lowering output power by reducing screen voltages

[shane]

Hi Folks

I'm about to build a low power plexi amp and was going to go the 6V6 route as many have done before.

But then I thought about using EL34's but at lower screen voltages and a higher Imp primary Output Tranny.

Looking at the EL34 curves If I use a 7k6 pri O/p tranny say a Hammond 1650F and keep the Screens at 250V and Anodes around 370V {I'll use a Hammond 370FX mains Trans). The loadline will pass just below the knee of the characteristic curves and will net me about 25W.

Only issue I see is that the sensitivity is a fair bit higher with the bias voltages only being around 15V or so. Then again no worse than a couple of EL84's!

Anyone tried this approach? Whaddya think?

Cheers
Shane

[jelle]

Marshall scaled the screens in the Slash amp. have fun!

[mhartman]

You could always build it at 50W and install powerscalling or VVR. That might give you more flexibility.

[martin manning]

...or Lar-Mar PPIMV is what I was thinking. It works very well and you still have the Plexi tone and full 50W power available.

[shane]

Well I'm a licenced power scaling installer for London Power and, yes I could scale a 50W But I dont want the extra knobs.

Also adding a Master produces the same problem.

Plus I really dont think the tone's the same, both methods are good and produce great tones in themselves with the power scaling being particularly useful.

But I want a good ol' straight forward plexi head I can crank without killing the front row or the sound engineer!

I'll have to decide if I go the well trodden 6V6 route or try the lowered screen method, just wondered if anyone had tried it.

I'll have to let you all know how it goes if I do try it.
Cheers
Shane

[jelle]

The lowered screen thing does not sound like a dimed Marshall to me. If you find a way to get that plexi tone at lower volumes and scalable..... Then I would patent it.

[martin manning]

Looking at the EL34 curves If I use a 7k6 pri O/p tranny say a Hammond 1650F and keep the Screens at 250V and Anodes around 370V {I'll use a Hammond 370FX mains Trans). The loadline will pass just below the knee of the characteristic curves and will net me about 25W.

Only issue I see is that the sensitivity is a fair bit higher with the bias voltages only being around 15V or so. Then again no worse than a couple of EL84's!

If you go this route (370V Va and 250V Vg2), I would say 6k6 for the OT Zpri, and bias at -18V.

[shane]

Hey Jelle I know what you mean! if I could do it it would be cool..better men than me have tried tho.....

Actually Martin you're right. Looking at it again the conditions with a 6K6 pri they are are better, only thing is it pushes the power up which is opposite of what I want plus I do have a couple of the higher imp trannies.

Maybe it's a dead end really I was just curious and thought it may keep more of the EL34 flavour to the tone.

I may just use a couple of JJ 6V6's and see how they go.
Since I was gonna build a standard PS and drop the screens via a regulator I could always put the reg in and try it then take it out and give the 6V6's a go see what I prefer.
Cheers
Shane

[martin manning]

I built a 50W last year with ~370V on the EL34 plates (JMP 50 PT) but otherwise 2204 (w/ circuit tweaks from TAG member Jana). It has pre-PI MV. That amp is capable of very good tones at low volumes, and is still the go-to amp in my house.

[shane]

Sounds like a very cool amp Martin. I'd be interested to know what those tweaks were!

Cheers
Shane

[cxx]

I built a 1987 (el34) out of the remnants of a crate vc3112 which was 4 el84's. The plate voltages were about 340. I did swap a different plexi output transformer later. This has about 25-30 watts and gets really good plexi tones on both channels. The low voltage power amp voltages allow the preamp to overdrive it further than usual, but clean to normal plexi overdrive is as close as you might want. I think the el34's are really necessary, especially in the clean to slightly hairy gain settings. I still have to use an attenuator when cranked.

[rogb]

I have a 2204 50w I just completed with PPIMV and an Airbrake, sounds like a really great Marshall to me

If you balance the PPIMV and Airbrake, it sounds very good.
Of course if you don't use that stuff, watch out walls - I love this amp

[martin manning]

Sounds like a very cool amp Martin. I'd be interested to know what those tweaks were!

It's here: https://tubeamparchive.com/viewtopic.ph ... highlight= There is a sound clip in the thread, which was recorded at pretty low volume... Nowhere near loud enough to keep up with a drummer.

[shane]

Nice one Martin. Good layoutdunno why we dont use it more often!
Cheers
Shane

[katopan]

Like some of the others I reckon keep the EL34s and just run it at lower B+ for lower power. Reduce the preamp supply dropping resistor to get preamp voltages back up and you won't even loose any treble.