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Messages - Loudthud

#211
Schematics and Layouts / Re: National GA920P Schematic
November 08, 2015, 11:18:55 PM
Quote from: galaxiex on November 02, 2015, 07:56:42 PM
Slightly OT...
I actually have one of those Heathkit combo amps (Heath TA-16) that is mentioned in the 2nd post of that thread.
It uses that mix of NPN si and PNP ge transistors in the output stage. TA2577 NPN si, 2N2148 PNP ge.

Most of the designs from the mid to late 60's didn't pay much attention to thermal feedback. Many times the bias diodes weren't mounted on the heatsink. A number of designs just used fixed bias like the Thomas/Vox amps. The Si/Ge designs were an attempt at a complementary output stage using parts that were actually available at reasonable prices.
#212
The Newcomer's Forum / Re: Marshall 5010
November 07, 2015, 08:04:49 PM
Often times when two pieces of equipment are connected together with single ended cables, a ground loop is created. The amount of hum picked up is highly variable and depends on many factors. Since the 5010 was not intended to receive an external signal other than at the guitar input, there may be significant hum between signal ground and the mains power input safety ground.

One thing to avoid is using the chassis of the 5010 as a ground for the new jack. Connect the jack's ground to a ground at the Master Volume pot on the PCB.
#213
The Newcomer's Forum / Re: Marshall 5010
November 06, 2015, 04:37:12 PM
You will probably need to locate and cut a trace on the PCB.

Edit: Bonus feature... Your amp will now have a low level hum.
#214
Schematics and Layouts / Re: National GA920P Schematic
November 02, 2015, 06:14:12 PM
R13 is not a feedback resistor, it's part of a bootstrap current source load for Q2. It uses the output capacitor and the speaker as the parts you normally see connected to the opposite rail of Q2's emitter which, because Q2 is a PNP, would be ground. A cute circuit that eliminates one resistor and one capacitor at the expense of a little DC current in the speaker. A similar circuit to the one in a recent MEF thread. Look at the last post here, link: http://music-electronics-forum.com/t40551/

The feedback network is R7, C2, and R5.
#215
Ouch, 59MB
#216
The link above redirects me to a page that looks like software sales. WTF??? The authors seem to be the guys who started Audio Precision, makers of audio test gear.
#217
Tubes and Hybrids / Re: Tino Zottola Tube amp book
October 02, 2015, 03:00:03 PM
If there is an official AC15 Top Boost schematic, I don't think it came from JMI. Perhaps there is a re-issue or something more contemporary. Here is a link to a thread on MEF where dchange0 cleaned up an AC15 schematic.

http://music-electronics-forum.com/t39856/
#218
Preamps and Effects / Re: Anyone remember TSM Kits?
June 26, 2015, 12:20:38 PM
Are you sure that is a piezo tank? The second pic shows what seem to be coil bobbins hanging off the bottom at each end.
#219
I'd be tempted to go with the smaller unit. Guitar amps sound better with weaker transformers.
#220
Q1 needs a ground reference for it's Gate. A 1Meg to ground or just short out C4.
#221
Could it be the wacky tone controls? When I first saw this thread I was thinking it was one of the contempory Traynor amps but I re-read the first post and the amp is a YGM3 from the 70's..
#222
The VBE multiplier is the circuit around the 2N3904 transistor in the schematic I posted in reply #7. For simplicity, replace the 68 Ohm resistor with a short. The circuit multiplys the Base to Emitter Voltage of the transistor so that it acts like several diodes in series. The pot adjusts the amount of multiplication so that the power transistors turn on just enough to eliminate crossover distortion. As the power transistors heat up, they need less Voltage to conduct the same current and the VBE multiplier (hopefully) reduces the Voltage applied to the Bases.

A power transistor could be used to make it easier to mount to the heat sink. You will need an insulator. This circuit works better with a high beta transistor, the beta interacts with the resistor values to some extent. It takes a little experimentation to get the thermal compensation just right. But this is a guitar amplifier, it doesn't need to be perfect.
#223
The output transistors 2N4921 and 2N4918 are (were) silicon. The bad news is they are only rated at 30W and 3A collector current (if you get Motorola parts). A  different version, MJE4921 and MJE4918 are a little beefier at 40W. But no matter, these parts haven't been around for probably 30 years. Note that those parts have a reverse pinout compared to most TO-220s. Like many parts of that era, the beta falls off substantially as maximum collector current is approached, less than 20 at 3A.

The two RCA diodes that mount on the heatsink were kind of cool in their day because you didn't have to insulate the package from the heat sink. They are long gone too I suspect. I just use a TO-92 VBE multiplier face down with an improvised clamp.

SWTP amps weren't known for their reliability. They would usually burn up if subjected to any serious use. Today we have much better parts and know how to beef up the weak points. But this amp does not need a Zobel, it doesn't have enough gain or bandwidth to need one.


#224
A little over a year ago that preamp was dissected here.

Link: http://www.ssguitar.com/index.php?topic=3373.0 

It should work great with this power amp, but it will want to oscillate (low frequency motorboating)through the power supply. Resist the temptation to use a Voltage regulator or zener diode. You will have to experiment to get the right Voltage and enough bypass capacitance to make it behave.
#225
In this thread a builder tried mixed mode feedback but later abandoned the idea.

http://www.diyaudio.com/forums/pass-labs/248030-aca-5w-guitar-power-amp.html

The ACA has an output impedance of several Ohms when the feedback is disconnected so it sounds just fine as a guitar amp.