A100 power amp question

[indianajo]

One is supposed to check for power transformer leakage before grounding the organ. I didn't worry about it on my H100, but it has a lot of pvc wiring and not so much cotton. If grounding the chassis makes that much difference, you might really have a power transformer problem. Measure the AC voltage between the chassis and the power strip neutral, not touching the chassis.
Potentiometers can be effectively measured with a DVM. The hum balance pot, if you just pull the tube you should get a good reading. Incidently, the two resistors connecting to the hum balance pot are ones you would replace by 1 kohm potentiometers to make adjustable gain on the AO39. Short the wiper to one of the end terminals to make a variable resistor out of it and limit the resistance to 1 kohm if the wiper loses contact.

[jdoc]

for more gain try boosting the amp with a cheap pawn shop solid state, i picked up a Fender 15w bass for $30 that gives you eq and lots of gain to drive a tube amp, even those little $10 Nova amps are fairly clean, I have seen them for $5 or 10, as to the hum......not sure, may be something in a connection or component

[bossbandbob]

This amp is not in any organ. it is a spare amp i have been messing with hoping to power my Leslie 130 with.
"The hum balance pot, if you just pull the tube you should get a good reading." Which tube are you referring to?

I'll measure the AC voltage between the chassis and the power strip neutral tonight or tomorrow.

[bossbandbob]

for more gain try boosting the amp with a cheap pawn shop solid state, i picked up a Fender 15w bass for $30 that gives you eq and lots of gain to drive a tube amp, even those little $10 Nova amps are fairly clean, I have seen them for $5 or 10

Mayswell give that a shot. Our former guitar player left his 15W crate amp in my studio. Just seems to me that a 13 W tube amp should be louder than this but i guess it needs some sort of additional preamp, which is also strange as the A101's preamp or out from one of its speakers does not boost it enough on its own. Indy may be right about a power transformer problem but I do have a spare one.

[indianajo]

Just seems to me that a 13 W tube amp should be louder than this but i guess it needs some sort of additional preamp, which is also strange as the A101's preamp or out from one of its speakers does not boost it enough on its own.

No it doesn't need an additional preamp. It needs two $.20 resistors, or for a complete experiment, two $2 potentiometers to replace R5 and R6. If R7 is messed up, you could buy the two 1 k pots and leave out R7 entirely, with both R5 and R6 adjustable. Just connect the bottom of the 1k pots to the chassis point where the wiper of R7 goes. This would make your voltage gain higher. The AO39 is designed to be a current amplifier with unity voltage gain, since the AO28 driving it puts out 7 VAC into 1000 ohms and the power tubes put out 7 VAC into 4 ohms at full power. There is tons of voltage gain available in the 12AX7, but hammond inhibited it due to the AO28 being a standard part. My dynakit PAS2 runs the phono input 12AX7 at 50X gain. Just a matter of what resistors are used.
Geo detailed some kind of Hammond picked limit on the current from the power transformer leakage. I think you put a 1 k ohm resistor between chassis and power strip neutral and measure the AC voltage. I forget what the upper limit voltage (current) was.

[bossbandbob]

No it doesn't need an additional preamp. It needs two $.20 resistors, or for a complete experiment, two $2 potentiometers to replace R5 and R6. If R7 is messed up, you could buy the two 1 k pots and leave out R7 entirely, with both R5 and R6 adjustable. Just connect the bottom of the 1k pots to the chassis point where the wiper of R7 goes. This would make your voltage gain higher.

The R7 pot (gain) is fine, it's the R22 pot(hum) that is suspect .That one has R20 and 21 connected to it it. In an earlier post you suggested doing this to the hum balance pot. Which pot do you mean?

If it's the gain pot, then it looks like I'd just change the resistors.

If it's the hum pot, please clarify "connect the bottom of the 1k pots to the chassis point where the wiper of R7 goes." I presume that is the middle terminal of the pot? If so easy enough.
If we are talking R22 pot(hum), on each new 1K Pot I presume one outer terminal would go to a pin on V2 and V3- where would the other two outer terminals connect? A diagram would be helpful here.

Going to either pick up a coupla 1K pots at Ratshack or get some resistors somewhere. My local store has these in stock http://www.radioshack.com/product/in...uctId=2062300#

[bossbandbob]

Actually my "parts" amp has 2 47K 1/2 W resistors. I'll just pull those out and replace R5 & 6?

