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Vintage threads from the first ten years

Homebrew

7/6/1998 4:18 PM
Lucas Smith	Homebrew Hi, I have the chassis and output tranny of an old Bogen PA amp. The amp originally had two 7868s and a smallish power tranny. The output tranny is none too big either. I'd like to put 6L6GCs on it and build a Marshall preamp onto a brownface power amp using Svetlanas at approximately 490 plate volts. Would the output transformer live very long? Is there a huge mismatch between the impedance of the tubes and the output tranny? I think not, as all of the info on the 7868s seems to indicate that the plate impedance was very similar to 6L6GCs. Thanks.

7/6/1998 4:18 PM

Lucas Smith

Homebrew

Hi, I have the chassis and output tranny of an old Bogen PA amp. The amp originally had two 7868s and a smallish power tranny. The output tranny is none too big either. I'd like to put 6L6GCs on it and build a Marshall preamp onto a brownface power amp using Svetlanas at approximately 490 plate volts. Would the output transformer live very long? Is there a huge mismatch between the impedance of the tubes and the output tranny? I think not, as all of the info on the 7868s seems to indicate that the plate impedance was very similar to 6L6GCs. Thanks.

7/8/1998 10:17 PM
Marc	Lucas: I've got a couple of the old Bogen PA's myself that are also in line for that resurrection in to a cooler form. Did you actually measure the 490V? I don't recall what I measured on my amps, but the schematic for the CHA33 notes 445V on the plates. Close enough I suppose. I want to try using the 6AJ8/ECH81 circuit that was outlined in Performer's Audio Last year..time flies by...has anyone out there tried that circuit by Rickard Bergland? Regards, Marc

7/8/1998 10:17 PM

Marc

Lucas:

I've got a couple of the old Bogen PA's myself that are also in line for that resurrection in to a cooler form. Did you actually measure the 490V? I don't recall what I measured on my amps, but the schematic for the CHA33 notes 445V on the plates. Close enough I suppose.
I want to try using the 6AJ8/ECH81 circuit that was outlined in Performer's Audio Last year..time flies by...has anyone out there tried that circuit by Rickard Bergland?

Regards,

Marc

7/9/1998 1:29 PM
Lucas Smith	Marc, So, is the model I am dealing with the CHA-33? I have no idea, although when I bought it, the tube complement was 1-12AX7, 1-6CG7, 1-6C4, and 2-7868s. The power transformer was small, and had a voltage doubler rectification scheme. As such, I figured that the power tranny didn't have much voltage, and replaced it with a Hammond, with 350-0-350 volts, htus the 490. 445 on the plates, huh? I hope that this is a CHA-33, as these readings would give similar readings to the 6L6GCs I hope to use. What do you have in mind for your amps? Thanks, Lucas

7/9/1998 1:29 PM

Lucas Smith

Marc,
So, is the model I am dealing with the CHA-33? I have no idea, although when I bought it, the tube complement was 1-12AX7, 1-6CG7, 1-6C4, and 2-7868s. The power transformer was small, and had a voltage doubler rectification scheme. As such, I figured that the power tranny didn't have much voltage, and replaced it with a Hammond, with 350-0-350 volts, htus the 490. 445 on the plates, huh? I hope that this is a CHA-33, as these readings would give similar readings to the 6L6GCs I hope to use. What do you have in mind for your amps?
Thanks,
Lucas

7/9/1998 7:55 PM
Marc	Lucas: Actually the schematic is not the right onefor the amps I have - they are CHB-35A models. Samr tube line up as yours. The original tranny is surely not as good as a Hammond, But it should handle the 6L6 conversion with ease. There is a bit of difference in the CHA33; it's tube line up is 12AX7, 6AV6, 12AX7, 6L6GB with a 5U4 rectifer. circa 1961 I haven't really put a solid plan together, it's more like a rainy day project or when the paying jobs thin out....I figure it'll make a decent 35W amp one day. Marc

7/9/1998 7:55 PM

Marc

Lucas:
Actually the schematic is not the right onefor the amps I have - they are CHB-35A models. Samr tube line up as yours. The original tranny is surely not as good as a Hammond, But it should handle the 6L6 conversion with ease. There is a bit of difference in the CHA33; it's tube line up is 12AX7, 6AV6, 12AX7, 6L6GB with a 5U4 rectifer. circa 1961
I haven't really put a solid plan together, it's more like a rainy day project or when the paying jobs thin out....I figure it'll make a decent 35W amp one day.

