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.

 

[AK #5]

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.

 

[AK #5]

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.

 

[AK #5]

@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.

 

[AK #5]

Well, I have replaced my antenna, that alone, eliminated the CMC problem.
All seems to be working well, except my QT 60 SSB audio sounds crunchy, and many have said that it sounds like the radio isn't getting the power it needs.
I'm using a 75A supply to power the unit, so that isn't the problem, so I'm left trying to pin down yet another issue.

 

[nomadradio]

Um, dare I ask is there fixed bias on the RF transistors? A lot of amplifiers are built for AM operators, and use the radio's carrier to turn on the transistors. Sideband requires that some DC current must flow in them when there is no drive.

Easy test for this if you have a second SSB radio handy is to monitor the amplifier's audio quality. Turn down the radio's mike gain as low as you can and still key the amplifier. If it sounds "less bad" with the mike gain turned up to full power, and sounds really bad at low drive, this indicates a bias issue.

73

 

[AK #5]

Um, dare I ask is there fixed bias on the RF transistors? A lot of amplifiers are built for AM operators, and use the radio's carrier to turn on the transistors. Sideband requires that some DC current must flow in them when there is no drive.

Easy test for this if you have a second SSB radio handy is to monitor the amplifier's audio quality. Turn down the radio's mike gain as low as you can and still key the amplifier. If it sounds "less bad" with the mike gain turned up to full power, and sounds really bad at low drive, this indicates a bias issue.

73

There is, but I honestly do not think it's working properly. I sent the QT 60 off to get checked out, and he cannot find an issue with the radio, so process of elimination dictates that there's a problem with the amplifier.
I will try out the test you suggested, but I think I already know what I'm gonna find.
Not a big deal really, since it's under warranty, but the waiting game will continue.
If I had all the money back that I spent on shipping this year, I would have almost enough to purchase a new Icom 7300.

 

[9C1Driver]

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.

Well said, this was mentioned in the first 14 posts. Hard to believe when the builder tested it with 50 watts drive it didn’t have issues right then before it was shipped. Never been a fan of 2x4 and 2x6 amplifiers.

 

[AK #5]

Well said, this was mentioned in the first 14 posts. Hard to believe when the builder tested it with 50 watts drive it didn’t have issues right then before it was shipped. Never been a fan of 2x4 and 2x6 amplifiers.

From what I understand, it's pretty simple to convert to a straight 8 pill amp.
I'm seriously considering it.