307 Amps 2X6 Base Amp Issue

[Shockwave]

Generally speaking, two driving six is just enough to get you in trouble. Plenty of drive to place the final stage into gain compression and clipped peaks. One of the best driver stages for 6 finals, is a single ended, tuned LC circuit, with a resonant input and output.

One of the most common problems with internal drivers, is RF feedback through the DC line. Without proper decoupling of the DC feeding the internal driver, the final stage RF will actually modulate the DC feeding the driver, across the voltage drop on the power wire feeding the amp.

Simply passing the driver DC through some ferrite and a .01 bypass cap to ground, usually solves this problem. Stubborn cases can benefit from an additional electrolytic. Caps are placed on the driver side of the ferrite.

If the final stage produces over a KW, I like to place a small metal shield, to block the drivers "view" of the final stage. That will prevent direct radiation back into the driver stage.

 

[AK321]

Generally speaking, two driving six is just enough to get you in trouble. Plenty of drive to place the final stage into gain compression and clipped peaks. One of the best driver stages for 6 finals, is a single ended, tuned LC circuit, with a resonant input and output.

One of the most common problems with internal drivers, is RF feedback through the DC line. Without proper decoupling of the DC feeding the internal driver, the final stage RF will actually modulate the DC feeding the driver, across the voltage drop on the power wire feeding the amp.

Simply passing the driver DC through some ferrite and a .01 bypass cap to ground, usually solves this problem. Stubborn cases can benefit from an additional electrolytic. Caps are placed on the driver side of the ferrite.

If the final stage produces over a KW, I like to place a small metal shield, to block the drivers "view" of the final stage. That will prevent direct radiation back into the driver stage.

I purchased an RF choke from Palomar Engineers, with a ground lug, to get rid of the RF. As far as placing a shield over the driver stage, that's a great idea. I Will get another RF choke to place being the amplifier if need be.

 

[techradioman]

Shockwave is correct and since Jim owner of 307 amps is an amp copier and NOT amp builder he really doesn't know much about RF and stuff like this nor how to isolate or at least help isolate RF problems like this. JIM just copies other good builders stuff and I've even seen dumb stuff by other builders and Nick copies that into some of his amps not even knowing what the stuff is or what it actually does.

 

[Shockwave]

Six 2SC2879s have a maximum peak drive capacity of 60 watts. Two 2SC2290s can easily hit them with more than double that amount. At these drive levels, you may not actually burn the transistor up however, you will begin to scrub gain off of the parts. In other words a transistor that starts off at 15.2 DB gain, may end up at below 10 DB after a period of time being driven like this.

That's when you can put them on the diode check scale and the base to collector junction, drops almost as much voltage as the base to emitter junction. In the absence of an HFE tester, this diode check gives some comparison of gain between parts. The more voltage dropped across the emitter junction, as compared to the collector junction, the more gain the part has. The greater the difference is between these two numbers, the "hotter" the part is.

 

[linearone]

The problem is likely the power supply. Did you replace it yet? 75a is marginal at best for that amp. Especially biased.

 

[AK321]

Shockwave is correct and since Nick owner of 307 amps is an amp copier and NOT amp builder he really doesn't know much about RF and stuff like this nor how to isolate or at least help isolate RF problems like this. Nick just copies other good builders stuff and I've even seen dumb stuff by other builders and Nick copies that into some of his amps not even knowing what the stuff is or what it actually does.

As far as I know, Jim Mc Knabb owns 307 Amps. No idea who this Nick guy is. That's not to say that you may be correct, as I have no idea what the depth of his knowledge is regarding building these amps.

Six 2SC2879s have a maximum peak drive capacity of 60 watts. Two 2SC2290s can easily hit them with more than double that amount. At these drive levels, you may not actually burn the transistor up however, you will begin to scrub gain off of the parts. In other words a transistor that starts off at 15.2 DB gain, may end up at below 10 DB after a period of time being driven like this.

That's when you can put them on the diode check scale and the base to collector junction, drops almost as much voltage as the base to emitter junction. In the absence of an HFE tester, this diode check gives some comparison of gain between parts. The more voltage dropped across the emitter junction, as compared to the collector junction, the more gain the part has. The greater the difference is between these two numbers, the "hotter" the part is.

Well, I have no idea what I can do about that. I am going to shield the driver section, and run it on the low side. I have replaced the power supply I'm using for the radio. I haven't had a chance to get audio reports, as I'm waiting for the RF choke, ground rod and 8 gusge wire to keep the RF out of my equipment.

 

[Shockwave]

As far as I know, Jim Mc Knabb owns 307 Amps. No idea who this Nick guy is. That's not to say that you may be correct, as I have no idea what the depth of his knowledge is regarding building these amps.

Well, I have no idea what I can do about that. I am going to shield the driver section, and run it on the low side. I have replaced the power supply I'm using for the radio. I haven't had a chance to get audio reports, as I'm waiting for the RF choke, ground rod and 8 gusge wire to keep the RF out of my equipment.

As long as you understand the potential for overdrive exists and you avoid that, you shouldn't have any problems.

 

[Dr_DX]

@AK321 - if you can leave your amp on high and turn down your radio, that would be better than running your amp on low side, if that is an option.

 

[AK321]

@AK321 - if you can leave your amp on high and turn down your radio, that would be better than running your amp on low side, if that is an option.

That's how I run it. 25-30W PEP max into the amp, and run the amp at 1/2 to 3/4 power on the amplifier.

 

[Shockwave]

That's how I run it. 25-30W PEP max into the amp, and run the amp at 1/2 to 3/4 power on the amplifier.

Things to keep in mind. That 30 watts is 50% of the drive that the six finals require, and it's arriving at the two drivers. I assure you the drivers have a power gain that's higher than two.

15 watts feeding the drivers should be more than enough. If it were me and I really wanted to run this radio with this amplifier, I might consider adding something like a 3db pad on the input of the amplifier. That would keep you safe.