[indianajo]

Sorry, we had a lot of lightning last night, had to turn off the lighning sensitive appliances and play the piano a while.
I don't know where you got the idea of two 47 K resistors for R5 and R6. That would give strange results.
If you do buy a pair of 1k or 2 k pots, they go between pin 3 and 8 of the 12AX7 tube and the ground point the old 250 k balance pot wiper went to. Hammond picks ground points for minimum hum, use the same one.
I don't recommend R**** S*****, the only non-garbage parts I have gotten from them are vacuum tubes, which they don't sell anymore, and clip leads. Diyaudio lists Action Electronics a store front in Cherry Hill NJ if that helps. (2003 listing, so call first). If you have a little time, I find USPS priority shipping cheaper than driving, usually. About $6.90 for small shipments.
See the following schematic of a dynakit ST35 amp which has about 50x gain. The 7247 is a later tube pin compatible with the 12AX7 with with higher plate current rating. That allows them to run the second half of the tube as a unity gain phase splitter with 27k on the plate and 33 k on the cathode. The 12AX7 would burn out with that low a plate resistor. But look at the 1.3 k resistor cathode to ground of the gain stage. That is what I am trying to get you to do. The 150 K from pin 3 to pin 8 is a linearizing negative feedback link, which I don't think you want for a guitar amp. I think you want the second harmonic distortion from too much gain, you don't want to damp it out with negative feedback.
It annoys me how much organforum shrank this schematic, the original is on http://www.diyaudioblog.com/2011/06/...-st35-kit.html it is a public domain schematic that was produced in 1964.
Note you don't want the 33 nf 470 k input filter, either, that is an RIAA phonograph rolloff filter.
If you're driving somewhere or making an order , I would also replace C2 and C3 with 0.47 uf 600 v capacitors instead of the 0.047 uf installed in the AO39. You could get away with 200 V ratings, but they tend to stock 600 v orange drops for guitar amp people. If they don't have those, 0.1 or 0.22 would be better than 0.047.
If you want to go whole hog and build one channel of the ST35 out of your AO39, I think triodeelectronics.com has the 7247. I think the power transformer would handle it. The 6BQ5 part of the circuit is about the same.

[bossbandbob]

To increase the gain of your experimental AO39 amp, change C2 and C3 to 0.47 uf 600v caps instead of 0.047 uf caps. Also Increase the value of the cathode resistors R5 and R6 to 1000 ohms or 4700 ohms. For permanent installation use resistors 2 cm long, (2 watt these days) but you can experiment with 1/2 watt resistors. It is the dust on them that makes the short ones arc over. Try to match the values, the 250 ohm balance pot is not going to be much good with these behind it. This should make it more suitable for lower level signals, not the 8v PP the AO28 GG terminals put out. All the other gain stages in the H100 schematic have bigger cathode resistors than this AO39. I can't use the dynakit PAS2 as a pattern, it has fixed cathode bias, with a transformer winding.

"I don't know where you got the idea of two 47 K resistors for R5 and R6. That would give strange results."

Uh, apparently you gave me the idea. Read the post of yours above. Actually, I meant 4700 ohm. I pulled off two 1000 ohm resistors off the parts amp anyways this AM that I'm going to replace R5 and R6 with. The 4700 ohms were too short. I replaced the hum balance pot with the one from the parts amp and I have 2 new 0.47 uf 600v caps that I never used from another project.

"If you do buy a pair of 1k or 2 k pots, they go between pin 3 and 8 of the 12AX7 tube and the ground point the old 250 k balance pot wiper went to. Hammond picks ground points for minimum hum, use the same one.
"I don't recommend R**** S*****, the only non-garbage parts I have gotten from them are vacuum tubes, which they don't sell anymore, and clip leads. Diyaudio lists Action Electronics a store front in Cherry Hill NJ if that helps."

Cherry Hill ,NJ is about 300 miles from my house in MA..not really worth the drive Looks like I'll skip the pots for now anyway.

[indianajo]

Sorry, I was thinking of someone in New Jersey when I looked that up. There is supposed to be a really cool walk in electronics supply in the ring around Boston somewhere, Good something, but I don't want to look that up. Won't be open at 1 PM on Saturday, anyway.
1 kohm cathode resistors should make a nice medium gain experiment. The price is just right- free.