Latest project, 80 watt valve amplifier

[nigelwright7557]

Here is my latest project.

12ax7 gain stage, into tone control, another 12ax7 gain stage.
12ax7 phase splitter into class ab quad el34 output.

Bias current is currently 80ma per el34 but I intend to decrease it to nearer 40ma.

It sounds very good, very detailed sound.

An externally hosted image should be here but it was not working when we last tested it.

 

[artosalo]

Can you show the schematic ? Seeing it would make this much more interesting.
Is this for HiFi or quitar use ?

 

[nigelwright7557]

It has a switch for high and low gain for hifi or guitar use.
The cct diagram is too big to show.

 

[TonyTecson]

what is your plate voltage? 80mA per tube seems rather high..

 

[nigelwright7557]

what is your plate voltage? 80mA per tube seems rather high..

Its quite a low B+ at 340VDC.

Yes I had a look around at other designs and 80mA is high.
I have ordered some more high power cathode resistors to reduce it to around 40mA.
Wen I designed it I wasn't sure of the cathode voltage so the cathode resistor was a best guess.
The cathode voltage is about 20 volts so I need a larger cathode resistor.

 

[funk1980]

It has a switch for high and low gain for hifi or guitar use.
The cct diagram is too big to show.

Cool build! Very nice and clean PCB. Do you have more pics showing the PCB itself? But I'd expect a guitar amp to be voiced different from a hifi amp, not just a difference in gain. The EQ even more so.

 

[nigelwright7557]

The 12ax7 and el34 are often used in guitar amplifiers.
There is no feedback so the valve sound isn't watered down.

The EQ is just a standard tone control with bass and treble controls.
I have a gain control as well as volume to alter the overdriving sound.

 

[funk1980]

The 12ax7 and el34 are often used in guitar amplifiers.
There is no feedback so the valve sound isn't watered down.

The EQ is just a standard tone control with bass and treble controls.
I have a gain control as well as volume to alter the overdriving sound.

I got the guitar part with the 12AX7 and EL34 , but am a bit reserved about the hifi part, since both tube types are rarely used in hifi, EQ center points differ from guitar and feedback is actually a good thing for hifi.

 

[nigelwright7557]

I see what you mean now.
I use it with my guitar but also like the same valve sound when listening to my mp3 player through it.

I should have said listening to music not hifi lol
Its not hifi with the amount of distortion it has.
I like the warm sound of valves and the detail.

 

[TonyTecson]

Its quite a low B+ at 340VDC.

Yes I had a look around at other designs and 80mA is high.
I have ordered some more high power cathode resistors to reduce it to around 40mA.
Wen I designed it I wasn't sure of the cathode voltage so the cathode resistor was a best guess.
The cathode voltage is about 20 volts so I need a larger cathode resistor.

yes, you are running the tube about 27 watts, higher than the tube's rating of 24 watts, your tubes will not last long that way...

no wonder your B+ is low, you can bias each tube to say 70% or 16 watts or about 40+mA per tube.

good luck...

 

[Parafeed813]

I got the guitar part with the 12AX7 and EL34 , but am a bit reserved about the hifi part, since both tube types are rarely used in hifi, EQ center points differ from guitar and feedback is actually a good thing for hifi.

I wouldn't say that they are rarely used in hifi: the famous Mullard 5-20 amp uses EL34 output valves and an ECC83 driver.
However, the ECC83 is not used for audio a lot. As a driver it is too light to drive heavy loads like output triodes.

My opinion about feedback is the opposite: I use it occasionally in guitar amps and try to avoid it in audio.

 

[Parafeed813]

yes, you are running the tube about 27 watts, higher than the tube's rating of 24 watts, your tubes will not last long that way...

no wonder your B+ is low, you can bias each tube to say 70% or 16 watts or about 40+mA per tube.

good luck...

The EL34s can handle 25W on the anode and 8W on the screen. This amp burns 25,6W total per EL34, so most likely within their rating.
It is still advisable to reduce the idle current considerably to improve life expectancy. EL34s burn up quickly anyways.

 

[MadsKaizer]

Is your output transformer only rated for guitar use? It seems to be a little small for 80 Watt audio use.

 

[nigelwright7557]

Is your output transformer only rated for guitar use? It seems to be a little small for 80 Watt audio use.

The spec says it s a 100 watt Fender transformer. One of the uses was with the bassman amp amongst a few others.

I gave it thorough testing tonight. I found I was getting some oscillation.
I have fitted grid stoppers so that wasnt the problem.
Certain combinations of volume and gain controls caused oscillation.
I also tried extra decoupling on the phase splitter in case of feedback through the B+ rail but it made no difference.
I looked around a few voltages and found the phase splitter feed was only 30 volts to the grid. I changed the anode resistor to give 80 volts to the phase splitter. Now hum and oscillation are gone.