLuvr wrote:
Sat 02/06/21 12:19 pm
My second guitar was a sunburst Jaguar, a '67 I think. Bought it secondhand when I was 19. Really a beautiful instrument, never should have let it go.

Jack
You betcha! The fiesta red jag is an original 1962.. first year of production with brazilian rosewood and clay dots
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Re: 36 to 18 switch--possible?

Post by cstenger5 »

JMPGuitars wrote:
Thu 02/04/21 4:27 pm
Bieworm wrote:
Thu 02/04/21 3:21 pm
Same here. But rather like 1 watt or something.. ofcourse with trem and verb :D
But first I'm going to do some guitar builds I've been thinking of. Right now I'm hotrodding my mustang build. Also resoldering the whole thing. I've learned a lot with the recent amp building .. so I will translate those into my guitars..
I'm likely to eventually make a low watt SE power section version that can be used on the Superlite TMB or Trem TMB.
Is there a way to simulate the LTP gain in a SE amp, without the PI? I have to imagine there is, using a single triode and some magical combination of caps and resistors. A SE el84-based power section is something I'd love to have. Did you end up working one up?
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