Bogen RP40A transformers....what to build with them? - Page 2

Eli Duttman · 21st April 2012, 03:02 AM

A choke decoupling g2 B+ from the main rail is pretty good. Talk to Jim McShane about the g2 B+ choke he installs, when refurbishing a H/K Cit 5. There's little room under that hood too.

Think about using the NFB setup in "El Cheapo". Instead of grounding the non-inverting grid of the LTP, via a cap., use that grid as the point NFB is applied. The CCS in the LTP's tail is essential to force symmetry upon both phases. So, equal value anode load resistors is correct.

deicide67 · 21st April 2012, 01:34 PM

Hi Eli,

Thanks for the diagram. I found another NFB example where the tap is at the VA stage on the cathode. Either look OK to me.

Looking at the original amp carcass, there is an unusual little cylinder labeled 500 coming off the B+ for the anodes and that feeds the G2 of the power tubes. It appears to be ceramic like a resistor, but mounts with a clamp like a through hole cap.

Is it a resistor?

Blair

rongon · 21st April 2012, 02:55 PM

Quote:

Originally Posted by deicide67

Hi Eli,
Looking at the original amp carcass, there is an unusual little cylinder labeled 500 coming off the B+ for the anodes and that feeds the G2 of the power tubes. It appears to be ceramic like a resistor, but mounts with a clamp like a through hole cap.

Is it a resistor?

Blair

Yes, it is very likely a 500 ohm power resistor for the G2's. If you measure it with a multimeter, do you get something like 500 ohms? I've seen resistors like that on old Scott integrated amps and the like.

nerdorama · 21st April 2012, 06:41 PM

rongon,

I have data for the RP-40a (original owner's manual with schematics), the RP-40 (SAM's), and the AP-60 (SAM's). If you give me an email I can send you the data you need. Just recently began to evaluate the AP-60 and found the OPT primaries were both bad. Oh well. Guess I'll start on the RP-40a's (I have a complete unit and one without OPT's but which is better cosmetically).

Best,
John

rongon · 21st April 2012, 06:57 PM

Thanks nerdorama, but I think deicide67 is the one who has the receiver.

Eli Duttman · 21st April 2012, 07:45 PM

Quote:

Thanks for the diagram. I found another NFB example where the tap is at the VA stage on the cathode. Either look OK to me.

Count the coupling caps. in the signal path, when you apply the NFB to the voltage gain stage's cathode. IMO, there are too many RC high pass poles present in the signal path, under those circumstances. It's all too easy to satisfy the Barkhausen criteria and end up with a phase shift oscillator, instead of an amp.

When the short loop to the non-inverting grid of the LTP is employed, it's nearly impossible to get phase shift oscillation.

The schematic provided shows an unbypassed cathode resistor, in the voltage gain setup. The local current NFB, AKA degeneration, inherent in that arrangement takes care of linearizing the stage.

nerdorama · 23rd April 2012, 03:58 AM

Quote:

Originally Posted by rongon

Thanks nerdorama, but I think deicide67 is the one who has the receiver.

10-4. I got an email and sent the manual.

John

deicide67 · 25th April 2012, 11:40 PM

How does this look? I will likely end up using 12AT7 PI and 6DJ8 VA though after digging in my garage

Thanks!

Blair

rongon · 26th April 2012, 02:49 PM

Quote:

Originally Posted by deicide67

How does this look? I will likely end up using 12AT7 PI and 6DJ8 VA though after digging in my garage

Wouldn't that be an awful lot of voltage gain for a pair of 6V6's? Or were you thinking 6L6's?

What about making the first stage a long-tailed-pair phase splitter (like the driver in Eli's El Cheapo) using a 6CG7 (or 12AT7 if you want higher gain), and then the second stage could be a push-pull driver using a tube with a low plate resistance (and low-ish mu)?

Something like the PP 6L6GC AB2 amp... ?

There are simpler versions too, without the MOSFET's, if you only need class A operation.

--

deicide67 · 26th April 2012, 03:30 PM

Hi Rongon,

I plan to use 6L6 power tubes.

I found a pair of 6CG7s to use for the PI, but I still plan to use the 6DJ8 if it will swing enough voltage. I have a pair of 6H30s I could use also, but I'd rather save them for a bigger amp that needs the current to drive it.

Do you think the schematic I provided is too much voltage gain for a 6L6?

Here is my progress so far. All I lack is the circuit. I'm sitting at 332vdc unloaded, so I am going to assume about a 35-40v drop after installing the tubes leaving me just shy of 300vdc.

Thanks for any assistance!!

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21st April 2012, 01:34 PM	#12
deicide67 diyAudio Member Join Date: Aug 2008	Hi Eli, Thanks for the diagram. I found another NFB example where the tap is at the VA stage on the cathode. Either look OK to me. Looking at the original amp carcass, there is an unusual little cylinder labeled 500 coming off the B+ for the anodes and that feeds the G2 of the power tubes. It appears to be ceramic like a resistor, but mounts with a clamp like a through hole cap. Is it a resistor? Blair

21st April 2012, 06:41 PM	#14
nerdorama diyAudio Member Join Date: Aug 2005	rongon, I have data for the RP-40a (original owner's manual with schematics), the RP-40 (SAM's), and the AP-60 (SAM's). If you give me an email I can send you the data you need. Just recently began to evaluate the AP-60 and found the OPT primaries were both bad. Oh well. Guess I'll start on the RP-40a's (I have a complete unit and one without OPT's but which is better cosmetically). Best, John

21st April 2012, 06:57 PM	#15
rongon diyAudio Member Join Date: Jul 2009 Location: Across the river from Rip's big old tree...	Thanks nerdorama, but I think deicide67 is the one who has the receiver.

25th April 2012, 11:40 PM	#18
deicide67 diyAudio Member Join Date: Aug 2008	How does this look? I will likely end up using 12AT7 PI and 6DJ8 VA though after digging in my garage Thanks! Blair

26th April 2012, 03:30 PM	#20
deicide67 diyAudio Member Join Date: Aug 2008	Hi Rongon, I plan to use 6L6 power tubes. I found a pair of 6CG7s to use for the PI, but I still plan to use the 6DJ8 if it will swing enough voltage. I have a pair of 6H30s I could use also, but I'd rather save them for a bigger amp that needs the current to drive it. Do you think the schematic I provided is too much voltage gain for a 6L6? Here is my progress so far. All I lack is the circuit. I'm sitting at 332vdc unloaded, so I am going to assume about a 35-40v drop after installing the tubes leaving me just shy of 300vdc. Thanks for any assistance!!