• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Conn Caprice organ

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#21
I think I stated before, but just in case I didn't they were 8 ohms each, and one was wired to each channel to the green tap on the OPT's.


Another weird thing! I retested my voltages with the amp running, and came up with 294v on the plates and 274 on the grids.

Quite a bit lower than my no-load voltage of 410v! The caps are all rated for 400v, so I would assume power supply voltage would not go above 350v.


294 is close enough to 300, and with the plate and screens @ 300v the datasheet says max power of about 24 watts @ 5%thd.

If I brought up my screen voltage to 300 ( and eliminated that HOT 10 watt resistor that is heat sinked to the chassis) and dropped my cathode resistors to about 170-180 ohms, would I be more in the 24watt range?

so far its like this :

Plates: 294v no signal
Grids: 274v no signal
Cathode resistor: 200R
Bias voltage (taken across cathode resistor) -11.0v
When clipping, the power supply drops about 10v or so.

I am starting to think the culprit is bad psu regulation. I have to figure out this power supply,its the weirdest setup I have seen. It appears to have a full wave voltage doubler in the power supply, with 100uf caps. I realize a voltage doubler could be ok for class A, but more current demanding applications will drain the caps too quickly.
 
#22
Voltage doublers work just fine in class AB tube amplifiers, often providing as good or better regulation than the tube based full wave rectifiers they replaced. Most likely the capacitors in the power supply circuitry are old and tired and need to be replaced. Am I correct in assuming you haven't replaced them? As an analogy this would be like taking an old corvette with 300K miles on the original engine and expecting it to accelerate to 60mph in 5 seconds - not gonna happen..😀

I bet with fresh caps the supply voltage will be considerably higher and stiffer. (Probably above 350V) What is the secondary AC voltage to the doubler? For example 140Vrms open circuit should result in something greater than 360V under load depending on winding resistance, cap esr and rectifier losses etc....
 
#23
I just rechecked my voltages..... something is going on, the plate voltage is now 385v! B+ is 390v, and I replaced the 2.25k 10w with a 390 ohm to bring the grid voltage closer to B+, but apparently forgot that I wasn't measuring plate voltage AT the plate. At the plate it is 385v, so I will have to replace the grid resistor again. It did have more power output with the 390 ohm...
 
#24
If it was my amp, I would stick with original design and component values until I had heard what it sounds like when working properly. There is clearly some sort of fault with yours since the voltage readings keep changing. I would check for dried out power supply capacitors as previously suggested and also look out for leaky coupling capacitors to the output tube grids which could cause too much current to be drawn. ( are any of the output tube plates starting to glow?)
At this age, pretty much every capacitor, power resistor, rectifier and tube is suspect.

Good luck
 
#25
The plates are not glowing, the tubes all tested at 97, and I'm sure the power supply caps are probably leaky, although there is little to no hum on the output. I have not checked the coupling caps... They might be a suspect of DC getting into the output stage and causing it to clip too early. I am in the process of tearing it down right now. By the time I remove all the un-needed sockets and things, it leaves 5 holes in the chassis :xeye: . I guess I could buy some chassis plugs and clean it up and paint it, or I could transfer all the parts over to a new hammond chassis.

On an interesting sidenote, I found out the McIntosh MC40 uses a full wave doubler in the power supply! I guess they can be used without problems in HiFi amps 😉

Edit: I have not replaced the diodes yet, They might also be leaky. Their old metal can diodes, so this is an earlier amp because newer versions use regular looking diodes. Suitable replacement , 1n4007?
 
#28
i know i am digging up an old thread here but i have a question for you all. i recently acquired a power transformer and 2 output transformers from one of these old conn minuet 431 organs and i am having a terrible time getting any data on them.

first off the 2 output transformers as near as i can tell they are push pull type with 4 and 8 ohm output 7.2K @ 8 ohms (31:1) or 4 (42:1).

next is the k70749 power transformer which i have found absolutley nothing on in regards to primary, secondary, 6.3, or 5v on. is ther eanyone who first off could tell me anything about it? more specifically i would like to know which color wire is what? i am wanting to build a tube guitar amp with the transformers and would like to know what wattage amp i could make, and if there is a 5v on it for a tube rectifier.

the wire colors are as follows:

black x2
green x2
green/yellow stripe ( or yellow/green stripe depending on how you are looking at it)
orange? x2
red
red/green stripe ( or green red stripe)
blue
yellow
yellow/red stripes ( red yellow stripes)

basically i was hoping to make somethign like a british 18 watt tube amp for guitar out of it at least, or maybe a fender clone of some sort since the output transformers did pop up a couple times as being used in some vintage fender amps.

anyway ANY help would be greatly appreciated at this point because if i cant make it work i will need to add it to the pile of transformers i need to sell in order to get some that will work for what i want to do LOL.
 
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