Marc

7/10/1998 6:27 AM
Graywater	Lucas: While the power tranny may be inadquate, the existence of a voltage doubler is not evidence of undersize - it is the power capacity of the transformer (voltagexcurrent) is important not just the available voltage. I use two 60w Bogens with a doubler that work fine and when I'm scratch building I'd rather have a lower voltage tranny with adequate current capacity than an over-voltage one. A doubler/tripler is a simple and effective way to get the voltages up to the where I want them - reducing voltages necessitate use of dropping resistors and the resultant voltage sag under high current draw. What makes the Hammond appropriate is the desired voltage and current capacity. GW

7/10/1998 6:27 AM

Graywater

Lucas: While the power tranny may be inadquate, the existence of a voltage doubler is not evidence of undersize - it is the power capacity of the transformer (voltagexcurrent) is important not just the available voltage. I use two 60w Bogens with a doubler that work fine and when I'm scratch building I'd rather have a lower voltage tranny with adequate current capacity than an over-voltage one. A doubler/tripler is a simple and effective way to get the voltages up to the where I want them - reducing voltages necessitate use of dropping resistors and the resultant voltage sag under high current draw.

What makes the Hammond appropriate is the desired voltage and current capacity.

GW

7/10/1998 10:24 AM
Steve F	GW - Do voltage multiplier schemes supply current that is proportionally the same that could be obtained with the same transformer operated without it? Meaning, if you double the voltage, can you still get half the current, or is there some other relationship at work? I tried a voltage doubler on a preamp design, and was never able to get it to put out very much current. It sounds like you use them even in fairly high power situations - do the sizes of the caps used have anything to do with this? Thanks, Steve Fillipp

7/10/1998 10:24 AM

Steve F

GW -

Do voltage multiplier schemes supply
current that is proportionally the same
that could be obtained with the same
transformer operated without it? Meaning, if
you double the voltage, can you still get
half the current, or is there some other
relationship at work?

I tried a voltage doubler on a preamp design,
and was never able to get it to put out very
much current. It sounds like you use them even in fairly high power situations - do the sizes of the caps used have anything to do with this?

Thanks, Steve Fillipp

7/10/1998 2:30 PM
Graywater	Steve: Well, I had to think about this one and consult reference texts - unfortunately, the texts I have handy don't really address available current but instead refer to regulation and note that all of the current supplied from a multiplier comes from the capacitors. So, it stands to reason that the larger capacitors will supply more current and I will continue to presume that the current is halved as the voltage is doubled ("no free lunch"). I am confused as to why your preamp doubler didn't put out enough current. Tube preamps usually draw extremely small currents, usually a few milliamps per stage, which should be fairly easy to supply. Tell me more about your preamp design. While I usually don't design around doublers for B+ I often use them for bias supplies. As I mentioned, Bogen used doublers in many amps - I have two 60w and one 100w that seem to work extremely well (had to replace caps in the 100w, the 60s still use the originals). GW

7/10/1998 2:30 PM

Graywater

Steve: Well, I had to think about this one and consult reference texts - unfortunately, the texts I have handy don't really address available current but instead refer to regulation and note that all of the current supplied from a multiplier comes from the capacitors. So, it stands to reason that the larger capacitors will supply more current and I will continue to presume that the current is halved as the voltage is doubled ("no free lunch").

I am confused as to why your preamp doubler didn't put out enough current. Tube preamps usually draw extremely small currents, usually a few milliamps per stage, which should be fairly easy to supply. Tell me more about your preamp design.

While I usually don't design around doublers for B+ I often use them for bias supplies. As I mentioned, Bogen used doublers in many amps - I have two 60w and one 100w that seem to work extremely well (had to replace caps in the 100w, the 60s still use the originals).

GW

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