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Vintage threads from the first ten years

Re: Homebrew

7/12/1998 9:33 AM
Steve F	Re: Homebrew Thanks, GW. The preamp was nothing special - one 12AXZ7 in a sort of Tweed Bassman arrangement. The whole thing draws about 3 ma. The voltage doubler was built using a couple of 200 uf caps from a computer power supply. (Is there anything I need to know about those caps?) Then it was filtered with a pi filter, I don't remember the specific values of R's and C's. The PT was a 125V, 15ma. I had no problem getting the doubler to deliver the current required to run the tube, but when I put a bleeder resistor into the circuit so the thing wouldn't store juice forever, the resulting voltage drop was quite a bit larger than I would have expected, and then when the tube would warm up it would drop some more. I don't remember specific values and calculations, it was over a year ago and my brain ain't what it used to be. I seem to remember figuring the bleeder should only be drawing about 2 ma. Could be I was off in my thinking on this one - as soon as I began to have problems with it, I took it out entirely and ran the preamp off the lower voltage. That's encouraging, that those Bogen amps are using doublers even for the output power supplies. What are the values of the caps they're using, and can you tell anything about the current capacity of the power trans.? Thanks, Steve

7/12/1998 9:33 AM

Steve F

Re: Homebrew

Thanks, GW.

The preamp was nothing special - one 12AXZ7 in a sort of Tweed Bassman arrangement. The whole thing draws about 3 ma.

The voltage doubler was built using a couple of 200 uf caps from a computer power supply.
(Is there anything I need to know about those caps?) Then it was filtered with a pi filter, I don't remember the specific values of R's and C's. The PT was a 125V, 15ma.

I had no problem getting the doubler to deliver the current required to run the tube, but when I put a bleeder resistor into the circuit so the thing wouldn't store juice forever, the resulting voltage drop was quite a bit larger than I would have expected, and then when the tube would warm up it would drop some more. I don't remember specific values and calculations, it was over a year ago and my brain ain't what it used to be. I seem to remember figuring the bleeder should only be drawing about 2 ma. Could be I was off in my thinking on this one - as soon as I began to have problems with it, I took it out entirely and ran the preamp off the lower voltage.

That's encouraging, that those Bogen amps are using doublers even for the output power supplies. What are the values of the caps they're using, and can you tell anything about the current capacity of the power trans.?

Thanks, Steve

7/12/1998 6:36 PM
Graywater	Steve: Took a while to find the schematics. Lessee, the Bogen CHB100 originally used two 100uf/300v capacitors for its voltage doubler with about 555v no-load to the output transformer center tap. Don't know the current but a reasonable guess would be 350-400 ma after the doubler for a 100w amp with 7868s. The Bogen M60A (my favorite) used two 8417 outputs (great tubes if you can find them) with over 600v on the plates. The doubler used two 40uf/350v caps with about 610v no-load at the output tranny center-tap. The current after the doubler would also be around 350-400 ma. As to your preamp - hmmm, my best guess is that either you miscalculated the resistor current draw or one of the resistors was actaully under value - did you actually measure the resistance of just go by the color code/markings? Usually carbon composition resistors are within 20% of the color code (I've seen 5% resistors 20% off) but occasionally I've seen ones that are 100% high - really low variances are more unusual but not suprising. GW

7/12/1998 6:36 PM

Graywater

Steve: Took a while to find the schematics. Lessee, the Bogen CHB100 originally used two 100uf/300v capacitors for its voltage doubler with about 555v no-load to the output transformer center tap. Don't know the current but a reasonable guess would be 350-400 ma after the doubler for a 100w amp with 7868s. The Bogen M60A (my favorite) used two 8417 outputs (great tubes if you can find them) with over 600v on the plates. The doubler used two 40uf/350v caps with about 610v no-load at the output tranny center-tap. The current after the doubler would also be around 350-400 ma.

As to your preamp - hmmm, my best guess is that either you miscalculated the resistor current draw or one of the resistors was actaully under value - did you actually measure the resistance of just go by the color code/markings? Usually carbon composition resistors are within 20% of the color code (I've seen 5% resistors 20% off) but occasionally I've seen ones that are 100% high - really low variances are more unusual but not suprising.

GW

7/13/1998 6:22 AM
Dave H	Steve, 200uF is a lot of capacitor for a supply of a few mA so I would suspect the poor regulation is down to that wimpy 15mA transformer winding. What sort of doubler was it ? Was it the Cockroft and Walton diode pump type (one end of winding grounded) or the dual half wave rectifier type (one end of winding at half output voltage) ? If you only need a doubler I would use the dual half wave rec type. It has better regulation and is easier to filter as it produces ripple at 2 x line frequency. BTW my Champ 12 uses the dual half wave rec doubler for B+. The schematic is in the Tube Amp Book. Dave

7/13/1998 6:22 AM

Dave H

Steve,

200uF is a lot of capacitor for a supply of a few mA so I would suspect the poor regulation is down to that wimpy 15mA transformer winding. What sort of doubler was it ? Was it the Cockroft and Walton diode pump type (one end of winding grounded) or the dual half wave rectifier type (one end of winding at half output voltage) ? If you only need a doubler I would use the dual half wave rec type. It has better regulation and is easier to filter as it produces ripple at 2 x line frequency.

BTW my Champ 12 uses the dual half wave rec doubler for B+. The schematic is in the Tube Amp Book.

Dave

7/13/1998 5:12 PM
Steve F	Hi, Dave. Thanks for your comments. I don't know the technical description of the circuit I used - it's the one with two caps in series, the CT of the pt secondary connects to the juncture between the caps, and the ends of the primary feed the positive and grounded ends of the capacitor "stack" through diodes of the appropriate polarity. In other words, not the one shown in the Champ 12 schematic. Thanks for the tip on that one, I'd not noticed that before - I'll have to try that out. Getting back to your opening comment, are you aware of some optimum relationship between the size of caps used and the amount of current the secondary can deliver? It seems, to my muddled mind, that there should be, but I've never seen it mentioned. Thanks again, Steve

7/13/1998 5:12 PM

Steve F

Hi, Dave. Thanks for your comments.

