Great work Marcel - it's been fascinating following along.
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Great work Marcel - it's been fascinating following along.
Thanks guys... building this amp was quite a lot of fun, and very rewarding...
I left a lot of the technical details out as I fear most of what I could have written would have gone straight over everyone's heads like a supersonic F1-11 on 50' TFR.
Suffice to say I managed to limit the number of metal film resistors to only 4 with the rest being the more era correct Carbon based or wire-wound types. Poly and Mica caps and nil Ceramic caps. Not that anyone will be able to pick or hear the difference.
Personally I still think it looks a bit of a rats nest mess, but at the same time it looks 'just right'. There are things that had to be done (like bending leads so they can expand when warm and not fatigue and break) and there are things I wish I had done (Like arrange the components better in neat little rows) which I suppose all add to the charm of the small tube amp that it is....
Looks like a really cool amp, very much looking forward to checking out the audio demo of it so I can hear what it sounds like.
It's all a bit of a numbers game when it comes to biasing amps.... and even then the tubes dictate which numbers are correct for them...
Let me start by saying that "plate dissipation" and "power out" are both expressed in Watts (W) and are NOT the same thing. One is how much heat is generated by the tube by just being there and switched on, the other is the energy being sent to the loud speaker....
My common cathode resistor on the EL84's was originally a 120 ohm 3W resistor... which turned out to be too low in value for the HT that the power supply was capable of delivering. With 330VDC on the plates my plate dissipation calculated out to be 14.9W per tube which is 2.9W more than the maximum allowable 12W for a EL84. So I had to increase the value of the cathode resistor.
I tried a 135 ohm 6W resistor. Everything got worse. My anode voltage had also increased (due to less current draw) to 355VDC and my dissipation had bumped up to 15.1W. Not a good place for the EL84's to operate, but the logic of increasing the cathode resistor to reduce anode dissipation is a long standing proven correct one, so I summarized that the operating curves of the power supply loading and of the tubes current draw were at odds and so I needed to go with an even higher cathode resistor.
In the end after various other failed attempts I decided to try a 270 ohm 3W resistor. This time things were far better. My anode voltage had sky rocketed to 373VDC and my cathode voltage was a staggering 15.8V but that was okay as my plate dissipation had now dropped to a mere 10.1W which is well within the 12W maximum....I noted down other voltages throughout the amp and was not surprised to all had significantly risen higher.... time to plug in a guitar...
Yeah... there is a tiny bit of hum.... and a tiny bit of noise at full gain...tremolo works well, can easily hear the preamp noise being modulated by the tremolo...
turn up the guitar....
:D ... :D ... :D .... :D .... :cool:
Why did I ever bother building a 2204 clone..... Damn this thing kicks !!! ... and the higher voltages on the preamp tubes has really made them come alive !!
Looks awsome Marcel !!!
Amen to that Dedman.... :)
Awesome thread Marcel I’ve just finished reading it great amp! This level of electronics is way over my head it’s quite an education for me lol
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I'm quite the opposite Andrew.
For me the theory and the build are the easy parts, the worst part is all the math needed to calculate plate dissipation which really boils down to only a bit of multiply and divide and occasional addition and subtraction...
Latest update, and probably the last few things to say about this amp build....
My stocks of parts is rather limited, and for this project I only ordered the essentials I needed at the time. As Murphy is always lurking, I had to put in another order for parts (mostly resistors) so that I could make proper bias or supply voltage adjustments on pretty well every tube.
Another long story short, I ended up making six major changes to the original AC15 circuit that I used to build this amp...being...
1/- Drop the reverb circuits... not required at this time.
2/- Used 'Colortone' power and output transformers which were readily available and are typically found in Marshall 18W combo amps.
3/- Increase the EL84's cathode resistor from 120 ohms (125 in a Marshall) up to 180 ohms. Gives a 11.3W anode dissipation which is perfect 95% biasing.
4/- Increase the power supply dropping resistor from 2W 270 ohms to 5W 1k ohms. Returns better voltages for all the 12AX7's
5/- Increase the EL84's screen resistors from 100 ohms to 1k2 ohms. Screen voltage now 10V lower than the anode and dissipation now just under 1W.
6/- Fit a 'Hi Cut' pot. Identical to as implemented in an AC30. Not normally installed in an AC15.
Incredibly the noise floor of the amp is lower now, but the loudness of the noise it can make is truly astounding...
I'm very happy with this build, and even learnt some things in the process... A re-visit to my 2204 build seems in order.