EF80 based power amp for guitar

[kuDo]

Hello all,

After I finished my Marshall JCM 800 inspired amp last year. I came to realise that 50 watts is an insane amount of power for using in the attic. Even at one millimeter of opening up the master volume pot, It quickly becomes loud.

My wish is to build more amps in the future. To be able the use them at a sane volume I thought I’d build myself an low power “power” amp.
That way I can build a couple of different preamp designs an utilise the power amp for al of my builds.
Furthermore I already have a Mesa Boogie Triaxis and only a IC based amp to use it. So with a small tube amp that would be much nicer to use.

My first project was a Mark IIc+ preamp which was also a pre-amp only.

Long story short, I would like to have a low power amp to suit my needs.

As I am relatively new in diy amp land I started looking for existing designs.
After a good search I discovered the micro amps that are on the website of Rob Robinette. One of the designs he came up with contained two EF80 tubes.

This idea was exactly wat I was looking for. So I decided to isolate the power amp section from the examples he shared on his site and give it a try.

[kuDo]

After working on it for the last months whenever I felt like some tinkering, I ended up with a nice little power amp which is awesome!

So happy with it. It is free of noise when the volumes are wide open en a pedal is connected at the input.
The volume is more then enough for my home playing.

Attached you can see some pictures.

[kuDo]

And to go with the pictures a little impresion via a youtube clip.



I recorded a guitar via a tube screamer into my looper. The looper is connected to the input of the power amp.
The amp has a pre phase inverter volume, a switchable presence control and a post phase inverter volume control.
Hope you guys like it. It has not jet have a house to live in but it is working.

[martin manning]

Looks great! Can you post the schematic here?

[kuDo]

Looks great! Can you post the schematic here?

Attached you can see the schematic I created bassed upon the schematic of Robs Robinette’s design.

Furthermore I created my own layout using DIY layout creator.

[martin manning]

Attached you can see the schematic I created..

Thanks. Should produce 3-4W, I think. Interesting MV design. Where did that come from? Also, there is no bypass capacitor on the 300R power tube cathode resistor. Is there a reason for that?

[Yoda]

Awesome thread!

I’ve built several amps with Rob Robinette’s EF80 power section, a couple of tweeds, a super lead, and a Matchless clone and they all churn out killer tones.

[martin manning]

I’ve built several amps with Rob Robinette’s EF80 power section...

I'm curious about the MV and the lack of a cathode bypass on the power stage. On his site sometimes those features are there, sometimes not. What was your experience?

[TUBEDUDE]

Congrats on the build. Seems like a lot of volume controls on a lowish power amp. Does it get a good clean tone?

[kuDo]

Attached you can see the schematic I created..

Thanks. Should produce 3-4W, I think. Interesting MV design. Where did that come from? Also, there is no bypass capacitor on the 300R power tube cathode resistor. Is there a reason for that?

Hello Martin,

According to my calculations this amp should make about 3,5 watt. That is with a supply at the anodes of 300V and a cathode resistor of 250 ohm. I don’t think my amp will reach that since my cathode resistor is at 320 ohm and my supply turned out to be a lot lower @ 253 measured Volts dc. I picked a larger cathode resistor to lower the maximum dissipation of the amp, because that is above the rated 2,5 watt max.

The master volume I have just copied and it works fine. I left it in for the option of driving the phase inverter harder and still have the option to controle the volume.

The reason that there is no bypass cap at the cathode is that I currently want to have the max dissipation on the save side.
I have jet to connect my real preamps so I am not sure If I would like to tweak the sound of the output by adding those.

I have to admit that currently I do not posses a guitar, but I am in the process of thinking of what to purchase.

[kuDo]

Congrats on the build. Seems like a lot of volume controls on a lowish power amp. Does it get a good clean tone?

Hi Tubedude,

The two volume controls are purely there for the options they provide. The first volume control is like a stock JCM Marshal might have it and the second one gives the chance of controlling the volume wile driving the phase inverter harder for a different sound.

I have jet to try this amp clean. As mentioned in the previous post in which I answered Martin.
I can’t wait to try it in the future.
My looper pedal in the youtube clip contained a tubescreamer distorted signal from my guitar, so it wasn’t possible to listen to it clean.

I hooked this amp up to a 8ohm Celestion Classic Lead 80 Watt loaded cab I made ages ago.
I think it really sounds good!

[kuDo]

Congrats on the build. Seems like a lot of volume controls on a lowish power amp. Does it get a good clean tone?

Hi Tubedude,

The two volume controls are purely there for the options they provide. The first volume control is like a stock JCM Marshal might have it and the second one gives the chance of controlling the volume wile driving the phase inverter harder for a different sound.

I have jet to try this amp clean. As mentioned in the previous post in which I answered Martin.
I can’t wait to try it in the future.
My looper pedal in the youtube clip contained a tubescreamer distorted signal from my guitar, so it wasn’t possible to listen to it clean.

I hooked this amp up to a 8ohm Celestion Classic Lead 80 Watt loaded cab I made ages ago.
I think it really sounds good!

[kuDo]

I just calculated that the amp is about 2,4 Watt as I build it.

I used an 125B Hammond output transformer wired to reflect 22.500 ohm impedance when a speaker of 8 ohm is connected.

The supply voltage that is available is 253 Volts rectified and filtered. This is measured.

Looking at the charts I think the amp is setup really save.

[martin manning]

I just calculated that the amp is about 2,4 Watt as I build it.

I used an 125B Hammond output transformer wired to reflect 22.500 ohm impedance when a speaker of 8 ohm is connected.

The supply voltage that is available is 253 Volts rectified and filtered. This is measured.

Looking at the charts I think the amp is setup really save.

What are your plate and screen voltages? Plate 253, screen ~250?

I would definitely try adding a bypass cap to the power stage cathode resistor. It won't affect the DC operating point.

[kuDo]

What are your plate and screen voltages? Plate 253, screen ~250?

I would definitely try adding a bypass cap to the power stage cathode resistor. It won't affect the DC operating point.

The plate voltage was about 253V and the screen around 251V

What would you think I would gain? More amplitude at given frequencies?