The Amp Garage

Just for info: I've just completed a AWV PA Amp Conversion. Being an Aussie Amp I wanted to keep an Oz flavour. Counter intuitively Oz flavour pentode front ends means English (Pommy) 6BR7 . Looking critically at the data sheets tells me that 6BR7 is closer to the 5879 then EF86 or other options. A ...

There are so many things wrong with that schematic. Wrong component values all over.

A quick search shows others have simply abandoned that schematic as being way wrong and just rebuilt the amp.

Here is one example of a rebuild.
http://www.blueguitar.org/new/articles/ ... kede30.pdf

Cheers,
Ian

THe B+5 power supply node supplies 3 cascaded gain stages - that makes it VERY prone to oscillation. This not the "normal" parasitic oscillation but a form of motorboating caused by feedback via the power supply node. Had that happen on a Fender a friend brought to me for a cure. In that unit I adde...

You need to consider the impedance being driven and the driving impedance. The bootstrap relies on providing a signal voltage the same (phase and voltage level) as at the anode to the top of the anode load resistor. The driver must be able to drive the required signal current into the resistor above...

If you want to go serious vintage then look at: http://www.diyaudio.com/forums/instruments-amps/204740-historic-harmonica-amplifier.html The tone control around the output tube works really well. The 6J7 triode strapped into the 6J7 pentode sounds lovely but 6J7 are a "bitch" for parasitic oscillati...

If they can match the quality of the original BRIMAR offerings that would be great. Now should any BRIMAR guys be looking in?? What is nobody making and is rapidly becoming rocking horse poo availability? We want a 6GW8/ECL86.. Stunning in both HiFi and Git Amps. Pull the triode section from your 12...

And as follow-up to gingertubes 6SQ7 suggestion: Any advantages/enhanced tone and/or qualities that using this tube would bring to a preamp bias-vary tremolo circuit over using a 12AX7/6SL7? NO -it is not handling the audio path. ONLY the control signal for the trem function. So I would have no hes...

Steve,
My experience too with U/L output stages.
I like U/L mode but only when I also disconnect any global feedback.
Otherwise too tight for a git amp.
Cheers,
Ian

Steve, Will be interested to here how that power section above works out. Would not be surprised if it gives a fair amount of SE sound. The output section with Ultralinear with cathode feedback will be very clean so with a bit of luck what will come thru' is the EL34 driving the interstage/phase spl...

The Merlin pages (see Martins link) say it all. Just a caution: the bootstrapping scheme shown on the bottom of the linked page works but has a very nasty and aggressive overdrive. Merlin actually says this in his 2nd Edition of his tube preamp book and says that he abandoned it in at least one prea...

THe suppressor tied to the bias supply instead of to the cathode is something IFAK Pete Traynor introduced. Garnet being a fellow Canadian probably copied from Pete. I know that Kevin O'Connor (another Canadian) is a big fan. He states that it does lower the anode current in a bias failure situation...

The 6SQ7 octal is very like the 6AV6 seven pin. It is a 12AX7'ish triode section with 2 detector diode plates. Doing a GA40 Les Paul Clone right now but as I am re-purposing an old Philips PA Amp, the 6SQ7 didn't physically fit so I'm substituting 1/2 a 12AX7 for the trem oscillator. This amp varies...

This twigged my interest because of the age old debate about whether the phase inverter should be perfectly balanced or should have some "deliberate" imbalance to promote some extra 2nd harmonic distortion for a "warm" tone. So I did a quick calc to see what would have given perfect PI balance I thi...

Green LEDs typically 1.8 to 2.0 Volts.
Winner is RED LED with voltage of about 1.4 Volts and lower dynamic resistance than green, use if you want the stage to run a little hotter.
Infra Red LEDS work really well too and the HIFI guys tend to use them.
Cheers,
Ian

Cathode bias resistors are typically 200 to 270 Ohms for each EL84 (6BQ5). The AC 30 used a single cathode bias resistor of 50 Ohms for all 4 EL84s bypassed with 220uF. BUT they ran the tubes really hot. You should use a resistor for each pair of output tubes or better yet a resistor on each EL84 fo...