I don't know the technical description of the
circuit I used - it's the one with two caps in series, the CT of the pt secondary connects to the juncture between the caps, and the ends of the primary feed the positive and grounded ends of the capacitor "stack" through diodes of the appropriate polarity.

In other words, not the one shown in the Champ 12 schematic. Thanks for the tip on that one, I'd not noticed that before - I'll have to try that out.

Getting back to your opening comment, are you aware of some optimum relationship between the size of caps used and the amount of current the secondary can deliver? It seems, to my muddled mind, that there should be, but I've never seen it mentioned.

Thanks again, Steve

7/14/1998 9:16 AM
Dave H	I sketched out your circuit and it looks like two half wave rect circuits stacked but each circuit only uses half of the transformer winding so you are only doubling half the available transformer voltage. You could get the same voltage and better regulation by putting a bridge rect across the whole winding. The centre tap isn’t used and only one cap is needed. If you want to split the voltage between two caps then wire the CT to the junction of the two caps. This is like your original circuit with two extra diodes. Too much hum from the doubler got through the single ended Champ 12 output stage so I had to double the caps and add a choke. It may work better with a push pull output. For guestimating capacitor value in rectifier circuits I use an equation that is technically inaccurate but it gets you in the ball park. I assume that - 1. The capacitor charges up instantly every time the rectifier turns on. ie. every 10mS for full wave rect or 20mS half wave rect on 50Hz mains. 2. The capacitor discharges through the load at constant current between charging pulses. So using Q = C V = I t The peak to peak ripple voltage is V = It/C It only works when the calculated ripple voltage is small compared to the output voltage and tends to give a value on the high side which is OK. For your pre-amp say the load current is 6mA, the rect is half wave from 60Hz mains and the cap is 50uF then the p-p ripple voltage is - V=It/C = 6mA x 16.6mS / 50E-6 = 2V 2V ripple is small compared to the DC output voltage so you are well in the ballpark. If you had used a 0.5uF cap and calculated 200V for the ripple then you would reject it, chuck in a bigger cap and recalculate. Dave

7/14/1998 9:16 AM

Dave H

I sketched out your circuit and it looks like two half wave rect circuits stacked but each circuit only uses half of the transformer winding so you are only doubling half the available transformer voltage. You could get the same voltage and better regulation by putting a bridge rect across the whole winding. The centre tap isn’t used and only one cap is needed. If you want to split the voltage between two caps then wire the CT to the junction of the two caps. This is like your original circuit with two extra diodes.

Too much hum from the doubler got through the single ended Champ 12 output stage so I had to double the caps and add a choke. It may work better with a push pull output.

For guestimating capacitor value in rectifier circuits I use an equation that is technically inaccurate but it gets you in the ball park. I assume that -

1. The capacitor charges up instantly every time the rectifier turns on. ie. every 10mS for full wave rect or 20mS half wave rect on 50Hz mains.

2. The capacitor discharges through the load at constant current between charging pulses.

So using Q = C V = I t

The peak to peak ripple voltage is V = It/C

It only works when the calculated ripple voltage is small compared to the output voltage and tends to give a value on the high side which is OK.

For your pre-amp say the load current is 6mA, the rect is half wave from 60Hz mains and the cap is 50uF then the p-p ripple voltage is -

V=It/C = 6mA x 16.6mS / 50E-6 = 2V

2V ripple is small compared to the DC output voltage so you are well in the ballpark. If you had used a 0.5uF cap and calculated 200V for the ripple then you would reject it, chuck in a bigger cap and recalculate.

Dave

7/16/1998 10:17 AM
Steve F	Dave, thanks for taking the time to show me the math. I know you post from work so available time must be limited. Jeez, what a bonehead I am. I took a look at the preamp - the PT has no center tap, it's just a 125V secondary. That's what I get for describing things from memory. (Told you my brain was full of cobwebs.) It was originally connected with two diodes of opposite polarities feeding the top and bottom of the cap stack from the same end of the secondary. The other end of the secondary connected to the juncture between the caps. The + side then went on to some more filtering. Now I remember why I wanted to do a doubler to begin with. This whole thing has a sort of bastardized history - the "first" original circuit was line connected (!) After blowing up a few diodes with ill-advised connections, (better them than me) I decided a transformer might be a good idea, and had this one that I had got from AES. Thanks again for the info. Steve

7/16/1998 10:17 AM

Steve F

Dave, thanks for taking the time to show me the math. I know you post from work so available time must be limited.

Jeez, what a bonehead I am. I took a look
at the preamp - the PT has no center tap, it's just a 125V secondary. That's what I
get for describing things from memory. (Told you my brain was full of cobwebs.)

It was originally connected with two diodes
of opposite polarities feeding the top and bottom of the cap stack from the same end of the secondary. The other end of the secondary
connected to the juncture between the caps.
The + side then went on to some more filtering.

Now I remember why I wanted to do a doubler to begin with. This whole thing has a sort of bastardized history - the "first" original circuit was line connected (!) After blowing up a few diodes with ill-advised connections,
(better them than me) I decided a transformer might be a good idea, and had this one that I had got from AES.

Thanks again for the info. Steve

7/21/1998 3:43 AM
JR	That 6AJ8/ECH81 circuit sounds interesting, using an IF mixing tube as an audio amplifier .I'd like to see the schematics too ,would you happend to have one ? or can you describe it a little ? Thank you JR

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