Marcel's scratch build AC15 based clone

[DrNomis_44]

I've not encountered many Genelec Gold Lion's, but the few I have they seem to be quite good in terms of gain and microphonics. Never tried the cheap Chinese tubes though I do have four of them in my stock. I do like the Telefunken tubes though one that I have could almost pretend to be a Shure SM58, and in the dim dark past the old Groove Tube was a popular choice though these days they seem to be out of favour.

These days my typical purchases for myself are JJ's. I figure if they are good enough for the likes of Angus Young then they will do me okay, and they are usually quite well priced.

Worst faults I typically encounter with tubes is low emission and/or low gain... and then heater flash...

FYI - Heater flash - A tube reliability issue - If there is this construction fault with the tubes heaters and when power is first applied to that tube's heaters there can be a bright flash of light from near the base of the tube. Happening only at power on, and for only a few seconds, the exposed section of heater at the base of the tube will light up like a light globe until the remainder of the heater that is surrounded by the cathode comes up to temperature. This 'flash' does not affect the operation of the tube as once warmed up the tube will work as normal, however it is a weak point in the heater that becomes more fragile with age. With repeated power on/off cycles the heater is likely to break at this increasingly fragile flash point and thus usually at a moment of least convenience rendering of the tube to a useless failed/dead state with nil 'Red glowy bits' in that tube and nil noises from the connected speakers for your listening pleasure.

Useless fact - I have an infrared temperature meter which I have used to measure the temperature of all sorts of things including various amplifier parts during operation.... On an average 30C day here at my tropical location and after 1hr of moderate use of this amp build I measured the following - Power transformer @44C, Output transformer @ 42C, ECC83/12AX7 RFI protective aluminium shields @67C, EL84 plates (cathode biased to 11.7W dissipation) @157C.

Those temperature readings look pretty normal to me, especially the 157C on the EL84 valves, I seem to remember reading somewhere that it's normal to bias output valves so that they're dissipating about 70% of their maximum plate dissipation.

[Marcel]

For cathode biased power tube up to 100% is permissible, though 95% is preferred. Some amp designs with certain tube brands and/or tube tolerances can default to almost 120% and is one big reason why trained techs should do power tube changes.

For fixed bias amps the bias is manually adjustable to taste. Typically 65% being cool biased, 70% being moderate bias, and 75% being hot biased. There are many videos on YouTube where they say "set your EL34's to 30mA" or "set the 6L6 to 50mA" which in certain amps may work as a generic setting, however the gain of the specific tubes being used and variances between chassis may result in a way too cool or way too hot a setting. For my 2204 build with EL34's and to get 70% dissipation the setting worked out to be 44mA, yet in my JVM210c which also uses EL34's the same 70% dissipation setting came in at 35mA, so tube type alone does not dictate the idle current through the power tubes.

Either way a cooler setting adds to the longevity of the tube. Hotter settings burn through the tubes quicker.... case in point - Angus Young likes his amps biased as hot as possible at 75 to 80% or more... His power tubes on all his dozen or so Marshall heads get changed after 3 gigs, that's if they last that long...

[Marcel]

Some more tube theory Gold...

1523 pages of heavy going tube and basic electronics theory from circa 1954...

https://ia601602.us.archive.org/18/i...dbook_1954.pdf

[Marcel]

Success at last... I have got my DAW setup working and have made some files that I can upload to soundcloud.

I do like my crunch, and Seymour Duncan rails pick ups in this partscaster Alder Strat build into the hi gain input of the amp do make the first stage JJ 12AX7 work pretty hard. Nil pedals and "All the gain !!!" is just the ticket to hide my horrible playing really well (I am more a tech than a musician after all).

The vented cab is loaded with 16 ohm versions of a V30 and a G12H75 wired in parallel to give an 8 ohm load to the amp. The cab itself is made from 17mm form ply from the big Green shed and is a HiFi ported bass reflex design modified for guitar use which is the reason for the massive bass roll off on the amp. One SM57 into the Behringer USB interface to the i5 laptop running Reaper 5.6 ...

I intend to play with microphone options latter but just getting one track down of something was the first objective...

https://soundcloud.com/user-26130539...tscaster-build

[dave.king1]

I can see it on SC but it's greyed out and won't play, have you got the settings right

[Marcel]

I believe so Dave. It plays fine for me. What settings do I need to set right?

[DrNomis_44]

I checked out the soundcloud demo and it played fine for me.

[DrNomis_44]

I believe so Dave. It plays fine for me. What settings do I need to set right?

You could check to see if it is set to grant public access.

[Marcel]

You could check to see if it is set to grant public access.

Don't know how or what it was set to before but SC is now public everything...
Done...

[dave.king1]

It's now visible Marcel but it's late and the www seems to be slow ( one handed typists late at night bogging it down I guess ) will check it out in the morning and also do a screen shot of the settings I use which do have a lot of limitations for copywrite purposes which obviously don't apply here